Phase-Selective Cathodic Stripping Voltammetry for Determination of Water-Soluble Mercaptans W. Michael Moore* and V. Frances Gaylor The Standard Oil Company (Ohio), 4440 Warrensville Center Road, Cleveland, Ohio 44 128
The appllcation of phase-selectlve cathodlc stripplng (PSCS) voltammetry to analyses of water-soluble mercaptans has been demonstrated uslng a micrometer hanglng mercury drop electrode. The sensltlvity llmlt was near 1 ppb for sodlum mercaptoacetate (SMA) analysls when a 2-mln deposltlon interval was employed. The average deviatlon for SIXIndependent experiments with 25 ppb SMA was 3.62%. The dependence of PSCS peak currents upon mercaptan concentration, applied ac voltage magnitude, signal frequency, and dc potential scan rate was investlgated. Because ac voltammetry discrlmlnates agalnst irreversible electrode processes, mercaptan PSCS experlments could be performed in air-saturated electrolytes. As a result, the method appears to be suitable for analyses of volatile mercaptans when Inert gas purging to remove oxygen Is not practical.
The search for more sensitive electroanalytical techniques led to the development of stripping voltammetry (SV) (1). The extraordinary sensitivity of SV arises from preconcentration of trace material into or onto a microelectrode by an electrochemical deposition before reversing the process and “stripping” the substance from the electrode. Anodic stripping voltammetry has been frequently utilized for the determination of trace metals (2,3). Cathodic stripping voltammetry has been employed for analyses of halide ions (4), other inorganic anions which form insoluble mercury compounds (5), and several mercaptans (6, 7). The resolution and sensitivity of SV can be improved when the stripping process is performed using ac-modulated or pulsed potential waveforms. Differential pulse cathodic stripping voltammetry a t a hanging mercury drop electrode (HMDE) has been employed to determine 2-mercaptopyridine-N-oxide ion with a sensitivity limit of 8 X 10-l’ M (8). Several studies have been devoted to phase-selective anodic stripping (PSAS) (9-11); however, little attention has been given to the cathodic counterpart. Several polarographic studies involving mercaptan compounds have been reported (12-14). The generally accepted electrode mechanism for the reaction of mercaptans a t a mercury electrode involves the formation of a mercurous mercaptide species in a reversible one-electron process RSH
+ Hg r? RSHg + H’ + e
(1)
which is represented by Equation 1. Unfortunately, oxygen cannot be removed by inert gas purging during analyses of volatile mercaptans. Sodium sulfite (15) and ascorbic acid (16) have been employed to destroy oxygen in situ prior to electrochemical analysis. Since ac voltammetry discriminates against irreversible electrode processes (13,it can occasionally be employed for analyses performed in the presence of oxygen if caution is exercised (18, 19). At present, there is no exact theoretical treatment to describe ac stripping currents obtained at the HMDE. Some features of the phase-selective cathodic stripping (PSCS) peak 1386
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
current are, however, suggested by theoretical treatments of the direct voltammetric process. Underkofler and Shain (20) derived an equation which describes the ac current obtained for a reversible process using the planar diffusion model; this equation was modified by Delmastro and Smith (21) to account for the geometry and spherical diffusion effects of a metal ion-amalgam system and found applicable to ac amalgam stripping by Moorhead and Davis (9). In the present study, use of PSCS for analyses of watersoluble mercaptans was investigated using a micrometer HMDE and the experimental variables affecting in-phase peak current were examined. Based upon results obtained, PSCS is a sensitive and reliable technique for mercaptan analyses.
EXPERIMENTAL Apparatus. PSCS experiments were performed using a Princeton Applied Research (PAR) Model 170 Electrochemistry System with PAR 9301 polargraphic cells. A calibrated Kemula-type Metrohm E-410 micrometer hanging mercury drop electrode [HMDE(K)] was employed as the working electrode for all cathodic stripping experiments. The reference electrode was a saturated calomel (SCE) which was separated from the cell solution by a salt bridge tube with porous cracked-glass tip (22) containing pH 5.5 sodium acetate solution. A PAR 9350 water-jacketed cell was used with a Haake Model S-84833 external-circulating constant-temperature bath for some thermostated (25 f 0.1 “C) experiments. The cell solution was stirred by a 3-mm X 12-mm Teflon-coated spin bar driven at 600 rpm by a larger (7-mm X 35-mm) spin bar mounted on the shaft of an inverted Sargent S76485 Synchronous Rotator. A stainless-steel cylinder with a 2-cm diameter hole cut in the top was mounted over the rotator shaft. The tapered PAR 9301 polargraphic cells fit nicely into the positioning device which ensured that the cell spin bar position remained invariant from run to run. Figure 1illustrates the electrochemical cell and stirring assembly employed for PSCS experiments. Mercaptan solutions were standardized by a coulometric titration procedure. A Brinkmann Model E524 Coulostat, Brinkmann E436 Potentiograph, E436D Multititration Stand, standard Metrohm cells, silver wire generating and auxiliary electrodes, and a dual-silver detecting electrode were employed in the coulometric titration procedure. Reagents. Electrolyte used for the electrochemical cell and SCE salt bridge compartment was sodium acetate solution which was prepared by adjusting 0.5 M acetic acid to pH 5.5 with 50% sodium hydroxide solution. Water of sufficient purity for these experiments was prepared by simple distillation, passage through a demineralizer (Bantam Model BD-1 Demineralizer), and a final distillation from a Corning AG-3 Still. High purity argon (Airco, Inc.) was used for experiments which required a purge gas. Argon was deoxygenated by passage through an acidic vanadous chloride solution (23) and humidified with a distilled water scrubber. Mercury for the HMDE and DME was triple-distilled product from Bethlehem Instrument Company. Sodium mercaptoacetate (SMA) and tert-butylmercaptan (TBM) were reagent-grade chemicals used without further purification. Standardization and Monitoring Procedures for Mercaptan Solutions. Mercaptan solutions used for calibration purposes were standardized coulometrically by titration with electrogenerated silver ion in acetate solution. Excess silver at the end point was detected amperometrically by a dual-silver electrode.
Figure 1. Lower portion of electrochemical cell and stirring assembly. (A) Salt bridge tube containing SCE, (e) hanging mercury drop electrode,
(C) platinum wire auxiliary electrode, (D) nitrogen inlet, (E) polarographic cell bottom, (F) cell spin bar, (G) driven spin bar, (H) stainless steel cell positioner, (I) synchronous rotator, (J) rotator shaft
A lo-’ M (1150 ppm) SMA solution was prepared in 0.1 M acetic acid and standardized by the coulometric titration procedure; a 20-fold dilution of this solution using degassed 0.1 M acetic acid as diluent yielded 58 ppm SMA. A portion of this solution was sealed under argon in a Hypo-Vial (Pierce Chemical) for syringe sampling. The 58-ppm SMA standard solution was not entirely stable and decreased about 5% per week. Since the coulometric titration procedure was rather time-consuming, differential pulse polarography (DPP) at a DME was employed as a quick method for monitoring SMA concentration. Aliquots from a freshly prepared 58-ppm SMA solution were added t o 10.0 mL pH 5.5 sodium acetate solution and DPP performed (100-mV pulse amplitude, 2-s mercury drop time, and potential scan rate of 5 mV/s). Peak currents obtained at -0.25 V vs. SCE for several concentrations were used to create a calibration curve which was subsequently used to monitor SMA concentration in standard solutions. Procedure for PSCS Experiments. To perform PSCS experiments, 10.0 mL of electrolyte solution was added to a cell and (for some experiments) deoxygenated for 5 min with humidified argon. Low-level mercaptan concentrations were generated by syringing aliquots from standard solutions into the electrolyte. Following addition of sample or standard, degassing was continued for 30 s before the argon was switched to blanket the solution. A mercury drop of area 0.035 cm2was obtained by dialing eight divisions of the E-410 HMDE assembly. Stirring was commenced 15 s before the applied potential was switched from -0.6 to 4-0.1 V vs. SCE to initiate the anodic plating process. After precisely 1.5 min, the stirrer was switched off to allow the solution to become quiescent for 30 s before the applied dc potential was scanned from +0.1 to -0.6 V vs. SCE to record either a blank or mercaptan stripping peak. In all cases, the rms ac current component in-phase with the superimposed ac voltage signal was recorded. For most experiments, the frequency and amplitude of the modulating ac voltage were 90 Hz and 10 mV peak-to-peak, respectively, and the potential scan rate was 100 mV/s.
RESULTS AND DISCUSSION Selection of pH. Selection of electrolyte pH for mercaptan stripping experiments was a compromise between peak resolution and mercaptan stability. The first oxygen reduction step resulted in a broad ac current response near -0.06 V vs. SCE in air-saturated sodium acetate solution. In order to resolve mercaptan stripping peaks sufficiently from the oxygen wave, pH 5.5 or greater was required. However, since the rate of mercaptan to disulfide oxidation is faster at high pH values (19), pH 5.5 was selected for the present study. All mercaptans examined by PSCS exhibited in-phase peak currents (i,) near -0.36 V vs. SCE in pH 5.5 sodium acetate solution. Peak potentials were shifted negatively by 58 mV per unit increase in pH; this result is predictable from the Nernst expression of the mercaptan electrode reaction (Equation 1). Selection of Deposition Potential. The value of deposition potential (Ei)for mercaptan PSCS experiments
in both deaorated and air-saturated sodium acetate solution. Experimental and conditions for the blank (A), and PSCS performed in deaerated (6) air-saturated (C) solution were: u = 100 mVls, w = 90 Hz, A€ = 10 mV, A = 0.035 cm2,T = 2.0 min, pH = 5.5, and E,= +0.1 V vs. SCE Figure 2. PSCS voltammograms for a blank and 25 ppb SMA
performed in air-saturated solution was critical. Initially, Ei = -0.1 V vs. SCE was employed, but stripping peak currents were erratic and sensitivity was poor. Although a relatively small ac response was obtained for oxygen reduction in air-saturated electrolyte, the dc reduction flux of oxygen to hydrogen peroxide was quite large a t -0.1 V vs. SCE. It appears that hydrogen peroxide reacted with mercaptans in the vicinity of the HMDE which resulted in the observed decrease in experimental sensitivity and precision. A background dc polarogram was obtained in air-saturated electrolyte solution. A narrow potential region centered about +0.1 V vs. SCE was revealed in which neither mercury oxidation nor oxygen reduction took place. PSCS results for mercaptans were greatly improved when the deposition step was performed at +0.1 V vs. SCE, a potential at which no extraneous electrode reactions occur. Results f o r Sodium Mercatoacetate (SMA). Figure 2 shows PSCS voltammograms for a blank and 25 ppb SMA (2.2 x lo-’ M) obtained in deaerated and air-saturated sodium acetate solution. Figure 2 is a graphic illustration of mercaptan sensitivity enhancement and oxygen discrimination by PSCS; although the molar concentration of oxygen was more than lo4times larger than SMA, the mercaptan stripping peak at -0.36 V in Figure 2C is larger than the oxygen peak at -0.06 V. The sensitivity limit for SMA was near 1ppb when a 2-min stirred deposition interval was utilized. Figure 3 illustrates calibration curve data obtained for PSCS experiments in both deaerated and air-saturated solutions using 0-100 ppb SMA. From Figure 3, it is clear that the presence of oxygen has only a small effect on the magnitude of i, obtained for a given SMA concentration. A linear response was obtained only for SMA concentrations below about 50 ppb when a 2-min stirred deposition interval was employed. Evidently, at higher sample concentrations, the HMDE was partially blocked by insoluble mercurous mercaptide deposit. The “electrode blockage” supposition is supported by the experimental observation that dc deposition currents were not constant, but declined with time when sufficiently large SMA concentrations were employed. In order to extend the concentration range over which a linear peak current response was obtained, a 60-s nonstirred ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
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20
40
60
5144
,
,
80
IC0
Figure 3. PSCS calibration plots for 0-100 ppb SMA in deaerated and air-saturated electrolyte using a 2.0-min stirred deposition Interval. Experimental conditions for data obtained in deaerated electrolyte solution (upper curve) and air-saturated solution (lower curve) were the same as in Figure 2 5.01
ro.1 I
-0.1
-0.3
-0.5
-0.7
E \ < SCE,\.It.
Figure 5. Differential pulse cathodic stripping voltammogram for 20 ppb SMA in air-saturated sodium acetate solution. Experimental conditions were: 100-mV pulse amplitude, pulse repetition time of 1 s, v = 5 mV/s, A = 0.035 cm2,T = 2.0 min, pH = 5.5, and E,= +0.1 V vs. SCE 14.01
12.0
J / ;o
,
,
,
TBM
deposition interval was employed. Figure 4 is a calibration plot obtained for experiments employing the 60-s quiescent plating interval; with this experimental modification, a linear current response was obtained for SMA concentrations up to 500 ppb, although the sensitivity limit increased to 20 ppb. Comparison of PSCS a n d DPCS Using SMA. PSCS was compared to differential pulse cathodic stripping (DPCS) using 20 ppb SMA and a 2-min deposition interval in both deaerated and air-saturated solutions. For each technique, electrical parameters (scan rate, etc.) were adjusted in order to optimize current response for the sake of comparison. In air-free electrolyte, PSCS and DPCS yielded peak currents of 0.87 and 2.51 PA, respectively. In air-saturated solution, however, PSCS yielded 0.91 MA, while accurate measurement of DPCS peak current was prevented by a severely sloped baseline due to superimposed oxygen reduction. Figure 5 illustrates the DPCS trace obtained for 20 ppb SMA in air-saturated solution; SMA peak current is located at -0.25 V and appears as a shoulder on the much larger oxygen peak. The above results indicate that DPCS is inherently more sensitive than PSCS for trace mercaptan determinations. PSCS is the superior technique for mercaptan analyses performed in the presence of oxygen, however, because of the ability to discriminate against oxygen reduction. Results for tert-Butylmercaptan (TBM). PSCS would appear to be a promising technique for determinations of 1388
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
9
i t
Figure 6. PSCS calibration plot for terf-butylmercaptan. Experimental conditions were the same as in Figure 2. Air-saturated electrolyte solutions were employed volatile mercaptans because it is not necessary to remove oxygen prior to performing the experiment. Therefore, the PSCS behavior of TBM (bp = 64 "C) was investigated. The procedure for TBM experiments was essentially the same as described for SMA. Stock solutions were more stable when prepared in ethanol because of increased TBM solubility in this solvent; these solutions were used immediately following standardization by coulometric titration. When not being sampled, TBM stock solutions were stored at Oo C. Figure 6 is a calibration plot obtained for TBM in airsaturated electrolyte; each data point was obtained by an independent experiment. Although a considerable degree of scatter was present in experimental i, data, the calibration curve shown for TBM is linear over the 0-200 ppb concentration range examined. A linear regression correlation coefficient of 0.979 was obtained for the data illustrated in Figure 6. Precision of P S C S Data. In order to evaluate the precision of stripping peak currents, six independent PSCS experiments were performed with 25 ppb SMA. Results of these experiments are shown in Table I. The average deviation, which generally decreased as mercaptan concentration increased, was 3.62% with 25 ppb SMA. Experimental
Table I . Reproducibility of PSCS Peak Currents Obtained with 25-ppb SMA in Deaerated Electrolyte' Run No.
2.0
1
ip'clA 1.42 1.24
1 2 3 4 5 6
1.44 1.41
1.38 1.42
Mean Av dev ' Experimental parameter values: Hz,A E = 10 m V , A = 0.035 cm2,T = %
1.39 3.62
100 mV/s, w = 90 2 . 0 min, p H = 5.5, El
v=
2.0
+0.1V vs. SCE. -
-:
precision was definitely improved through use of the cylindrical cell positioning device which ensured reproducible convection during the deposition interval, although this apparatus required working at ambient temperature (25 f 0.5 "C). Reproducibility of experiments performed using the cell positioner was better than for temperature-thermostated experiments in which the cell and rotator shaft were aligned by sight only. The greatest obstacle to achieving reproducible results with SMA and TBM was the reactive nature of the mercaptans. Although not particularly volatile, SMA appeared to react slowly during PSCS experiments as evidenced by a steady decline of stripping peak currents for several stripping scans employing the same test solution. Therefore, to achieve best results, determinations were performed without delay after SMA was syringed into the cell. The extreme volatility of TBM rendered reproducible results difficult to achieve. The average per cent deviation of the 17 data points in Figure 6 from the linear regression straight line was determined to be 10.1%, an acceptable figure considering the nature of the analysis. Effect of Frequency upon Peak Current. It was deemed desirable to determine the effects of various electrical parameters upon i, for PSCS experiments to compare with functional dependencies observed in phase-selective anodic stripping (PSAS) and predicted by ac polarographic theory. Therefore, the effects of signal frequency ( w ) , peak-to-peak ac voltage magnitude (AE), and dc potential scan rate (v) upon i, were determined using 25-30 ppb SMA with a 2-min stirred deposition interval. The effect of changes in signal frequency was examined over the interval 25-500 Hz. In-phase stripping peak currents were proportional to d2 in this frequency range (linear regression correlation coefficient = 0.9996) which is predicted by basic ac polarographic theory (23). Moorhead and Davis also found that in-phase stripping peak currents for PSAS experiments using Cd(I1) (9) and Ga(II1) (IO)were proportional to w1/2. A frequency of 90 Hz was employed for most experiments. Although stripping peak currents increased linearly as a function of a'/', extremely high signal frequencies were avoided because of the presence of large background currents. Effect of A E u p o n P e a k Currents. Figure 7 illustrates the linear dependence of i, upon peak-to-peak ac voltage magnitude (U) obtained for sufficiently small hE values; the linear relationship is in agreement with theory for ac polarography (24) and is similar to results obtained by PSAS for Cd(I1) reduction (9). A 10-mV ac modulating voltage was employed for most of this work because of the sensitivity enhancement achieved. Effect of S c a n R a t e upon Peak Current. The effect of potential scan rate ( u ) upon stripping peak currents was investigated using a 25-ppb SMA solution. Figure 8 shows that a linear plot was obtained when i, was plotted vs. ~ ' 1 ' . A linear regression correlation coefficient of 0.9980 was ob-
4.0
'EPP,
6.0
."
8.0
10.0
Figure 7. PSCS peak current (in-phase) vs. peak-to-peak ac signal magnitude (A€). Except for 30 ppb SMA and variable A€, experimental conditions were the same as In Figure 2
I .8
I .6 .-
1.
h
m
1.4
,,
/
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stirred deposition interval. A linear dependence of peak current upon scan rate was obtained (linear regression correlation coefficient = 0.9997) so long as the quantity of coulombs involved in the stripping process remained constant. The dependence of first and second harmonic responses in ac voltammetry on experimental parameters was recently outlined by Smith et al. (26). The effect of dc scan rate was not described explicitly in this study, although the basic equations involved in ac voltammetry were presented which show that the alternating current response is proportional to surface concentrations of oxidized and reduced forms. Since deposited mercurous mercaptide is insoluble in the solution and electrode phases, its activity should be near unity throughout the stripping scan (27) and the resulting PSCS current response proportional to mercaptan concentration a t the electrode surface. Thus, the square root relationship of PSCS peak currents and scan rate results primarily from the dependence of generated mercaptan surface concentrations upon dc scan rate.
ACKNOWLEDGMENT The authors are grateful to Donald E. Smith for aid in understanding the scan rate dependence of PSCS peak currents.
(3) I. Shain in “Treatise on Analytical Chemistry”, Part I, Vol. 4, I. M. Koltoff and P. J. Eiving, Ed., Interscience, New York, N.Y., 1963, Chap. 50. (4) G. Colovos, G. S. Wilson, and J. L. Moyers, Anal. Chem.,46, 1051 (1974). (5) Kh. Z. Brainina, Talanfa, 18, 513 (1971). (6) H. Berge and P, Jeroschewskl, Fresenius’ 2.Anal. Chem., 212, 278 (1965). (7) M. J. D. Brand and B. Fleet, Analyst(London), 93, 498 (1968). (8) D. A. Csejka, S.T. Nakos, and E. W. DuBord, Anal. Chem., 47, 322 (1975). (9) E. D. Moorhead and P. H. Davis, Anal. Chem., 45, 2178 (1973). (IO) E. D. Moorhead and P. H. Davis, Anal. Chem., 47, 622 (1975). (11) E. D. Moorhead and G. A. Forsberg, Anal. Chem., 46, 751 (1976). (12) M. L. Mittal and A. V. Pandey, J . Electroanal. Chem., 36, 249 (1972). (13) R. S.Saxena and U. S.Chaturvedi, J. Electroanal. Chem..36, 515 (1972). (14) W. Strlcks, J. K. Frischman, and R. G. Mueller, J . Nectrochem. Soc., log, 518 (1962). (15) S.Kukuchi, Bull. Chem. SOC. Jpn., 27, 65 (1954). (16) T. M. Florence and Y. J. Farrar. J. Nectroanal. Chem., 41, 127 (1973). (17) A. M. Bond, Anal. Chem., 44, 315 (1972). (18) A. M. Bond and J. H. Canterford, Anal. Chem.. 43, 228 (1971). (19) A. M. Bond, Talanta, 20, 1139 (1973). (20) W: L. Underkofler and I. Shain, Anal. Chem., 37, 216 (1965). (21) J. R. Delmastro and D. E. Smith, Anal. Chem., 38, 169 (1966). (22) J. J. Lingane, J . Electroanal. Chem., 1, 379 (1960). (23) Princeton Applied Research Corporation, Prlnceton, N.J., Application Note 108. (24) D. E. Smith in “Electroanalytical Chemistry-A Series of Advances”, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1966, Chap. 1. (25) M. Brezlna and P. Zuman, “Polarography in Medicine, Biochemistry, and Pharmacy”, Interscience, New York, N.Y., 1956, pp 470-478. (26) A. M. Bond, R. J. O’Halloran, I. Ruzic, and D. E. Smith, Anal. Chem., 48, 872 (1976). (27) T. Berzins and P. Delahay, J . Am. Chem. Soc., 75, 555 (1953).
LITERATURE CITED (1) E. Barendrecht in “Electroanalytical Chemistry-A
Series of Advances”, Voi. 2, A J. Bard, Ed., Marcel Dekker, New York, N.Y., 1967, pp 53-109, (2) W. Kemula and 2 . Kubik in “Advances In Analytical Chemistry and Instrumentation”, Vol. 2, C N. Reiliey, Ed., Interscience, New York, N.Y., 1963, Chap 3.
RECEIVED for review February 22,1977. Accepted April 27, 1977. This material was presented in part at the 3rd Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies in Philadelphia, Pa., November 1976.
Peak Shapes in Semidifferential Electroanalysis Penny Dalrymple-Alford Trent University, Peterborough, Canada
Masashi Goto Nagoya University, Japan
Keith 6. Oldham”’ Bristol University, United Kingdom
A derlvative neopolarogram Is the peaked curve that is generated by semidlfferentlatlon of the current that flows In response to an imposed ramp slgnal on a stationary electrode. The predicted dependences of peak height, peak potentlal, and peak wldth on concentration, electron number, scan rate, etc., are confirmed experimentally uslng one established and two new clrcuits to monitor the electroreductlon of several metal Ions in aqueous solutlon. Features of the technique that make it attractlve for qualltatlve and quantltatlve chemlcal analysis are cited, and comparlson Is made with pulse polarography.
Semidifferential electroanalysis was first described by Goto and Ishii (I). More recently, the present authors (2) have ‘Permanent address, T r e n t University, Peterborough, Canada.
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discussed some theoretical aspects of this voltammetric technique. The purpose of the current article is to compare the theoretical predictions with experimental data, to demonstrate that semidifferentiation may be effected digitally or by simpler circuitry than that used hitherto, and to discuss the analytical potentialities of the method. In semidifferential electroanalysis, a cathodic-going ramp signal is applied to a working electrode immersed in a solution containing, in addition to excess supporting electrolyte, one or more species that are electroreducible. The semiderivative e of the cathodic current that flows in consequence of the electroreduction is displayed as a function of the applied potential E. Figures 1 and 2 show examples of the resulting curves, which are termed “derivative neopolarograms” (2). A derivative neopolarogram consists of one or more peaks, each of which corresponds to a single reduction process. In Figure 1,for example, the two peaks correspond to the reductions of the In3+and Zn2+ions. Each peak is independent of the others; thus when In3+was omitted from the solution used
