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Voltammetric study of humic and fulvic substances
J. Electroanal. Chem., 101 (1979)211--229 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
211
VOLTAMMET R I C STUDY OF HUMIC AND FU L V IC SUBSTANCES P A R T I. STUDY OF T H E F A C T O R S I N F L U E N C I N G T H E M E A S U R E M E N T OF T H E I R COMPLEXING P R O P E R T I E S WITH LEAD
F.-L. GRETER, J. BUFFLE * and W. HAERDI Department of Inor~,anic and Analytical Chemistry, University of Geneva, Sciences II, 30 quai Ernest Ansermet, CH-1211 Geneva 4 (Switserland) (Received 14th August 1978; in revised form 17th November 1978) ABSTRACT A detailed, qualitative, investigation of the behaviour of lead complexes with fulvic substances has been carried out using several polarographic techniques, e.g.d.c, polarography, a.c. polarography, normal and differential pulse polarography and cyclic voltammetry. The mercury drop electrode was used for this study. It is shown that under the conditions used (pH - 6) the charge transfer process is reversible, the complexes are labile and the adsorption of fulvic acids and fulvic complexes occurs on the electrode. The implications of these results, with regard to the use of the various amperometric titrations for the measurement of the complexing capacity of natural waters, are discussed. INTRODUCTION In r ecen t years several authors have tried to measure the com pl exi ng capacity o f natural waters for a particular metal by means of different kinds o f amperometric titrations [1--6]. However, it has been r e p o r t e d [7--10] t h a t this t y p e o f measurements may sometimes lead t o large errors particularly when t h e w at er contains some c o m p o u n d s which are adsorbable on the electrode. This is specially true for waters containing humic and fulvic substances (which will be globelly referred to as HA and FA hereafter). The results published in this field are o f ten difficult to compare, mainly because: (a) the interpretations of the amperornetric titrations are based on di fferent assumptions according t o t he nature o f the reaction mechanism (labile or inert complexes, reversible or irreversible electrode process) a n d / o r (b) the humic and fulvic acids are n o t sufficiently well-defined. However, since the voltarnmetric techniques are amongst t he few m e t h o d s which allow the measurements of the com pl exi ng properties o f waters w i t h o u t modifying t o o m u c h their com pos i t i on [8,9,11], it is very i m p o r t a n t t o k n o w h o w to use t h e m under best conditions. In the present paper, an a t t e m p t has been made t o define the nat ure o f t h e m o s t i m p o r t a n t properties of humic and fulvic substances and their complexes o f lead at the m e r c u r y electrode, t o assess their influence on the measured polarographic parameters (current and potential), and t o discuss t he validity o f * To whom correspondence should be addressed.
212 the polarographic techniques for measuring the complexing capacity of natural waters. It should be n o t e d that the organic c o m p o u n d s used for this study were "dissolved" humic and fulvic substances of fresh water (Section I). The same sample of fresh water was used for most of this work. Anyway, it must be n o t e d that other w o r k s [12,13] seem to show that for any natural unpolluted fresh water, the most important properties of FA and HA such as fluorescence spectra, u.v. absorption, adsorption on mercury electrode, complexation properties, or mean molecular weight, are very similar. Hence, the results reported here should be also valid for any other organic c o m p o u n d s of the same kind, i.e. those of more or less eutrophicated fresh waters located in a non-peaty region. (I) APPARATUS AND REAGENTS
(I. 1) Apparatus Polarographic measurements were performed with PAR 170 and Tacussel PRT 30 and UAP 4. Metrohm E 500 pH meters were used for pH measurements. The polarographic working electrodes used were: (a) classical dropping mercury electrode (typical parameters were: m = 1.35 mg s-l; drop time = 2 s), (b) Kemula t y p e hanging mercury drop Metrohm E 503 (area of the drop ~ 2 mm2), (c) stationary or rotating electrode with a deposit of mercury film on Pt disk (diameter of the disk = 3 mm). The deposition was done as described in ref. 14. Unless otherwise stated, Ag/AgCI/sat.KC1//NaNO3 0.1 M/[ was used as the reference electrode (Metrohm EA 420). M e t r o h m No. EA 876-20 polarographic cell was used for all polarographic measurements.
(I. 2) Reagen ts Unless otherwise stated all measurements were carried o u t in 0.1 M NaNO3. The solutions were deoxygenated with 99.995% pure N2 delivered b y Bertholet SA. All reagents were pro analysis Merck products. The location and composition of the waters containing the humic and fulvic substances and their properties are described in refs. 12 and 13. These works showed that 60--70% of the organic matter of these waters may be considered as fulvic acids (molecular weight < 1 0 0 0 0 ) . However, hereafter no distinction will be made between humic and fulvic, and the global concentration of FA and HA will be denoted by either [HA] or [L]. These waters were concentrated ~o a b o u t 40 times by freezing concentration and stored at 4 ° C in the dark. It was shown [ 12] that, during storage, this water initially loses 10--20% of their fulvic and humic substances and then remains stable for more than one year. The concentrated samples were filtered on 0.2 # m filters, decarbonated by acidifying with HC104 to pH ~ 4, diluted to the desired concentration with 0.1
213
M NaNO3 and reneutralized to the required pH. Unless otherwise stated, the pH was adjusted to 6.0. Most of this w o r k was done with sample No. 50 [12]. In some of the experiments, sample l a was used. (II) P R E L I M I N A R Y
STUDIES
(II. 1) Shape o f the polarographic curves The shapes of the reduction waves of Pb(II) in the presence of excess of fulvic acids, by d.c., normal pulse and differential pulse polarography are shown in Figs. 1--3. It can be seen in this Figures that the polarograms for the reduction of lead complexes have a classical profile. However, t w o points must be noted: The addition of HA causes an increase in the capacitive current represented by sc = di/dE for E >> E1/2 (Figs. 2 and 4). This effect, studied in detail in ref. 15, is large compared to the low concentration of HA (see particularly Fig. 2) and therefore, complicates greatly the measurement of the faradaic current by d.c. or normal pulse polarography. Hence the titrations were done using differential pulse polarography. In some cases, for pH > 7, the reduction wave splits into two waves whose nature is n o t y e t well understood (for discussion see ref. 16). For pH ~< 6.5, only one wave with classical shape was observed. However, this wave is n o t perfectly symmetrical (see particularly Fig. 3) and is more spread out than the reduction wave of the hydrated Pb 2*. This may be due to the same effect, b u t to a lesser extent as that which causes the splitting of the wave. Because of this effect and the possibility of hydrolysis of Pb(II) for pH > 7, all the experiments were done at pH ~< 6.5.
current
///
I .
~
E/mY -30o
-3so
-4oo
-4 o
Fig. 1. Shapeof the d.c. pola~og~amsobtainedin the presence and in the absenceof fulvic a c i d s . T = 2 5 ° C ; d r o p t i m e = 5 . 3 s; s c a n r a t e = 2 m V s -1. ( . . . . . . ) [Pb] t = 2 x 10 -s M; [ H A l t = 0, p H = 4 . 5 0 . ( ) [ H A l t = 0 . 0 7 0 g 1-1, p H = 6 . 0 0 [ P b ] t = 5 X 1 0 - s M.
214
current
j
J
III ~ / /I//~/F / / ~
E/mV
- 350 -~00 -~s0 Fig. 2. Shape o f the normal pulse p o l a r o ~ a m s o b t a i n e d in the presence and in the absence o f fulvie acids. [ P b ] t = 1 x 10 .6 M; p i t = 6 . 0 0 ; drop time = 2 s; l e n g t h o f the pulse = 65 ms. ( . . . . . . ) [HAlt ; 0; ( ) [HA]t = 9 . 0 6 mg 1-1.
(II.2) Titration experiments An example of the titration o f HA with Pb 2+ is given in Fig. 5a, where the peak current o f differential pulse polarograms, ip, is recorded versus the total concentration o f lead, [ P b ] t :
current / /
[
I
5nA
'
\ \
,\
/
\
/
\
E/mY I
-3oo Fig. 3.
-3;0
-~;o
- 450 '
- 500 ~
ShaDe of the differential pulse po]aro~rams obtained in the presence and in the absence
o f fulvic acids. [ P b ] t = 5 X 10 -6 M; p H = 5 . 5 ; drop time ffi 1 . 0 0 s; scan rate = 2 m V s -1 ; h e i g h t o f the pulse = 10 m Y . ( . . . . . . ) [ H A ] t = 0; ( ) [ H A ] t = 0 . 0 6 g 1-1.
215 sc(H A / , O ) / s c ( H A = O)
(HA) T/g 0
* 0
*
* 0.05
~
' *
'
'
1-1
J 0.10
Fig. 4. Change in the capacitive current measured by normal pulse polarography, with increasing concentrations of HA. Experimental conditions: see Fig. 9 in ref. 9.
If the Pb-HA complexes were inert, as it is sometimes assumed, then the observed current at the beginning of the plot should have been zero and, towards the end, it should increase linearly, yielding a slope equal to that obtained in the absence of HA [3,6]. Moreover the peak potential should remain unchanged during the titration. Figure 5a shows a completely different behaviour. There is no range of [Pb] t for which ip = 0 and there is no break point in the titration curve. Moreover the peak potential of lead, E~ b, varies continuously during the Pb(II) titration (Fig. 5b). It is interesting to note that the same behaviour was observed for the titration of HA with Cu 2÷ and of salicylic acid with Fe(III). These results seem to indicate that the c o m p l e x e s formed are labile. In this case, by assuming a reversible reduction process, an estimation of the degree of complexation ~ = [ P b ] t / [ P b 2÷] may be made, by taking into account the change in ip and Ep (eqn. 4.33, ref. 17), at the beginning of the titration, when there is an excess of HA compared to [Pb]t. However, the values obtained in this way are a b o u t 4 orders of magnitude greater than those obtained by ion-selective electrode measurements [ 18]. This result is caused by a shift of Ep which is t o o high to be explained by eqn. (4.33) of ref. 17 (see Section III.3). Ernst et al. [19] tried to take this fact into account b y assuming that the reduction of complexes was irreversible. However, we obtained the same kind of titration curves, and the same order of magnitude for a, by using a.c. polarography (frequency 80 Hz; amplitude 10 mV; phase angle 0 ° ) instead of differential pulse. This result seems to indicate t h a t the charge transfer is reversible. Finally, we tried to carry o u t the titrations of AH with Pb 2÷, Cu 2÷ and T1÷ b y
216 ip/,uA
(o)
/ ,
./ lOb(Pb)T/mol [-I 5
10
*
15
Ep/mV
t
20
(b)
-400
..........
t ....... _
_
-
~
.... 105(Pb)T/mot I-I
-200 5
, 10
, 15
20
Fig. 5. (a) Change in the peak current obtained by differential pulse polarography, during the titration of HA with Pb 2+. T = 25°C; pH = 7.00; scan rate = 2 mV s -1 ; drop time = 1 s; length of the pulse = 50 ms; height of the pulse = 25 mV; sampling duration = the last 10 ms of the pulse duration. [HAlt = (1) 0; (2) 0.036 g 1-1 . (b) Change in the peak potential obtained by differential pulse polarography, during the titration of HA with Pb 2+. Experimental conditions: see (a). Potential of the ref. electrode = +100 mV vs. NHE.
differential pulse p o l a r o g r a p h y ( c a t h o d i c stripping), o n H M D E a n d m e r c u r y film d e p o s i t e d o n Pt disk. In all these cases, the p o i n t s o f the curves ip = f ( [ M e l t ) ( [ M e l t = t o t a l c o n c e n t r a t i o n o f m e t a l ) were relatively scattered and t h e r e p r o d u c i b i l i t y was low. F o r Cu 2+ a n d Pb 2÷, the shift in Ep was always m u c h t o o high c o m p a r e d with t h a t e x p e c t e d f o r labile c o m p l e x f o r m a t i o n with reversible r e d u c t i o n . These results p o i n t o u t t h a t , b e f o r e using t h e a m p e r o m e t r i c t i t r a t i o n techniques f o r the m e a s u r e m e n t o f t h e c o m p l e x i n g c a p a c i t y o f waters, m o r e i n f o r m a tion is r e q u i r e d a b o u t (a) the degree o f reversibility o f t h e e l e c t r o d e process,
217
(b) the degree of lability of the complexes, and (c) how correct values of a and of the complexing capacity may be obtained from polarographic data. In the following part of the paper we have endeavored to define the important steps of the electrode process and their influence on the voltammetric parameters. (III) NATURE OF T H E FACTORS INFLUENCING THE ELECTRODE PROCESS
(111.1) Role o f diffusion in the mass transfer process For a diffusion controlled reduction wave, at any potential, the following relationship will be obeyed [20]: log(if) = const. + ½ log(h) where h is the height of the mercury column. Figure 6 shows that the preceding equation holds for potentials situated close to the top of the wave and for sufficiently large values of h. Indeed, in the latter conditions, the slope of the curve at E = --0.420 V, which corresponds to the limiting current, is 0.43 which agrees reasonably with the theoretical one. For increasingly positive potentials and decreasing h the curve tends to be more and more rounded and the slope of the tangent at the curvature increases for low values of h. This effect shows that the polarographic wave is influenced log ( i f / n A )
2,0 o ~ ; ~ " ~ ~' _..-~;-". ~
6
1.5
1.0 i
/ x
0.5'
Iog(h/crn) 0.0
~ 1.4
~ 1.6
~ 1.8
~ 2.0
Fig. 6. Influence of the height of the mercury column, h, o n t h e faradaic c u r r e n t if measured by d.c. polaro~aphy. T = 25°C; pH = 6.0; [Pb]t = 2 × 10 -s M; [HAl t = 0.070 g 1-1 ; scan rate = 1 mV s- . E/mV: (1) --350; (2) --360; (3) --370; (4) --380; (5) --390; (6) --400; (7) --410; (8) --420.
218 by a secondary p h e n o m e n o n which will be discussed in Section III.3 and in Part II (ref. 16, Section II.4: this issue, pp.231--251). Because of the low concentration of Pb(II) that one has to use for the ligand to be present in excess, the capacitive c o m p o n e n t of the current measured on one falling drop is not completely negligible. Then if ic and if are the capacitive and faradaic currents respectively, the measured current, i, is given by: i = if + i¢
(1)
In the absence of HA, at constant potential, ic = k c t -1/3 and for a reversible, diffusion controlled electrode process, if = kft 1/6 [20]. However, in presence of HA, it was shown [15], that for t < 3 s, i¢ ~- k~t -~/3 + ko t÷2/3. Moreover, by taking into account the secondary p h e n o m e n o n mentioned above, it may be shown [ref. 16, paragraph IV.3.1) that: if = k~ t 1/6 __ k~ [Pb 2÷] ot ( p - r - 1/3) where k f , k'~, k~, k o , p a n d r are constants and [Pb2+]0 is the concentration of Pb at the electrode surface. For potentials corresponding to the limiting current [Pb2÷]0 = 0. Hence eqn. (1) becomes: it 1/3 = kc + k o t + k f t 1/2
(2)
Since the term k e t becomes important only at the end of the drop time, a plot of it 1/3 versus t ~/2 allows us to obtain k¢ by extrapolation to t -* 0. Then, a plot of (it -2/3 ~ k c t -1) as a function of t -1/2 should give a straight line: i t -2/3 - - k c t - I = k e + k f t -1/2
(3)
Figure 7 shows that this is indeed the case. The value of 37.3 nA s -1/6 was obtained for k~ at a potential corresponding to the limiting current, which allowed us to c o m p u t e [20], for the diffusion coefficient of the reducing species, the value of 4.7 X 10 -6 cm 2 s -~. This result shows that the diffusion coefficient of the c o m p l e x is significantly lower than that of free Pb 2÷. However, because of the various corrections made for its derivation, this value must only be taken as an order of magnitude. Finally, the polarograms obtained by normal pulse polarography have a classical shape (Fig. 2). N o w this technique is a kind of very fast relaxation technique. Hence, the similar results obtained b y b o t h this m e t h o d and d.c. polarography indicate that any coupled chemical reaction and particularly complex formation can be considered as labile from the point of view of overall redox process [21]. T h i s is confirmed by Fig. 9 in ref. 9 which shows that the decrease in the reduction current of Pb(II) observed in the presence of HA is independant of its concentration. If the complex was n o t labile~ this decrease would have been more and more p r o n o u n c e d with increasing [HA]t. However, because of the secondary phenomenon, mentioned above (see also Section III.3 and ref. 16, paragraph II.4), it is n o t possible to c o m p u t e the diffusion coefficient of the complex directly from the limiting current obtained in normal pulse polarography. All the preceding results show that the limiting current of the reduction wave of the complex is diffusion controlled and is n o t influenced by the rate o f the preceding chemical reactions. However, they also show that, on the rising portion of the wave, the mass transfer process is n o t due only to diffusion.
219 (i t "m- k¢ t -I ) / n A s"m 30 ¸
./
./ /
x
cot
10
t~
~
a
2 \t
1
~..._.__.. . . . . . . . . .
.-...,.....
.....~~° "~ ~, o 0
-10 i
i
i
0.4
0.6
0.8
E/v
i -&s
- so
•
1~0
Ep (0 °)
Fig. 7. L i n e a r i s a t i o n o f t h e i--t c u r v e s o b t a i n e d b y d.c. p o l a r o g r a p h y , a c c o r d i n g e q n . (3). ]~xp e r i m e n t a l c o n d i t i o n s : (it) - - 5 0 0 m V ; (X) - - 4 7 5 m V ; (A) - - 4 5 0 m V ; (~J) - - 4 2 5 i n V . D r o p t i m e tg = 5.5 s; f l o w r a t e o f m e r c u r y = 1 . 3 5 m g s -1 ; T = 2 5 ° C ; p H = 6; [ H A ] t = 1 0 0 m g 1-1 ; [ P b ] t = 10 - s M. Fig. 8. V a l u e s o f c o t ¢ as a f u n c t i o n o f E a l o n g a n a.c. p o l a r o g r a p h i c c u r v e . T = 2 5 ° C ; p H -7 . 0 0 ; [ P b ] t = 5 X 10 - s M , [ H A ] t = 0 , 0 3 6 g 1-1 ; d r o p t i m e = 1.0 s; ~ = 1 8 0 H z ; s c a n r a t e = 2 m V s -1 ; a m p l i t u d e = 10 i n V . R e f . e l e c t r o d e : A g / A g C I / 0 . 1 M K C I / / 0 . 1 M N A N 0 3 / / .
(III. 2) Nature of the charge transfer process The behaviour of the a.c. polarographic current for the reduction of Pb(II)-HA complexes, is shown in Figs. 8--10. Cot ¢, given in Figs. 8 and 10, is c o m p u t e d from the ratio of the currents at ¢ = 0 ° and ¢ = 90 ° , respectively, where ¢ is the phase angle between current and potential. Figure 8 shows that for w = 180 Hz, cot ~ is close to 1.0 at any potential, which is expected for a reversible and fast charge transfer process [22]. This behaviour was observed for all frequencies < 2 0 0 Hz as is shown in Fig. 10, where (cot ¢)p is the value of cot ¢ at the peak potential. The same result is obtained when the peak current, recorded at ¢ = 0 ° is plotted as a function of ~ (Fig. 9). The linearity of the curve, for w ~< 400 Hz, is an indication for the reversibility of the charge transfer process [22] and the fact that the curves obtained in the presence and in the absence of HA are linear up to approximately the same frequency (about 350 Hz) is an indication that the lack of reversibility for higher frequencies is due to the lead system itself and n o t to the presence of HA. As in pulse polarography, it is n o t possible to c o m p u t e the diffusion coefficient of the complex by comparing the peak current of the a.c. polarographic curves of the complex and of the free metal. For a sim-
220
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20
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Fig. 9. I n f l u e n c e o f f r e q u e n c y o n t h e p e a k c u r r e n t o b t a i n e d b y a.c. p o l a r o g r a p b y . T = 2 5 ° C ; e x p e r i m e n t a l c o n d i t i o n s : see Fig. 8. C o t ~b = 0; [ H A ] t = ( 1 ) 0; ( 2 ) 0 . 0 3 6 g 1-1 .
ple reversible case, the ratio of the currents is equal to the square r o o t of the diffusion coefficient of the two particles. However, in the present case, it will be seen (ref. 16, paragraph IV.I) that this relationship is strictly valid only at a potential corresponding to the limiting current in d.c. polarography (where [Pb2*]0 ~-- 0), which is not the case for the peak currents in a.c. polarography. The reversibility of the Pb-HA system is also supported by differential pulse polarography, another fast relaxation technique. For reversible systems by applying periodically, on a linear function E = f(t), square pulses with constant amplitude, AE, a symmetrical peak (1), i = f(E), is obtained. If the amplitude of the pulses is --AE, an identical peak (2) is obtained, but shifted by --AE when compared to peak (1) [23]. Figure 11 shows the ratio of the current of peak (1) at potential E to the current of peak (2) at E -- AE. The fact that this ratio is 1, regardless of the potential, is an additional indication for the overall reversibility of the electrode process.
221
(cot ~)p
10
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..// 1
...,-"':"
o
10
J~/H~ ~ 20
30
Fig. 10. I n f l u e n c e o f t h e f r e q u e n c y o n t h e value of c o t ~ aS t h e p e a k , p o t e n t i a l o f t h e a.c. p o l a r o g r a p h i c curves. E x p e r i m e n t a l c o n d i t i o n s : see Fig. 8.
(III.3) Nature of the secondary phenomenon mentioned in Section III.1 As can be seen in Fig. 12, in d.c. polarography, the half-wave potential, Eln, shifts towards more negative values when [HA]t increases which is expected for the reversible reduction of labile complexes [17,20]. Figure 9 in ref. 9 shows that a similar shift in the half-wave potential, Eu2, np is observed for normal pulse polarography. However, as in the case of titration experiments (Section II.2) this shift is much too large compared with that observed with i.s.e. Moreover
i1 I i2
E/mY o
'
Fig. 11. R a t i o o f t h e c u r r e n t s o f t w o d i f f e r e n t i a l pulse p o l a r o g r a p h i c waves ( a t t h e p o t e n t i a l E ( i l ) a n d E - - A E (i2)), for w h i c h t h e a m p l i t u d e o f pulses A E = - - 1 0 m V a n d - - A E are used, respectively. T = 2 5 ° C ; p H = 6 . 0 0 ; [ P b ] t = 5 × 10 -6 M ; [ H A ] t = 0.1 g 1-z ; scan r a t e = 1 m V s - l ; length o f t h e pulse = 50 m s ; s a m p l i n g t i m e = f r o m 75 t o 9 5 % o f t h e pulse d u r a t i o n ; d r o p time = 2 s. Ref. e l e c t r o d e : A g / A g C l / 0 . 1 M KC1//0.1 M N a N O 3 / / .
222
AEll 2 /mY 100
j/
~0 60 40
20
./
log [HAlt/g 1-1 ) 0
-2.0
l
l
-1.5
-1.0
i
-0.5
Fig. 12. Influence of [ H A ] t o n the change in El~ 2 m e a s u r e d by d.c. polarography. T = 25°C; p H = 6.00; [Pb]t = 2 × 10 -s M ; scan rate = 1 m V s -I ; drop time = 5.8 s. 2~kE1/2 = E 1 / 2 ( [ H A ] t = 0 ) - - E 1 / 2 ( [ H A ] t :/= e).
this Figure also shows that EI"~p strongly depends on Pb(II) concentration, even when [HA]t is in excess. Finally, other results [16] obtained with DME also show that the potential and the width of the peak (or wave) are influenced by time-dependent parameters such as the height of the mercury column, h, in d.c. polarography, the drop time in pulse polarography, or the frequency, co, in a.c. polarography. This effect is still much more pronounced by using HMDE. The shape of the curves i = f(E) obtained by cyclic voltammetry on HMDE, for uncomplexed Pb 2÷ and Pb(II)-HA complexes, are shown in Fig. 8, in ref. 9. This Figure shows that the curves obtained in the presence of HA are strongly affected by the time, ta, elapsed at the initial potential Ei, before the beginning of the scanning. The longer is this duration, the larger are the anodic and cathodic peak currents (i~ and i~, respectively) and the cathodic peak potential E~ is shifted towards more negative values. Because of experimental difficulties, t~ = 0, is not known with great accuracy, and since, at low values of ta, ~p/Ipand Ep Ep vary sharply, it is quite difficult to extrapolate accurately these two parameters at ta -~ 0. However, from Fig. 13, it can be seen that the extrapolated values are close to 1.0 and 30 m V , respectively, which are the theoretical values expected for the reversible reduction of a labile complex (E~ = potential of the anodic peak). Figure 8 in ref. 9 also shows that the cathodic and anodic peaks of the Pb-HA complex are much more rounded and lower than those obtained in the absence of HA, for the same concentration of Pb(II). This confirms that the diffusion coefficient of Pb-HA is lower than that of Pb ~÷, which causes a decrease in the reduction current of Pb(II) when complexed by HA as compared with that of free Pb 2÷. "C
*R
&
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C
(III. 4) Influence of pH Figure 14 shows the influence of pH on some parameters of the cyclic voltammetric curves obtained in the presence of FA.
223
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ic
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tads/min 0
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1
2
3
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4
5
6
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......... • --
90
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60
30
tods/rain 0
, 0
1
' 2
3
4
5
6
Fig.
13. I n f l u e n c e o f the time, ta, elapsed at the initial potential before scanning, in cyclic v o l t a m m e t r y . T = 25°C; p H = 8.0; [ P b ] t = 2 x 10 - s M; [ H A l t : (o) s a m p l e 50, 34 m g 1-1 ; surface of the drop = 2 m m 2 ; scan rate = 100 m V s -1 . (a) I n f l u e n c e o f tad s on the ratio o f the cathodic over a n o d i c p e a k currents. (b) I n f l u e n c e o f tad s on the difference b e t w e e n the a n o d i c and t h e c a t h o d i c p e a k potentials.
According to the results obtained in Section III.3, the system seems to behave as the reversible reduction of labile complexes when ta ~ 0. Hence, in this Section, the electrochemical parameters were either extrapolated to ta = 0, or measured by using the minimum value of t~. The ratio of the cathodic peak current in the presence and in the absence of FA (Fig. 14b) confirms the interpretation of Section III.3: for low pH (pH < 4), where the mean number of ligands per metal ion, ~, tends to zero, the ratio tends to 1, while for higher pH (pH > 6), for which ~ tends to its maximum
224
6C
o
2
o. o
-o
•
c
•
30 . . . . . . . . . . . . . . . . . . .
I
-
o
=. . . . . . . . . . . . . . . . . . . . pH
4
5
7
6
ip (HA ~-0)/ip (HA = 0)
8
9
(b)
~°~O,~o
03
pH o.0 4
5
6
7
8
Fig. 14. I n f l u e n c e o f p H o n t w o parameters o f the c y c l i c v o l t a m m e t r i c curves. T = 25°C; scan rate = 100 m V s -1 ; [ P b ] t = 2 × 10 -s M; surface o f the drop = 2 m m 2 ; [ H A ] t : (1) 0; (2) (o) sample 50, 34 m g 1-1, (o) s a m p l e l a , 18 m g 1-1. (a) I n f l u e n c e o f p H o n the d i f f e r e n c e b e t w e e n the anodic and the c a t h o d i c peak potentials, t a = 5 s. (b) I n f l u e n c e o f pH o n the ratio o f the c a t h o d i c peak current in the presence and in the a b s e n c e o f HA. Currents extrap o l a t e d to t a ~ 0.
value [ 1 6 ] , the ratio tends to a value lower than 1. From Fig. 14b this value is found to be 0.38. Since in this experiment, the peak currents were extrapolated to ta -* 0 (see also Section IV) and since, in this technique, the peak potential is not very far from the potential corresponding to the top of the d.c. polarographic wave (where [Pb2+]0 -~ 0 and (dN/dt)a -~ 0; see eqn. 26 in ref. 16), the ratio of the peak current in the presence and in the absence of FA may be considered as equal to the square root of the ratio of the diffusion coefficients of the complex and the free Pb 2+. From the value of this ratio at pH 6.0, the value of De = 1.2 × 10 -6 cm 2 s-' was obtained. A discussion about the validity of this value is given in Section IV. Figure 14a shows the influence of pH on the values of E~ - - E ~ , measured at ta = 5 s. It is seen that E~ -- E~ is independent of pH and relatively close to the theoretical value (30 mV) expected for a reversible redox system coupled with a labile chemical reaction, particularly considering that t~, although small, is not zero. Finally the same type of curves as those shown in Fig. 13a were obtained for any pH, in the pH range 3--7, so that, for t~ -* 0, the ratio of the cathodic
225
to the anodic peak current is close to 1.0 and independent of pH. Hence, in the pH range studied, Fig. 14 indicates that pH has no effect on the lability of the complexes and the rate and reversibility of the redox system.
(III. 5) Inferences from these results Because Pb(II) is n o t reducible at Ei (cyclic voltammetry, Section III.3) and because it was ensured that HA is n o t reduced nor oxidized at this potential, the most probable effect of waiting at Ei is the adsorption of HA on the electrode. Indeed this adsorption might explain the shift in Epc with ta (and El/2 np with HA) towards t o o negative values, as well as the time-dependence of the potential of the waves obtained with DME. The adsorption of HA on mercury electrode has been proved and studied in more detail in other works [15,16]. Hence, the results of the present study indicate that, on the mercury electrode, the reduction process of the complexes of Pb(II) with the fulvic and humic substances of fresh waters involves: a reversible charge transfer process -- the limiting current is diffusion controlled; labile complexes, even for fast polarographic techniques; an adsorption p h e n o m e n o n of HA on the electrode. This reaction influences particularly strongly the results of the techniques involving long time of contact between the electrode and the solution (HMDE), b u t it also explains the t o o large negative shift in E l n observed with dropping mercury electrode and the unexpected dependency of E~n with other factors [16]. (IV) APPLICATION TO THE MEASUREMENT OF THE COMPLEXING PROPERTIES OF FA BY POLAROGRAPHIC TITRATIONS
(IV. 1) Diffusion coefficient of the Pb(II)-fulvic complexes On the basis of the results of Section III.3, a systematic quantitative study was made to investigate the exact nature of the adsorption p h e n o m e n a by varying drop time in differential pulse polarography [16]. For a reversible process, x/-Dc/Dn¢ is equal to ~php'.n¢, C where ~p'Cand ~p.neare the peak currents in the presence and absence of FA, and De and Dnc are the diffusion coefficients of the complexed and uncomplexed lead, respectively. Obviously, in such a system, tp/~p -c .nc is independent of tg, since i~ ¢ and ~p .c are both proportional to the surface area of the drop, i.e. to t~~a. In fact this was f o u n d to be almost true for tp •.e b u t n o t for ~p. "¢ Hence, the true value of ~/-D¢/Dn¢ m a y be obtained only by extrapolating i~/i~e to zero drop time (ref. 16, Fig. 1). From these results, and by using the value o f D ~ e = 8.28 × 10 -6 cm 2 s -1 [20], the value of De was found to be statistically between 1 × 10 -4 and 3 × 10 -6 cm 2 s -1, regardless of the concentration of FA. The low precision on this value of De is attributed partly to the experimental difficulties of recording polarograms at very short drop times, and partly to a possible change in composition of the solution during the experiment, particularly b y the formation of aggregates of FA [12]. A n y w a y the order of magnitude of the value of De found here is the same as that which we obtained by cyclic voltammetry (Section III.4) and dif-
226
fusion through membranes. In b o t h cases a value of 1.2 X 10 -6 cm 2 s -~ was obtained. On the other hand, the value of D¢ obtained by classical polarography (Section III.1) was higher than those reported above. This discrepancy is attributed to the fact that, in this case D¢ was n o t measured by extrapolation to zero adsorption time b u t by using a mathematical correction, which might be inaccurate for this t y p e of system. (IV. 2) The use o f " a m p e r o m e t r i c " titration curves
Most of the measurements of complexing capacity of natural waters are based on the hypothesis that the complexes formed are inert. All the results obtained here show that this hypothesis is not at all justified for FA and HA and that, on the contrary, these complexes are labile. However, due to the much lower value of the diffusion coefficient of the complexes, compared to that of free lead, an amperometric titration may be used to obtain some rough indication of the complexing capacity. Indeed, if only one complex, PbL, is formed, the peak current obtained in differential pulse polarography, at constant drop time is given by: i t = knc[Pb 2÷] + kc[PbL]
(4)
with [Pb]t = [Pb 2÷] + [ P b a ] where i t is the measured current, and kn~ and k¢ are the proportionality constants for the reduction of free and complexed lead, respectively. By combining these t w o equations with a Scatchard type equation (eqn. 8 in ref. 18) one obtains: {L}t
it~tic -- [Pb], _ Meq [Pb] t -- it/knc 1 -- kc/knc
Y
__
+ Meu K
(5)
X
where {L}t is the total concentration of FA in g 1-1, K is the stability constant of PbL, in mol 1-1, and Meq is the equivalent weight [18]. knc may be obtained from a calibration plot with {L}t = 0 and k-c from the slope of the straight line obtained at the beginning of the titration curve, when [ L ] t > > [Pb]t (Fig. 5a) ([L] t = {L}t/Meq ). Hence by plotting Y versus X (eqn. 5) a straight line m a y be obtained (Fig. 15) from which Meq and K can be computed. From the results discussed in Section IV.l, it is clear that kc is dependent on drop time. However, Fig. 9 in ref. 9 shows that, at constant drop time k¢ m a y be considered to be independent of the concentration of L in solution. Hence the m e t h o d may be used even if the adsorption effect is n o t completely negligible. The values of Meq and log K obtained from Fig. 15 are 260 and higher than 4.4, respectively. The reason w h y this value of Meq is lower than that obtained with i.s.e. (1800) can be partly explained as due to the fact that these were obtained from the portion of the titration curve where [Pb]t and [ L ] t were com-
227 Y.//gtool-1 8.(
x 10-2
J 6.0
4.0 2.0
~ X / I tool -1
0
'
0.5
1,0
,
I
1-5
2.0
,
x 10- ~
Fig. 15. Relationship between Y and X (see eqn. 5). Experimental conditions: see Fig. 5a.
parable whereas in the case of i.s.e, measurements [18] [L]t > [Pb]t. Anyway, the method discussed above gives only an estimation of the complexation parameters. Indeed, it cannot distinguish between the formation of PbL and PbL2. Moreover, in the region of the curve where FA is not in excess compared to Pb(II), the concentration of free ligand and the value of ~ at the surface of the electrode can no longer be considered as equal to those in the solution and this might produce an error on the computation of K and Meq. Finally, this method is rather time-consuming, since the shift in Ep values necessitates the recording of the entire polarographic wave for each titration point to ensure that the peak current is measured. Moreover, to be rigorous, it would be necessary to use an extrapolation technique such as that of Fig. 1 in ref. 16, to compute the peak current without adsorption effect. CONCLUSION The preceding results make possible to compare the limitations and advantages of the various electrochemical methods usable for the measurement of the complexing properties of FA and HA. Because of the fact that they form reversibly reduced labile complexes with lead, the complexation ability may be determined polarographically by: (1) titrating FA with the metal and recording the resulting current by any type of (a) direct (reduction) polarography, (b) anodic stripping technique; (2) measuring the change in E1/2 (or Ep) with the concentration of FA, at constant concentration of lead. As there is a shift in Eln, the method l a will be valid only if the curve i = f(E) is recorded, all along the titration. Furthermore, the complexing capacity has to be computed by assuming the formation of labile complexes. Hence sufficiently precise measurements will be possible only if the difference between the diffusion coefficient of free and complexed lead is sufficiently large. Finally, this method gives only rough values of complexation parameters which moreover,
228 are strictly valid only in conditions where [Pb]t and [L]t are similar. From the results of Section III.3, and of Part II it may be deduced that the methods lb, although more sensitive than direct dop.p., might be not better because of the possibility of important adsorption process occurring during the pre-eleCtrolysis step. Although this process depends on the concentration of HA [15,24], our results indicate that it occurs even for few mg 1-1 of HA. As will be shown in Part II, the determination of true stability constants is possible by method 2, but only if the values of E l n are extrapolated to zero drop time (or zero adsorption time) for each change in the concentration of HA or lead. Then, because short drop times (0.3--1.0 s) have to be used, the sensitivity of the method is limited to about 10 -6 M of Pb(II) which in turn necessitates to Work with relatively high concentrations of FA, since the interpretations of the potential data becomes simpler only when there is an excess of ligand as compared to the metal [20]. Because the i.s.e, have about the same sensitivity limit and do not require an excess of ligand [18] for making measurements, and since, moreover, they are easier to use and the reading of the potential is more precise, the direct d.p.p, actually does not offer any advantage over i.s.e. Hence, the results in this Section mainly point out that although the measurement of c0rnplexation properties of FA is possible, in principle, by polarographic techniques~ some practical reasons actually greatly limit its usefullness, partictflarly when compared with i.s.e. Therefore, the development of a new polarographic technique, which is sensitive enough as well as usuable in the presence of natural surface active agent, is greatly needed. Part II of the series tries to emphasize the main factors which have to be taken into account to develop such a technique. ACKNOWLEDGEMENTS
We wish to thank J. Mallevialle from the "Societal Lyonnaise des Eaux et de t'Eclairage" (Le Pecq/France) for having provided us with the humic and fulvic substances used in this study. This work is part of the project No. 2.587-0.76 supported by the Swiss National Foundation for Scientific Research. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
Y.K. Chau, R. G6chter and K. Lumm-Shue-Chan, J. Fisheries Res. Board Can., 31 (1974) 1515, K.H. Mancy, Progr. Wate~ Technol., 3 (1973) 63. K.W. Hahck and J.W. DfUard, Anal. Chim. Acta, 89 (1977) 329. Y.K. Chau and K. Lum-Shue-Chan, Water Res., 8 (1974) 383. M.S. Shuman and G.P. Woodward, Anal. Chem., 45 (1973) 2032. T.A. O'Shea and K.H. Mancy, Anal. Chem., 48 (1976) 1603. P.L. Brezonik, P. Brauner and W. Stumm, Water Res., 10 (1976) 605. W. Davison and M. Whitfield, J. Electroan~d. Chem., 75 (1977) 763. J. Buffle, F.L. Gretar, G. Nembrini, J. Paul and W. Haerdi, Z. Anal. Chem., 282 (1976) 339. H. Siegerman and G. O'Dom, Amer. Lab., 4 (59) (1972). H.W. Niirnberg, P. Valenta, L. Mart, B. Raspor and L. Sipos, Z. Anal. Chem,, 282 (1976) 357. J. Buffle, P. Deladoey and W. Haerdi, Etude de Caract~risation et de Comparaison des Propri~t~s des Mati~res Humiques et Fulviques de Diff~rentes Eaux, R a p p o r t du projet No. 2.587-0,76, F onds National Suisse p o u r la Recherche Scientifique, 13 J. Buffle, P. Deladoey and W. Haerdi, Application d ' u n Syst~me de Titration A u t o m a t i q u e ~ la Mesure de la C o m p l e x a t i o n du Cuiv~e par les Substances Fulviques e t Humiques et Interpretation des R~sultats, R a p p o r t du projet No. 2.587-0.76, Fonds National Suisse pour la Recherche Seientifique.
229 14 15 16 17. 18 19 20 21
J.A. Cox, The A n a l y t i c a l Utility of the Mercury Film Electrode, Thesis, University of Illinois, 1967. J. Buffle and A. Cominoli, to be published. J. Buffle and F.L. Greter, Part II of this work, J. Electroanal. Chem., 101 (1979) 231 (this issue). D.R. Crow, Polarography of Metal Complexes, Academic Press, New York, 1969, J. Buffle, F.L. G~ete¢ and W. Haerdi, Anal. Chem., 49(2) (1977) 216. R. Ernst, H.E. Allen and K.H. Mancy, Water Res., 9 (1975) 969. J. Hey ro wsky and J. Kuta, Principles of Polarography, Academic Press, New York, 1966. H.P. van Leuwen, Pulse Polarog~aphy of Quasi-Labile Metal Complexes; the Cd-EDTA System at l ow pH, lecture at Euchem Conference, Lerum, Sweden, 1977. 22 A. Weissberger (Ed.), Techniques of Chemistry. Vol. I, Part II. Electrochemical Methods, Wiley-Interscience, New York, 1971. 23 R. Rosset, Les M~thodes Polarographiques R~centes, Polycopid de l'Ecole de Physique et de Chimie de Paris. 24 J. Buffle, A. Cominoli, F.L. Greter and W. Haerdi, l~oc. Anal. Div. Chem. Soc., 1978, pp. 59--61.
211
VOLTAMMET R I C STUDY OF HUMIC AND FU L V IC SUBSTANCES P A R T I. STUDY OF T H E F A C T O R S I N F L U E N C I N G T H E M E A S U R E M E N T OF T H E I R COMPLEXING P R O P E R T I E S WITH LEAD
F.-L. GRETER, J. BUFFLE * and W. HAERDI Department of Inor~,anic and Analytical Chemistry, University of Geneva, Sciences II, 30 quai Ernest Ansermet, CH-1211 Geneva 4 (Switserland) (Received 14th August 1978; in revised form 17th November 1978) ABSTRACT A detailed, qualitative, investigation of the behaviour of lead complexes with fulvic substances has been carried out using several polarographic techniques, e.g.d.c, polarography, a.c. polarography, normal and differential pulse polarography and cyclic voltammetry. The mercury drop electrode was used for this study. It is shown that under the conditions used (pH - 6) the charge transfer process is reversible, the complexes are labile and the adsorption of fulvic acids and fulvic complexes occurs on the electrode. The implications of these results, with regard to the use of the various amperometric titrations for the measurement of the complexing capacity of natural waters, are discussed. INTRODUCTION In r ecen t years several authors have tried to measure the com pl exi ng capacity o f natural waters for a particular metal by means of different kinds o f amperometric titrations [1--6]. However, it has been r e p o r t e d [7--10] t h a t this t y p e o f measurements may sometimes lead t o large errors particularly when t h e w at er contains some c o m p o u n d s which are adsorbable on the electrode. This is specially true for waters containing humic and fulvic substances (which will be globelly referred to as HA and FA hereafter). The results published in this field are o f ten difficult to compare, mainly because: (a) the interpretations of the amperornetric titrations are based on di fferent assumptions according t o t he nature o f the reaction mechanism (labile or inert complexes, reversible or irreversible electrode process) a n d / o r (b) the humic and fulvic acids are n o t sufficiently well-defined. However, since the voltarnmetric techniques are amongst t he few m e t h o d s which allow the measurements of the com pl exi ng properties o f waters w i t h o u t modifying t o o m u c h their com pos i t i on [8,9,11], it is very i m p o r t a n t t o k n o w h o w to use t h e m under best conditions. In the present paper, an a t t e m p t has been made t o define the nat ure o f t h e m o s t i m p o r t a n t properties of humic and fulvic substances and their complexes o f lead at the m e r c u r y electrode, t o assess their influence on the measured polarographic parameters (current and potential), and t o discuss t he validity o f * To whom correspondence should be addressed.
212 the polarographic techniques for measuring the complexing capacity of natural waters. It should be n o t e d that the organic c o m p o u n d s used for this study were "dissolved" humic and fulvic substances of fresh water (Section I). The same sample of fresh water was used for most of this work. Anyway, it must be n o t e d that other w o r k s [12,13] seem to show that for any natural unpolluted fresh water, the most important properties of FA and HA such as fluorescence spectra, u.v. absorption, adsorption on mercury electrode, complexation properties, or mean molecular weight, are very similar. Hence, the results reported here should be also valid for any other organic c o m p o u n d s of the same kind, i.e. those of more or less eutrophicated fresh waters located in a non-peaty region. (I) APPARATUS AND REAGENTS
(I. 1) Apparatus Polarographic measurements were performed with PAR 170 and Tacussel PRT 30 and UAP 4. Metrohm E 500 pH meters were used for pH measurements. The polarographic working electrodes used were: (a) classical dropping mercury electrode (typical parameters were: m = 1.35 mg s-l; drop time = 2 s), (b) Kemula t y p e hanging mercury drop Metrohm E 503 (area of the drop ~ 2 mm2), (c) stationary or rotating electrode with a deposit of mercury film on Pt disk (diameter of the disk = 3 mm). The deposition was done as described in ref. 14. Unless otherwise stated, Ag/AgCI/sat.KC1//NaNO3 0.1 M/[ was used as the reference electrode (Metrohm EA 420). M e t r o h m No. EA 876-20 polarographic cell was used for all polarographic measurements.
(I. 2) Reagen ts Unless otherwise stated all measurements were carried o u t in 0.1 M NaNO3. The solutions were deoxygenated with 99.995% pure N2 delivered b y Bertholet SA. All reagents were pro analysis Merck products. The location and composition of the waters containing the humic and fulvic substances and their properties are described in refs. 12 and 13. These works showed that 60--70% of the organic matter of these waters may be considered as fulvic acids (molecular weight < 1 0 0 0 0 ) . However, hereafter no distinction will be made between humic and fulvic, and the global concentration of FA and HA will be denoted by either [HA] or [L]. These waters were concentrated ~o a b o u t 40 times by freezing concentration and stored at 4 ° C in the dark. It was shown [ 12] that, during storage, this water initially loses 10--20% of their fulvic and humic substances and then remains stable for more than one year. The concentrated samples were filtered on 0.2 # m filters, decarbonated by acidifying with HC104 to pH ~ 4, diluted to the desired concentration with 0.1
213
M NaNO3 and reneutralized to the required pH. Unless otherwise stated, the pH was adjusted to 6.0. Most of this w o r k was done with sample No. 50 [12]. In some of the experiments, sample l a was used. (II) P R E L I M I N A R Y
STUDIES
(II. 1) Shape o f the polarographic curves The shapes of the reduction waves of Pb(II) in the presence of excess of fulvic acids, by d.c., normal pulse and differential pulse polarography are shown in Figs. 1--3. It can be seen in this Figures that the polarograms for the reduction of lead complexes have a classical profile. However, t w o points must be noted: The addition of HA causes an increase in the capacitive current represented by sc = di/dE for E >> E1/2 (Figs. 2 and 4). This effect, studied in detail in ref. 15, is large compared to the low concentration of HA (see particularly Fig. 2) and therefore, complicates greatly the measurement of the faradaic current by d.c. or normal pulse polarography. Hence the titrations were done using differential pulse polarography. In some cases, for pH > 7, the reduction wave splits into two waves whose nature is n o t y e t well understood (for discussion see ref. 16). For pH ~< 6.5, only one wave with classical shape was observed. However, this wave is n o t perfectly symmetrical (see particularly Fig. 3) and is more spread out than the reduction wave of the hydrated Pb 2*. This may be due to the same effect, b u t to a lesser extent as that which causes the splitting of the wave. Because of this effect and the possibility of hydrolysis of Pb(II) for pH > 7, all the experiments were done at pH ~< 6.5.
current
///
I .
~
E/mY -30o
-3so
-4oo
-4 o
Fig. 1. Shapeof the d.c. pola~og~amsobtainedin the presence and in the absenceof fulvic a c i d s . T = 2 5 ° C ; d r o p t i m e = 5 . 3 s; s c a n r a t e = 2 m V s -1. ( . . . . . . ) [Pb] t = 2 x 10 -s M; [ H A l t = 0, p H = 4 . 5 0 . ( ) [ H A l t = 0 . 0 7 0 g 1-1, p H = 6 . 0 0 [ P b ] t = 5 X 1 0 - s M.
214
current
j
J
III ~ / /I//~/F / / ~
E/mV
- 350 -~00 -~s0 Fig. 2. Shape o f the normal pulse p o l a r o ~ a m s o b t a i n e d in the presence and in the absence o f fulvie acids. [ P b ] t = 1 x 10 .6 M; p i t = 6 . 0 0 ; drop time = 2 s; l e n g t h o f the pulse = 65 ms. ( . . . . . . ) [HAlt ; 0; ( ) [HA]t = 9 . 0 6 mg 1-1.
(II.2) Titration experiments An example of the titration o f HA with Pb 2+ is given in Fig. 5a, where the peak current o f differential pulse polarograms, ip, is recorded versus the total concentration o f lead, [ P b ] t :
current / /
[
I
5nA
'
\ \
,\
/
\
/
\
E/mY I
-3oo Fig. 3.
-3;0
-~;o
- 450 '
- 500 ~
ShaDe of the differential pulse po]aro~rams obtained in the presence and in the absence
o f fulvic acids. [ P b ] t = 5 X 10 -6 M; p H = 5 . 5 ; drop time ffi 1 . 0 0 s; scan rate = 2 m V s -1 ; h e i g h t o f the pulse = 10 m Y . ( . . . . . . ) [ H A ] t = 0; ( ) [ H A ] t = 0 . 0 6 g 1-1.
215 sc(H A / , O ) / s c ( H A = O)
(HA) T/g 0
* 0
*
* 0.05
~
' *
'
'
1-1
J 0.10
Fig. 4. Change in the capacitive current measured by normal pulse polarography, with increasing concentrations of HA. Experimental conditions: see Fig. 9 in ref. 9.
If the Pb-HA complexes were inert, as it is sometimes assumed, then the observed current at the beginning of the plot should have been zero and, towards the end, it should increase linearly, yielding a slope equal to that obtained in the absence of HA [3,6]. Moreover the peak potential should remain unchanged during the titration. Figure 5a shows a completely different behaviour. There is no range of [Pb] t for which ip = 0 and there is no break point in the titration curve. Moreover the peak potential of lead, E~ b, varies continuously during the Pb(II) titration (Fig. 5b). It is interesting to note that the same behaviour was observed for the titration of HA with Cu 2÷ and of salicylic acid with Fe(III). These results seem to indicate that the c o m p l e x e s formed are labile. In this case, by assuming a reversible reduction process, an estimation of the degree of complexation ~ = [ P b ] t / [ P b 2÷] may be made, by taking into account the change in ip and Ep (eqn. 4.33, ref. 17), at the beginning of the titration, when there is an excess of HA compared to [Pb]t. However, the values obtained in this way are a b o u t 4 orders of magnitude greater than those obtained by ion-selective electrode measurements [ 18]. This result is caused by a shift of Ep which is t o o high to be explained by eqn. (4.33) of ref. 17 (see Section III.3). Ernst et al. [19] tried to take this fact into account b y assuming that the reduction of complexes was irreversible. However, we obtained the same kind of titration curves, and the same order of magnitude for a, by using a.c. polarography (frequency 80 Hz; amplitude 10 mV; phase angle 0 ° ) instead of differential pulse. This result seems to indicate t h a t the charge transfer is reversible. Finally, we tried to carry o u t the titrations of AH with Pb 2÷, Cu 2÷ and T1÷ b y
216 ip/,uA
(o)
/ ,
./ lOb(Pb)T/mol [-I 5
10
*
15
Ep/mV
t
20
(b)
-400
..........
t ....... _
_
-
~
.... 105(Pb)T/mot I-I
-200 5
, 10
, 15
20
Fig. 5. (a) Change in the peak current obtained by differential pulse polarography, during the titration of HA with Pb 2+. T = 25°C; pH = 7.00; scan rate = 2 mV s -1 ; drop time = 1 s; length of the pulse = 50 ms; height of the pulse = 25 mV; sampling duration = the last 10 ms of the pulse duration. [HAlt = (1) 0; (2) 0.036 g 1-1 . (b) Change in the peak potential obtained by differential pulse polarography, during the titration of HA with Pb 2+. Experimental conditions: see (a). Potential of the ref. electrode = +100 mV vs. NHE.
differential pulse p o l a r o g r a p h y ( c a t h o d i c stripping), o n H M D E a n d m e r c u r y film d e p o s i t e d o n Pt disk. In all these cases, the p o i n t s o f the curves ip = f ( [ M e l t ) ( [ M e l t = t o t a l c o n c e n t r a t i o n o f m e t a l ) were relatively scattered and t h e r e p r o d u c i b i l i t y was low. F o r Cu 2+ a n d Pb 2÷, the shift in Ep was always m u c h t o o high c o m p a r e d with t h a t e x p e c t e d f o r labile c o m p l e x f o r m a t i o n with reversible r e d u c t i o n . These results p o i n t o u t t h a t , b e f o r e using t h e a m p e r o m e t r i c t i t r a t i o n techniques f o r the m e a s u r e m e n t o f t h e c o m p l e x i n g c a p a c i t y o f waters, m o r e i n f o r m a tion is r e q u i r e d a b o u t (a) the degree o f reversibility o f t h e e l e c t r o d e process,
217
(b) the degree of lability of the complexes, and (c) how correct values of a and of the complexing capacity may be obtained from polarographic data. In the following part of the paper we have endeavored to define the important steps of the electrode process and their influence on the voltammetric parameters. (III) NATURE OF T H E FACTORS INFLUENCING THE ELECTRODE PROCESS
(111.1) Role o f diffusion in the mass transfer process For a diffusion controlled reduction wave, at any potential, the following relationship will be obeyed [20]: log(if) = const. + ½ log(h) where h is the height of the mercury column. Figure 6 shows that the preceding equation holds for potentials situated close to the top of the wave and for sufficiently large values of h. Indeed, in the latter conditions, the slope of the curve at E = --0.420 V, which corresponds to the limiting current, is 0.43 which agrees reasonably with the theoretical one. For increasingly positive potentials and decreasing h the curve tends to be more and more rounded and the slope of the tangent at the curvature increases for low values of h. This effect shows that the polarographic wave is influenced log ( i f / n A )
2,0 o ~ ; ~ " ~ ~' _..-~;-". ~
6
1.5
1.0 i
/ x
0.5'
Iog(h/crn) 0.0
~ 1.4
~ 1.6
~ 1.8
~ 2.0
Fig. 6. Influence of the height of the mercury column, h, o n t h e faradaic c u r r e n t if measured by d.c. polaro~aphy. T = 25°C; pH = 6.0; [Pb]t = 2 × 10 -s M; [HAl t = 0.070 g 1-1 ; scan rate = 1 mV s- . E/mV: (1) --350; (2) --360; (3) --370; (4) --380; (5) --390; (6) --400; (7) --410; (8) --420.
218 by a secondary p h e n o m e n o n which will be discussed in Section III.3 and in Part II (ref. 16, Section II.4: this issue, pp.231--251). Because of the low concentration of Pb(II) that one has to use for the ligand to be present in excess, the capacitive c o m p o n e n t of the current measured on one falling drop is not completely negligible. Then if ic and if are the capacitive and faradaic currents respectively, the measured current, i, is given by: i = if + i¢
(1)
In the absence of HA, at constant potential, ic = k c t -1/3 and for a reversible, diffusion controlled electrode process, if = kft 1/6 [20]. However, in presence of HA, it was shown [15], that for t < 3 s, i¢ ~- k~t -~/3 + ko t÷2/3. Moreover, by taking into account the secondary p h e n o m e n o n mentioned above, it may be shown [ref. 16, paragraph IV.3.1) that: if = k~ t 1/6 __ k~ [Pb 2÷] ot ( p - r - 1/3) where k f , k'~, k~, k o , p a n d r are constants and [Pb2+]0 is the concentration of Pb at the electrode surface. For potentials corresponding to the limiting current [Pb2÷]0 = 0. Hence eqn. (1) becomes: it 1/3 = kc + k o t + k f t 1/2
(2)
Since the term k e t becomes important only at the end of the drop time, a plot of it 1/3 versus t ~/2 allows us to obtain k¢ by extrapolation to t -* 0. Then, a plot of (it -2/3 ~ k c t -1) as a function of t -1/2 should give a straight line: i t -2/3 - - k c t - I = k e + k f t -1/2
(3)
Figure 7 shows that this is indeed the case. The value of 37.3 nA s -1/6 was obtained for k~ at a potential corresponding to the limiting current, which allowed us to c o m p u t e [20], for the diffusion coefficient of the reducing species, the value of 4.7 X 10 -6 cm 2 s -~. This result shows that the diffusion coefficient of the c o m p l e x is significantly lower than that of free Pb 2÷. However, because of the various corrections made for its derivation, this value must only be taken as an order of magnitude. Finally, the polarograms obtained by normal pulse polarography have a classical shape (Fig. 2). N o w this technique is a kind of very fast relaxation technique. Hence, the similar results obtained b y b o t h this m e t h o d and d.c. polarography indicate that any coupled chemical reaction and particularly complex formation can be considered as labile from the point of view of overall redox process [21]. T h i s is confirmed by Fig. 9 in ref. 9 which shows that the decrease in the reduction current of Pb(II) observed in the presence of HA is independant of its concentration. If the complex was n o t labile~ this decrease would have been more and more p r o n o u n c e d with increasing [HA]t. However, because of the secondary phenomenon, mentioned above (see also Section III.3 and ref. 16, paragraph II.4), it is n o t possible to c o m p u t e the diffusion coefficient of the complex directly from the limiting current obtained in normal pulse polarography. All the preceding results show that the limiting current of the reduction wave of the complex is diffusion controlled and is n o t influenced by the rate o f the preceding chemical reactions. However, they also show that, on the rising portion of the wave, the mass transfer process is n o t due only to diffusion.
219 (i t "m- k¢ t -I ) / n A s"m 30 ¸
./
./ /
x
cot
10
t~
~
a
2 \t
1
~..._.__.. . . . . . . . . .
.-...,.....
.....~~° "~ ~, o 0
-10 i
i
i
0.4
0.6
0.8
E/v
i -&s
- so
•
1~0
Ep (0 °)
Fig. 7. L i n e a r i s a t i o n o f t h e i--t c u r v e s o b t a i n e d b y d.c. p o l a r o g r a p h y , a c c o r d i n g e q n . (3). ]~xp e r i m e n t a l c o n d i t i o n s : (it) - - 5 0 0 m V ; (X) - - 4 7 5 m V ; (A) - - 4 5 0 m V ; (~J) - - 4 2 5 i n V . D r o p t i m e tg = 5.5 s; f l o w r a t e o f m e r c u r y = 1 . 3 5 m g s -1 ; T = 2 5 ° C ; p H = 6; [ H A ] t = 1 0 0 m g 1-1 ; [ P b ] t = 10 - s M. Fig. 8. V a l u e s o f c o t ¢ as a f u n c t i o n o f E a l o n g a n a.c. p o l a r o g r a p h i c c u r v e . T = 2 5 ° C ; p H -7 . 0 0 ; [ P b ] t = 5 X 10 - s M , [ H A ] t = 0 , 0 3 6 g 1-1 ; d r o p t i m e = 1.0 s; ~ = 1 8 0 H z ; s c a n r a t e = 2 m V s -1 ; a m p l i t u d e = 10 i n V . R e f . e l e c t r o d e : A g / A g C I / 0 . 1 M K C I / / 0 . 1 M N A N 0 3 / / .
(III. 2) Nature of the charge transfer process The behaviour of the a.c. polarographic current for the reduction of Pb(II)-HA complexes, is shown in Figs. 8--10. Cot ¢, given in Figs. 8 and 10, is c o m p u t e d from the ratio of the currents at ¢ = 0 ° and ¢ = 90 ° , respectively, where ¢ is the phase angle between current and potential. Figure 8 shows that for w = 180 Hz, cot ~ is close to 1.0 at any potential, which is expected for a reversible and fast charge transfer process [22]. This behaviour was observed for all frequencies < 2 0 0 Hz as is shown in Fig. 10, where (cot ¢)p is the value of cot ¢ at the peak potential. The same result is obtained when the peak current, recorded at ¢ = 0 ° is plotted as a function of ~ (Fig. 9). The linearity of the curve, for w ~< 400 Hz, is an indication for the reversibility of the charge transfer process [22] and the fact that the curves obtained in the presence and in the absence of HA are linear up to approximately the same frequency (about 350 Hz) is an indication that the lack of reversibility for higher frequencies is due to the lead system itself and n o t to the presence of HA. As in pulse polarography, it is n o t possible to c o m p u t e the diffusion coefficient of the complex by comparing the peak current of the a.c. polarographic curves of the complex and of the free metal. For a sim-
220
ip/.}4A
1.o
0.9
/("
. •
0-8
; ,
° x N
•
i
/ 0-7'
°'
'
\
, -,.
///"
".
0-5
0-3
ol
7
,.v2/.~,12
0
~ 0
10
20
30
Fig. 9. I n f l u e n c e o f f r e q u e n c y o n t h e p e a k c u r r e n t o b t a i n e d b y a.c. p o l a r o g r a p b y . T = 2 5 ° C ; e x p e r i m e n t a l c o n d i t i o n s : see Fig. 8. C o t ~b = 0; [ H A ] t = ( 1 ) 0; ( 2 ) 0 . 0 3 6 g 1-1 .
ple reversible case, the ratio of the currents is equal to the square r o o t of the diffusion coefficient of the two particles. However, in the present case, it will be seen (ref. 16, paragraph IV.I) that this relationship is strictly valid only at a potential corresponding to the limiting current in d.c. polarography (where [Pb2*]0 ~-- 0), which is not the case for the peak currents in a.c. polarography. The reversibility of the Pb-HA system is also supported by differential pulse polarography, another fast relaxation technique. For reversible systems by applying periodically, on a linear function E = f(t), square pulses with constant amplitude, AE, a symmetrical peak (1), i = f(E), is obtained. If the amplitude of the pulses is --AE, an identical peak (2) is obtained, but shifted by --AE when compared to peak (1) [23]. Figure 11 shows the ratio of the current of peak (1) at potential E to the current of peak (2) at E -- AE. The fact that this ratio is 1, regardless of the potential, is an additional indication for the overall reversibility of the electrode process.
221
(cot ~)p
10
/
..// 1
...,-"':"
o
10
J~/H~ ~ 20
30
Fig. 10. I n f l u e n c e o f t h e f r e q u e n c y o n t h e value of c o t ~ aS t h e p e a k , p o t e n t i a l o f t h e a.c. p o l a r o g r a p h i c curves. E x p e r i m e n t a l c o n d i t i o n s : see Fig. 8.
(III.3) Nature of the secondary phenomenon mentioned in Section III.1 As can be seen in Fig. 12, in d.c. polarography, the half-wave potential, Eln, shifts towards more negative values when [HA]t increases which is expected for the reversible reduction of labile complexes [17,20]. Figure 9 in ref. 9 shows that a similar shift in the half-wave potential, Eu2, np is observed for normal pulse polarography. However, as in the case of titration experiments (Section II.2) this shift is much too large compared with that observed with i.s.e. Moreover
i1 I i2
E/mY o
'
Fig. 11. R a t i o o f t h e c u r r e n t s o f t w o d i f f e r e n t i a l pulse p o l a r o g r a p h i c waves ( a t t h e p o t e n t i a l E ( i l ) a n d E - - A E (i2)), for w h i c h t h e a m p l i t u d e o f pulses A E = - - 1 0 m V a n d - - A E are used, respectively. T = 2 5 ° C ; p H = 6 . 0 0 ; [ P b ] t = 5 × 10 -6 M ; [ H A ] t = 0.1 g 1-z ; scan r a t e = 1 m V s - l ; length o f t h e pulse = 50 m s ; s a m p l i n g t i m e = f r o m 75 t o 9 5 % o f t h e pulse d u r a t i o n ; d r o p time = 2 s. Ref. e l e c t r o d e : A g / A g C l / 0 . 1 M KC1//0.1 M N a N O 3 / / .
222
AEll 2 /mY 100
j/
~0 60 40
20
./
log [HAlt/g 1-1 ) 0
-2.0
l
l
-1.5
-1.0
i
-0.5
Fig. 12. Influence of [ H A ] t o n the change in El~ 2 m e a s u r e d by d.c. polarography. T = 25°C; p H = 6.00; [Pb]t = 2 × 10 -s M ; scan rate = 1 m V s -I ; drop time = 5.8 s. 2~kE1/2 = E 1 / 2 ( [ H A ] t = 0 ) - - E 1 / 2 ( [ H A ] t :/= e).
this Figure also shows that EI"~p strongly depends on Pb(II) concentration, even when [HA]t is in excess. Finally, other results [16] obtained with DME also show that the potential and the width of the peak (or wave) are influenced by time-dependent parameters such as the height of the mercury column, h, in d.c. polarography, the drop time in pulse polarography, or the frequency, co, in a.c. polarography. This effect is still much more pronounced by using HMDE. The shape of the curves i = f(E) obtained by cyclic voltammetry on HMDE, for uncomplexed Pb 2÷ and Pb(II)-HA complexes, are shown in Fig. 8, in ref. 9. This Figure shows that the curves obtained in the presence of HA are strongly affected by the time, ta, elapsed at the initial potential Ei, before the beginning of the scanning. The longer is this duration, the larger are the anodic and cathodic peak currents (i~ and i~, respectively) and the cathodic peak potential E~ is shifted towards more negative values. Because of experimental difficulties, t~ = 0, is not known with great accuracy, and since, at low values of ta, ~p/Ipand Ep Ep vary sharply, it is quite difficult to extrapolate accurately these two parameters at ta -~ 0. However, from Fig. 13, it can be seen that the extrapolated values are close to 1.0 and 30 m V , respectively, which are the theoretical values expected for the reversible reduction of a labile complex (E~ = potential of the anodic peak). Figure 8 in ref. 9 also shows that the cathodic and anodic peaks of the Pb-HA complex are much more rounded and lower than those obtained in the absence of HA, for the same concentration of Pb(II). This confirms that the diffusion coefficient of Pb-HA is lower than that of Pb ~÷, which causes a decrease in the reduction current of Pb(II) when complexed by HA as compared with that of free Pb 2÷. "C
*R
&
__
C
(III. 4) Influence of pH Figure 14 shows the influence of pH on some parameters of the cyclic voltammetric curves obtained in the presence of FA.
223
p/i a
(a)
ic
P
I I
tads/min 0
""
0
~
1
2
3
Q
4
5
6
(b)
......... • --
90
o / ° /
60
30
tods/rain 0
, 0
1
' 2
3
4
5
6
Fig.
13. I n f l u e n c e o f the time, ta, elapsed at the initial potential before scanning, in cyclic v o l t a m m e t r y . T = 25°C; p H = 8.0; [ P b ] t = 2 x 10 - s M; [ H A l t : (o) s a m p l e 50, 34 m g 1-1 ; surface of the drop = 2 m m 2 ; scan rate = 100 m V s -1 . (a) I n f l u e n c e o f tad s on the ratio o f the cathodic over a n o d i c p e a k currents. (b) I n f l u e n c e o f tad s on the difference b e t w e e n the a n o d i c and t h e c a t h o d i c p e a k potentials.
According to the results obtained in Section III.3, the system seems to behave as the reversible reduction of labile complexes when ta ~ 0. Hence, in this Section, the electrochemical parameters were either extrapolated to ta = 0, or measured by using the minimum value of t~. The ratio of the cathodic peak current in the presence and in the absence of FA (Fig. 14b) confirms the interpretation of Section III.3: for low pH (pH < 4), where the mean number of ligands per metal ion, ~, tends to zero, the ratio tends to 1, while for higher pH (pH > 6), for which ~ tends to its maximum
224
6C
o
2
o. o
-o
•
c
•
30 . . . . . . . . . . . . . . . . . . .
I
-
o
=. . . . . . . . . . . . . . . . . . . . pH
4
5
7
6
ip (HA ~-0)/ip (HA = 0)
8
9
(b)
~°~O,~o
03
pH o.0 4
5
6
7
8
Fig. 14. I n f l u e n c e o f p H o n t w o parameters o f the c y c l i c v o l t a m m e t r i c curves. T = 25°C; scan rate = 100 m V s -1 ; [ P b ] t = 2 × 10 -s M; surface o f the drop = 2 m m 2 ; [ H A ] t : (1) 0; (2) (o) sample 50, 34 m g 1-1, (o) s a m p l e l a , 18 m g 1-1. (a) I n f l u e n c e o f p H o n the d i f f e r e n c e b e t w e e n the anodic and the c a t h o d i c peak potentials, t a = 5 s. (b) I n f l u e n c e o f pH o n the ratio o f the c a t h o d i c peak current in the presence and in the a b s e n c e o f HA. Currents extrap o l a t e d to t a ~ 0.
value [ 1 6 ] , the ratio tends to a value lower than 1. From Fig. 14b this value is found to be 0.38. Since in this experiment, the peak currents were extrapolated to ta -* 0 (see also Section IV) and since, in this technique, the peak potential is not very far from the potential corresponding to the top of the d.c. polarographic wave (where [Pb2+]0 -~ 0 and (dN/dt)a -~ 0; see eqn. 26 in ref. 16), the ratio of the peak current in the presence and in the absence of FA may be considered as equal to the square root of the ratio of the diffusion coefficients of the complex and the free Pb 2+. From the value of this ratio at pH 6.0, the value of De = 1.2 × 10 -6 cm 2 s-' was obtained. A discussion about the validity of this value is given in Section IV. Figure 14a shows the influence of pH on the values of E~ - - E ~ , measured at ta = 5 s. It is seen that E~ -- E~ is independent of pH and relatively close to the theoretical value (30 mV) expected for a reversible redox system coupled with a labile chemical reaction, particularly considering that t~, although small, is not zero. Finally the same type of curves as those shown in Fig. 13a were obtained for any pH, in the pH range 3--7, so that, for t~ -* 0, the ratio of the cathodic
225
to the anodic peak current is close to 1.0 and independent of pH. Hence, in the pH range studied, Fig. 14 indicates that pH has no effect on the lability of the complexes and the rate and reversibility of the redox system.
(III. 5) Inferences from these results Because Pb(II) is n o t reducible at Ei (cyclic voltammetry, Section III.3) and because it was ensured that HA is n o t reduced nor oxidized at this potential, the most probable effect of waiting at Ei is the adsorption of HA on the electrode. Indeed this adsorption might explain the shift in Epc with ta (and El/2 np with HA) towards t o o negative values, as well as the time-dependence of the potential of the waves obtained with DME. The adsorption of HA on mercury electrode has been proved and studied in more detail in other works [15,16]. Hence, the results of the present study indicate that, on the mercury electrode, the reduction process of the complexes of Pb(II) with the fulvic and humic substances of fresh waters involves: a reversible charge transfer process -- the limiting current is diffusion controlled; labile complexes, even for fast polarographic techniques; an adsorption p h e n o m e n o n of HA on the electrode. This reaction influences particularly strongly the results of the techniques involving long time of contact between the electrode and the solution (HMDE), b u t it also explains the t o o large negative shift in E l n observed with dropping mercury electrode and the unexpected dependency of E~n with other factors [16]. (IV) APPLICATION TO THE MEASUREMENT OF THE COMPLEXING PROPERTIES OF FA BY POLAROGRAPHIC TITRATIONS
(IV. 1) Diffusion coefficient of the Pb(II)-fulvic complexes On the basis of the results of Section III.3, a systematic quantitative study was made to investigate the exact nature of the adsorption p h e n o m e n a by varying drop time in differential pulse polarography [16]. For a reversible process, x/-Dc/Dn¢ is equal to ~php'.n¢, C where ~p'Cand ~p.neare the peak currents in the presence and absence of FA, and De and Dnc are the diffusion coefficients of the complexed and uncomplexed lead, respectively. Obviously, in such a system, tp/~p -c .nc is independent of tg, since i~ ¢ and ~p .c are both proportional to the surface area of the drop, i.e. to t~~a. In fact this was f o u n d to be almost true for tp •.e b u t n o t for ~p. "¢ Hence, the true value of ~/-D¢/Dn¢ m a y be obtained only by extrapolating i~/i~e to zero drop time (ref. 16, Fig. 1). From these results, and by using the value o f D ~ e = 8.28 × 10 -6 cm 2 s -1 [20], the value of De was found to be statistically between 1 × 10 -4 and 3 × 10 -6 cm 2 s -1, regardless of the concentration of FA. The low precision on this value of De is attributed partly to the experimental difficulties of recording polarograms at very short drop times, and partly to a possible change in composition of the solution during the experiment, particularly b y the formation of aggregates of FA [12]. A n y w a y the order of magnitude of the value of De found here is the same as that which we obtained by cyclic voltammetry (Section III.4) and dif-
226
fusion through membranes. In b o t h cases a value of 1.2 X 10 -6 cm 2 s -~ was obtained. On the other hand, the value of D¢ obtained by classical polarography (Section III.1) was higher than those reported above. This discrepancy is attributed to the fact that, in this case D¢ was n o t measured by extrapolation to zero adsorption time b u t by using a mathematical correction, which might be inaccurate for this t y p e of system. (IV. 2) The use o f " a m p e r o m e t r i c " titration curves
Most of the measurements of complexing capacity of natural waters are based on the hypothesis that the complexes formed are inert. All the results obtained here show that this hypothesis is not at all justified for FA and HA and that, on the contrary, these complexes are labile. However, due to the much lower value of the diffusion coefficient of the complexes, compared to that of free lead, an amperometric titration may be used to obtain some rough indication of the complexing capacity. Indeed, if only one complex, PbL, is formed, the peak current obtained in differential pulse polarography, at constant drop time is given by: i t = knc[Pb 2÷] + kc[PbL]
(4)
with [Pb]t = [Pb 2÷] + [ P b a ] where i t is the measured current, and kn~ and k¢ are the proportionality constants for the reduction of free and complexed lead, respectively. By combining these t w o equations with a Scatchard type equation (eqn. 8 in ref. 18) one obtains: {L}t
it~tic -- [Pb], _ Meq [Pb] t -- it/knc 1 -- kc/knc
Y
__
+ Meu K
(5)
X
where {L}t is the total concentration of FA in g 1-1, K is the stability constant of PbL, in mol 1-1, and Meq is the equivalent weight [18]. knc may be obtained from a calibration plot with {L}t = 0 and k-c from the slope of the straight line obtained at the beginning of the titration curve, when [ L ] t > > [Pb]t (Fig. 5a) ([L] t = {L}t/Meq ). Hence by plotting Y versus X (eqn. 5) a straight line m a y be obtained (Fig. 15) from which Meq and K can be computed. From the results discussed in Section IV.l, it is clear that kc is dependent on drop time. However, Fig. 9 in ref. 9 shows that, at constant drop time k¢ m a y be considered to be independent of the concentration of L in solution. Hence the m e t h o d may be used even if the adsorption effect is n o t completely negligible. The values of Meq and log K obtained from Fig. 15 are 260 and higher than 4.4, respectively. The reason w h y this value of Meq is lower than that obtained with i.s.e. (1800) can be partly explained as due to the fact that these were obtained from the portion of the titration curve where [Pb]t and [ L ] t were com-
227 Y.//gtool-1 8.(
x 10-2
J 6.0
4.0 2.0
~ X / I tool -1
0
'
0.5
1,0
,
I
1-5
2.0
,
x 10- ~
Fig. 15. Relationship between Y and X (see eqn. 5). Experimental conditions: see Fig. 5a.
parable whereas in the case of i.s.e, measurements [18] [L]t > [Pb]t. Anyway, the method discussed above gives only an estimation of the complexation parameters. Indeed, it cannot distinguish between the formation of PbL and PbL2. Moreover, in the region of the curve where FA is not in excess compared to Pb(II), the concentration of free ligand and the value of ~ at the surface of the electrode can no longer be considered as equal to those in the solution and this might produce an error on the computation of K and Meq. Finally, this method is rather time-consuming, since the shift in Ep values necessitates the recording of the entire polarographic wave for each titration point to ensure that the peak current is measured. Moreover, to be rigorous, it would be necessary to use an extrapolation technique such as that of Fig. 1 in ref. 16, to compute the peak current without adsorption effect. CONCLUSION The preceding results make possible to compare the limitations and advantages of the various electrochemical methods usable for the measurement of the complexing properties of FA and HA. Because of the fact that they form reversibly reduced labile complexes with lead, the complexation ability may be determined polarographically by: (1) titrating FA with the metal and recording the resulting current by any type of (a) direct (reduction) polarography, (b) anodic stripping technique; (2) measuring the change in E1/2 (or Ep) with the concentration of FA, at constant concentration of lead. As there is a shift in Eln, the method l a will be valid only if the curve i = f(E) is recorded, all along the titration. Furthermore, the complexing capacity has to be computed by assuming the formation of labile complexes. Hence sufficiently precise measurements will be possible only if the difference between the diffusion coefficient of free and complexed lead is sufficiently large. Finally, this method gives only rough values of complexation parameters which moreover,
228 are strictly valid only in conditions where [Pb]t and [L]t are similar. From the results of Section III.3, and of Part II it may be deduced that the methods lb, although more sensitive than direct dop.p., might be not better because of the possibility of important adsorption process occurring during the pre-eleCtrolysis step. Although this process depends on the concentration of HA [15,24], our results indicate that it occurs even for few mg 1-1 of HA. As will be shown in Part II, the determination of true stability constants is possible by method 2, but only if the values of E l n are extrapolated to zero drop time (or zero adsorption time) for each change in the concentration of HA or lead. Then, because short drop times (0.3--1.0 s) have to be used, the sensitivity of the method is limited to about 10 -6 M of Pb(II) which in turn necessitates to Work with relatively high concentrations of FA, since the interpretations of the potential data becomes simpler only when there is an excess of ligand as compared to the metal [20]. Because the i.s.e, have about the same sensitivity limit and do not require an excess of ligand [18] for making measurements, and since, moreover, they are easier to use and the reading of the potential is more precise, the direct d.p.p, actually does not offer any advantage over i.s.e. Hence, the results in this Section mainly point out that although the measurement of c0rnplexation properties of FA is possible, in principle, by polarographic techniques~ some practical reasons actually greatly limit its usefullness, partictflarly when compared with i.s.e. Therefore, the development of a new polarographic technique, which is sensitive enough as well as usuable in the presence of natural surface active agent, is greatly needed. Part II of the series tries to emphasize the main factors which have to be taken into account to develop such a technique. ACKNOWLEDGEMENTS
We wish to thank J. Mallevialle from the "Societal Lyonnaise des Eaux et de t'Eclairage" (Le Pecq/France) for having provided us with the humic and fulvic substances used in this study. This work is part of the project No. 2.587-0.76 supported by the Swiss National Foundation for Scientific Research. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
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