Modulated polarographic and voltammetric techniques in the study of natural water chemistry

J. Electroanal. Chem., 75 (1977) 763--789 © Elsevier Sequoia S.A., Lausanne - - Printed in The Netherlands

MODULATED POLAROGRAPHIC AND VOLTAMMETRIC IN THE STUDY OF NATURAL WATER CHEMISTRY *

763

TECHNIQUES

W. DAVISON Freshwater Biological Association, The Ferry House, Ambleside, Cumbria L A 2 2 OLP (England) M. WHITFIELD Marine Biological Association, The Laboratory, Citadel Hill, Plymouth PL1 2PB (England) (Received 26th May 1976)

ABSTRACT Modulated polarographic and voltammetric techniques are of particular importance in natural water chemistry because of their sensitivity not only to very low concentrations of electroactive components but also to their chemical form. Direct polarographic techniques are most useful for the analysis of non-metallic components at low concentrations since metal levels are only rarely high enough for the analysis of untreated samples. Preconcentration by chemical or electrochemical techniques have both been employed. Potentially the most productive field of application of polarographic and voltammetric methods is in determining the chemical speciation of electroactive components in natural waters. Some clarification is required of the chemical and biological significance of operational classifications currently employed. Intermetallic interferences and the influence of surface films on electrode behaviour need to be more thoroughly investigated before analyses or speciation studies on untreated samples can be routinely undertaken. Chemical and electrochemical understanding rather than increased sophistication in the instrumentation is required at this stage if full advantage is to be taken of the capabilities of modulated polarographic and voltammetric methods in natural water chemistry.

INTRODUCTION Over the past decade the emphasis of natural water chemistry has shifted considerably. As a consequence of increased concern over environmental pollution there has been a growing demand for accurate, precise and simple means for the chemical analysis of natural waters. Because of the expense of collecting samples (oceanic sea water samples cost more per litre than high q u a l i t y c h a m p a g n e ) a n d t h e t i m e d e l a y s i n v o l v e d in l a b o r a t o r y - b a s e d a n a l y s e s , i n t e r e s t is t u r n i n g t o w a r d s a u t o m a t e d in s i t u o r o n s i t e p r o b e s t h a t c a n give i n f o r m a t i o n a b o u t c h e m i c a l c h a n g e s as t h e y o c c u r . T h i s is p a r t i c u l a r l y t r u e * In honour of Dr. G.C. Barker's 60th birthday.

Principl e

Sampled c current measurement

Sampled c current measurement

Sampled c current measurement

Technique a

S q u a r e wave polarography s.w.p,

Pulse polarography p.p.

Differential pulse polarography d.p.p. 1st derivative

d.c. p o l a r o g r a m

1st derivative

Wave s h a p e

Characteristics of modulated voltammetric techniques

TABLE 1

8.0

7.3

6.7--8.0

Concentration limit b

Small a m p l i t u d e ( < 100 m V ) pulses s u p e r i m p o s e d o n linear r a m p a n d a p p l i e d to c o n s e c u t i v e Hg drops. Current sampled immediately b e f o r e pulse applied a n d j u s t b e f o r e e n d o f pulse life. Difference between currents measured.

4 0 - - 1 0 0 ms pulses o f successively increasing a m p l i t u d e are applied f r o m a c o n s t a n t base p o t e n t i a l to c o n s e c u t i v e Hg drops. T h e c u r r e n t is sampled just before end of pulse life.

225 Hz s q u a r e wave ( < 1 0 0 m V amplitude) superimposed on linear r a m p . C u r r e n t s a m p l i n g s y n c h r o n i s e d 1.75 s i n t o d r o p life a n d 2 ms a f t e r p o t e n t i a l change.

T y p i c a l details o f t e c h n i q u e

6,7,10

3,4,5,9

Ref.

5 x 104 t o l × 105 6,7,9, tolerance to preceding 10, 11 p e a k s at 0.2 V s e p a r a t i o n . Sensitive t o irreversible processes a n d s u r f a c t a n t s . Cell r e s i s t a n c e m u s t b e > 5 0 0 0 ,.Q.

S e n s i t i v i t y t o irreversible r e a c t i o n s a n d tole r a n c e t o high cell resist a n c e b e t t e r t h a n s.w.p. Capillary noise r e d u c e d .

40,000 : 1 tolerance to p r e c e d i n g p e a k s at 0.2 V s e p a r a t i o n . Insensitive t o irreversible r e a c t i o n s . Cell resistance m u s t b e > 5 0 ~ . Sensitive t o s u r f a c t a n t s .

Application notes

Continuous e current measurement

Continuous e curr~nt measurement

Continuous e current measurement

A.c.p, w i t h second harmonics

Intermodulation polarography

Radiofrequency polarography (r.f~p.) 2 n d derivative

2 n d derivative

8.3

7.7

7.7

7.7

1st derivative

2 n d derivative

6.0

1st derivative

13, 14

G o o d . r e s o l u t i o n . Curr e n t p r o p o r t i o n a l to (voltage) 2 a n d n 3.

Small a m p l i t u d e (100 kHz t o 6.4 MHz) radio signal s u p e r i m p o s e d o n linear r a m p m o d u l a t e d w i t h 225 Hz square wave.

R e c o r d a.c. signal corres p o n d i n g t o d i f f e r e n c e in frequency between two modulations

O1

1, 13, 14

6, 15

13, 14

I m p r o v e d sensitivity b y i m p r o v i n g i F : ic. i F p r o p o r t i o n a l to voltage and t o n 2 f

L o c k - i n a m p l i f i e r r e c o r d s curr e n t 90 ° o u t o f p h a s e w i t h applied voltage and r e m o v e s m o s t o f ic R e c o r d 2 n d h a r m o n i c resulting f r o m n o n - l i n e a r i t y o f F a r a d a i c process.

30--40 m V resolution. 12--14 Insensitive t o irreversible processes. F r e q u e n c y can be a l t e r e d t o shift o r o b l i t e r a t e peaks.

10 m V sinusoidal a.c. signal, 3 0 - - 1 0 0 0 Hz, s u p e r i m p o s e d o n linear r a m p . A.c. c o m p o nent measured.

The a b b r e v i a t i o n s s h o w n will be u s e d in s u b s e q u e n t tables a n d , o n o c c a s i o n , in t h e t e x t . E x p r e s s e d a s - - l O g l 0 [Cd 2+] at t h e l i m i t [ 1 ] . Uses t h e fact t h a t i c d e c a y s m o r e q u i c k l y t h a n i F t o i m p r o v e i F : i c. Uses p h a s e d i f f e r e n c e o f i F and ic to i m p r o v e i F : i c. Uses n o n - l i n e a r i t y o f Faradaic p r o c e s s t o i m p r o v e i F : ic. n = s t o i c h i o m e t r i e n u m b e r o f e l e c t r o n s t r a n s f e r r e d in e l e c t r o d e process:

Continuous d current measurement

A.c.p. w i t h p h a s e sensitive detection

a b c d e f

Continuous current measurement

Alternating current polarography a.c.p.

766

in water quality control where a continuous watch must be kept to maintain the concentrations of toxic or potentially toxic materials within legally defined limits. As studies of chemical cycling in natural waters progress it is also becoming increasingly evident that total concentration measurements are only of limited use and that toxicity and geochemical activity are critically dependent on the chemical form of the various solutes. Electrochemical techniques promise both a facility for automation and a sensitivity towards chemical form that make them particularly attractive for natural water analysis [1,2]. In classical d.c. polarography a linear voltage scan is applied between a dropping mercury electrode (DME) and a reference electrode. The current steps accompanying the successive discharge of electroactive components are recorded. The discharge potential of each step is characteristic of the c o m p o n e n t being reduced and the limiting current attained is proportional to its concentration. The sensitivity of d.c. polarography is restricted by the presence of extraneous currents (notably the capacitance charging current (ic)) which can mask the Faradaic current (iF) characterising a particular component. Modulation techniques [3,4] (Table 1) were pioneered in large measure by Barker and his coworkers [5--8], and a t t e m p t to separate iF from ic by superimposing a non-linear signal on the classical linear ramp. The main purpose of this review is to see h o w far the tremendous potential of modulated polarographic and voltammetric techniques has actually been exploited in natural water chemistry and to emphasise areas of particular difficulty and particular promise. ELECTROCHEMICAL CHARACTERISTICS OF NATURAL WATERS

Natural waters are characterised by their variability (Table 2) and complexity (Table 3). From the voltammetric point of view it is important to establish h o w well the background ionic composition can perform as a supporting electrolyte and to try and identify any interferences that are likely to be significant.

Major ion composition The composition of rainwater (Table 2) provides a guide to the lower level of ionic constituents in surface waters. The additional dissolved mineral matter in surface run-off (Table 2) originates primarily from the weathering of rocks. Ionic strengths are also low in these surface waters b u t once the water begins to percolate through the strata (ground waters, Table 2) very high concentrations of dissolved salts can be accumulated. The chemistry of lake waters is strongly influenced by the local mineralogy and by the drainage pattern. Closed basin lakes in h o t climates tend to become very alkaline and hypersaline (Table 2). Sea water itself is characterised by a remarkable constancy of composition of the major ions which enables their concentrations to be specified to better than 0.1% simply by measuring some gross physical property such as

767 TABLE 2 Natural waters -- The composition of the supporting electrolyte Property a

Rainwater b

Fresh water lakes

Run off c

d

e

f

g

h

i

[Ca 2+ ] [Mg 2+ ] [K +] [Na + ] [HCO3-- ] [C1--] [8042--1 pH Ionic a strength

0.008 0.008 0.005 0.083 0.007 0.093 0.033 4.5

0.374 0.169 0.059 0.274 0.958 0.220 0.117 7.0

0.126 2.45 0.058 0.10 0.011 0.03 0.223 0.45 0.194 p 4.60 p 0.240 0.30 0.075 0.30 6.6 8.0

0.09 0.013 0.007 0.139 0.061 0.151 0.060 6.7

0.59 0.12 0.04 0.43 1.08 0.42 0.19 --

0.12 3.58 2.49 5.29 12.5 1.55 0.25 --

0.17 1.73 0.79 2.83 6.24 0.62 0.04 --

0.21

2.08

0.85

0.51

2.5

18.8

9.1

Property a

Rainwater b

[8042--1 pH Ionic a strength

0.008 0.008 0.005 0.083 0.007 0.093 0.033 4.5

Groundwaters

Closed basin lakes j

[Ca 2+ ] [Mg2+] [K+] [Na + ] [HCO3-- 1 [C1--]

8.4

k 0.03 0.02 19.23 956.32 254.88 P 544.4 10.31 9.6

1142.6 q

l

13.42 18.02 41.17 4391.3 8.75 4372.4 87.60 7.5 4644.9

a Concentrations in mmol 1-1. b Ref. 16, varies considerably from sample to sample [17,18].~ c Mean river water, Livingstone [ 19 ]. d Crosby Gill !Tributary of R. Duddon, U.K.) [20]. e Bere stream, U.K. [21]. f Thirlmere, U.K. [22]. g Whin's Pond, U.K. [22].

m

0.11 1.40 29.92 934.78 260.36 p 380.82 -

-

9.6

1045.3 q

n

1400 2675.9 553.7 914.3 9.0 9625.5 -3.5 13703

Sea water o

258.2 •1614.2 655.9 1894.5 12.5 6261.8 5.6 6.6

10.57 54.72 10.49 481.78 2.39 560.97 29.05 8.2

8168.3

716.5

h Lake Kivu, East Africa [231. i Lake Tanganyika, East Africa [24]. / Abert Lake, U.S.A. [25]. k Saline Valley, U.S.A. [25]. I Mono Lake, U.S.A. [25]. m N.E. Mecklenburg, N. German Plain [26]. n Lausitz, N. German Plain [26]. o 35%osea water [27]. P Carbonate and bicarbonate both expressed as [HCO3--I. q Allowing for predominance of CO32- at pH 9.6.

c o n d u c t i v i t y or refractive i n d e x . This c o n s t a n c y of c o m p o s i t i o n plus the high i o n i c s t r e n g t h m a k e sea w a t e r a n i d e a l s u p p o r t i n g e l e c t r o l y t e f o r p o l a r o g r a p h i c and voltammetric measurements. S o m e d i f f i c u l t i e s are e x p e r i e n c e d w i t h n a t u r a l f r e s h w a t e r s b e c a u s e t h e i r l o w i o n i c s t r e n g t h s ( 1 0 - 3 t o 1 0 - 2 M, T a b l e 2) c o n t r a s t w i t h t h e c o n c e n t r a t i o n

768 of supporting electrolyte traditionally required by polarography (0.1 to 1 M). As the effective cell resistance increases so does the time for capacitance decay. Hence the ability to enhance i F by modulation is reduced (Table 1). Modern potentiostats provide good control of potential by employing a three electrode system in which no appreciable current flows at the reference electrode. This substantially reduces the effective ohmic (iR) drop and improves the i F : ic ratio (Table 1). Of the modulated techniques, pulse polarography is especially well suited to low ionic strength solutions (down to 10 -3 M) since relatively long duration pulses (40 to 100 ms) are used so that ic has time to decay to negligible proportions before i F is measured [6,30,31]. Near the limits of detection, non-linear backgrounds make it more difficult to distinguish and measure the peaks [ 11 ]. Instrumental techniques to circumvent the iR drop problem have mainly involved manual application of positive feedback compensation [8,14,32]. A new instrument [33] uses automatic dynamic compensation with an overall response rate of a few microseconds, well within the required range for pulse techniques. The maximum cell resistance can typically be increased from 10 k~2 to 0.5 M ~ [33]. An alternative approach to iR compensation might be to use differential coulostatic polarography [34] which can work down to 1 0 - 4 M supporting electrolyte concentrations. L o w concentrations of supporting electrolytes do n o t always create problems. Some irreversible processes show a marked increase in reversibility at lower concentrations of the supporting electrolyte [35]. Trace constituents The concentrations of electroactive trace constituents in sea water (Table 3) typically represent the lower levels found in natural waters. These low levels, several orders of magnitude lower than can be attained by preparing artificial sea waters from analytical grade salts, are a tribute to the very efficient biological and geological scavenging processes that occur in estuaries and in the open oceans. They also indicate that great care must be taken to avoid accidental contamination of the samples, particularly via any chemicals introduced as a supporting electrolyte [36,37]. In open ocean waters it is unlikely that concen. trations will vary by more than an order of magnitude a b o u t the levels shown (Table 3). Fresh waters and estuarine waters will show much greater variability because their chemistry depends, to a large extent, on local mineralogy and can be influenced markedly by, for example, the presence of mine drainage waters or industrial effluents. Because the trace composition of natural waters is so variable and diverse, interferences between various electroactive species are a c o m m o n feature of the polarographic and voltammetric analysis of natural waters. The c o m m o n e s t and most readily countered problems occur when two metals are deposited from the untreated sample at similar potentials onto the mercury electrode. These interferences are most simply overcome by altering the chemistry of the supporting electrolyte (see Tables 4 and 5). For

769 TABLE 3 Typical concentrations of electroactive components in sea water (35%o salinity) a Element

Chemical species b

Total concentrations ([X ]/nmol 1--1)

pX value (--lOgl0 [X])

Ag As Bi Cd Co Cr Cu Fe Hg I In Mn Ni Pb Sb Sn T1

AgCl~HAsO42-, H2AsO 4 BiO +, Bi(OH)2 + CdC120 Co 2+ Cr(OH)3 , C r O 4 2 CuCO30, CuOH + Fe(OH)2+, Fe(OH)4-HgC142-- , HgC120 IO3--, I--, In(OH)2 + Mn 2+, MnC1+ Ni 2+ PbCO30, Pb(CO3)22Sb(OH)6-SnO(OH)3-T1+ UO2(CO3)22WO42ZnOH +, Zn 2+, ZnCO3°

0.4 50 0.1 1.0 0.8 5.7 8.0 35 0.02 500 0.0008 3.6 28 0.2 2.0 0.05 0.05 14 0.5 76

9.40 7.30 10.00 9.00 9.10 8.24 8.10 7.46 10.70 6.30 12.10 8.44 7.55 9.70 8.70 10.30 10.30 7.85 9.30 7.12

U

W Zn

Relevant properties

c,f e c,e c,e d,g e c,f,g g c c d,g d,f c~e d,f c,f c,f d c,f,g

a Selected from Brewer [ 28 ]. b 25 ° C, 1 atmosphere pressure, 35%0 salinity, pH 8.2, oxygen saturated water. c Amalgam forming metals. d Form very dilute amalgams <10 - 3 wt. %. e Toxic at levels sometimes found in other natural waters [29 I. f Potentially toxic but usually too dilute [29 ]. g Nutrient metals.

example EDTA has been used in direct polarography to remove zinc [38,39], lead [40] or copper [41] interferences. Interfering a.c. currents resulting n o t from coincident deposition but from the adsorption and desorption of chloride or bromide ions can affect the determination of copper in sea water [42 ]. Differential pulse techniques are also sensitive to adsorption interferences on occasion [43]. When lead is present at very low levels in unbuffered natural waters, traces of residual oxygen can cause localised increases in pH near the electrode surface giving a depressed signal for the metal ion [44]. This problem can be overcome by using well buffered or acidic supporting electrolytes. Most natural waters are also culture media for living organisms and consequently contain variable quantities of dissolved organic material [45,46]. Additional organic loads may result from sewage or industrial effluent. Only a small proportion of dissolved organic material has been identified (in sea-

770 water [46] less than 10%) and this characterised fraction consists mainly of urea, long chain hydrocarbons, fatty acids, carbohydrates, amino acids and vitamins. Surfactants present may aid some voltammetric determinations by suppressing polarographic maxima and interfere in other cases by coating the electrode surface and affecting the efficiency of metal reduction or oxidation [29]. The formation of strong complexes (e.g. with humic acids) may render many metals electrochemically inactive. Most voltammetric procedures for the quantitative analysis of dissolved metals consequently include a strong oxidation step to destroy organic material [47--51].

p H and redox potential [52] The pH of most natural waters is controlled, over millenia at least, by the carbon dioxide system. For surface waters the normal pH range is 6.5 to 8.5. Sea water rarely falls outside the range pH 8.0 to 8.3. Such deviations that do occur are most c o m m o n l y associated with the evolution or uptake of CO2 during algal respiration or photosynthesis. Surface waters with a high calcium bicarbonate content (Table 2) may be above pH 8 and waters subjected to high partial pressures of carbon dixide (ground waters, Table 2) m a y fall below pH 5. On o c c a s i o n t h e dissolution of silicate minerals can cause an increase to pH 10. Values below pH 4.5 are usually due to the presence of free mineral acids supplied from volcanic gases or from the oxidation of sulphides. Ferric and aluminium salts and organic acids are also sometimes responsible for low pH values. The pH buffer capacity of most natural waters is low. Attempts to define the redox potential of natural waters [ 52] have been largely unproductive because of the presence of a whole range of redox couples that are n o t very efficiently linked [53,54]. Nonetheless, conditions range from oxygen supersaturation to complete oxygen depletion. Anoxic conditions c o m m o n l y occur in ground waters, in h o t springs, in poorly flushed basins, fjords and lakes and in the interstitial waters of many marine and fresh water sediments. Such environments can create an unusual and interesting water chemistry. Anoxic waters are, in a sense, ideally suited for in situ voltammetric work because they have already been de-oxygenated. ANALYTICAL APPLICATIONS OF DIRECT POLAROGRAPHY

Analysis of samples at natural concentrations The sensitivity of modulated polarographic techniques (Table 1) enables them to be used directly in natural samples for the estimation of a few constituents ( F - , IO3-, Zn (II), silica, Table 4). In principle, direct analysis greatly reduces the number of chemical manipulations involved, and hence minimises the risk of contamination. However, some sample pretreatment is usually required (Table 4) to avoid interferences in the polarographic analysis and to ensure that the c o m p o n e n t being analysed is quantitatively converted to the electroactive form. Most procedures involve the reduction of the c o m p o n e n t

771 being analysed. However the c o m p o n e n t of interest can also be made to react with an electro-active ligand whose concentration is monitored (e.g. the determination of Al(III) [47] and F - [58], Table 4). The determination of sulphide is unusual because it depends on the formation of an insoluble mercuric sulphide film on the electrode surface [ 64]. Determinations by both differential pulse [65] and normal pulse polarography [66] have been reported but the only practical determination in water samples has used square wave polarography [40]. Direct polarographic methods are not always simpler than more conventional techniques. A systematic study [47] of the application of d.p.p, to the analysis of some metal ions in fresh waters resulted in satisfactory and sensitive analyses for Cu(II), Pb(II), Mn(II) and Al(III) (Table 4). The polarographic methods were, however, more time consuming than conventional colorimetric methods because of the tedious sample pretreatment (wet oxidation) required [47]. The same m a y be said for the analysis of fluoride in sea water using a.c. polarography [58]. A prolonged digestion is required and correction must be made for sulphate interference. The precision achieved is little better than that obtained with the more convenient potentiometric procedure [67]. A technique for determining silica in fresh waters using a.c. polarography and d.p.p. [62] also appears to be more tedious than alternative colorimetric procedures since it requires prior extraction of the molybdosilicate complex into ethyl acetate. The technique is more sensitive than the standard colorimetric procedures but the only c o m m o n l y occurring natural fresh water that would have sufficiently low silica concentrations to warrant the added effort would be rain water. In summary, direct polarography has not yet found its metier in the analysis of natural water samples. The technique is most useful for the determination of non-metallic components that might be difficult to preconcentrate [38--40, 59,61]. Recent work [53,68] has suggested that hydrogen peroxide might be a key c o m p o n e n t in fixing the redox potential of natural waters. Direct polarographic analysis using radiofrequency polarography or intermodulation techniques [1,3] should prove useful here. The unique versatility of polarographic techniques -- their sensitivity to chemical form and their ability to analyse a number of components in a single, small sample -- could be more fully exploited. Where several oxidation states are present it is sometimes possible to discern the toxic from the non-toxic forms (e.g. Cr(VI) from Cr(III) [41], or As(III) from As(V) [55,56] ) or to determine the ratio of the oxidised to the reduced form (e.g. I O 3 - / I - [38, 39] ). Natural anoxic waters often contain unusually high concentrations of electroactive components and, since they are already de-oxygenated, they provide ideal polarographic media. Recent work on anoxic (hypolimnetic} lake waters [69,70] has shown that d.p.p, can be used without pretreatment to determine Fe(II), Mn(II) and S 2- in a single scan in the range 2 to 100 pmol 1-1. Good agreement was obtained between these direct determinations and conventional colorimetric techniques.

5.5 t o 7.5

a.c.p,

d.p.p.

s.w.p.

Cu(II)

a.c.p.

7.8 t o 8.1

d.p.p,

Cr(VI)

F--

t o 7.3

d.p.p,

As(III)

4 t o 5.3

t o 6.70

t o 8.0

to 7.7

d.p.p,

Al(III)

Concentration range b

Technique a

Component

sea water

fresh water river water

sea water

sewage effluent stream water

fresh water

Sample

Direct p o l a r o g r a p h i c analyses o f n a t u r a l w a t e r s using m o d u l a t e d t e c h n i q u e s

TABLE 4

1 M NaC1 0.07 M HC104 pH < 1 with HNO 3 Acidify with H2SO 4

KCNS

to pH 5

0.1 M N H 4 acetate to pH 7 5 x 10 - 3 M e t h y l ene d i a m i n e

1 M HC1

Sample treatment

Indirect method via z i r c o n i u m Alizarin S complex. De-oxygenation unnecessary.

Good agreement w i t h flamel#ss a.a.s, c Only applicable to grossly p o l l u t e d waters Stability constants o f Cu c o m p l e x e s also e s t i m a t e d R e m o v e organics b y acid o x i d a t i o n 20-fold preconcentration recommended

Indirect method via S o l o c h r o m e V i o l e t R.S. Can also b e used t o determine Fe(III)

Notes

58

32 d

47

57

41

55,56 d

47

Ref.

b~

a.c.p./d.p.p.

d.p.p.

Silica

Zn(II)

5.7 t o 6.4

5.8 t o 6.8

4 to 6

fresh water industrial effluent rain water sea water

sewage a n d surface waters river water

fresh water

sea w a t e r sea w a t e r

sea w a t e r

Using a b b r e v i a t i o n s given in T a b l e 1. E x p r e s s e d as p X = - - l O g l 0 [ X ] : l o w e r limits usually given ( c o m p a r e T a b l e 3). Atomic absorption spectrometry. C a l i b r a t i o n curve used. In all o t h e r cases s t a n d a r d a d d i t i o n is used.

s.w.p.

S2-

a b c d

- 6.3

7.6 to 8.3

a.c.p.

NTA ( n i t r i l o triacetic acid) Pb(II)

6.7 to 7.1

d.p.p.

d.p.p.

Mn(II)

6.0 to 6.7 6.8 t o 8.1

6.5 t o 8.0

d.p.p. d.p.p.

IO 3 I--

6.5 to 6.7

s.w.p.

d.p.p.

IO 3 -

Adjust to pH 3

1 M NaOH 0.09 M N a 4 E D T A See t e x t

Ammoniacal Cd a c e t a t e b u f f e r , pH 8 0.5 M NaC104 0.005 M NaF p H 3 w i t h HNO 3 0.1 M HC1

Add Na4EDTA t o 10 - 4 M A d j u s t to p H 3 Irradiate sample w i t h u.v. for 2 h, add Na4EDTA t o 10 - 4 M 0.1 M BaC12

47

R e m o v e organics b y acid o x i d a t i o n

Can be d e t e r m i n e d without pH a d j u s t m e n t using reverse sweep

48

R e m o v e organics b y acid o x i d a t i o n

63

62

40 d

61

47

59,60 38,39

Rapid drop d e t a c h m e n t used

D e t e r m i n e as difference after estimating IO 3 - o n s u b s a m p l e R e m o v e organics b y acid o x i d a t i o n .

38,39

TABLE 5

sea w a t e r a n d freshwater freshwater freshwater

sea w a t e r a n d freshwater freshwater freshwater freshwater freshwater sea w a t e r a n d freshwater freshwater freshwater sea w a t e r a n d freshwater freshwater sea w a t e r

400 10--20 10--20 10--20 10--20 400 10--20 10--20 400 10--20 800 400 10--20 10--20

evaporation evaporation evaporation evaporation ion e x c h a n g e evaporation evaporation ion e x c h a n g e evaporation solvent extraction ion e x c h a n g e evaporation evaporation

200

ion e x c h a n g e

coprecipitation

sea w a t e r

400

10--20

10--20

400

Concentration factor

0.5 M NH4C1/NH 3 a n d 0.2 M CaC12 NH4C1 p y r i d i n e + HC1

0.5 M NH4C1/NH 3 a n d 0.2 M CaC12 excess d i m e t h y l glyoxime 0.5 M NH4C1/NH 3 excess d i m e t h y l glyoxime 0.2 M CaCl 2 a n d 0.5 M HC1 NH4C1 p y r i d i n e + HC1 NH4C1 NH4C1 0.5 M N H 4 C I / N H 3 a n d 0.2 M CaC12 NH4Cl p y r i d i n e + HC1 0.2 M CaCl 2 a n d 0.5 M HC1 NH4Cl HC104[tartrate

p y r i d i n e + HC1

NH4C1

0.2 M CaC12 a n d 0.5 M HC1

Supporting electrolyte

to to to to

7.4 7.3 7.4 7.3

7.1 t o 7.4 7.1 t o 7.3

6.7

7.6 t o 7.9 7.8

7.1 t o 7.4 7.1 to 7.3 8.4

7.1 7.1 7.1 7.0 7.9

7.5

9.4 t o 9.9

9.8

7.1 to 7.3

7.4 t o 7 . 7

7.9

Sample a concentration

a E x p r e s s e d as p X = --log10 [ X ] in t h e original s o l u t i o n . b Using a.c.p., all o t h e r e x a m p l e s use d.p.p. c C a n n o t be p r e - c o n c e n t r a t e d t o a n y e x t e n t b y a m a l g a m f o r m a t i o n . All m e t h o d s r e c o m m e n d d e s t r u c t i o n o f organic m a t t e r p r i o r t o

Zn(II)

U(VI) c

Pb(II)

Fe(III) c Mn(II) c Ni(II) c

Cu(II)

ion e x c h a n g e

sea w a t e r a n d freshwater

evaporation evaporation

freshwater freshwater

Co(II) c

ion exchange

sea w a t e r a n d freshwater

Cd(II)

Concentration technique

Sample

Component

P o l a r o g r a p h i c analysis o f n a t u r a l w a t e r s f o l l o w i n g c h e m i c a l p r e c o n c e n t r a t i o n

71 72 b

49

71 73

71 72 b 49

71 72 b 71 71 49

49

37

49

72 b

71

49

Ref.

775

Quantitative analysis after chemical preconcentration Polarographic methods are attractive for the analysis of chemically preconcentrated samples because of their ability to determine several elements in a single aliquot if the supporting electrolyte is carefully selected (Table 5). This is well illustrated by a technique used for the analysis of marine and freshwater samples preconcentrated on a chelating resin [49,74]. Six metals were determined in a single 4 ml sample, simply by adding reagents to modify the supporting electrolyte between sweeps (Table 5, see also refs. 71,72). Chemical preconcentration techniques are especially useful for the determination of trace metals that do n o t form amalgams (Tables 3 and 5). Procedures have been described for the determination of Mn(II) and Fe(III) in fresh waters [71] and for the determination of Co(II) [37,49], Ni(II) [49] and U(VI) [73] in sea water (Table 5). The uranium procedure, though tedious, produced results of a high precision that compared well with estimates made using isotope dilution analysis. The ion exchange procedure for preconcentrating cobalt [49] is more convenient than the coprecipitation technique [37] and has been used successfully on board ship. Since comparable precisions are obtained in the t w o analyses, the individually tailored technique [37] does not seem to represent a significant advance. Recent criticisms of the ion-exchange procedure [ 75,76] might alter this perspective. STRIPPING ANALYSIS -- VOLTAMMETRIC ANALYSIS FOLLOWING ELECTROLYTIC PRECONCENTRATION

The technique Many electroactive constituents in natural waters are present at concentrations near or below the detection limits of even the most sensitive direct polarographic techniques (compare Tables 1 and 3). For amalgam-forming metals (Table 3) a convenient form of preconcentration and subsequent voltammetric analysis is provided b y anodic stripping voltammetry (a.s.v.) as suggested by Barker and Jenkins [5]. The working electrode is usually a hanging mercury drop electrode (HMDE) or a thin film electrode (TFE) which consists of a carbon substrate (glassy carbon, wax impregnated grahite, carbon paste) on to which a thin mercury film is deposited, preferably directly from the sample solution [77,78]. Metals are concentrated from the solution into the mercury by cathodic deposition and the anodic signal, corresponding to the subsequent stripping of the metals back into the solution, is recorded. The overall sensitivity of the analysis is increased by two or three orders of magnitude over direct polarographic methods [1,2,79,80]. A.s.v. suffers the same interferences as direct polarography. Overlapping peaks cause problems [81] in the determination of lead with tin, bismuth with antimony, lead with thallium (HMDE only) and thallium with cadmium (TFE only). The stripping potentials can be modified to some extent by the choice

776 of electrode [81]. Intermetallic c o m p o u n d s are also formed in the mercury prior to stripping giving rise to low stripping currents [ 51,82,83]. These effects are most notiCeable in TFE systems. Metals with the most negative deposition potentials (e.g. Zn, Cd, In) are most severely affected. Modulated stripping procedures help to reduce this problem because their enhanced sensitivity enables lower amalgam concentrations to be used. Other devices to reduce this form of interference are (i) careful selection of the plating potential to exclude the interfering element (e.g. removal of zinc interference in copper analysis [51]) or (ii) the purposeful addition of a metal that forms a more stable comp o u n d with the interfering metal (e.g. addition of gallium in acid solution to prevent copper interference with zinc analysis [84,85]). A.s.v. is also subject to interferences from organic materials in the sample, particularly surface active components, in both the deposition and stripping steps. The need for the removal of organic matter prior to analysis in industrial effluent using d.p.a.s.v. at the HMDE has been emphasised [ 50] although no pretreatment is required for the analysis of tap water.

Modulation techniques The utility of modulation techniques in a.s.v, depends, to a large extent, on the working electrodes used. Modulation techniques during anodic stripping increase the analytical sensitivity at the HMDE by an order of magnitude and greatly improve the selectivity of the stripping step by providing narrower current peaks and a more uniform baseline [ 50,86]. The sensitivity obtained with the HMDE approaches that obtained with the TFE using a glassy carbon [81] or carbon paste [1] substrate. The use of modulation techniques with the glassy carbon TFE results in a relatively small increase in sensitivity [81] over linear scan a.s.v, and broader peaks, more sensitive to surfactants, are obtained. A more noticeable improvement is observed with graphite electrodes using d.p.a.s.v. [87,88] and phase selective a.c.a.s.v. [42]. For laboratory-based analyses the HMDE has a number of distinct advantages over the TFE [89,90]. The deposition and stripping processes at the HMDE £re less sensitive to changes in the composition of the supporting electrolyte [81]. Intermetallic interferences are also greatly reduced at the HMDE because of the larger mercury volumes used and clearer resolution of zinc from the hydrogen evolution wave and copper from the mercury oxidation wave can be obtained in acid solutions. The HMDE is also less sensitive to iR drop problems when very dilute supporting electrolytes are used [ 91]. For field work and kinetic studies [1] the TFE has some advantages. Although the HMDE has been used at sea [92] it is obviously more sensitive to vibration than the TFE. Greater uniformity of stirring and insensitivity to ship movement is obtained using a rapidly rotating TFE. The rotating glassy carbon TFE seems to provide the most sensitive system for a.s.v, at the present

Cu(II)

9.1 6.7--8.0

S.W.

HMDE

a.c.

7.2

fresh a n d saline river water sea w a t e r a n d estuarine water

8--9.3

a.c.

HMDE

tapwater

6.7

acid m i n e water

d.p. HMDE

HMDE

8.5 sea w a t e r

fresh a n d saline river water sea water, estuarine water sea w a t e r

8--9.3

7--9

"natural" water stream waters

sea w a t e r

fresh w a t e r

Sample

6.4--8.4

HMDE d.p. tubular TFE

d.p. T F E (glassy C) d.p. TFE (graphite) a.c. HMDE

Cd(II)

9.3--10.1

d°p.

d.p. T F E (glassy C)

Bi(III)

8.8

d.p. gold electrode

As(III)

Concentration b

a.c. HMDE

Technique a

Cornponent

0.015 M HNO 3

none

0.4 M KC1 0.3 M HCI

none

none

0.015 M HNO 3

acetate buffer to pH 5 acetate buffer t o p H 5,7

0.05 M HC1

2 M HC104/Na2SO 3 at 80 t o 1 0 0 ° C t o r e d u c e As(V). 1 M HC104 s u p p o r t i n g electrolyte

Sample treatment

M o d u l a t e d v o l t a m m e t r i c analysis of n a t u r a l w a t e r s f o l l o w i n g e l e c t r o l y t i c p r e - c o n c e n t r a t i o n (a.s.v.)

TABLE 6

(Continued

Fair a g r e e m e n t w i t h a.a.s, c

Good agreement with a.a.s, c

Fair a g r e e m e n t w i t h a.a.s, c

Good agreement w i t h c a r b o n r o d a.a.s, c Good agreement w i t h flameless a.a.s, e

W(VI) and Mo(VI) interference

100

99

50

104

88,103

101,102

100

99

83 d

87

98

97

Ref.

o n p. 7 7 8 )

Careful p r e t r e a t m e n t of Au electrode required

Notes

Pb(II)

d.p. HMDE a.c. HMDE a.c. HMDE d.p. HMDE

industrial effluent

5.0--6.3

fresh and saline river w a t e r sea w a t e r and estuarine w a t e r sea w a t e r

8--9.3

8.1

8.2

tap w a t e r

8.2

7--9

"natural" water s t r e a m waters

5.6--8.7

d.p. T F E (.glassy C) d.p. T F E (graphite) d.p. HMDE

sea w a t e r

--

acid m i n e water

sea w a t e r

7.4

Sample

4.2--5.5

d.p. HMDE d.p. tubular TFE

Cu(II)

Concentration b

s.w. HMDE

Technique a

Cornponent

TABLE 6 (continued)

none

0,015 M HNO 3

none

acetate buffer to p H 5 acetate buffer to pH 5.7 digest w i t h HC104/HNO 3

0.4 M KC1 0.3 M HC1

none

none

Sample t r e a t m e n t

Fair a g r e e m e n t w i t h a.a.s, c

Good agreement with c a r b o n r o d a.a.s, c Good agreement with flameless a.a.s, c Good agreement with c o l o r i m e t r i c results. a.a.s, c results h i g h e r

P r o b l e m s w i t h Ag contamination from reference electrode Good agreement w i t h a.a.s, c

Notes

101,102

100

99

50

50

83

87

104

88,103

101,102

Ref.

00

stream waters tapwater f r e s h a n d saline river w a t e r sea w a t e r sea water sea w a t e r acid m i n e water

10.2--10.7

6.1 4.6 8--9.3

7.1 7.5 3.8--5.2

d.p. HMDE

d.p. TFE (graphite) d.p. HMDE a.c. HMDE a.c. HMDE d.p. HMDE d.p. tubular TFE s.w. HMDE

Zn(II)

0.4 M KCI 0.3 M HC1

none

none

0.015 M HNO 3

acetate buffer to p H 5.7 none

0.09 M acetate buffer t o p H 4 . 6 , 2.9 x 10 - 3 M EDTA

0.4 M KCI 0.3 M HCI

none

104 76

83

Good agreement with a.a.s, c 140-fold preconcent r a t i o n b y ion e x c h a n g e Good agreement w i t h f l a m e l e s s a.a.s, c

88 104

Flow through system Good agreement with a.a.s, c

101,102

100

99

50

88,103

Flow through system

¢D

a (i) m o d u l a t i o n o f s t r i p p i n g s t e p : d.p. = d i f f e r e n t i a l p u l s e ; a.c. = a.c. w i t h p h a s e s e n s i t i v e d e t e c t i o n ; s.w. = s q u a r e wave. (ii) e l e c t r o d e : T F E = thin film electrode; HMDE = hanging m e r c u r y drop electrode. b E x p r e s s e d as p X = - - l O g l 0 [ X ] . c Atomic absorption spectrometry. d Uses radio tracers to s t u d y interference effects.

7.3

river w a t e r a n d sea w a t e r

acid m i n e waters

6.1--7.6

Tl(III)

sea w a t e r

9.2

d.p. tubular TFE s.w. HMDE

Pb(II)

780

time [77,84]. Sensitivity might be increased still further using rotating ringdisc electrodes [94--96].

Analytical applications The omnipresence of potential interferences in natural samples is largely responsible for the rather conservative range of analyses so far attempted with modulated a.s.v. (Table 6) and for the reluctance to adopt these methods as routine procedures. Most applications have involved the quartet of metals Zn(II), Cu(II), Cd(II) and Pb(II). Usually the application of a differential pulse [87,89,90,101,102] or a.c. [42] modulated stripping step results in significant improvement in the sensitivity and resolution of the analyses of natural samples. However some analyses of sea water samples using the glassy carbon TFE [81,98] (particularly the determination of bismuth [78] ) suggest that little is gained by using d.p.a.s.v. In fact the slowness of the stripping step in the modulated mode is seen as a disadvantage when many samples are being analysed [84]. The analysis of thallium in sea water [76] is unusual in that a chemical preconcentration step is required to analysis by d.p.a.s.v, at the HMDE. Thallium is preconcentrated as the anionic chlorocomplex (TIC163-) on an anion exchange resin. Nearly three days are required for each sample to pass through the column. In fresh water analyses, good agreement has frequently been observed between results obtained by modulated a.s.v, techniques and those obtained using atomic absorption spectrometry [50,83,87,100,104] or colorimetric techniques [3,50] following chemical preconcentration (Table 6). This agreement is sometimes surprising [87] since it is expected that a.s.v, analysis on untreated samples should be sensitive to only a fraction of the total metal present (see Speciation Studies). The experience of intercalibration exercises in marine chemistry is n o t so good [36,105,106] and a.s.v, methods frequently give anomalously high results. Some cold comfort may be drawn from the fact that poor performance in intercalibration procedures is a c o m m o n fault of most procedures used for the analysis of trace metals in sea water [105,107] and arises mainly from the difficulties in preventing contamination and in preparing suitable standards when such low metal concentrations are determined. Similar problems might give rise to the relatively poor reproducibility obtained with replicate samples when a.c.a.s.v, is used to analyse saline waters [100]. IN SITU AND ON SITE ANALYSIS

The most obvious way of overcoming contamination problems is to remove the sampling step completely and lower the cell directly into the water. The complexity and variability of natural waters (Tables 2 and 3) suggests that extreme caution must be taken in interpreting such direct measurements. The possibility of direct polarographic measurement in anoxic waters has already been mentioned [67,70] and high frequency (500--1000 Hz) short controlled

781

drop time a.c. polarography is capable of making useful measurements in the presence of oxygen provided that oxygen reduction products do n o t interfere [108,109]. Much of the hardware needed is already available [2] and compact systems have been described for the direct and accurate digital recording of voltammetric curves under field conditions [110--112]. Voltammetric analyses have been carried out successfully on board ship [92,113] and on the river bank [114] and a variety of flow cells have been designed [93,115--117] that should enable the scope of on site analyses to be widened by the introduction of auto-analyser techniques [115]. Zirino and his coworkers [88,93, 103] have used a TFE system based on a tubular graphite electrode and a tubular silver/silver chloride reference electrode to investigate the on site analysis of sea water samples by a.s.v. Deoxygenated sea water is p u m p e d through the electrodes during the plating step and during the stripping step, which is continued until the mercury film itself is stripped off. A mercury plating step is interposed between the analyses and the whole procedure is a u t o m a t e d [103]. The application of d.p.a.s.v, enhances the zinc analyses fifty-fold and improves the resolution of the peak from the hydrogen evolution wave. Although initial results [88,93,103] showed rather small peaks against a strongly sloping background, later improvements have given better resolution and results in good agreement with atomic absorption spectrometry on preconcentrated samples [ 118]. The agreement between replicate samples [ 93 ] is significantly better than that achieved with more conventional methods [92,100]. A similar flow through system has been described for the determination of thallium in sea water [119]. SPECIATION STUDIES

Voltammetric techniques are uniquely suited for speciation studies because Of their sensitivity both to low metal concentrations and to the chemical forms of metals in solution. Speciation studies, potentially the most useful applications of voltammetric techniques to natural waters, are progressing on two levels [2,88,120].

Operational classifications On the coarse level the soluble forms (particle size ~ 0 . 4 5 pm) of the amalgam-forming metals are subdivided according to whether they can be detected by a.s.v, analysis on the untreated sample (electro-active metal) or whether some vigorous treatment such as u.v. oxidation or acidification to pH 3 or less is required to release the metal for a.s.v, analysis (electro-inactive metal). The electro-active metal consists mainly of inorganic complexes and labile (kinetically mobile) organic complexes while the electro-inactive fraction will include colloidal material and metal b o u n d in refractory (kinetically immobile) complexes. Most investigations of this coarse fractionation in sea water have used a.s.v.

782 analysis with a d.c. stripping step [2,88]. D.p.p. has been used to study the fractionation of zinc in sea water in some detail [120]. Only 10 to 15% of the zinc in the samples studied was electro-active. The electro-active c o m p o n e n t was further subdivided by observing the apparent release of metal as the pH was gradually decreased. D.p.p. has also been used to illustrate the removal of electroactive components from natural waters by sewage [121]. D.p.a.s.v. at the HMDE has been used to determine labile zinc, copper, cadmium and lead in lake water [122] adjusted to pH 6.8 to 7.8 with an acetate buffer. Oxidation with sulphuric acid and potassium persulphate was used to release the electro-inactive metal ions. A survey of seventeen different lake waters suggested that most of the copper and lead were strongly b o u n d while zinc was usually predominantly in a labile form. The labile c o m p o n e n t tended to be more significant in lakes of lower pH. Concentrations of cadmium were t o o low ( ~ 2 nmol 1-1) for fractionation to be studied. D.p.a.s.v. has also been used to determine electro-active copper in model systems [123] containing representative solid phases (bentonite, manganese dioxide) and organic material (tannic acid or humic acid). Total soluble copper was determined by atomic absorption spectrometry. The fraction o f soluble copper that was electro-active varied from 100% at pH 3.6 to 0% at pH 6. A similar distribution was observed in systems containing only copper and the organic components. Amperometric titrations, using d.p.a.s.v., can be used to estimate the complexing capacity of a natural water for a particular metal [2,124--126]. In a study of the complexing capacity of lake water [127] the electro-active copper concentration was measured by d.p.a.s.v. 2 h after an aliquot of ionic copper was added. A plot of peak height versus spiked copper concentration gave a straight line whose intercept on the concentration axis was interpreted as the complexation capacity of the lake water. Using EDTA as a model complexing ligand theoretical results were achieved [127]. Copper was able to displace ligands from existing labile metal complexes and only Pb(II) and Fe(III) complexes were likely to remain unaffected. The complexing capacities of various lake waters varied from undetectable to 0.7 pmol 1-1 copper equivalent. The same technique has been used [128] to investigate the toxic effect of copper on p h y t o p l a n k t o n production in waters having different complexing capacities. While micromolar concentrations of copper have been used in most toxicity studies the indications are that ionic copper is already toxic to planktonic algae below 0.1 nmol 1-1. A slightly different approach was used [125] to determine the apparent complexing capacity of sea water for ionic lead. A break in the curve of peak height versus added lead concentration was used to estimate a complexing capacity of 0.3 pmol 1-1. These procedures are time consuming and an ingenious "single p o i n t " proce. dure has been suggested [129]. An excess of Co(II) was added to the sample and mild oxidising conditions at pH 7 were used that enabled Co(III) to be formed, provided it is stabilised by an organic ligand. The residual Co(II) was

783

determined by d.p.p, in 0.05 M ethylene diamine and the complexing capacity estimated. The detection limit was 0.6 pmol 1-1 and correlations were observed between the complexing capacity and the presence of industrial and sewage effluents. Clearly this operational subdivision can go on indefinitely and molecular size has recently been suggested [130] as an additional criterion. More emphasis should n o w be given to establishing the chemical and biological significance of the various fractions. In particular it is important to bear in mind the sensitivity of the a.s.v, system itself to the various chemical manipulations used and to the presence of surface active constituents [29,84,131].

The determination of stability constants On the fine level, speciation studies have been directed at the determination of stability constants for clearly defined inorganic complexes in artificial, organic-free replicas of natural waters [1,88,132]. Generally the metal levels employed have been several orders of magnitude higher than those c o m m o n l y observed in natural systems. In a particularly detailed study [133] the stability of copper, lead, cadmium and zinc complexes of relevance to fresh water systems were determined. A 0.1 M KNO3 supporting electrolyte with enriched metal concentrations of 2.5 pmol 1-1 were used so that d.p.a.s.v, and d.p.p. could be used to investigate the dependency of peak potential and current on alkalinity and pH. Both hydroxide and carbonate were important in regulating the copper species present. For lead the dominant species found was PbCO30 while the zinc equilibrium was dominated by insoluble hydroxide formation above pH 7.5. Cadmium remained almost exclusively in the soluble form. Stability constants for PbCO3 ° and CuCO3 ° formation were determined from shifts in the peak potential using the Lingane equation for both the d.p.p, and d.p.a.s.v, measurements [132,134] and were found to agree with previous determinations. Glycine formed an irreversibly reducible copper complex b u t did n o t significantly complex lead, cadmium or zinc. Addition of humic acid showed evidence of strong complex formation in all cases and produced irreversible chemical reactions. Accurate measurements of stability constants were n o t possible as was evidenced by the completely different estimates obtained from d.p.p, and d.p.a.s.v. This emphasises tl:e usefulness of using two distinct techniques to check on the self-consistency of a determination. Chemical models for sea Water are notorious for their inconsistency in predicting the chemical speciation of the most c o m m o n l y studied electroactive metals [120,135] and direct experimental investigations are sorely needed [1]. D.p.p. has been used on enriched sea water samples (1 pmol 1-1 metal) to determine the speciation of zinc [120,136] and indium [137] using the procedure of DeFord and Hume [138]. The dominant species suggested were ZnOH ÷ and In(OH)2 ÷ respectively. In the indium study [137] solutions in the range 1 to 100 pmol 1-1 w e r e analysed by d.p.p, and those in the range 10 to 1 0 - 3 pmol 1-1 by a.s.v. Phase sensitive a,c. polarography has been used [ 57] to

784

determine the stability constants of copper complexes at natural copper levels in artificial ionic media (NaC1, NaC104, NaHCO3, artificial sea water). The dominant inorganic copper complexes suggested in natural sea water are CuC1 + and Cu(HCO3)2OH-. Recently, an important and subtly different approach has been proposed [1,137] for the determination of stability constants by a.s.v, at natural concentrations. The peak potentials observed in the stripping trace are plotted as a function of the deposition potential and a step-shaped curve is obtained analogous to the classical d.c. polarogram. By plotting the apparent half-wave potential of this curve (E*/2) or the tangent potential at the f o o t of the curve (E*) versus ligand concentration the stability constants and stoichiometry of the chloro-complexes of cadmium in sea water were evaluated [139]. CONCLUSIONS

Almost all of the applications of modulation techniques to natural waters have appeared within this decade. This would suggest that the techniques are in their infancy and y e t examination of the paper by Barker and Jenkins [5] published in 1952 clearly shows that this is n o t so. As well as introducing square wave polarography they outlined the principles of ohmic compensation, introduced modulated a.s.v, techniques and provided instrumental refinements that have only recently been incorporated in commercial instruments. A reviewer at that time could reasonably have concluded that numerous applications would be shortly forthcoming. That this was not so was perhaps due in part to the advent of atomic absorption spectroscopy in 1955 [140] and in part to the lack of any commercial instrumentation of modest price and robust design. Although some advance might be expected in the development of more rapid modulated voltammetric procedures [ 109,141] and improved rejection of ic [15,142] the most significant changes are likely to result from the introduction of computer controlled and processed systems. Here again Barker [143] has provided a lead by developing a multimode polarograph (square wave, radio frequency and square wave intermodulation polarography) coupled with a purpose built data processor. This processor enables baselines to be stored and later substracted, standard peaks to be stored and used to separate overlapping peaks and consecutive polarograms to be averaged to eliminate random noise. In this instance commercial sensitivities have kept pace with events and Princeton Applied Research Corporation have recently introduced a combined polarograph and microprocessor for d.p.p, and d.p.a.s.v, analysis with processing capabilities similar to those provided by Barker's instrument [143] but with the addition of an improved, automated electrode system. A number of instruments have been described [9,144--146] where the electrochemical cell is controlled by a larger c o m p u t e r of more conventional design The versatility of this approach is well illustrated by a system recently decribed [84,147,148]. The whole analytical procedure, including calibration by

785 standard addition and selection of pulse duration and sampling interval, is computer controlled. A rotating glassy carbon TFE was used. The system was used very effectively for the analysis of zinc, cadmium, copper and lead in sea water using a new multiple scan a.s.v, procedure [84]. With computer controlled experiments it was quicker and more precise to average a large number of rapid d.c. scans [143] than use data accumulated by the much slower differential pulsed scan. The use of rapid pulsed scanning [141] or rapid a.c. scanning [109] might combine the best features of both techniques. Current integration techniques [149] can be conveniently incorporated into such systems to enhance the accuracy, precision and sensitivity of modulated voltammetric procedures. Computer controlled systems of this kind will be particularly useful since they will enable the use of voltammetric techniques in combination to be explored -- both in the sense of using several techniques in succession to look at a particular sample [135] and in the development of intermodulation techniques [6,15]. A number of improvementsin cell design should also be forthcoming in the near future. Flow-throughsystems have already been mentioned [88,73,112] in connection with in situ analysis. They could be used to investigate the application of medium exchange [150] to the study of natural samples. Microcells [151] mightprove useful for the analysis of pore waters and interstitial waters where only small samples can be obtained. A developmentof importance to oceanographywould be the application of polarographic and voltammetric techniques to study the effect of pressure and temperature on the chemical speciation of trace metals [152 ]. The basic experimental techniques have already been developed [153--155]. The introduction of more sophisticated instrumentation, however, will not produce any real advancesin our understanding of natural waters unless it is preceded by, or at least accompaniedby, a more rigorous investigation of the chemical and electrochemicalvariables that can affect the half wave potentials and peak currents observed in polarographic and voltammetric analyses. Predictions from theoretical analyses of the various voltammetric procedures [13,14,81,145,156,157] should be compared whereverpossible with the observed response of the signal to variations in the electrochemicaland chemical parameters. Such analyseswill be particularly useful as the potentiality of modulated voltammetrictechniques for the study of kinetic parameters are more fully realised [1,93]. Additional information about the kinetic stability of the various complexes,could be obtained using rapid modulated or stripping steps [109,141] in conjunction with rotating disc, or ring-disc electrodes [94--96]. Organic interferenceswitl~ vo]tammetricanalyses must also be given greater attention in the future [1,29]. The Kalousek commutator technique, in which the polarisation potential is discontinuouslychanged from the ramp potential to the potential of maximum adsorption (Eecm), has been refined to the point where surfactant concentrations can be determined in natural sea water [158--160]. More studies of the distribution of surfactants in natural waters

786

are required and more care must be taken in interpreting voltammetric measurements in natural samples where adsorption of organic material is likely to be significant. If we are to take full advantage of the new perspectives provided by Barker's sensitive electrochemical techniques we must ensure that the line of sight is not obscured by the many possible interferences.

REFERENCES 1 H.W. Nilrnberg and P. Valenta, The Nature of Seawater, Dahlem Konferenzen, Berlin, 1975, p. 87. 2 M. Whitfield, Chemical Oceanography, Vol. 4, Academic Press, London, 2nd edn., 1975, p. 1. 3 H. Schmidt and M.J. von Stackelberg, Modern Polarographic Methods, Academic Press, New York, 1963 4 T. Kambara, Modern Aspects of Polarography, Plenum Press, New York, 1966. 5 G.C. Barker and J.L. Jenkins, Analyst, 77 (1952) 685. 6 G.C. Barker, Anal. Chim. Acta, 18 (1958) 118. 7 G.C. Barker and A.W. Gardner, Z. Anal. Chem., 173 (1960) 79. 8 G.C. Barker, G.W.C. Milner and H.I. Shalgosky, Proc. Congr. Modern Anal. Chem., Soc. Anal. Chem., Heffer, Cambridge, 1957. 9 P.E. Sturrock and P.J. Carter, CRC Crit. Rev. Anal. Chem., 5 (1975) 201. 10 A.A.M. Brinkman, Thesis, Free University of Amsterdam, 1968. 11 J.H. Christie and R.A. Osteryoung, J. Electroanal. Chem., 49 (1974) 301. 12 B. Breyer and H.H. Bauer, Alternating Current Polarography and Tensammetry, Interscience, New York, 1963. 13 D.E. Smith, Electroanalytical Chemistry, Vol. 1, Marcel Dekker, New York, 1966. 14 D.E. Smith, CRC Crit. Rev. Anal. Chem., 2 (1971) 247. 15 A.M. Bond and R.J. Halloran, Anal. Chem., 47 (1975) 1906. 16 E. Gorham, Geochim0 Cosmochimo Acta, 7 (1955) 231. 17 F.J.H. Mackereth, Proc. Linnean Soc. London, 167 (1957) 159. 18 E. Gorham, Bull. Geol. Soc. Amer., 72 (1961) 795. 19 D.A. Livingstone, Data of Geochemistry, U.S. Geol. Surv. Prof. Paper 440-G, U.S. Govt. Printing Office, Washington D.C., 6th edn., 1963, ch. G47. 20 D.W. Sutcliffe, private communication. 21 H. Casey, private communication. 22 J. Heron, private communication. 23 E.T. Degens, R.P. von Herzen, H.-K. Wong, W.G. Dueser and H.W. Jannasch, Geol. Rundschau, 62 (1973) 245. 24 E.T. Degens, R.P. von Herzen and H.-K. Wong, Naturwiss., 58 (1971) 229. 25 B.F. Jones, Proc. 2nd N. Ohio Geol. Soc. Symp., (1966) 181. 26 H.J. RSsler and H. Lange, Geochemical Tables, Elsevier, Amsterdam, 1973, p. 320. 27 J.P. Riley and G. Skirrow, Chemical Oceanography, Vol. 4, Academic Press, London, ~nd end., 1975, p. 320. 28 P.G. Brewer, Chemical Oceanography, Vol. 1, Academic Press, London, 2nd edn., 1975, p. 417 29 P.L. Brezonic, Aqueous Environmental Chemistry of Metals, Ann Arbor Science, Ann Arbor, Mich., 1974, p. 167. 30 E.P. Parry and R.A. Osteryoung, Anal. Chem., 37 (1965) 1634. 31 G.D. Christian, J. Electroanal. Chem., 33 (1967) 1.

787 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

B. Novosel and E.B. Buchanan, Z. Anal. Chem., 262 (1972) 100. C. Yarnitsky and N. Klein, Anal. Chem., 47 (1975) 880. T.M. Katzenberger and P.H. Daum, Anal. Chem., 47 (1975) 1887. F. Sturm and M. Ressel, Microchem. J., 5 (1961) 53. Interlaboratory Lead Analysis of Standardised Samples of Sea Water, Mar. Chem., 2 (1974) 69. B.R. Harvey and J.W.R. Dutton, Anal. Chim. Acta, 67 (1973) 377. P.S. Liss, J.R. Herring and E.D. Goldberg, Nature Phys. Sci., 242 (1973) 108. J.R. Herring and P.S. Liss, Deep-Sea Res., 21 (1974) 777. H. Kakiyama and K. Komatsu, Japan Anal., 19 (1970) 902. S.T. Crosmun and T.R. Mueller, Anal. Chim. Acta, 75 (1975) 199. W.R. Seitz, Ph.D. Thesis, Mass. Inst. of Technol°, Cambridge, Mass., 1970. F.C. Anson, J.B. Flanagan, K. Takahashi and A. Yamada, J. Electroanal. Chem., 67 (1976) 253. J.G. Osteryoung and R.A. Osteryoung, Amer. Lab., 4 (1972) 8. P.A. Cranwell, Specialist Periodical Reports, Environmental Chemistry, Vol. 1, The The Chemical Society, London, 1975. P.J. Le B. Williams, Chemical Oceanography, Vol. 2, Academic Press, London, 2nd edn., 1975, p. 301. N.J. Nicholson, Tech. Memo Wat. Res. Ass. U.K., TM59, 1970. E.B. Buchanan, T.D. Schroeder and B. Novosel, Anal. Chem., 42 (1970) 370. M.I. Abdullah and L.G. Royle, Anal. Chim. Acta, 58 (1972) 283. H. Siegerman and G.O'Dom, Amer. Lab., 4 (1972) 59. W.L. Bradford, Tech. Rep. Chesapeake Bay Inst., No. 76, 1972. L.G.M. Baas-Becking, I.R. Kaplan and D. Moore, J. Geol., 48 (1960) 243. R. Parsons, The Nature of Sea Water, Dahlem Konferenzen, Berlin, 1975, p. 505. W. Stumm, Adv. Water Pollut. Res., 1 (1967) 283. D.J. Myers and J. Osteryoung, Anal. Chem., 45 (1973) 267. D.J. Myers, H.E. Heimbrook, J. Osteryoung and S.M. Morrison, Environ. Letters, 5 (1973) 53. M. Odier and V. Plichon, Anal. Chim. Acta, 55 (1971) 209. H. Berge and L. Briigmann, Beitr. Meeresk., 29 (1972) 115. M. Petek and M. Branica, Thalassia Jugosl., 5 (1969) 257.. M. Branica and M. Petek, Rapp. Comm. Int. Met M~dit., 19 (1969) 767. J. Wernet and K. Wahl, Zh. Anal. Chem., 251 (1970) 373. A. Suzanne, O. Vittori and M. Porthault, Anal. Chim. Acta, 75 (1975) 486. A. Piro, M. Bernhard, M. Branica and M. Verzi, Radioactive Contamination of the Marine Environment, IAEA, Vienna, 1973. J.A. Turner, R.H. Abel and R.A. Osteryoung, Anal. Chem., 47 (1975) 1343. D.R. Canterford and A.S. Buchanan, J. Electroanal. Chem., 44 (1973) 291. D.R. Canterford, J. Electroanal. Chem., 52 (1974) 144. T.B. Warner, Science, 165 (1969) 178. W.G. Breck, The Sea, Vol. 5, Wiley, New York, 1974, p. 153. W. Davison, J. Electroanal Chem., in press. W. Davison, in preparation. R.E. Frazier, J. Amer. Water Works Assoc., 64 (1972) 391. F. Ebner, H. Gams and L.J. Ottendorfer, Oesterr. Abwasser Rundsch., 17 (1972) 53. G.W.C. Milner, J.D. Wilson, G.A. Barnett and A.A. Smales, J. Electroanal. Chem., 2 (1961) 25. M.I. Abdullah and L.G. Royle, Nature, 238 (1972) 329. T.M. Florence and G.E. Batley, Talanta, 23 (1976) 179. G.E. Batley and T.M. Florence, J. Electroanal. Chem., 61 (1975) 205. T.M. Florence, J. Electroanal. Chem., 27 (1970) 273. R.G. Clem, G. Litton and L.D. Ornelas, Anal. Chem., 45 (1973) 1306.

788 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121

E. Barendrecht, Electroanalytical Chemistry, Vol. 2, Edward Arnold, London. I. Shain, Treatise on Analytical Chemistry, Part I, Vol. 4, J. Wiley, New York, p. 2533 G.E. Batley and T.M. Florence, J. Electroanal. Chem., 55 (1974) 23. L. Huynn-Ngoc, Ph.D. Thesis, Institut de Math6matiques et Sciences Physique, L'Universit6 de Nice, 1973. S.T. Crosmun, J.A. Dean and J.R. Stokely, Anal. Chim. Acta, 75 (1975) 421. D. Jagner and L. Kryger, Anal. Chim. Acta, 80 (1975) 255. T.R. Copeland, R.A. Osteryoung and R.K. Skogerboe, Anal. Chem., 46 (1974) 2093. J.B. Flato, Anal. Chem., 44 (1972) 75A. T.R. Copeland, J.H. Christie, R.A. Osteryoung and R.K. Skogerboe, Anal. Chem., 45 (1973) 2171. A. Zirino, S. Lieberman and M.L. Healy, Marine Electrochemistry, Electrochem. Soc., Princeton, N.J., 1973, p. 319. A.M. Bond, Anal. Chim. Acta, 74 (1975) 163. G.E. Batley and T.M. Florence, J. Electroanal. Chem., 55 (1974) 23. T.R. Copeland, J.H. Christie, R.K. Skogerboe and R.A. Osteryoung, Anal. Chem., 45 (1973) 995. A. Zirino and M.L. Healy, Envir. Sci. Technol., 6 (1972) 243. S.H. Lieberman and A. Zirino, Anal. Chem., 46 (1974) 20. W.J. Albery and M.L. Hitchman, Ring Disc Electrodes, Oxford University Press, Oxford, 1971. D. Laser and M. Ariel, J. Electroanal. Chem., 49 (1974} 123. F. Opekar and F. Beran, J. Electroanal. Chem., 69 (1976) 1. G. Forsberg, J.W. O'Laughlin and R.G. Megargle, Anal. Chem., 47 (1975) 1586. T.M. Florence, J. Electroanal. Chem., 49 (1974) 225. H. Blutstein and A.M. Bond, Anal. Chem., 46 (1974) 1531. T. Rojahn, Anal. Chim. Acta, 62 (1972) 438. G. Donadey and R. Rosset, Rapp. Comm. Int. Mer M6dit., 20 (1972) 657. G. Donadey, Ph.D. Thesis, Facult6 des Sciences, Universit6 de Paris, 1969. A. Zirino and S.H. Lieberman, Analytical Methods in Oceanography, Amer. Chem. Soc., Washington, 1975. M. Komatsu, T. Matsueda and H. Kakiyama, Japan Anal., 20 (1971) 987 P.G. Brewer and D.W. Spencer, Woods Hole Oceanogr. Inst., Tech., Rept., 70-62, 1970. J. Duinker, I. Elskens and P.G.W. Jones, ICES C.M. Papers and Reports, No. E:27, 1975. J.P. Riley, Chemical Oceanography, Vol. 3, Academic Press, New York, 2nd edn., 1975. A.M. Bond, Talanta, 20 (1973} 1139. A.M. Bond, Anal. Chem., 45 (1973) 2026. W.W. Goldsworthy and R.G. Clem, Anal. Chem., 43 (1971) 1718. W.W. Goldsworthy and R.G. Clem, Anal. Chem., 44 (1972) 1360. R.G. Clem and W.W. Goldworthy, Anal. Chem., 43 (1971) 918. G.C. Whitnack, Marine Electrochemistry, Electrochem. Soc., Princeton, N.J., 1973, p. 342. D.M. Coleman, R.E. VanAtta and L.N. Klatt, Environ. Sci. Tech., 6 (1972) 452. W. Lund and L.-N. Opheim, Anal. Chim. Acta, 79 (1975) 35. J. Janata and H.B. Mark, Jr., Anal. Chem., 39 (1967) 1896. E. Scarano, M.G. Bonicelli and M. Farina, Anal. Chem., 42 (1970) 1470. A. Zirino, personal communication. R. Seitz, R. Jones, L.N. Klatt and W.D. Mason, Anal. Chem., 45 (1973) 840. M. Bernhard, E.D. Goldberg and A. Piro, The Nature of Sea Water, Dahlem Konferenzen, Berlin, 1975, p. 43. W.T. Donaldson, Int. J. Environ. Anal. Chem., 3 (1973) 1.

789 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 159 159 160

Y.K. Chau and K. Lum-Shue-Chan, Water Res., 8 (1974) 383. R.D. Guy, C.L. Chakrabarti and L.L. Schramm, Can. J. Chem., 53 (1975) 661. W.R. Matson, Ph.D. Thesis, Mass. Inst. Technol., Cambridge, Mass., 1968. M.S. Shuman and G.P. Woodward, Jr., Anal. Chem., 45 (1973) 2032. R.G. Clem, McKee-Pedersen Instruments, Application Notes, 8 (i), 1973. Y.K. Chau, R. Gachter and K. Lum-Shue-Chan, J. Fish. Res. Bd. Can., 31 (1974) 1515. R. Gachter, K. Lum-Shue-Chan and Y.K. Chau, Schweiz. Z. Hydrol., 35 (1973) 252. K.W. Hanck and J.W. Dillard, U.S. Nat. Tech. Inform. Serv., P.B. Report No. 227874/5GA, 1973. R.G. Smith, Anal. Chem., 48 (1976) 74. W. Stumm and P. Brauner, Chemical Oceanography, Vol. 1, Academic Press, London, 2nd edn., 1975, ch. 3. D.R. Crow, Polarography of Metal Complexes, Academic Press, London, 1969. R. Ernst, H.E. Allen and K.H. Mancy, Water Res., 9 (1975) 969. J.J. Lingane, Chem. Rev., 29 (1941) 1. M. Whitfield, Mar. Chem., 4 (1976) in press. A. Piro, M. Bernhard, M. Branica and M. Verzi, Radioactive Contamination of the Marine Environment, IAEA, Vienna, 1973. A. Barid and M. Branica, Limnol. Oceanogr., 14 (1969) 796. D.D. Deford and D.N. Hume, J. Amer. Chem. Soc., 75 (1951) 5321. S. Budi~ and M. Branica, Thalassia Jugosl., 9 (1973) 47. A, Walsh, Spectrochim. Acta, 7 (1955) 108. H. Blutstein and A.M. Bond, Anal. Chem., 48 (1976) 248. B.H. Vassos and R.A. Osteryoung, Chem. Instr., 5 (1974) 257. G.C. Barker, A.W. Gardner and lVLJ. Williams, J. Electroanal. Chem., 42 (1973) App 21 M.W. Overton, L.L. Alber and D.E. Smith, Anal. Chem., 47 (1975) 363A. H.E. Keller and R.A. Osteryoung, Anal. Chem., 43 (1971) 342. Q.V. Thomas, L. Kryger and S.P. Perone, Anal. Chem., 48 (1976) 761. L. Kryger, D. Kagner and H.J. Skov, Anal. Chim. Acta, 78 (1975) 241. L. Kryger and D. Jagner, Anal. Chim. Acta., 78 (1975) 251. L.C. Thomas, G.D. Christian and J.D.S. Danielson, Anal. Chim. Acta, 77 (1975) 163. L. Zieglerov~, K. Stulfk and J. Dolezal, Talanta, 18 (1971) 603. L. Huderov~ and K. Stulik, Talanta, 19 (1972) 1285. M. Whitfield, The Nature of Sea Water, Dahlem Konferenzen, Berlin, 1975, p. 137. G.J. Hills, Talanta, 12 (1965) 1317. G.J. Hills and P. Ovenden, Adv. Electrochem., 4 (1966) 185. G.J. Hills, Symp. Soc. Exp. Biol., 26 (1972) 1. H.E. Keller and R.A. Osteryoung, Anal. Chem., 48 (1973) 210. J.W. Dillard and K.W. Hanck, Anal. Chem., 48 (1976) 218. K. Kozarac, B. Cosovid and M. Branica, J. Electroanal. Chem., 68 (1976) 75. B. Cosovi~ and M. Branica, J. Electroanal. Chem., 46 (1973) 63. J. Radej, I. Ruzid, D. Konrad and M. Branica, J. Electroanal. Chem., 46 (1973) 261.