Anal. Chem. 1986, 58, 1383-1389 (3) Sawynok, J.; Dawborn, J. K. Clin. Exp. Pharmacol. Physiol. 1975, 2 , 1-15. (4) Lazdlns, 1.; Dawborn. J. K. Clin. Exp. Pharmacol. Physlol. 1978, 5 , 75-80. ( 5 ) Baker. M. D.; Mohammed, H. Y.; Veening, H. Anal. Chem. 1981, 53, 1858- 1862. (6) Yamamoto, Y.; Saito, A.; Manjl, T.; Maeda, K.; Ohta, K. J . Chromatogr. 1979, 162, 23-29. (7) Yamamoto, Y.; ManJI, T.; Salto, A.; Maeda, K.; Ohta, K. J. Chromafogr. 1979, 162, 327-340. (8) Hlraga, Y.; Kinoshlta, T. J. Chromafogr. 1981, 226, 43-51. (9) Kai, M.; Mlyazaki. T.; Yamaguchi, M.; Ohkura, Y. J . Chromafogr. 1983, 268, 417-424. (10) Kai, M.; Miyazaki, T.; Ohkura, Y.; J. Chromafogr. 1984, 311, 257-266. (11) Hung, Y-L.; Kal, M.; Nohta, H.; Ohkura, Y. J. Chromatogr. 1984, 305, 28 1-289.
(12) (13) (14) (15) (16)
1383
Engelhardt, H.; Neue, U. D. Chromatographia 1982, 15, 403-408. Mori, A.; Hosotani, M.; Choong, T. L. Blochem. M e d . 1974, 10, 8-14. Angyal, S. J.; Warburton, W. K. J. Chem. SOC. 1951, 2492-2494. Failey, C. F.; Brand, E. J. Biol. Chem. 1933, 102, 767-771. Ratner, S.; Petrack, B.;Rochovansky, 0. J. Biol. Chem. 1953, 204, 95-113.
RECEIVED for review September 17,1985. Accepted January 15,1986. H.V. thanks the National Science Foundation (Grant INT-8317607) and Bucknell University for financial support while on sabbatical leave in The Netherlands. This work was supported by the Dutch Foundation for Technical Sciences, Grant VCH 33.0436.
Simultaneous Determination of Arsenic( I 11) and Arsenic(V) in Metallurgical Processing Media by Ion Chromatography with Electrochemical and Conductivity Detectors Liang K. Tan* and John E. Dutrizac Mineral Sciences Laboratories, C A N M E T , Energy, Mines and Resources Canada, 555 Booth Street, Ottawa, Ontario K I A OG1, Canada
As( II I ) and As(V) In metallurglcai processlng solutions are determlned slmultaneously by Ion chromatography with electrochemlcal and conductivlty detectors, respectlvely. The effects of acld, base, or alkall salts on the ion chromatograms obtained from low concentratlons of As( I 1I ) are descrlbed. Cations such as Co(II), Cu(II), Fe(I1 and HI), and NI(I1) must be preseparated wlth an H+ cation exchanger to avoid precipltatlon of arsenlc wlth the metal hydroxides formed during dllutlon. Na+ or K+ In the sample must be preseparated slmllarly because of thelr effects on the current response of the arsenlte. The effect of H2S04on the current sensltlvlty of the arsenlte was determlned. The detectlon llmlts for arsenlte In 0 and 5 mM H,S04 are 0.005 and 0.025 mg/L As( III),respectlvely. The detectlon llmit for arsenate is 0.022 mg/L As(V). As(III)/As(V) ratlos as low as 0.025 can be determlned quantitatively.
The determination of As(II1) and As(V) in ferric chloride-hydrochloric acid leaching media (1)and in ferric sulfate-sulfuric acid processing solutions (2) by ion chromatography (IC) with conductivity detection was reported previously. Although arsenite and arsenate ions can be separated by the IC column, arsenite is not detected by the conductivity detector because of the low dissociation constant of arsenous acid (3). In the above studies, therefore, As(II1) was determined by difference following oxidation of the test solution with aqua regia ( I , 2). This method was useful for moderately high As(III)/As(V) ratios and had an accuracy of 5-10% at the 0.2-10 mg/L level. Although such accuracies are acceptable for most metallurgical applications, many processing solutions contain low As(II1) concentrations relative to the total arsenic concentration. In such solutions, it is impossible to determine As(II1) accurately by difference. Kinetic studies aimed a t examining the reduction of As(V) also require accurate As(II1) concentrations at low levels. A sensitive and 0003-2700/86/0358-1383$01 SO/O
direct method of measurement of As(III), such as by electrochemical detection, would be desirable and, ideally, such methods should also permit simultaneous measurement of As(V). The measurement of arsenic(II1) by polarography is wellknown, and the complex polarographic behavior of As(II1) in acidic, neutral, and alkaline media has been reviewed (4). A polarographic detector for flowing liquid has been investigated ( 5 , 6 ) . Although the dropping mercury electrode provides a constantly renewed surface, it presents several disadvantages as a detector for liquid chromatography that include oscillation of the measured current due to the growth and fall of the mercury drop and a complicated cell design. Recent studies have indicated that solid electrodes (Pt, Au, glassy carbon) for flowing liquid generally have greater sensitivity and simpler cell design than dropping mercury electrodes (5-9). Anodic detection of arsenic(II1) in a flow-through platinum-wire electrode for flow-injection analysis has also been reported (10, 11). The arsenic(V) concentration in the sample was, however, obtained by difference after the reduction of arsenic(V) to arsenic(II1) with hydrazine sulfate (11). The present study reports a method for the simultaneous determination of As(II1) and As(V) by IC with two detectors. After the separation of arsenite and arsenate by the IC separator column, the effluent containing the analytes passes through a flow-through electrochemical cell with a platinum working electrode which measures the arsenite concentration. It then flows through a suppressor column and finally passes through a conductivity cell which measures the arsenate concentration. The ion chromatograms resulting from the conductivity detection of arsenate-containing solutions have been documented previously (1-3). The ion chromatograms from the electrochemical detection of arsenite-containing solutions, however, have not been reported in the literature. Therefore, the purpose of the present work is to discuss the various ion chromatograms of arsenite in solutions containing acid, base or alkali salts, and to develop an analytical method
Published 1986 by the Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
for the simultaneous determination of As(1II) and As(V) in typical metallurgical processing solutions. EXPERIMENTAL SECTION Apparatus. The ion chromatograms were obtained with a Dionex Model 2020i ion chromatograph equipped with a 50-pL sample loop, HPIC-AG4 guard column, HPIC-AS4 separator column, and AFSl anion hollow fiber suppressor, which was continuously regenerated at 2.3 mL/min flow rate using 0.025 N H@O4 The electrochemicaldetector consisted of a potentiostat with a digital current readout and a three-electrode flow-through cell having a volume of 26 pL. Its configuration has been documented previously (12). This cell was located between the separator column and the suppressor column. An Ag/AgCl reference electrode was separated from the flowing stream by a Nafion membrane, and the counter electrode was made of stainless steel. A platinum electrode (1.3 cm long X 0.178 cm diameter) served as the working electrode. It was occasionally cleaned by polishing with fine alumina paste. Unless otherwise mentioned, an applied potential of 0.40 V was used for the electrochemicalmeasurements. The platinum working electrode was reconditioned following each sample run: while eluant was flowing, the applied potential was set at -1.50 V for 30 s, and then the potential was reset to 0.40 V and the electrode was equilibrated for 15 min. The conductivity flow-through cell was located after the suppressor column and was connected to a digital readout. The experiments were run at 2.0 mL/min eluant flow rate using a predegassed solution of 2.80 mM NaHCO3-2.25 mM NaZCO3(pH 10.4). Output data were recorded on a Dionex Model 4270 integrator equipped with a dual channel option cartridge. One of the channels was used to plot the chromatogram as well as to integrate and analyze the output data from one of the detectors. The other channel integrated and analyzed the output data from the other detector but could not plot the chromatogram. Nevertheless, a simultaneous plot of the two chromatograms from the two detectors was obtained by the addition of a Cole-Palmer Model 8373-20 dual pen plotter. Cation Exchange Column. Standard Econo borosilicate glass columns (1cm X 10 cm) were used for the separation of cobalt, copper, iron, nickel, uranium, sodium and potassium from the sample solutions. The bottom end was sealed with a glass frit, and the upper end was connected to a sample receiver of 8 mL volume. The column was prepared for use by pouring an aqueous slurry of Bio,Rad AG50W-X8 H+-form, 100-200 mesh resin, to a bed height of 8 cm (resin volume, 6.3 cm3). Regeneration of the resin was done by passing two 8-mL aliquots of 3.0 M HzS04 through the column, followed by four 8-mL aliquots of water. All samples and eluants were passed dropwise through the columns at a rate of 1.5 mL/min. Reagents. All solutions were prepared in doubly deionized water. Unless otherwise mentioned, reagent grade chemicals were used. A primary As(II1) standard was prepared from As203 (Fisher, Certified ACS) and a primary As(V) standard was prepared by oxidizing the primary As(II1) standard with aqua regia as mentioned previously ( I , 2). Secondary As(II1) and As(V) standards were prepared from sodium arsenite (NaAs02) and sodium monohydrogen orthoarsenate (NazHAs04.7H20),respectively. They were standardized against the primary As(II1) or As(V) standard solutions by ion chromatography (2). Solutions of As(V) for the calibration of the ion chromatograph were prepared according to the previously described procedures ( 2 ) . Procedure. A 0.1 mL aliquot of sample was placed on top of the resin bed of the cation exchange column. The solution was then passed through the column followed by four 8-mL aliquots of water. The effluent was collected and diluted with water to give As(II1) and As(V) concentrations less than approximately 1and 20 mg/L, respectively. This volume of water elutes 100% of the arsenic species and gives an acceptable H2S04concentration in the final solution for accurate analysis. Four solutions were prepared in this manner for each sample. At the final dilution step, known quantities of the As(II1) standards were added to three of the four solutions to give concentrations of 0.105,0.210, and 0.315 mg/L As(II1). During the final dilution step, NaHC03 and Na2C03were added such that the final concentrations of these salts were exactly equal to those of the IC eluant. Each solution was then injected immediately into the ion chromatograph. Arsenate concentrations were determined from the peak heights
CH2S041, m M o 0
260
A 0
2.0 3.5
PH
10.4 7.0 4.6
i ,nA
28
-i
0
.2
.4
Eaw
.6
.8
.9
,v
Flgure 1. Dependence of the measured anodic current on the applied
potential for arsenite solutions containing varlous concentrations of H,S04 in eluant.
using the linear calibration routine of the ion chromatograph; standard As(V) solutions of three different concentrations were used for this calibration and these bracketed the concentration of the test solutions (2). Arsenite concentrationswere determined from the peak heights by the standard addition method. Metallurgical Processing Solutions. The developed analytical method for As(II1)and As(V) was evaluated by use of three types of simulated metallurgical processing solution. These solutions were (1)a sulfate-based leaching solution from the leaching of impure arsenical copper (such solutions typically contain 20-57 g/L Cu, 2-23 g/L Ni, 0-10 g/L total As, and 50-200 g/L HZSO,), (2) solutions from the oxygen-sulfuric acid pressure leaching of cobalt concentrates containing cobaltite (CoAsS),arsenopyrite (FeAsS), and pyrite (FeS2)(such solutions typically contain 35-53 g/L Co, 17-50 g/L Fe, 0-10 g/L total As, and 50-100 g/L HzS04), and (3) a sulfate-based solution from the leaching of uranium oxide ores containing significant arsenide mineralization as niccolite (NiAs) and gersdorffite (NiAsS) (such solutions typically contain 5-23 g/L U, 9-23 g/L Ni, 5-33 g/L Fe, 0-10 g/L total As, and 20-100 g/L H,SO,).
RESULTS AND DISCUSSION Applied Potential for the Electrochemical Detection of Arsenite. Figure 1shows the dependence of the measured anodic current on the applied potential (Eapp) for arsenite in solutions containing 2.80 mM NaHCO,-2.25 mM Na2C03. The optimum current sensitivity for arsenite occurs a t the same applied potential regardless of the pH (10.4, 7.0, and 4.6) of the test solution that was varied by the addition of various concentrations of sulfuric acid (0, 2, and 3.5 mM). A possible explanation is as follows. The injected test solution (50 KLonly) enters the IC separating column where the anionic species are separated. The arsenite is then eluted very early (-0.86 min) by the eluant and reaches the electrochemical cell -6 min earlier than the sulfate. Consequently, the electrochemical measurement of arsenite takes place in a solution consisting mainly of eluant (pH 10.4) and in the absence of sulfate. In the vicinity of the electrode, the pH and the ionic strength of the eluant are the important factors. Dependence of the Anodic Current of Arsenite on the pH. Figure 1 shows that the anodic current of arsenite decreases as the p H of the test solution is reduced. This fact
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
1385
280 270
260 250
240
i, nA
230 220
210 200 190 180
7
5
9
11
PH
Figure 2. Dependence of the measured anodic current on the pH of arsenite solution. 32
12
1
-
0
20
40
60
80
100
NUMBER OF INJECTIONS
Figure 3. Current response of arsenite solutions in eluant as a function of the number of successive injections.
is due to the presence of less dissociated arsenite ion in the test solution at lower pH (pK, of arsenous acid is 9.2). A better illustration of the anodic current dependence on the pH at a fixed E,, = 0.40 V is depicted in Figure 2. Data for pH 10.4 cannot be realized because the addition of base affects the peak current as will be described later. The dependence of the conductivity of arsenate on pH has been reported previously (2). Anodic Current Response of Arsenite. Figure 3 illustrates the anodic current response of 0.210 mg/L As(II1) solutions in eluant at Eapp = 0.40 V as a function of the number of successive injections. For the first 10 injections, the anodic currents are progressively reduced with a decrease of 15% occurring after the tenth injection. The current drifts slowly downward during the following injections but is more reproducible after -40 injections. The presence of H2S04does not alter the general form of the current response profile, nor does the use of higher applied potentials. After the electrode was reconditioned by setting E,, at -1.50 V for 30 s and then the electrode was reequilibrated a t 0.40 V for 15 min, the test solution gives exactly the same current value as for the first injection. Solid electrodes are subject to complicated surface renewal problems, which have been reviewed (13). Oxide film for-
-
t As(lll)l , mg/ L
Figure 4. Dependence of the anodic current on the As(II1) concentration of solutions containing 0, 2, and 4 mM H2S04.
mation caused nonreproducible currents during the electrolytic oxidation of arsenic solutions (14). Chemical tests proved that the film was PtO and PtOz (15),which could be removed by prolonged exposure to the As(II1) solution (14). The oxidized electrode has also been viewed as possessing a variable “effective area” (16). The electrochemical formation and growth of oxide films on platinum electrodes have been the subject of continuing debate (17,18). The initial film growth has been considered as an electrochemically reversible chemisorption process although an irreversible component also was observed at higher formation potentials or long formation times (19,20). Mechanisms for the electrocatalyzed oxidation of As(II1) at a platinum electrode have recently been proposed (21, 22). The present results, which show a current decay for the first 10 successive injections (Figure 3), are in agreement with the findings of previous workers (11)who recorded the decrease of the anodic current for a continuous flow of As(II1) solution in a platinum-wire flow-through detector. Although the present results seem to indicate current reproducibility after 40 injections, a decay in current was still apparent if the flow of eluant was stopped for a day before the next measurement. In this regard, the present results differ from the flow injection measurements (11), which showed a reproducible anodic current for As(II1) when the oxide film reached a constant thickness. In the present work, reproducible currents (or peak heights) are attained by electrochemical reduction of the interfering species on the electrode surface. This reconditioning of the electrode is performed after each injection of arsenite-containing solution. Arsenate is not electrochemically active and does not affect the platinum electrode of the electrochemical cell. The conductivity detection system for arsenate was found to be both accurate and reproducible, as noted in previous studies (1-3). Dependence of the Anodic Current on As(II1) Concentration. Figure 4 shows the relationship of anodic current and As(II1) concentration for solutions in eluant with or without H2S04. In general, linear relationships are obtained from 0.01 to 4 mg/L As(II1). At higher As(II1) concentrations, the curves begin to level off. This phenomenon also was observed previously in the electrochemical analysis of arsenic with a platinum electrode (23). The leveling off can be attributed to the inhibition of the arsenite oxidation reaction by an interfering film on the electrode surface. The linear portions of the curves of Figure 4 have slopes of 108,101, and 94.4 nA L mg-l As(II1) for solutions containing 0,2, and 4 mM H2SO4, respectively. These slopes represent the sensitivities
1386
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
r?
ELECTROCHEMlCAl
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0 Time.mtn 4
f
80
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6 or
8
K,SQI x
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1
lo3, M
Flgure 7. Anodic current response of 0.2 10 mg/L As( I I I) in solutions
containing various concentratlons of Na2S04(triangles) or K,S04 (circles). Closed symbols are data without cation preseparation; open symbols are data with cation preseparation.
ionA
89
a
2
[ Na,SO,
Figure 5. Ion chromatograms obtained simultaneously from the electrochemical and conductivii detectors for a test solution Containing 0.210 mg/L As(III), 5.25 mg/L As(V), and 1.8 mM H2S04. All cations in the orlginal sample were preseparated using cation exchange resin.
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Figure 8. Ion chromatograms obtained from the electrochemical detection of 0.210 mg/L As(II1) In various media.
of the anodic currents (or peak heights) due to the oxidation of arsenite on the platinum electrode under the experimental conditions used. Ion Chromatograms. Figure 5 shows two ion chromatograms obtained simultaneously from the electrochemical and conductivity detectors for a test solution containing 0.210 mg/L As(III), 5.25 mg/L As(V), and 1.8 mM HzS04. All of the cations present in the original sample were preseparated with the cation exchange resin. The ion chromatogram from the conductivity detection of arsenate in acidic or basic media is simple (1-3). However, the ion chromatogram from the electrochemical detection of low concentrations of arsenite is relatively complex in either acidic or basic solutions. Except for chromatograms 6a and 6e, all the chromatograms in Figure 6 were obtained for 0.210 mg/L As(II1) in various media. The chromatogram of arsenite in water is shown in Figure 6b. The negative peak and the peak at retention time 1.03 rnin are both due to water as can be deduced by comparison with the chromatogram of water alone (Figure 6a). The chromatogram of arsenite containing the same concentrations of NaHC0, and Na2C03as the eluant is simple (Figure 6d) and resembles the detection peak for As(II1) reported previously in flow injection analysis (11). When 3.0 mM NaZCO3is added to the arsenite solution in eluant, the apparent current response (or peak height) is -1.5 times higher (Figure 6f) than that obtained in eluant alone. Also, another peak at 1.00 rnin is detected. Identical chromatograms were obtained by the addition of the same concentration of either Na2S04or K2S04 (Figure 6g and Figure 6h). These observations are consistent
with the fact that sodium or potassium salts in eluant, in the absence of arsenite, give chromatograms with a major peak at retention time 0.79 rnin and a minor peak at 1.04 min (Figure 6e). The peak at 0.79 min, of course, cannot be resoIved from that of arsenite at 0.86 min. Hence, the two peaks overlap and are observed as a single peak at 0.80-0.82 min retention time and with a higher current sensitivity (Figure 6f-h). Because only one Na ion is available in NaHCO,, adding the same molarity of NaHC0, increases the peak height by only 1.1times (Figure 6i). Clearly, the final diluted test solution injected into the ion chromatograph must contain exactly equal concentrations of Na2C03and NaHCO, as the eluant. Figure 6j and Figure 6k were obtained from solutions of arsenite in eluant containing 3.0 mM NaOH (or KOH) and 3.0 mM HzS04,respectively. The above observations are very important as concerns the accurate measurement of As(II1) in metallurgical processing solutions. Considerable amounts of sodium or potassium salts can be present in processing media, and clearly Na' or K+ must be preseparated from the sample solution prior to measurement. Also, leaching solutions commonly contain relatively large amounts of sulfuric acid, and the analytical procedure must take into account such high and variable acidities. Preseparation of Metal and Sodium/Potassium Cations. Data represented by the closed symbols in Figure 7 show the progressive increase of the anodic current response of the 0.210 mg/L As(1II) standard solution in eluant containing increasing amounts of Na2S04(triangles) or K2S04(circles). When these solutions are passed through a cation exchange column containing H+-form resin, sodium or potassium ions undergo exchange with the hydrogen ions on the resin. The arsenite is subsequently eluted with water. Analysis of the effluents after ion exchange yielded the anodic currents indicated by the open symbols in Figure 7. An initial slight decrease in anodic current is followed by a slight increase a t greater salt concentrations. This behavior is attributable to variations in the sulfuric acid concentration of the effluent collected. The initial decrease of the current is due to the increase of the acid concentration resulting from ion exchange. The subsequent increase of current is due to partial overlapping of the arsenite peak with the peak at retention time 1.0 min caused by the high acid concentration (Figure 6k). Arsenic forms precipitates with the hydroxides of cobalt, copper, iron, or nickel during dilution with either water or eluant, Direct dilution and injection, therefore, yields low arsenic levels as measured by IC. Therefore, these metal cations are preseparated from the sample solution using the
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
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Table I. Recovery of As(II1) and As(V) from Synthetic Acid Leaching Solutions Containing Copper Sulfate or Nickel Sulfate mean % recovery f % std dev
[As(III)I, g/L
[As(V)I, g/L
[CUSO~I, M
[NiS041, M
[H8041, M
As(II1)
As(V)
0 0.105 0.210 0.420 0.525 0.525 0.525 1.050
5.250 2.100 2.380 1.050 1.050 1.050 15.750 10.500
0.200 0.700 0.200 0.700 0.200 0.700 0.800 0.400
0.400 0.400 0.100 0.400 0.200 0.200 0.300 0.100
2.0 0.6 1.0 1.0 2.0 1.5
0 104 f 4.6 112 f 12 92.1 f 12 100 f 2.8 107 f 6.8 109 f 9.1 101 f 11
107 f 2.2 106 f 3.0 110 f 1.4 108 f 5.1 92.6 f 2.3 110 f 6.2 102 f 0.8 108 f 3.1
0.8 0.6
Table 11. Recovery of As(II1) and As(V)'from Synthetic Acid Leaching Solutions Containing Cobalt Sulfate and Ferric Sulfate mean '70 recovery f % std dev
[As(III)l, g/L 0 0.105 0.210 0.210 0.210 4.200 10.500 10.500
[AsWI, g/L
[CoSO41, M
[F~z(SO,),I,M
[HzSO~I, M
As(II1)
AsW)
4.200 4.200 4.200 4.200 4.200
0.600 0.600 0 0.600 0.900 0.600 0.600 0.900
0.300 0.300 0.300 0.300
1.0 1.0 0.5 1.0 0.5 1.0 1.0 1.0
0 104 f 2.9 102 f 6.6 109 f 1.4 112 f 8.3 103 f 6.7 93.8 f 5.2 105 f 14
96.5 f 4.4 1.4 96.5 101 f 0.9 99.9 f 1.0 95.9 f 2.8 0 109 f 2.8 109 f 1.9
0 10.500 10.500
0 0.300 0.300 0.450
cation exchange column. Although precipitation of As(II1) and h ( V ) with uranium species upon dilution is not observed, the same treatment should be accorded to uranium processing solutions because such media frequently contain considerable amounts of iron and nickel. When a 0.1-0.2 mL sample aliquot is used in the preseparation step, samples containing 0.1 M H2S04and 21.0 g/L As(II1) or As(V) with 0.3 M Fez(SO,),,0.9 M C0S04, 0.9 M CuS04,or 0.9 M NiS04 yield only 88-9070 arsenic recovery. For such arsenic and metal sulfate concentrations, at least 0.2 M HzS04must be present in the sample. For samples containing 0.105-10.5 g/L As(II1) or As(V) and the above mentioned concentrations of metal sulfates, 0.1 M H2S04in the sample is adequate to yield total arsenic recovery. Fortunately, metallurgical processing solutions commonly contain high acid concentrations. During the ion exchange preseparation step, cations are adsorbed on the resin and H+ ions are liberated to combine with existing SO4&anions. These H+ and SO?- ions plus those originating from the sulfuric acid in the original sample determine the pH and the concentration of total sulfuric acid in the collected effluent. Solutions of pH 4-11 are necessary for the accurate measurement of arsenate by IC with conductivity detection (2). In addition, sulfate concentrations >500 mg/L in the diluted test solution affect the conductivity response of the arsenate (2). Therefore, effluent collected from the cation exchange column is diluted to increase the pH and to reduce the sulfate concentration. Adjustment of the pH of the effluent with dilute Na2C03or NaOH solution must not be used for the simultaneous determination of As(II1) and As(V) because of the effect of sodium salts or sodium hydroxide on the measurement of arsenite. NaHC03 and NaZCO3are added to the final diluted effuent, however, to make their concentrations exactly equal to those of the eluant. Accuracy and Precision. Because of the effect of sulfuric acid on the measurement of arsenite, the standardization method can be employed only using standard solutions with a sulfuric acid concentration equal to that of the test solution. The effect of sulfuric acid can be eliminated by using the standard addition method. Known quantities of As(II1) standard solution were added to the solutions after the
*
preseparation of cations. The addition of As(II1) to the original sample can cause precipitation of the arsenic with the metal cations present in the leaching solution. The detection limit of arsenite in eluant is 0.005 mg/L As(III), where the detection limit is defined as the concentration that gives a peak intensity twice the base line noise. When the diluted test solution contains relatively high sulfuric acid concentrations (e.g., 5 mM), however, the detection limit is 0.025 mg/L because of peak overlap. Arsenate is measured by use of the standardization method. The linearity of the conductivity vs. As(V) concentration relationship has been demonstrated in a previous report (2). Under the present experimental conditions, the sensitivity of the arsenate peak is 0.410 wmho L mg-l As(V), and the detection limit is 0.022 mg/L As(V). Synthetic solutions having various concentrations of As(II1) and As(V) in metal sulfate-sulfuric acid media were prepared. These solutions were analyzed four times with each analysis being carried through all the analytical procedures. The accuracy of the method is reflected by the percent recoveries of As(II1) and As(V), which are shown in Tables 1-111. Since dilution is often required to reduce the concentration of sulfuric acid, low concentrations of the analytes are produced in the final diluted test solution, and these low concentrations affect the accuracy of the analysis. Also, because the two detectors have different sensitivities, measurement of arsenite requires greater dilution than measurement of arsenate. Depending on the ratio of As(III)/As(V) in the sample, a given dilution factor may be suitable for the measurement of one of the analytes but may be too large for the measurement of the other. In general, the data in Tables 1-111 indicate recoveries of 92-112% for As(II1) and 93-110% for As(V). Percent standard deviations from four determinations are 1-13 for As(II1) and 1-6 for As(V). Subsequent reanalysis of the above synthetic solutions after a storage period of several months gave the same values for As(II1) and As(V) listed in Tables 1-111. Clearly, the redox reaction As(II1) F! As(V) in metal sulfate-sulfuric acid media is extremely slow at room temperature, and both As(II1) and As (V) enjoy a considerable degree of metastability although
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JUNE 1986
Table 111. Recovery of As(II1) and As(V) from Synthetic Acid Leaching Solutions Containing Uranium Sulfate and Various Base Metal Sulfates mean % recovery f %
[As(III)I, g/L 0 0
[As(V)l,
g/L
[UOZSO~I, [NiS041, M M
10.500 0.210 0.210
10.500 10.500 0 5.250 5.250
0.020 0.060 0.060 0.060 0.060
0.525 1.050 2.100
5.250 10.500 2.100
0.060
0.100 0.100
0
[FeS041, M
[Fez(S04)91, [other metal M sulfate], M
0.150 0.150 0.300 0.150
0 0 0 0 0.020
0.050 0.050 0.300 0.050
0.400 0.400 0.200
0 0.100 0.100
0.200 0.200 0.200
0
0 0 0 0 0.019" 0.082* 0 0 0
std dev As(II1) AdV)
[HzS041, M 0.2 0.2 0.2 0.5 0.5
0 0 109 f 0.01 104 f 5.7 105 f 9.8
98.0 f 2.0 96.7 f 1.5 0 95.7 f 0.8 92.8 f 1.7
1.0 0.5 1.0
94.1 f 2.7 94.1 f 12 110 f 12
94.7 f 2.3 96.2 f 1.0 99.3 f 3.0
"Al&304)p * MgSO4. Table IV. Comparison of the Recovery of Total Arsenic by Aqua Regia Oxidation and by Simultaneous Measurement of As(II1) and As(V)
[As(III)I, gl 10.500 0.210 0.525 1.050 10.500
[As(V)I, g lL 0 2.100 5.250 10.500
10.500
[CuSOd, M
[NiSOA, M
0.300 0.200 0.100 0.400 0.300
0 0.100 0.100
0.100 0.300
[H,SO41, M 0.3 0.6 0.3 0.6 0.6
the reasons for this have not yet been elucidated. Comparison of the Determination of Total Arsenic by Aqua Regia Oxidation a n d by Simultaneous Measurement of As(II1) and As(V). The introductory section of this paper describes the determination of total arsenic by conductivity detection following aqua regia oxidation of As(II1) (I,2). The simultaneous determination of As(II1) and As(V) with two detectors was presented above. Table IV indicates that both methods are in good agreement within the range of concentrations studied. Despite the necessity to recondition the platinum electrode for each analysis, simultaneous measurement of As(II1) and As(V) is preferred over the oxidation method for the following reasons. Simultaneous measurement is applicable to diluted solutions with
