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Electroanalytical chemistry of cadmium complexes of amino acids at the ionic strength of seawater (0.70 M NaCLO4)
J Eiectroanal Chern., 187 (1985) 333448 Elsewer Sequoia .%A, Lausanne - Printed
ELECTROANALYTK4L AMINO ACIDS AT THE PART
333 m The Netherlands
CHEMISTRY OF C4DRTIUM COMPLJXES OF IONIC STRENGTH OF SEAWATER (0.70 M NaCLO,)
II
M L S SIMdES
GONCALVES
Cenrro de Qurnuca (Recewd
Esirutural,
and MM Complexo
15th August 1984,
CORREIA
Inrerdtscrplmar,
DOS SANTOS Imtliuto
In reblsed form 18th December
Supenor
Trcnrco,
Lrsboa
(Portugal)
1984)
ABSTRACT The complexauon of cadrmum wnb naturally occwrmg ammo acids ~+as studied at the 1omc strength of -water (0 70 M m sodmm perchlorate me&urn) and at 25 f0 1°C by tiferenti pulse polarography and potenuometry WI& a glass electrode The h-quad program was used to analyse and refins the results of the potenhometry. Although the stabhty of the complexes IS kery sumlar smce the chelate groups of the hgands are the same, the rate of dlssoclahon of the complexes can be drfferent The kmeuc aspect has been stuled by cychc voltammetry which IS und x the mfluence of the type of adsorpllon When this IS strong the tifnslon and adsorphon p& are we3 separated and, m prmclple. the rate constant of tisonauon of the complex can be deterslned If there 1s weak adsorphon the tifuslon peak IS dmorted by adsorption and kmenc stu&es cannot be done The adsorpbon of the bgands themselveslms hem studd by ac polarography at a 90’ angle phase and It has been nottced that hgands with 5 bonds, sulphur groups ok wtth long hydrocarbon chams are more adsorbed than those WthOUt.
INTRODUCI-ION
The
toxrcity
speciahon
of cadmium
111 the mar-me envrromnent
that 1s also a funchon
1s strongly
of the orgamc and morgaruc
dependent
on
hgands ~11the natural
medmm. If the level of orgarucs present in the ocean is negligible, over 97% of the cadmium exrsts as chlorocomplexes, especially in the form CdClz [l]. The concentratton of the macroconstrtuents in seawater 1s roughly constant ah over the world, contrary to other natural waters which are very much dependent on the enwonment [2]. According to the hterature, ammo acids are only a relatrvely small fraction of the total DOM (dissolved orgaruc matter), not more than 0.04 mg/l[3]. and so cadrmum ammo acid complexes are not important in seawater condmons. However, these complexes can exist 111regions of higher ammo acid levels, as m some coastal waters, estuaries, pore waters of certain se&ments and zones of high phytoplankton acnvlty, ds extrudates and decomposrtion products of orgamsms. So a study was undertaken 0022-0728/85/%03
30
0 1985 Ekewer
Se+oia
5%.
334
of the complexes of cadnuum with naturally occurring ammo acids, the ftrst part of thrs study havmg already been presented [4] for the complexes with alanine, serine, vahne and glutamtc acid The amino acids considered in thrs paper are proline, rsoleucine, aspartic acrd and phenylglutanune, asparagrne, lysme, methionine, alanine. Some adsorption studres were also done wrth hrsttdine. Some of these armmo acrds are also important from the biochemical point of vtew, smce some enzymes have heavy metals such as copper, nickel or zmc rn thetr structure bound to some anuno actd groups of the type of lysine, methronine, etc. The stabihty of cadnuum complexes wrth these annnoactds was detemuned in the non-complexmg medium of 0.70 M NaCIO, (sirmlar to -the ~onrc strength of seawater) by potentrometry and the Ivimrquad program [5] when there were no values m the hterature [6] for tlus ~onrc strength. Some of these values have also been conftrmed by drfferentral pu!se polarography (DPP) wrth lower metal concentration, closer to those met tn the real system. The determmation of the stabllrty constants by DPP m artrficial and genuine unpolluted seawater has not been done smce the agreement was good for the stabrhty constants of cadtruum glyctne complexes [7] and for the cadmium aspartrc acrd complexes deternuned by anodrc stnpping voltammetry (ASV) [S]. Kmetrc studies for media closer to seawater, considering all the stabrlrty constants for other possible srde reactions, will be camed out m the near future. A cot-relatron has been estabbshed between the stabrhty constants of 1.1 complexes and the drssoctatton constant of the protonated amino group of the hgand. The rate constants of dissocration of the complexes have been determmed by cyclic voltammetry (CV) where adsorpuon probiems on the hanging mercury drop electrode do not exist, or at least can be well separated from the kmetic mecharusm The adsorptron of the ammo acids has been studied rn qualitative terms by ac polarography. EXPERl?viENTAL
Chemicals
and sohrtlons
The reagents used were of analyttcal grade, wrth the exceptron of sodmm perchlorate prepared from Merck suprapur sodmm hydroxrde and perchlonc acrd. The content of heavy metals was tested by DPP and found to be negligible. The standard solutron of cadnuum rutrate was prepared from Merck P.A. salt and checked wrth a standard solutton of EDTA. The &stilled and detonized water used had a conducuvlty value < 0.1 @. In the polarographrc expenments the pH was adJusted wrth a borate buffer (5 X 10m3 M H,BO, + 5 x iOm4 M NaOH) in sodnun perchlorate solution. Instruments For the potenttometry an ORION 701A digttal pH Meter was used with an ORION 91-01 glass electrode and a calomel electrode filled with a saturated sodmm
335
chloride solution. Polarograp-mc mezuurements were carried out with a PAR 174 A polarograph under potentiostatic control, srlth a three-electrode system: droppmg mercury electrode, platinum electrode and calomel electrode filled with saturated sodium chloride The differential pulse mode was used with a pulse height of 50 mV, scan rate of 2 mV s-l, drop time of 2 s and a sensitivity of 0.5 PA over the full scale. The cyclic voltammetnc expenments were performed with the hanging mercury electrode (Metrchm model EA 290) and the potentiostat/g.aIvanostat PAR 173 coupled to the universal programmer PAR 175. Scan rates between 200 and 20000 mV/s were used and the voltammograms were stored 111 a digital storage oscllloscope from Nicolet Model 3091 and then registered m a Houston Instruments Omrugraphm X-Y recorder. For the alternating current expenments the basrc model PAR i74 A mas operated m conJunction with the model 174/50 AC Polarographlc Interface Accessory, connected with the Lock-m Amphfier Model 122 and the Lock-in AnaIyser Model 5204 from Pnnceton Applied Research. The three-electrode system was again used with the hanging mercury electrode An alternating potential of amphtude 10 mV was superimposed on the dc ramp voltage from 0 mV to -1000 mV, and the frequency was changed from 100 to 1000 Hz All the experiments were camed out at 20 + 0 1°C and at 25 f O.l”C m a thermostated glass cell Procedure
Potentiometnc, DPP and cychc voltammetnc measurements were camed out as mclicated 111 Part I of tlus work [4]. The altemaung current polarography runs were performed in ammo acid solutions ~t.h concentrations of 5.0 x lo-” A3 at iomc strength 0.70 M and borate buffer @H = 8.0 -t_ 0 2). The solutions were always deaerated wtth U-type pure nitrogen under stnrmg for 15 mm RESULTS
AND
DISCUSSION
Stab&y
constants
The evaluation by potentlometry of the stablhty constants and the coordmatlon number of the species m solution were obtamed by a least square refinement usmg the Mnnquad program that also calculates the standard deviation of the constants and an agreement factor R
R==&
CM, + CL, + cli,
Y
1=l
where U is the sum of the square
residuals
u=~[h -Cir)S+(Cr-C;)~+(CH-cC;l)?] 1=1
m represents
the number
of expenmental
points
and the values cM, cL and cH are
the expenmental values for the total conczntrauon of metal, hgand and hydrogen ton, c’,,, c; and c;l bemg the same concentratrons calculated from the mass balance equation. The best set of species is chosen according to the values of the statistical outputs of the program. An agreement factor R less than 0.004 is usually acceptable as is a deviatton of c 10% m the formation constants, although when many species are present in solution, values of up to 25% may stiIl be acceptable. Three titrations are generally done m these potenuometnc expenments wrth drfferent metal and hgand concentrations in the range 1: 1 and 1: 2, and all potentrometric curves have been worked together wrth the Mnuquad program, several hypotheses having been consrdered. The expenmental results already presented m Part I for the cadmium complexes wrth alamne, serme, valme and glutamic acid were also refined by the computer program. In Table 1 only the most probable models (ML + ML,, ML + ML2 + ML, and ML + ML, + MLOH) and the respextrve statistical models have been considered. One can see that for asparhc acid and Isoleucine, the species ML, or MLOH in the last two models do not unprove the results stat~tlcaIly, smce the R parameter does not change withm the experunental errors and the values of log 8, and log & (the global formation constants of ML and ML, specres) do not change wrthm the experimental errors. So the system can be well characterized wrth only the two are the only species parameters pi and & and the ML and ML z complexes detectable in the expenrnental condmons used. Glutamic acrd gives snmlar results, the ML, species being rejected and the error m /3 hlLOH ( = iMLoHi tHi/[Mi iLl) far beyond the acceptable values. For the hgands prolme and &rune the model with the MLOH species IS rejected and wrth the set of specres ML + ML, + ML, the & value has an error of about 40% wluch is not acceptable This is due to the fact that the ML, complex, if rt exists in solution, occurs only 111the last part of the titration curve. and so very close to the precipitation; thus the species cannot be well characterized m the expenmental conditions. With vahne, the model, ML, ML2 and MLOH can be consIdered but agam we can see that the error in & and m pNOH LS not acceptable. For glutamine the two models wrth MLOH and wrth ML, have been rejected For the senne/cadmium system, however, the MLOH specres can also be considered together wrth ML and ML2 complexes smce the parameter R decreases constderably, the stability constants of ML, decreases stgnificantly and the error of P MLoH is about 5% which is reasonable. So although the specres MLOH IS not as well defined as ML and ML, (the dommant complex m solutron) it seems that rt exists in solutron m a small concentration as can be seen m Fig 1. Thrs seems reasonable in prmciple because of the negative induchve effect of the hydroxyl group 0: serme, which mcreases the positive charge on the metal ion, and so the dissocration of the proton of the coordinated water molecule becomes easier.
337
TABLE
1
Stablty
constants
Ltgand Aspartlc
aad
Isoleucme
Glutanuc
acid
obtamed
wth
the Muuquad
for Uferent
modek
Model
log
loi% fi ML,
1%
MLtML, ML+ML,+MLOH ML-I- ML, + ML,
4537kOOO3 4533+0002 4544*0004
8220~0007 8152-t_0009 818 *002
109502
-s -
ML+ML, ML+ML,+MLOH ML+ML,+ML,
3635fOOO2 363SfOOO2 3642&OCO3
6833-cOOO5 681 *OOl 6.79 &002
-
ML+ ML, ML+ML,+MLOH
383
683
-
ML+ ML2 + ML, ML+ML, ML+ ML, + MLOH
Prohe
Program
ML+ML2
+ML,
flML
_tOOl
3 83 +002 383 It001 4274&0005
log
2001
+002
91*01
-69 -
BhlLOH
R
132005
0002 0001 0002
f02
-
0005
reJected -
-
73
*to6
121*02
-
00009
-
000s -
-
7
x:1
-
ML+ML, ML+ ML2 + MLOH ML+ML2+ML,
387S~Ooo7 reJected 410 &OOS
721
*001
-
70
-to9
114*02
Vahe
ML+ML, ML+ML,+MLOH ML+ ML, + ML,
368 rtOO3 372 1004 reJected
7.00 -coos 6 +2 -
-
-56 -
ML+ML, ML+ ML? + MLOH ML+ML2+ML,
362 5002 reJected
666
-
-
ML+ML, ML-+ML,+MLOH ML+ML,+ML,
365 fO02 372?&0005 374 *002
720 702 695
&OOl 2001 i-009
108_+01
-S -
ML + ML2
344 348 344
643 59 643
*002 501 &002
reJected
-568~004 -
&MC?
Phenylalarune
ML+ML,4MLOH ML+ML,+ML,
77
Ftg
80
83
rgected
+004
0 004s +04
000.5 0005 0 00s -
-
k-002 &to01 *002
0004 0005 0001
Ahune
Glutamme
0 cm 0001 0001
681 5005 683 *OO;! 7-94 kOO2
K?JCXtzd
430
flML:
-
83kOO2
000s 0001 0004 0006 0003 0006
pn
1. DlstnbuUon of the various species of cadnuum correspondmg to the ratlo q,,/c,_ = 0 75/l 00
with
set-me for a potentiomelnc
V.rauon
338
Fmally, for phenylalanme the model wrth ML, IS reJected and although the parameter R decreases by one half when one considers the MLOH species, the ML2 complex becomes ill-defmed. It therefore seems probable that, wrth the exceptton of the cadrmum senne system, only the ML and ML, species exrst m solutton in detectable concentratrons for all the other ammo acids studied It IS rmportant to pomt out that, in the whole treatment, the different trtratron curves used rn the calculatrons have no systematrc errors since the stabihty constants deternuned for each tttratron curve agree within the experunental errors. So all the data can be used on a sunple batch for the refmement of the stabrhty constants and the standard devratrons presented by the Munquad program are representative of the errors of the constant The polarograms obtamed by DPP with the cadnuum complexes of the several ammo acrds studied show, wrth the exceptron of the glutanuc and aspartrc acid systems. a constant peak height wrtlun an error of 5% and a cathodic sluft of potentral with the ligand concentrations Tlus means that m the scaletrme of the techruque the species are iabrle and smce the systems can be considered reversible (peak wrdth at half he&t of about 60-65 mV), the stability constants have been determmed by the DeFord-Hume method [9] in the usual way. The free hgand concentratton has bezn determmed accordmg to the system [L] = cJ(1 + /3y[H+]) as pomted out tn ref. 7. The values of the stabthty constants obtamed by DPP as well as the potentrometnc results after refinement by the Miniquad program are presented in Table 2 where one can see that the results agree withm the expenmental errors. The only chfferences are the MLOH species, winch do not exist 111the polarographrc expenmental conchtrons tlus IS reasonable smce there IS a big excess of hgand wluch also explains the ML, species for the cadmmm isoleucme system. Indeed this complex could already be detected rn potentrometnc condrtrons and the stability constant, although of the same order of magmtude as the values obtamed by DPP, had a hrgher error due to the smaller concentratton m solution. So although in the polarographrc expertmental condrtrons the ML, species are probable for all the ammo acids m the absence of a stereochenucal effect, only for rsoleucme has the ML, complex been detected The values of stabrhty constants determined by DPP have not been refined by computer calculations and the constants have only been determmed 111 order to check the accuracy of the results ana whether the same species are berg formed at lower concentrations closer to the values m natural waters In order to check, wtthin the expenmental errors, whether the same sort of complexes are being formed wrth cadmmm and the several ammo actds, I.e., If the chelatron rmgs are sun&r and there are nerther 7~ bonds nor stenc hmdrances but only mductrve effects, the experimental points log & (= [ML]/[M] [L]) vs. p#y (= WI tWW-I) have been plotted (Fig 2). From the figure where only the refined values of potentrometnc data a-e presented, as well as some hterature values, one can see that there is a hnear rela*tonshrp of equation log & = (4.3 &- 0.8) x 10-l
339
TABLE
2
Srablllty constants polarography
of cadrruum ammoacld
complexes
obtamed
by potenbometry
and differential
pulse
Lgand
Method
1% BhJL
1%
T/"C
I/M
References
Alanme
POT DPP
9 698
3 875&0007 375+005 3 80
728 2001 720 -1-004 7 10
20 20 2.5
07 07 10
This uork Thus work 6
Glycme
POT DPP
9544
3 88&001 3 9620 03 441
735 *002 725 I_003 7 60
20 20 25
07 07 10
7 7 6
Valme
POT DPP
9 54 -
368+003 3 67&003 346
700 699 646
*oos -co02
20 20 25
37 07 05
ThJs work Ttus work 6
4 274&0005 426+004
794 7.71
&032 f003
25 25
07 07
‘l?us work T!us work
8 943
344~002 3 832004 3 87 37
643 ~902 698 2’102 6 73 69
25 25 20 25
07 07 0 37 01
Tlx work This Nork 6 6
9 526
3635kOOO2 3 43
6 830&0005 6 70 70
25 2.5 25
07 07 10
Thus work Tlus worh 6
8 98
3 728+0005 3455002
702 680
2001 fool
20 20
07 07
This work Thus uorh
5004
PK,”
Prolme
POT DPP
10 41 -
Phenylalanme
POT DPP
-
Isoleucme
’
Senne ’ Gldtamme Aspartlc
acid
POT DPP
-
-
-
POT DPP
-
POT
9 031
362+002
666
25
07
This work
POT
9 54
4 537+0003 4 35 43
8 22O_cOOO7 7 55 81
25 25 25
07 01 0.7
TINS work
90s
367
7 03
25
01
6
940
3 53+001 39
683 -
25 25
07 ‘il
m 6
LS ASV MeUuonme Glu~an~c acid
Igh,L2
POT
B log BUL, = 9 5 (thus work) b 1% Bh4LOH = -5 83&002
+-002
6 8 work
(this work)
PK,~ - (3 & 8) x 10-l wrth a correlatron coefficient of 0.87 higher than the theoretrcal value of 0.632 for 8 degrees of freedom at the 95% level of srgmficance. So 111 spite of the simtlar order of magmtude of the pKIH and log j3, values, rt can be concluded that the last proton to be dissociated and the cadmnun ion are mvo!ved m the same type of chelate ring. The only exception is the complex with aspar!~ acid wkch probably behaves as a tndentate hgand, since there is the possrb5ty of coordination of a second group formmg a six-ring chelate. However, the mcrease of stabrlity IS small because the contnbution of - COO- relatrve to H,O is also small. Wtth the values of the stab&y constants for cadnuum and the amino actds studied, we could calculate the drstnbutron of species in seawater contammg ammo acids as the only organic ligands. Thrs has already been done for alamne and glycine
PK:
2 Correlation between log of the stability constants of 1 1 cadtruum complexes, log Ip,. with the .. dlssoclatlon of the ammo group of the hgand, pK,” (1) Phenylalanme. (2) serme, (3) methonme, (4) glutamme. (5) glutamtc acid. (6) Isoleuctne, (7) vahne. (8) glycme, (9) alanme, (10) prohe. (11) asparm acid
and the distribution wih be snndar smce the values of the stabllrty constants of ML and ML2 species, as well as the pKIH are of the same order of magnitude. So, once more, we could conclude that the dommant cadmmm species m seawater condrttons are the chlorocomplexes, specrahy CdCl, [l], that the calcmm and magnesium ammo acid compounds can be neglected and that the cadmmm complexes with ammo actds d be more important in less saline waters where the competition wrth cbIonde 1s smaller. Assuming that there ts eqmhbnum m seawater, the determined stablhty constants of cadmmm ammo acrd complexes can be used m computer calculations, together with interactron parameters with the solid phase, m order to obtain a real prcture of the situation in the open sea [4,7]
Klnetlc
of dssoczatron of the complexes
In spectation studies the kmeuc point of vrew IS also Important. In this context the electrochenucal mterface wdl produce the same type of drsturbance as the biologtcal membrane III the reaction zone, rf the rate constant of assurulatron 1s run&u to the rate constant of the electrochemical process [2]. In Part I of thrs work ]4] wc have seen that the cadmium complexes with glycine [7], alamrte, serme and vahne are completely Iabrle wrthin the tune scale of DPP which means that k, > lo4 s-l. Now we have notrced the same type of behaviour with phenylalamne, prohne, Isoleucme, lysine, meth.iomne and glutamine The cadmmm complexes wtth glutamic and aspartic acrds seem to have a kinetrc behaviour and/or adsorption problems since the peak current obtamed by DPP
341
decreases with the ligand concentrahon and the peak potentral has a cathodic shrft. In order to study the real mcchamsm u-t solution we tried to study the systems by cyclic voltamrnetry (CV) with different scan rates between 200 and 20,000 mV/s and different hgand concentrahons for the several cadmium ammo acrd complexes The total cadmium concentration was always equal to 2.0 x 10m4 M m order to achteve good sensrtivity with the techmque, the pH tn all the experiments was always about 8.0 f 0.2 and the ionic strength 0.70 M to simulate water condtttons Although there 1s always a negattve shtft of the cathodic peak with the ligand concentration due to cadmtum reduction from the dissociatron of the complex, the behaviour in terms of CV for the complexes with the several ammo actds cbffers considerably and so we have drvrded them mto three groups: (1) Cadnuum complexes with gfutanune, asparagine, alanme and prolme where there 1s only very weak adsorptron superimposed on the kinetrc effect Thrs means
that the peak current normalized to the scan rate (r,/fi) 1s approximately constant but varymg a little more than for the cyclic voltammogram of cadmium ton Itself, and the ratro of both peaks lE/i”, is nearly one. For the range of scan rates between 200 and 20,000 mV/s the potent& of the peak E: shifts only shghlly G 15 mV. The time of contact of the electrode with the solution we have used 1s about 2 s but we have also done expenments with higher values to study the effect on the voltammogram. So when the tune of contact increases, the cathodic peak current slightly mcreases as weLl for ah the complexes, which shows that these amino acids stti have weak adsorption probably super-unposed on the kmettc effect of the slow drssociation of the complex. (2) Cadmium complexes nlth lysme, methrorune, htstidme and rsoleucme where there IS weak adsorption of the reagent, or at least weak tnfluence due to adsorptron of the amino acrd as an mert spectes on the reduction of cadmium ion (Fig. 3) Indeed for all the complexes there is an enhancement of the normalized cathodtc peak current (1,/6) with the scan rate, because of electron transfer mvolvmg the adsorbed matenal (or mfluenced by the adsorbed material) at nearly the same potentral as “normal” electron transfer. For the htghest scan rates the cathodic peak sometunes seems to be broader due to a shoulder. In fact, as 1s weLl known, at higher scan rates the amount of diffusing material IS small relatrve to the adsorbed material reduced at the electrode So adsorptton is predominant tn relatron to the dtffusron mechanism. This behavrour IS charactensuc of weak adsorptton when there 1s reversible charge transfer, the rate of adsorption-desorption IS suffrctently fast m
terms of the time scale of the technique, and the mrtial potentral IS at least 200/n mV more positive than the ftrst peak [lo] The anodrc peak is not much influenced by the scan rate because the swrtchmg potentral 1s about 400 mV more negative than its potential The ratto I:/I; decreases significantly wrth the scan rate. For the range of scan rates between 200 and 20,000 mV/s the potenttal of the peak Ei shrfts catholcally by about 15 mV for the complexes with rsoleucine and by about 4.0-50 mV for the other complexes. On the other hand, the difference E; - E; tends to a slight increase with the scan rate. For the complexes with histidme, lysme and methionine the cathodic peaks are perturbed dependmg on the tune of contact of the
342
electrode, specially in terms of wrdth since they are broader. So, m tlus case, although there IS probably adsorptron superimposed on a kmetrc mechamsm, the ftrst effect 1s more pronounced. (3) Cadnuum complexes with serine, glutan~c and aspartic acids where there 1s strong adsorption of the reagent, or strong influence of the amino acrds in excess as mert species, on the reductton of cadmtum Ion. For +hese complexes there are always two reductron peaks: one that corresponds to the reduction of free cadmmm due to the dissoctation of the complex and another that is due to the reduction of the complex adsorbed or not on the electrode. Thrs last peak increases with the scan rate, which can be due to a kmetrc mechanism smce there IS less time for the drssocratton of the complex, or due to adsorptton which IS more stgnifrcant for higher scan rates, or to boL b effects . Wtth glutamic acid systems we also have a tid cathodic peak for the h&rest scan rates whtch means that in thrs situation the two phenomena can be separated The second cathodic peak for the cadmmm serine complexes can only be nottced d one increases the time of contact of the electrode with the solution, which means that the kinetics of the mechanism IS slower than in the other systems (Frg. 4). The drfference between the last two groups 1s that- A Er, of the cathodic peaks is greater than 70/n mV for the complexes wrth serme, gluttic and aspartrc acids and so the chffusron and adsorption peaks are well defined. So, for the second group, the drffuston peak of cadrmum reductton due to the drssociahon of the complex IS influenced by adsorption, contrary to thts last group where 111 prmciple the first cathodic peak can be used for kinetrc determmation of the drssocration rate of the complexes. In fact, the fust cathodtc peak Ei shrfts shghtly m the positive direction wrth the scan rate, followed by a sun&r shrft of the anodic peak and ~E/fi
POTENTIAL
VS
SCE/V
Ftg 3 Cyclic voltammograms of cadnuum-tsoleucme complexes(I) ( cM = 2 0 x 10-j M. cL = 5 0 X lo-’ M) and cadrmum(l1) (c, = 2 Ox 10m4 M) at ~oruc strength 0 70 M, pH = 7 92 and scan rate 10000 mV/s
343
-0
4
-0
6
-0
POTENTIAL
-1
6 VS
0
ScE/V
fg 4 Cychc voltammograms of cadnuum senne complexe; ( cM = 2 0 X 10m4 Al, cL = 5 0 x lo-’ M. / = 0 70 M, pH = 7 87, u = 10000 mV/s) correspondrag to drffcrent Me delays between the fom~~tron of the mercury
drop and the potenual
scan,
ft = 0 5. rtt = 5 S, fIII = 10 s. lIv = 45 s
decreases sigrnficantly wtth the increase of the scan rate of the method as ts supposed to happen in slow precedmg reactrons ]ll]_ However, if we use a time of contact between electrode and solution hrgher than 2-3 s, the three complexes of this group have a different behaviour which is especially related to the kinettcs of the adsorphon process. So, as we have already pomted out, the second cathodic peak of senne IS only noticed for fugher times of contact (Fig. 4) whereas the drffusion peak is kept constant, thrs berg the system where the adsorption process IS slower. For cadnuum glutannc acid complexes there is a tendency for the diffusron peak to drsappear with the permanency of the electrode m solutton before the scan, as well as for the second pe&, whereas the third adsorption peak increases. This phenomenon IS clear for
344
cadmtum asparttc acid systems, smce even for the shortest tune possible (2 s) only for the scan rates between 200 mV/s and 1500 mV/s IS the dtffusion peak clearly noticed For the highest scan rates there is oniy a shoulder due to the diffusion and a peak with adsorptton characteristrcs. So the adsorption peak, although dtstmct, distorts the diffusion peak and tt IS not possible to determme the rate constant of the dtssoctatton of cadmmm aspartic acid complexes. It seems that the tridentate complex ML, where there is not a net dipole, ts more easily adsorbed on the mercury. For the cadmium serine system the rate constant has been determmed m a stmllar way as for cadmmm glutarmc acid complexes [4] according to the semi-emp~ncal expresstons [ll] adapted to the existence of two species m solutton. = 1.02 + 0.471
IJI~
/-
L-F 6 ~~ 0~7
where I~ and lk are the diffusion I =
pd
+
pr
and kmetlc currents
wtth
WI/~
Pd =
klP1
P‘=
wwl
and
a=&[Ll+P#-I’ The mathematical calculatton has been done assummg that the slow step is the dtssoctatton of ML g M + L, as we have postulated for cadmium glutamrc actd complexes, after checkmg that the calculated rate constants are sumlar within the expenmental errors for pH 7.18 and 8.05 [4], whtch invalidates the h-ypothests of the slow step being ML + H+* M + HL The constancy of the values of the rate constant obtamed for several hgand concentratrons (Table 3) leads to the concluston that the hypothesis of the slow step IS valid as well as the proposed mechanism From this result we can see that the rate constants of cadrmum serine complexes are of the same order of magnitude and about lo5 s-‘, winch could be expected accordmg to Etgen’s results 1121, since k, should be wtthm 109-1010 M-r s-l, tf the substttutton of water molecules of the hydration sphere of metal ion IS the rate determmmg step
TABLE
3
Rate constant of dlssoclatlon of cadrmum senne complex (ML) at 25 50 l°C, IO2 Q/M
IO-%,/S-’
2 4 6
25 32 45
Average
value
34+-10
I = 0 70 M and pH = 7 87
345
For the ftrst and second group of hgands tt 1s not posstble to determine the rate constants smce the drffusron peaks are affected by adsorptron. The dc voltammograms of cadrruum glutamic and aspartrc acrd complexes using a dropping mercury electrode also show the influence of adsorptton, since the rate constant of the drssociatton of the cadmium glutamtc acrd complex could not be determined by this techmque 141. On the other hand, the cadmium serine complex, although wrth a rate constant of dissocratlon of the same order as the complex cadrmum/glutamic acid, has no apparent kmetrc behavlour in terms of DPP and dc polarography. In fact, wrth serine, where adsorptron is a slower process, the complex behaves as bemg completely !abile m terms of the last two techniques. So, the “pseudo” kme!ic effects observed m DPP can be due to adsorption problems Smce thts techmque is not qtnte suitable to study either adsorptron or kinetrc effects [13], the results have not been used m quantttatrve detetmmations. The complexatton of cadmmrn with aspartrc acid was also studred by hnear sweep anodrc stnppmg voltammetry (LSASV) [8], but no kmetrc problems have been notrced due to the drfferent tune scale of thrs techmque m comparison ~th DPP In order to determine thrs value for ASV, the thickness of the drffusron layer, 6, should be known [14]. As the mterface mercury/aqueous solutron can be considered a model of the mterfaces hydrophobic substances/aqueous solutton m natural waters, we tned to understand better what 1s being adsorbed on the electrode and the reason for the different behaviour of the several groups of ammo acids. If the ammo acids can be adsorbed as electromactrve substances, they may mfluence the rate of the reductron process by a stenc effect There is also the possrbrhty of adsorption of the complexes themselves on the electrode A compehtron between the two mechamsms IS also possrble. Several authors have studred adsorptron of ammo acids [15-171 and have noticed that when there are short hydrocarbon chams as. for example, m glycme and alanme, the adsorphon near the potential of zero charge (pzc) is negheble and there is an mcrease of capacity at both ends of the curve, as opposed to what happens wrth the ammo acids wrth long hydrocarbon chams where there IS a decrease m capacrty near the pzc. At higher posrtive or negatrve potenttal the amino actd molecule 1s adsorbed with the zwrttenon group more or less oriented, with the positive end (at negative potenttal), or Its negattve end (at positive potentral) toward the electrode surface. It has also been nottced that the mcrease of the polar character of amtnes or acids results m a decrease m their surface actrvity. So m a general way we can conclude that for the orgamc compounds the neutral species are more adsorbed than the charged ones, since mercury 1s a hydrophobic substance and so chemrcally the uncharged species have more afhmty for it. On the other hand, physical forces of an electrostahc character have to be constdered smce the electrode has a charge and the molecules have posrtrve or negative groups. In order to study the aclsorptton of HL species of the aminoacids we have determmed the capacrty of the double layer for solutions of the amino acids wrthout cadmium m the same expenmental condrhons. The values have been determined
346
from the results of ac polarography currents at 90’ and O” phase angle sh_tft [US]. It was apparent that the results were not s@ficantly altered when the time of contact (tc) of the electrode with the solution increased up to 5 min and in the ac frequency studred (loo-1009 I%). Accordmgly, only the capacrty values obtamed at 600 Hz and t, = 30 s are presented. It has been noticed that all the aminoacrds are bemg adsorbed mainly on the of the electrocapillary curve (Frg. 5), wluch IS positive side of the maximum consrstent with general adsorphon behaviour, that E, the anions are preferentially adsorbed However, wrth the exceptton of the aminoaclds metluonme, lysme, ~ISUdme and rsoleucme, the capacitance curves (C, vs. E) (Frg. 5) cannot be distmguished from each other wrtlnn the experimental errors. This shows that all the ammo acrds erther of HL or HL- type (for glutamrc and aspartrc acrd) are weakly adsorbed on the mercury, as also observed by Baugh and Parsons [16], unless there IS a sulphur group in the structure (methionme) or ~7 bonds (brstidme) or a longer hydrocarbon cham (lysine and isoleucme). These results can help to understand the drfferent behavlour of the cadmium aminoacrd complexes m terms of adsorptron. The cadmmm complexes of glutarmc and aspartic acrd have strong adsorption stnce the ML complex is neutral, and the adsorption peak is more separated from the diffusron one On the other hand the HL- species IS only weakly adsorbed. The cadmmm complex wrth set-me IS of the ML+ type, but due to the negatrve mductrve effect of the OH group there 1s strll strong adsorption. The cadmmm complexes wrth the ammo acrds alanine, prohne, glutamine and asparagme are only very weakly adsorbed smce the 1: 1 complexes (ML+ type) are charged and the hgands (I-IL form) are only very weakly adsorbed on the mercury mtiy on +?e posrtrve side of the electrocapillary curve. The ML2
I
I
-01
-03
I -05
I
1
-07 POTENTIAL
-09 VS
SCE/V
Fig 5 Differential double layer capacity vs potential curves for mercury rn sodmm perchlorate 0 70 M and borate buffer at 25°C and frequency of 600 Hz (a) Supportmg electrolyte (4) soleucme, (A) lysme, (P) hsrldme. (s) metiorune. (0) glutanuc acid
347
species, although neutral, probably have more problems of stenc hmdrance. The 1 - 1 cadnuum complexes wrth methtomne, histrdme, lystne and tsoleucine are not strongiy adsorbed on mercury since they are charged. However, there IS weak but net adsorption probably due to the adsorption of the ligand itself through the sulphur group, the r bonds or the hydrocarbon cham. The adsorpuon of the hgand itself can influence the reduction process of the cadmium ion from the dissocration of the complex by a stenc effect, or, as the ammo acrd group IS drrectcd to the aqueous solutron, the cadmium complex can be formed and directly reduced on the mercury So the different
behavrour acrds can be due to drfferent
of the three groups of cadmium complexes mecharusms of adsorptron on the e&trade.
wrth ammo
CONCLUSIONS
The present study leads to the followmg conclusrons: (1) The cadrruum complexes of the type ML and ML, ulth studied are stmtlar, the hgands being bidentate wrth the exception which has a tndentate character. (2) The rate constants
of the reaction
CdL
2
Cd’+
for
the ammoacrds of aspartic actd
the cadnuum
glutanuc
acrd and cadmium serme systems have been determmed by CV and are of the order of (3-4) x lo5 s-l. (3) The ammo acids themselves can be adsorbed at the mercury/solutton interface whtch can be considered a model of the hydrophobtc substance/aqueous solutron rnterface in natural waters. However, due to the zwittenon form, the adsorptron 1s weak unless the ammo acids have a long hydrocarbon cham, sulphur groups, or rr bonds wrth high interactron wrth the mercury mterface (4) For cadnuum glutamic and asparnc actd complexes there is a strong adsorpnon of the neutral specres ML. For the cadmium senne complex there 1s also strong, although slow, adsorption probably due to *he existence of OH group m the molecule_ (5) For cadmnun methionine, lysrne, hrstrdme and rsoleucme complexes there 1s weak adsorptton due to a drfferent mechamsm, stnce the hgands themselves are betng adsorbed on the mercury. (6) For cadmium glutanune, asparagme, alanme and proline complexes the adsorptron of the hgands and complexes IS very weak smce the complex ML+ 1s charged and the hgands are only weakly adsorbed. (7) The adsorption and lanetrc effects are being superrmposed tn cychc voltammograms for the first two groups of complexes ACKNOWLEDGEMENT
This work was partially supported by Junta Nacronal de Investrga$io Tecnologica under the research contract No. 31581.57. We are indebted to Prof. Werner Stumm for valuable drscussions
Crentifrca
e
REFERENCES 1 hi L Sun&s Gonqalvrs, MC T Abreu Vaz and J J. Frausto da Sdva, TaIanta, 28 (1981; 237 2 M Wtutfield m RM. Pytkow~a (Ed ). Acuvlty Coefficrents in Electrolyte Solut~oq Vol ?, CRC Press Boca Raton, 1979, Ch. 3 3 M L S Sun&s Gocqalves and P Valema, J Ekctroanal Chem. 132 (1982) 357 4 M L S Sun&s GonqaJve~ and M M Cm-rem dos Santos. J Electroanal Cheru. 163 (1984) 315 5 P Cans, A_ Sabatuu and A. Vacca, Inorg. Chun Acta, 18 (1976) 237 6 RM Smlh and A E. Martell, Cnucal Stabtity Constants. Plenum Press, New York, 1976. suppl 1982 7 M L S Sunk Gon~ves, P Valenta and H W. Numberg. J EIectroa& Chem , 149 (1983) 249 8 P Valenta, M L S Sun&s Gonqalves and M Sugawara, m C J M Kramer and J C Dumker (Eds ). Proceedmgs of the International Symposmm on Complexatlon of Trace Metals m Natural Water (Netherlands), 1983, M NlJhoff and Dr. W Junk, The Hague, 1984. p 357 9 D D DeFord and D N Hume. J Am. Chem Sot, 73 (1951) 5321 10 RH Wopschall and I Sham, Anal Chem 39 (1967) 1514 11 RS Nicholson and I Sham, Anal Chea 36 (1964) 706 12 M Ejgen, Ber Bunsenges Phys Chem, 67 (1963) 753 13 Herman P Van Leeuwen. J Electroanal Chent, 133 (1982) 201 14 W Damson. J ElectroanaL Chem. 87 (1978) 395 15 G.J HIUS. DJ Sctuffrm and T Solomon. J Ekctroanal Chem., 41 (1973) 41 16 L M Baugh and R Parsons, J Electroanal Chem .41 (1973) 311 17 T K&v&u and M Senda, J Ekctroanal. Chem, 88 (1978) 219 18 M Sluyters-Rehbach and J H Sluyters in A.J Bard (Ed ). Ekctroanzdytlcal Chenustry, Vol 4, Marcel Dekker, New York, 1982. Ch ?
ELECTROANALYTK4L AMINO ACIDS AT THE PART
333 m The Netherlands
CHEMISTRY OF C4DRTIUM COMPLJXES OF IONIC STRENGTH OF SEAWATER (0.70 M NaCLO,)
II
M L S SIMdES
GONCALVES
Cenrro de Qurnuca (Recewd
Esirutural,
and MM Complexo
15th August 1984,
CORREIA
Inrerdtscrplmar,
DOS SANTOS Imtliuto
In reblsed form 18th December
Supenor
Trcnrco,
Lrsboa
(Portugal)
1984)
ABSTRACT The complexauon of cadrmum wnb naturally occwrmg ammo acids ~+as studied at the 1omc strength of -water (0 70 M m sodmm perchlorate me&urn) and at 25 f0 1°C by tiferenti pulse polarography and potenuometry WI& a glass electrode The h-quad program was used to analyse and refins the results of the potenhometry. Although the stabhty of the complexes IS kery sumlar smce the chelate groups of the hgands are the same, the rate of dlssoclahon of the complexes can be drfferent The kmeuc aspect has been stuled by cychc voltammetry which IS und x the mfluence of the type of adsorpllon When this IS strong the tifnslon and adsorphon p& are we3 separated and, m prmclple. the rate constant of tisonauon of the complex can be deterslned If there 1s weak adsorphon the tifuslon peak IS dmorted by adsorption and kmenc stu&es cannot be done The adsorpbon of the bgands themselveslms hem studd by ac polarography at a 90’ angle phase and It has been nottced that hgands with 5 bonds, sulphur groups ok wtth long hydrocarbon chams are more adsorbed than those WthOUt.
INTRODUCI-ION
The
toxrcity
speciahon
of cadmium
111 the mar-me envrromnent
that 1s also a funchon
1s strongly
of the orgamc and morgaruc
dependent
on
hgands ~11the natural
medmm. If the level of orgarucs present in the ocean is negligible, over 97% of the cadmium exrsts as chlorocomplexes, especially in the form CdClz [l]. The concentratton of the macroconstrtuents in seawater 1s roughly constant ah over the world, contrary to other natural waters which are very much dependent on the enwonment [2]. According to the hterature, ammo acids are only a relatrvely small fraction of the total DOM (dissolved orgaruc matter), not more than 0.04 mg/l[3]. and so cadrmum ammo acid complexes are not important in seawater condmons. However, these complexes can exist 111regions of higher ammo acid levels, as m some coastal waters, estuaries, pore waters of certain se&ments and zones of high phytoplankton acnvlty, ds extrudates and decomposrtion products of orgamsms. So a study was undertaken 0022-0728/85/%03
30
0 1985 Ekewer
Se+oia
5%.
334
of the complexes of cadnuum with naturally occurring ammo acids, the ftrst part of thrs study havmg already been presented [4] for the complexes with alanine, serine, vahne and glutamtc acid The amino acids considered in thrs paper are proline, rsoleucine, aspartic acrd and phenylglutanune, asparagrne, lysme, methionine, alanine. Some adsorption studres were also done wrth hrsttdine. Some of these armmo acrds are also important from the biochemical point of vtew, smce some enzymes have heavy metals such as copper, nickel or zmc rn thetr structure bound to some anuno actd groups of the type of lysine, methronine, etc. The stabihty of cadnuum complexes wrth these annnoactds was detemuned in the non-complexmg medium of 0.70 M NaCIO, (sirmlar to -the ~onrc strength of seawater) by potentrometry and the Ivimrquad program [5] when there were no values m the hterature [6] for tlus ~onrc strength. Some of these values have also been conftrmed by drfferentral pu!se polarography (DPP) wrth lower metal concentration, closer to those met tn the real system. The determmation of the stabllrty constants by DPP m artrficial and genuine unpolluted seawater has not been done smce the agreement was good for the stabrhty constants of cadtruum glyctne complexes [7] and for the cadmium aspartrc acrd complexes deternuned by anodrc stnpping voltammetry (ASV) [S]. Kmetrc studies for media closer to seawater, considering all the stabrlrty constants for other possible srde reactions, will be camed out m the near future. A cot-relatron has been estabbshed between the stabrhty constants of 1.1 complexes and the drssoctatton constant of the protonated amino group of the hgand. The rate constants of dissocration of the complexes have been determmed by cyclic voltammetry (CV) where adsorpuon probiems on the hanging mercury drop electrode do not exist, or at least can be well separated from the kmetic mecharusm The adsorptron of the ammo acids has been studied rn qualitative terms by ac polarography. EXPERl?viENTAL
Chemicals
and sohrtlons
The reagents used were of analyttcal grade, wrth the exceptron of sodmm perchlorate prepared from Merck suprapur sodmm hydroxrde and perchlonc acrd. The content of heavy metals was tested by DPP and found to be negligible. The standard solutron of cadnuum rutrate was prepared from Merck P.A. salt and checked wrth a standard solutton of EDTA. The &stilled and detonized water used had a conducuvlty value < 0.1 @. In the polarographrc expenments the pH was adJusted wrth a borate buffer (5 X 10m3 M H,BO, + 5 x iOm4 M NaOH) in sodnun perchlorate solution. Instruments For the potenttometry an ORION 701A digttal pH Meter was used with an ORION 91-01 glass electrode and a calomel electrode filled with a saturated sodmm
335
chloride solution. Polarograp-mc mezuurements were carried out with a PAR 174 A polarograph under potentiostatic control, srlth a three-electrode system: droppmg mercury electrode, platinum electrode and calomel electrode filled with saturated sodium chloride The differential pulse mode was used with a pulse height of 50 mV, scan rate of 2 mV s-l, drop time of 2 s and a sensitivity of 0.5 PA over the full scale. The cyclic voltammetnc expenments were performed with the hanging mercury electrode (Metrchm model EA 290) and the potentiostat/g.aIvanostat PAR 173 coupled to the universal programmer PAR 175. Scan rates between 200 and 20000 mV/s were used and the voltammograms were stored 111 a digital storage oscllloscope from Nicolet Model 3091 and then registered m a Houston Instruments Omrugraphm X-Y recorder. For the alternating current expenments the basrc model PAR i74 A mas operated m conJunction with the model 174/50 AC Polarographlc Interface Accessory, connected with the Lock-m Amphfier Model 122 and the Lock-in AnaIyser Model 5204 from Pnnceton Applied Research. The three-electrode system was again used with the hanging mercury electrode An alternating potential of amphtude 10 mV was superimposed on the dc ramp voltage from 0 mV to -1000 mV, and the frequency was changed from 100 to 1000 Hz All the experiments were camed out at 20 + 0 1°C and at 25 f O.l”C m a thermostated glass cell Procedure
Potentiometnc, DPP and cychc voltammetnc measurements were camed out as mclicated 111 Part I of tlus work [4]. The altemaung current polarography runs were performed in ammo acid solutions ~t.h concentrations of 5.0 x lo-” A3 at iomc strength 0.70 M and borate buffer @H = 8.0 -t_ 0 2). The solutions were always deaerated wtth U-type pure nitrogen under stnrmg for 15 mm RESULTS
AND
DISCUSSION
Stab&y
constants
The evaluation by potentlometry of the stablhty constants and the coordmatlon number of the species m solution were obtamed by a least square refinement usmg the Mnnquad program that also calculates the standard deviation of the constants and an agreement factor R
R==&
CM, + CL, + cli,
Y
1=l
where U is the sum of the square
residuals
u=~[h -Cir)S+(Cr-C;)~+(CH-cC;l)?] 1=1
m represents
the number
of expenmental
points
and the values cM, cL and cH are
the expenmental values for the total conczntrauon of metal, hgand and hydrogen ton, c’,,, c; and c;l bemg the same concentratrons calculated from the mass balance equation. The best set of species is chosen according to the values of the statistical outputs of the program. An agreement factor R less than 0.004 is usually acceptable as is a deviatton of c 10% m the formation constants, although when many species are present in solution, values of up to 25% may stiIl be acceptable. Three titrations are generally done m these potenuometnc expenments wrth drfferent metal and hgand concentrations in the range 1: 1 and 1: 2, and all potentrometric curves have been worked together wrth the Mnuquad program, several hypotheses having been consrdered. The expenmental results already presented m Part I for the cadmium complexes wrth alamne, serme, valme and glutamic acid were also refined by the computer program. In Table 1 only the most probable models (ML + ML,, ML + ML2 + ML, and ML + ML, + MLOH) and the respextrve statistical models have been considered. One can see that for asparhc acid and Isoleucine, the species ML, or MLOH in the last two models do not unprove the results stat~tlcaIly, smce the R parameter does not change withm the experunental errors and the values of log 8, and log & (the global formation constants of ML and ML, specres) do not change wrthm the experimental errors. So the system can be well characterized wrth only the two are the only species parameters pi and & and the ML and ML z complexes detectable in the expenrnental condmons used. Glutamic acrd gives snmlar results, the ML, species being rejected and the error m /3 hlLOH ( = iMLoHi tHi/[Mi iLl) far beyond the acceptable values. For the hgands prolme and &rune the model with the MLOH species IS rejected and wrth the set of specres ML + ML, + ML, the & value has an error of about 40% wluch is not acceptable This is due to the fact that the ML, complex, if rt exists in solution, occurs only 111the last part of the titration curve. and so very close to the precipitation; thus the species cannot be well characterized m the expenmental conditions. With vahne, the model, ML, ML2 and MLOH can be consIdered but agam we can see that the error in & and m pNOH LS not acceptable. For glutamine the two models wrth MLOH and wrth ML, have been rejected For the senne/cadmium system, however, the MLOH specres can also be considered together wrth ML and ML2 complexes smce the parameter R decreases constderably, the stability constants of ML, decreases stgnificantly and the error of P MLoH is about 5% which is reasonable. So although the specres MLOH IS not as well defined as ML and ML, (the dommant complex m solutron) it seems that rt exists in solutron m a small concentration as can be seen m Fig 1. Thrs seems reasonable in prmciple because of the negative induchve effect of the hydroxyl group 0: serme, which mcreases the positive charge on the metal ion, and so the dissocration of the proton of the coordinated water molecule becomes easier.
337
TABLE
1
Stablty
constants
Ltgand Aspartlc
aad
Isoleucme
Glutanuc
acid
obtamed
wth
the Muuquad
for Uferent
modek
Model
log
loi% fi ML,
1%
MLtML, ML+ML,+MLOH ML-I- ML, + ML,
4537kOOO3 4533+0002 4544*0004
8220~0007 8152-t_0009 818 *002
109502
-s -
ML+ML, ML+ML,+MLOH ML+ML,+ML,
3635fOOO2 363SfOOO2 3642&OCO3
6833-cOOO5 681 *OOl 6.79 &002
-
ML+ ML, ML+ML,+MLOH
383
683
-
ML+ ML2 + ML, ML+ML, ML+ ML, + MLOH
Prohe
Program
ML+ML2
+ML,
flML
_tOOl
3 83 +002 383 It001 4274&0005
log
2001
+002
91*01
-69 -
BhlLOH
R
132005
0002 0001 0002
f02
-
0005
reJected -
-
73
*to6
121*02
-
00009
-
000s -
-
7
x:1
-
ML+ML, ML+ ML2 + MLOH ML+ML2+ML,
387S~Ooo7 reJected 410 &OOS
721
*001
-
70
-to9
114*02
Vahe
ML+ML, ML+ML,+MLOH ML+ ML, + ML,
368 rtOO3 372 1004 reJected
7.00 -coos 6 +2 -
-
-56 -
ML+ML, ML+ ML? + MLOH ML+ML2+ML,
362 5002 reJected
666
-
-
ML+ML, ML-+ML,+MLOH ML+ML,+ML,
365 fO02 372?&0005 374 *002
720 702 695
&OOl 2001 i-009
108_+01
-S -
ML + ML2
344 348 344
643 59 643
*002 501 &002
reJected
-568~004 -
&MC?
Phenylalarune
ML+ML,4MLOH ML+ML,+ML,
77
Ftg
80
83
rgected
+004
0 004s +04
000.5 0005 0 00s -
-
k-002 &to01 *002
0004 0005 0001
Ahune
Glutamme
0 cm 0001 0001
681 5005 683 *OO;! 7-94 kOO2
K?JCXtzd
430
flML:
-
83kOO2
000s 0001 0004 0006 0003 0006
pn
1. DlstnbuUon of the various species of cadnuum correspondmg to the ratlo q,,/c,_ = 0 75/l 00
with
set-me for a potentiomelnc
V.rauon
338
Fmally, for phenylalanme the model wrth ML, IS reJected and although the parameter R decreases by one half when one considers the MLOH species, the ML2 complex becomes ill-defmed. It therefore seems probable that, wrth the exceptton of the cadrmum senne system, only the ML and ML, species exrst m solutton in detectable concentratrons for all the other ammo acids studied It IS rmportant to pomt out that, in the whole treatment, the different trtratron curves used rn the calculatrons have no systematrc errors since the stabihty constants deternuned for each tttratron curve agree within the experunental errors. So all the data can be used on a sunple batch for the refmement of the stabrhty constants and the standard devratrons presented by the Munquad program are representative of the errors of the constant The polarograms obtamed by DPP with the cadnuum complexes of the several ammo acrds studied show, wrth the exceptron of the glutanuc and aspartrc acid systems. a constant peak height wrtlun an error of 5% and a cathodic sluft of potentral with the ligand concentrations Tlus means that m the scaletrme of the techruque the species are iabrle and smce the systems can be considered reversible (peak wrdth at half he&t of about 60-65 mV), the stability constants have been determmed by the DeFord-Hume method [9] in the usual way. The free hgand concentratton has bezn determmed accordmg to the system [L] = cJ(1 + /3y[H+]) as pomted out tn ref. 7. The values of the stabthty constants obtamed by DPP as well as the potentrometnc results after refinement by the Miniquad program are presented in Table 2 where one can see that the results agree withm the expenmental errors. The only chfferences are the MLOH species, winch do not exist 111the polarographrc expenmental conchtrons tlus IS reasonable smce there IS a big excess of hgand wluch also explains the ML, species for the cadmmm isoleucme system. Indeed this complex could already be detected rn potentrometnc condrtrons and the stability constant, although of the same order of magmtude as the values obtamed by DPP, had a hrgher error due to the smaller concentratton m solution. So although in the polarographrc expertmental condrtrons the ML, species are probable for all the ammo acids m the absence of a stereochenucal effect, only for rsoleucme has the ML, complex been detected The values of stabrhty constants determined by DPP have not been refined by computer calculations and the constants have only been determmed 111 order to check the accuracy of the results ana whether the same species are berg formed at lower concentrations closer to the values m natural waters In order to check, wtthin the expenmental errors, whether the same sort of complexes are being formed wrth cadmmm and the several ammo actds, I.e., If the chelatron rmgs are sun&r and there are nerther 7~ bonds nor stenc hmdrances but only mductrve effects, the experimental points log & (= [ML]/[M] [L]) vs. p#y (= WI tWW-I) have been plotted (Fig 2). From the figure where only the refined values of potentrometnc data a-e presented, as well as some hterature values, one can see that there is a hnear rela*tonshrp of equation log & = (4.3 &- 0.8) x 10-l
339
TABLE
2
Srablllty constants polarography
of cadrruum ammoacld
complexes
obtamed
by potenbometry
and differential
pulse
Lgand
Method
1% BhJL
1%
T/"C
I/M
References
Alanme
POT DPP
9 698
3 875&0007 375+005 3 80
728 2001 720 -1-004 7 10
20 20 2.5
07 07 10
This uork Thus work 6
Glycme
POT DPP
9544
3 88&001 3 9620 03 441
735 *002 725 I_003 7 60
20 20 25
07 07 10
7 7 6
Valme
POT DPP
9 54 -
368+003 3 67&003 346
700 699 646
*oos -co02
20 20 25
37 07 05
ThJs work Ttus work 6
4 274&0005 426+004
794 7.71
&032 f003
25 25
07 07
‘l?us work T!us work
8 943
344~002 3 832004 3 87 37
643 ~902 698 2’102 6 73 69
25 25 20 25
07 07 0 37 01
Tlx work This Nork 6 6
9 526
3635kOOO2 3 43
6 830&0005 6 70 70
25 2.5 25
07 07 10
Thus work Tlus worh 6
8 98
3 728+0005 3455002
702 680
2001 fool
20 20
07 07
This work Thus uorh
5004
PK,”
Prolme
POT DPP
10 41 -
Phenylalanme
POT DPP
-
Isoleucme
’
Senne ’ Gldtamme Aspartlc
acid
POT DPP
-
-
-
POT DPP
-
POT
9 031
362+002
666
25
07
This work
POT
9 54
4 537+0003 4 35 43
8 22O_cOOO7 7 55 81
25 25 25
07 01 0.7
TINS work
90s
367
7 03
25
01
6
940
3 53+001 39
683 -
25 25
07 ‘il
m 6
LS ASV MeUuonme Glu~an~c acid
Igh,L2
POT
B log BUL, = 9 5 (thus work) b 1% Bh4LOH = -5 83&002
+-002
6 8 work
(this work)
PK,~ - (3 & 8) x 10-l wrth a correlatron coefficient of 0.87 higher than the theoretrcal value of 0.632 for 8 degrees of freedom at the 95% level of srgmficance. So 111 spite of the simtlar order of magmtude of the pKIH and log j3, values, rt can be concluded that the last proton to be dissociated and the cadmnun ion are mvo!ved m the same type of chelate ring. The only exception is the complex with aspar!~ acid wkch probably behaves as a tndentate hgand, since there is the possrb5ty of coordination of a second group formmg a six-ring chelate. However, the mcrease of stabrlity IS small because the contnbution of - COO- relatrve to H,O is also small. Wtth the values of the stab&y constants for cadnuum and the amino actds studied, we could calculate the drstnbutron of species in seawater contammg ammo acids as the only organic ligands. Thrs has already been done for alamne and glycine
PK:
2 Correlation between log of the stability constants of 1 1 cadtruum complexes, log Ip,. with the .. dlssoclatlon of the ammo group of the hgand, pK,” (1) Phenylalanme. (2) serme, (3) methonme, (4) glutamme. (5) glutamtc acid. (6) Isoleuctne, (7) vahne. (8) glycme, (9) alanme, (10) prohe. (11) asparm acid
and the distribution wih be snndar smce the values of the stabllrty constants of ML and ML2 species, as well as the pKIH are of the same order of magnitude. So, once more, we could conclude that the dommant cadmmm species m seawater condrttons are the chlorocomplexes, specrahy CdCl, [l], that the calcmm and magnesium ammo acid compounds can be neglected and that the cadmmm complexes with ammo actds d be more important in less saline waters where the competition wrth cbIonde 1s smaller. Assuming that there ts eqmhbnum m seawater, the determined stablhty constants of cadmmm ammo acrd complexes can be used m computer calculations, together with interactron parameters with the solid phase, m order to obtain a real prcture of the situation in the open sea [4,7]
Klnetlc
of dssoczatron of the complexes
In spectation studies the kmeuc point of vrew IS also Important. In this context the electrochenucal mterface wdl produce the same type of drsturbance as the biologtcal membrane III the reaction zone, rf the rate constant of assurulatron 1s run&u to the rate constant of the electrochemical process [2]. In Part I of thrs work ]4] wc have seen that the cadmium complexes with glycine [7], alamrte, serme and vahne are completely Iabrle wrthin the tune scale of DPP which means that k, > lo4 s-l. Now we have notrced the same type of behaviour with phenylalamne, prohne, Isoleucme, lysine, meth.iomne and glutamine The cadmmm complexes wtth glutamic and aspartic acrds seem to have a kinetrc behaviour and/or adsorption problems since the peak current obtamed by DPP
341
decreases with the ligand concentrahon and the peak potentral has a cathodic shrft. In order to study the real mcchamsm u-t solution we tried to study the systems by cyclic voltamrnetry (CV) with different scan rates between 200 and 20,000 mV/s and different hgand concentrahons for the several cadmium ammo acrd complexes The total cadmium concentration was always equal to 2.0 x 10m4 M m order to achteve good sensrtivity with the techmque, the pH tn all the experiments was always about 8.0 f 0.2 and the ionic strength 0.70 M to simulate water condtttons Although there 1s always a negattve shtft of the cathodic peak with the ligand concentration due to cadmtum reduction from the dissociatron of the complex, the behaviour in terms of CV for the complexes with the several ammo actds cbffers considerably and so we have drvrded them mto three groups: (1) Cadnuum complexes with gfutanune, asparagine, alanme and prolme where there 1s only very weak adsorptron superimposed on the kinetrc effect Thrs means
that the peak current normalized to the scan rate (r,/fi) 1s approximately constant but varymg a little more than for the cyclic voltammogram of cadmium ton Itself, and the ratro of both peaks lE/i”, is nearly one. For the range of scan rates between 200 and 20,000 mV/s the potent& of the peak E: shifts only shghlly G 15 mV. The time of contact of the electrode with the solution we have used 1s about 2 s but we have also done expenments with higher values to study the effect on the voltammogram. So when the tune of contact increases, the cathodic peak current slightly mcreases as weLl for ah the complexes, which shows that these amino acids stti have weak adsorption probably super-unposed on the kmettc effect of the slow drssociation of the complex. (2) Cadmium complexes nlth lysme, methrorune, htstidme and rsoleucme where there IS weak adsorption of the reagent, or at least weak tnfluence due to adsorptron of the amino acrd as an mert spectes on the reduction of cadmium ion (Fig. 3) Indeed for all the complexes there is an enhancement of the normalized cathodtc peak current (1,/6) with the scan rate, because of electron transfer mvolvmg the adsorbed matenal (or mfluenced by the adsorbed material) at nearly the same potentral as “normal” electron transfer. For the htghest scan rates the cathodic peak sometunes seems to be broader due to a shoulder. In fact, as 1s weLl known, at higher scan rates the amount of diffusing material IS small relatrve to the adsorbed material reduced at the electrode So adsorptton is predominant tn relatron to the dtffusron mechanism. This behavrour IS charactensuc of weak adsorptton when there 1s reversible charge transfer, the rate of adsorption-desorption IS suffrctently fast m
terms of the time scale of the technique, and the mrtial potentral IS at least 200/n mV more positive than the ftrst peak [lo] The anodrc peak is not much influenced by the scan rate because the swrtchmg potentral 1s about 400 mV more negative than its potential The ratto I:/I; decreases significantly wrth the scan rate. For the range of scan rates between 200 and 20,000 mV/s the potenttal of the peak Ei shrfts catholcally by about 15 mV for the complexes with rsoleucine and by about 4.0-50 mV for the other complexes. On the other hand, the difference E; - E; tends to a slight increase with the scan rate. For the complexes with histidme, lysme and methionine the cathodic peaks are perturbed dependmg on the tune of contact of the
342
electrode, specially in terms of wrdth since they are broader. So, m tlus case, although there IS probably adsorptron superimposed on a kmetrc mechamsm, the ftrst effect 1s more pronounced. (3) Cadnuum complexes with serine, glutan~c and aspartic acids where there 1s strong adsorption of the reagent, or strong influence of the amino acrds in excess as mert species, on the reductton of cadmtum Ion. For +hese complexes there are always two reductron peaks: one that corresponds to the reduction of free cadmmm due to the dissoctation of the complex and another that is due to the reduction of the complex adsorbed or not on the electrode. Thrs last peak increases with the scan rate, which can be due to a kmetrc mechanism smce there IS less time for the drssocratton of the complex, or due to adsorptton which IS more stgnifrcant for higher scan rates, or to boL b effects . Wtth glutamic acid systems we also have a tid cathodic peak for the h&rest scan rates whtch means that in thrs situation the two phenomena can be separated The second cathodic peak for the cadmmm serine complexes can only be nottced d one increases the time of contact of the electrode with the solution, which means that the kinetics of the mechanism IS slower than in the other systems (Frg. 4). The drfference between the last two groups 1s that- A Er, of the cathodic peaks is greater than 70/n mV for the complexes wrth serme, gluttic and aspartrc acids and so the chffusron and adsorption peaks are well defined. So, for the second group, the drffuston peak of cadrmum reductton due to the drssociahon of the complex IS influenced by adsorption, contrary to thts last group where 111 prmciple the first cathodic peak can be used for kinetrc determmation of the drssocration rate of the complexes. In fact, the fust cathodtc peak Ei shrfts shghtly m the positive direction wrth the scan rate, followed by a sun&r shrft of the anodic peak and ~E/fi
POTENTIAL
VS
SCE/V
Ftg 3 Cyclic voltammograms of cadnuum-tsoleucme complexes(I) ( cM = 2 0 x 10-j M. cL = 5 0 X lo-’ M) and cadrmum(l1) (c, = 2 Ox 10m4 M) at ~oruc strength 0 70 M, pH = 7 92 and scan rate 10000 mV/s
343
-0
4
-0
6
-0
POTENTIAL
-1
6 VS
0
ScE/V
fg 4 Cychc voltammograms of cadnuum senne complexe; ( cM = 2 0 X 10m4 Al, cL = 5 0 x lo-’ M. / = 0 70 M, pH = 7 87, u = 10000 mV/s) correspondrag to drffcrent Me delays between the fom~~tron of the mercury
drop and the potenual
scan,
ft = 0 5. rtt = 5 S, fIII = 10 s. lIv = 45 s
decreases sigrnficantly wtth the increase of the scan rate of the method as ts supposed to happen in slow precedmg reactrons ]ll]_ However, if we use a time of contact between electrode and solution hrgher than 2-3 s, the three complexes of this group have a different behaviour which is especially related to the kinettcs of the adsorphon process. So, as we have already pomted out, the second cathodic peak of senne IS only noticed for fugher times of contact (Fig. 4) whereas the drffusion peak is kept constant, thrs berg the system where the adsorption process IS slower. For cadnuum glutannc acid complexes there is a tendency for the diffusron peak to drsappear with the permanency of the electrode m solutton before the scan, as well as for the second pe&, whereas the third adsorption peak increases. This phenomenon IS clear for
344
cadmtum asparttc acid systems, smce even for the shortest tune possible (2 s) only for the scan rates between 200 mV/s and 1500 mV/s IS the dtffusion peak clearly noticed For the highest scan rates there is oniy a shoulder due to the diffusion and a peak with adsorptton characteristrcs. So the adsorption peak, although dtstmct, distorts the diffusion peak and tt IS not possible to determme the rate constant of the dtssoctatton of cadmmm aspartic acid complexes. It seems that the tridentate complex ML, where there is not a net dipole, ts more easily adsorbed on the mercury. For the cadmium serine system the rate constant has been determmed m a stmllar way as for cadmmm glutarmc acid complexes [4] according to the semi-emp~ncal expresstons [ll] adapted to the existence of two species m solutton. = 1.02 + 0.471
IJI~
/-
L-F 6 ~~ 0~7
where I~ and lk are the diffusion I =
pd
+
pr
and kmetlc currents
wtth
WI/~
Pd =
klP1
P‘=
wwl
and
a=&[Ll+P#-I’ The mathematical calculatton has been done assummg that the slow step is the dtssoctatton of ML g M + L, as we have postulated for cadmium glutamrc actd complexes, after checkmg that the calculated rate constants are sumlar within the expenmental errors for pH 7.18 and 8.05 [4], whtch invalidates the h-ypothests of the slow step being ML + H+* M + HL The constancy of the values of the rate constant obtamed for several hgand concentratrons (Table 3) leads to the concluston that the hypothesis of the slow step IS valid as well as the proposed mechanism From this result we can see that the rate constants of cadrmum serine complexes are of the same order of magnitude and about lo5 s-‘, winch could be expected accordmg to Etgen’s results 1121, since k, should be wtthm 109-1010 M-r s-l, tf the substttutton of water molecules of the hydration sphere of metal ion IS the rate determmmg step
TABLE
3
Rate constant of dlssoclatlon of cadrmum senne complex (ML) at 25 50 l°C, IO2 Q/M
IO-%,/S-’
2 4 6
25 32 45
Average
value
34+-10
I = 0 70 M and pH = 7 87
345
For the ftrst and second group of hgands tt 1s not posstble to determine the rate constants smce the drffusron peaks are affected by adsorptron. The dc voltammograms of cadrruum glutamic and aspartrc acrd complexes using a dropping mercury electrode also show the influence of adsorptton, since the rate constant of the drssociatton of the cadmium glutamtc acrd complex could not be determined by this techmque 141. On the other hand, the cadmium serine complex, although wrth a rate constant of dissocratlon of the same order as the complex cadrmum/glutamic acid, has no apparent kmetrc behavlour in terms of DPP and dc polarography. In fact, wrth serine, where adsorptron is a slower process, the complex behaves as bemg completely !abile m terms of the last two techniques. So, the “pseudo” kme!ic effects observed m DPP can be due to adsorption problems Smce thts techmque is not qtnte suitable to study either adsorptron or kinetrc effects [13], the results have not been used m quantttatrve detetmmations. The complexatton of cadmmrn with aspartrc acid was also studred by hnear sweep anodrc stnppmg voltammetry (LSASV) [8], but no kmetrc problems have been notrced due to the drfferent tune scale of thrs techmque m comparison ~th DPP In order to determine thrs value for ASV, the thickness of the drffusron layer, 6, should be known [14]. As the mterface mercury/aqueous solutron can be considered a model of the mterfaces hydrophobic substances/aqueous solutton m natural waters, we tned to understand better what 1s being adsorbed on the electrode and the reason for the different behaviour of the several groups of ammo acids. If the ammo acids can be adsorbed as electromactrve substances, they may mfluence the rate of the reductron process by a stenc effect There is also the possrbrhty of adsorption of the complexes themselves on the electrode A compehtron between the two mechamsms IS also possrble. Several authors have studred adsorptron of ammo acids [15-171 and have noticed that when there are short hydrocarbon chams as. for example, m glycme and alanme, the adsorphon near the potential of zero charge (pzc) is negheble and there is an mcrease of capacity at both ends of the curve, as opposed to what happens wrth the ammo acids wrth long hydrocarbon chams where there IS a decrease m capacrty near the pzc. At higher posrtive or negatrve potenttal the amino actd molecule 1s adsorbed with the zwrttenon group more or less oriented, with the positive end (at negative potenttal), or Its negattve end (at positive potentral) toward the electrode surface. It has also been nottced that the mcrease of the polar character of amtnes or acids results m a decrease m their surface actrvity. So m a general way we can conclude that for the orgamc compounds the neutral species are more adsorbed than the charged ones, since mercury 1s a hydrophobic substance and so chemrcally the uncharged species have more afhmty for it. On the other hand, physical forces of an electrostahc character have to be constdered smce the electrode has a charge and the molecules have posrtrve or negative groups. In order to study the aclsorptton of HL species of the aminoacids we have determmed the capacrty of the double layer for solutions of the amino acids wrthout cadmium m the same expenmental condrhons. The values have been determined
346
from the results of ac polarography currents at 90’ and O” phase angle sh_tft [US]. It was apparent that the results were not s@ficantly altered when the time of contact (tc) of the electrode with the solution increased up to 5 min and in the ac frequency studred (loo-1009 I%). Accordmgly, only the capacrty values obtamed at 600 Hz and t, = 30 s are presented. It has been noticed that all the aminoacrds are bemg adsorbed mainly on the of the electrocapillary curve (Frg. 5), wluch IS positive side of the maximum consrstent with general adsorphon behaviour, that E, the anions are preferentially adsorbed However, wrth the exceptton of the aminoaclds metluonme, lysme, ~ISUdme and rsoleucme, the capacitance curves (C, vs. E) (Frg. 5) cannot be distmguished from each other wrtlnn the experimental errors. This shows that all the ammo acrds erther of HL or HL- type (for glutamrc and aspartrc acrd) are weakly adsorbed on the mercury, as also observed by Baugh and Parsons [16], unless there IS a sulphur group in the structure (methionme) or ~7 bonds (brstidme) or a longer hydrocarbon cham (lysine and isoleucme). These results can help to understand the drfferent behavlour of the cadmium aminoacrd complexes m terms of adsorptron. The cadmmm complexes of glutarmc and aspartic acrd have strong adsorption stnce the ML complex is neutral, and the adsorption peak is more separated from the diffusron one On the other hand the HL- species IS only weakly adsorbed. The cadmmm complex wrth set-me IS of the ML+ type, but due to the negatrve mductrve effect of the OH group there 1s strll strong adsorption. The cadmmm complexes wrth the ammo acrds alanine, prohne, glutamine and asparagme are only very weakly adsorbed smce the 1: 1 complexes (ML+ type) are charged and the hgands (I-IL form) are only very weakly adsorbed on the mercury mtiy on +?e posrtrve side of the electrocapillary curve. The ML2
I
I
-01
-03
I -05
I
1
-07 POTENTIAL
-09 VS
SCE/V
Fig 5 Differential double layer capacity vs potential curves for mercury rn sodmm perchlorate 0 70 M and borate buffer at 25°C and frequency of 600 Hz (a) Supportmg electrolyte (4) soleucme, (A) lysme, (P) hsrldme. (s) metiorune. (0) glutanuc acid
347
species, although neutral, probably have more problems of stenc hmdrance. The 1 - 1 cadnuum complexes wrth methtomne, histrdme, lystne and tsoleucine are not strongiy adsorbed on mercury since they are charged. However, there IS weak but net adsorption probably due to the adsorption of the ligand itself through the sulphur group, the r bonds or the hydrocarbon cham. The adsorpuon of the hgand itself can influence the reduction process of the cadmium ion from the dissocration of the complex by a stenc effect, or, as the ammo acrd group IS drrectcd to the aqueous solutron, the cadmium complex can be formed and directly reduced on the mercury So the different
behavrour acrds can be due to drfferent
of the three groups of cadmium complexes mecharusms of adsorptron on the e&trade.
wrth ammo
CONCLUSIONS
The present study leads to the followmg conclusrons: (1) The cadrruum complexes of the type ML and ML, ulth studied are stmtlar, the hgands being bidentate wrth the exception which has a tndentate character. (2) The rate constants
of the reaction
CdL
2
Cd’+
for
the ammoacrds of aspartic actd
the cadnuum
glutanuc
acrd and cadmium serme systems have been determmed by CV and are of the order of (3-4) x lo5 s-l. (3) The ammo acids themselves can be adsorbed at the mercury/solutton interface whtch can be considered a model of the hydrophobtc substance/aqueous solutron rnterface in natural waters. However, due to the zwittenon form, the adsorptron 1s weak unless the ammo acids have a long hydrocarbon cham, sulphur groups, or rr bonds wrth high interactron wrth the mercury mterface (4) For cadnuum glutamic and asparnc actd complexes there is a strong adsorpnon of the neutral specres ML. For the cadmium senne complex there 1s also strong, although slow, adsorption probably due to *he existence of OH group m the molecule_ (5) For cadmnun methionine, lysrne, hrstrdme and rsoleucme complexes there 1s weak adsorptton due to a drfferent mechamsm, stnce the hgands themselves are betng adsorbed on the mercury. (6) For cadmium glutanune, asparagme, alanme and proline complexes the adsorptron of the hgands and complexes IS very weak smce the complex ML+ 1s charged and the hgands are only weakly adsorbed. (7) The adsorption and lanetrc effects are being superrmposed tn cychc voltammograms for the first two groups of complexes ACKNOWLEDGEMENT
This work was partially supported by Junta Nacronal de Investrga$io Tecnologica under the research contract No. 31581.57. We are indebted to Prof. Werner Stumm for valuable drscussions
Crentifrca
e
REFERENCES 1 hi L Sun&s Gonqalvrs, MC T Abreu Vaz and J J. Frausto da Sdva, TaIanta, 28 (1981; 237 2 M Wtutfield m RM. Pytkow~a (Ed ). Acuvlty Coefficrents in Electrolyte Solut~oq Vol ?, CRC Press Boca Raton, 1979, Ch. 3 3 M L S Sun&s Gocqalves and P Valema, J Ekctroanal Chem. 132 (1982) 357 4 M L S Sun&s GonqaJve~ and M M Cm-rem dos Santos. J Electroanal Cheru. 163 (1984) 315 5 P Cans, A_ Sabatuu and A. Vacca, Inorg. Chun Acta, 18 (1976) 237 6 RM Smlh and A E. Martell, Cnucal Stabtity Constants. Plenum Press, New York, 1976. suppl 1982 7 M L S Sunk Gon~ves, P Valenta and H W. Numberg. J EIectroa& Chem , 149 (1983) 249 8 P Valenta, M L S Sun&s Gonqalves and M Sugawara, m C J M Kramer and J C Dumker (Eds ). Proceedmgs of the International Symposmm on Complexatlon of Trace Metals m Natural Water (Netherlands), 1983, M NlJhoff and Dr. W Junk, The Hague, 1984. p 357 9 D D DeFord and D N Hume. J Am. Chem Sot, 73 (1951) 5321 10 RH Wopschall and I Sham, Anal Chem 39 (1967) 1514 11 RS Nicholson and I Sham, Anal Chea 36 (1964) 706 12 M Ejgen, Ber Bunsenges Phys Chem, 67 (1963) 753 13 Herman P Van Leeuwen. J Electroanal Chent, 133 (1982) 201 14 W Damson. J ElectroanaL Chem. 87 (1978) 395 15 G.J HIUS. DJ Sctuffrm and T Solomon. J Ekctroanal Chem., 41 (1973) 41 16 L M Baugh and R Parsons, J Electroanal Chem .41 (1973) 311 17 T K&v&u and M Senda, J Ekctroanal. Chem, 88 (1978) 219 18 M Sluyters-Rehbach and J H Sluyters in A.J Bard (Ed ). Ekctroanzdytlcal Chenustry, Vol 4, Marcel Dekker, New York, 1982. Ch ?