ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
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Determination of Trace Level Arsenic(III), Arsenic(V), and Total Inorganic Arsenic by Differential Pulse Polarography F. T. Henry, T. 0. Kirch, and T. M. Thorpe"' Department of Chemistry, Miami University, Oxford, Ohio 45056
Speciation of As(", As(V), and total inorganic arsenic (As(tot)) was achieved by differential pulse polarography. As(II1) was determined directly in 1 M HC104 or 1 M HCI. Total inorganic arsenic was measured in either of these supporting electrolytes after prereduction of electroinactive As( V ) with a boiling solution of NaHS03. As(V) was evaluated by difference. The efficiency of reduction ranged from 93% to 109% for concentrations of As(V) ranging from 24 ppb to 4.9 ppm. Standard deviations for the procedure were less than 5.2 YO. The detection limit in the HCIO,-HSO,- reduction medium was 20 ppb; in HCI-HS03- it was 7 ppb. Relative errors for the determinations of As(II1) and As(tot) ranged from 0 % to 19.2%. Interferences from Pb, Sn, and TI were accounted for by a blank determination employing Ce( IV); interference from the breakdown of monomethylarsonic acid (MMA) during the preliminary reduction was not significant at concentrations of MMA which normally occur in natural waters.
Traditionally, determinations of the total content of arsenic in environmental specimens have been considered adequate to assess the presence, amount, and behavior of this element. Spectrophotometry, atomic absorption, and neutron activation have been employed as the analytical techniques after pretreating samples to convert all of t h e arsenic present to inorganic arsenic (1-3). Recent research (4-7) has shown that t h e predominant arsenic-containing species found in natural aqueous systems are: inorganic arsenate and arsenite, and organic dimethylarsinic a n d monomethylarsonic acids. Furthermore, the distribution of arsenic among these chemical forms is dynamic with interconversions between the species taking place via chemical and biochemical oxidation-reduction reactions, and by means of biochemical methylation-demethylation (4,8-10). These observations, coupled with recognition that the toxicity, carcinogenicity, transport, and bioavailability of As are highly dependent on t h e chemical form of the element, have prompted development of analytical methods capable of distinguishing between species of arsenic which exist as part-per-billion levels in environmental media. Spectroscopic techniques, when used with preliminary oxidation or reduction reactions, hydride generation, or liquid-liquid extraction yield speciation data on arsenic (1-3, 11-1 7). T h e preliminary operations make it difficult to quantitatively recover very small amounts of As and they may be cumbersome when dealing with large numbers of samples. Gas chromatographic procedures have been reported for the selective determination of organic and inorganic arsenicals (5, 18-23). These methods require preparation of volatile, thermally stable derivatives. While all of t h e arsenic-containing species mentioned previously have been determined chromatographically in nanogram a n d sub-nanogram quantities, molecular rearrangements of derivatives and 'Present address: The Procter & Gamble Company, Sharon Woods Technical Center, 11530 Reed Hartman Highway, Cincinnati, Ohio 45241. 0003-2700/79/035 1-0215$01.OO/O
consequent losses of accuracy have been observed (21). Forsberg et al. ( 2 4 ) , Davis et al. (25), Sulek, Zink, and Delude (26), and Holak (27) have used anodic stripping voltammetry (ASV) t o determine As(II1). Direct current polarographic measurement of arsenic has been reviewed extensively (28). Myers and Osteryoung (29) described the differential pulse polarographic ( D P P ) determination of as little as 0.22 wg/L As(II1) in a 1 M HCl supporting electrolyte. This method has been employed to measure As in raw sewage and sewage sludge (30),fish (27),and foodstuffs (27). DPP is a n advantageous technique because of its high sensitivity for elements, such as arsenic, which d o not form mercury amalgams readily (29). The polarographic determination of As(V) or total inorganic arsenic requires preliminary reduction of the electroinactive As(V) to electroactive As(II1). Reducing agents which have been suggested for this purpose include LiAlH4 ( 3 2 ) , Znamalgam (32),cuprous ion, hydrazine salts, and acidic solutions of iodide ion (24, 28, 29). Sulfur dioxide, derived from an aqueous solution of HS0,- has also been recommended as a reductant. However, detailed reports describing the range of conditions over which bisulfite reduction is applicable to the determination of trace level As(V) have not been presented, nor have data on the use of such a prereduction to speciate As(II1) and As(V). I n this paper we report a method for determination of As(III), As(V), and total inorganic arsenic by D P P . As(1II) is measured directly in 1 M HC104 or 1 M HC1. Total inorganic arsenic is determined in either of these supporting electrolytes after prereduction of electroinactive As(V) with a solution of sodium bisulfite. As(V) is evaluated by difference. Bisulfite was selected as the reductant since it reduces As(V) rapidly and quantitatively, and excess HSO? is readily removed from the reaction mixture.
EXPERIMENTAL Reagents. High purity arsenic trioxide was obtained from ROC/RIC (Belleville, N.J.) and ultrapure arsenic pentoxide was purchased from Alfa Inorganics (Danvers, Mass.). Standard M As(II1) or As(V) were prepared solutions containing 1 X by dissolving an appropriate amount of oxide in a minimal amount of 3 N NaOH. The solution was acidified to pH 2 and diluted to the desired volume with triply distilled deionized water. These stock solutions were stable for a t least three months. Working M As(II1) or As(V) were prepared weekly. solutions of 1 x High purity dimethylarsinic acid and monomethylarsonic acid were obtained from Ansul Company (Weslaco, Texas). A solution of 0.10 M Ce(1V) was prepared from reagent grade cerric ammonium nitrate. Sixty-three grams of Ce(NH4)2(N03)6 were mixed with 30 mL of concentrated sulfuric acid. The paste formed from this mixture was dissolved by slowly adding 500 mL of triply distilled deionized water. After cooling the solution to room temperature, it was filtered through a fine porosity qintered glass filter. The filtrate was diluted to 1 I,. U.S. Environmental Protection Agency water reference standards were prepared according to the directions supplied with those samples. Solutions of a commercially available trace element standard (Eastman Kodak gelatin multicomponent trace element reference material TEG-50-B, Rochester, N.Y.) were prepared by addition of a weighed amount of the standard to 25 mL of triply C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
distilled deionized water. This mixture was heated gently until solution was complete. All other chemicals were reagent grade. All solutions were prepared with triply distilled deionized water. Glassware was leached for 24 h with 1:l HN03. Instrumentation. A Princeton Applied Research Corporation (Princeton, N.J.) Model l i 4 A polarographic analyzer and a Hewlett-Packard (Avondale, Pa.) Model 7040A X-Y recorder were used for all D P P determinations. A Hach (Ames, Iowa) Model 8596 expanded scale pH meter was used for all pH measurements. Reduction of As(V) with Bisulfite. The optimized procedure for determination of A s ( W or for measurement of total inorganic arsenic is described below. The pH of an aqueous sample is adjusted to 3, either with dilute HC1 or with dilute NaOH. Equal volumes of the sample and 1 M NaHS03 are mixed to yield an HS03- concentration of 0.50 M. Solid NaHS03 may be substituted for the solution of the reducing agent, if desired. The pH of the mixture is readjusted to a value of 3. The solution is boiled for 30 min with continuous stirring. An air condensor is used to minimize evaporation (33). After the reduction step is complete, the solution is cooled briefly and sufficient HCIOl or HCI is added to neutralize the excess HS03- and to provide a 1G07’‘ excess of acid. The sample is boiled for 15 min, with the air condensor removed, and with nitrogen bubbling through the solution to facilitate removal of SO2. After the purging step, the solution is cooled and the acid concentration adjusted to 1 M with HCIO4 or HCI. Samples are split into two portions for the determinations of As(II1) and As(II1) plus As(V) (As(tot)). The first portion is used for determination of As(II1) after it has been made 1 M in HC104 or HCl. The second portion is carried through the bisulfite reduction procedure to measure As(tot). Quantitation is performed by the method of standard additions. Pb(II), Sn(II), Sn(IV), TI(I), and Tl(II1) are potential interferences to the D P P measurements. After recording polarograms for As(II1) or As(tot) and for the standard additions used for quantitation, sufficient Ce(1V) is added to the solution to oxidize As(II1) to polarographically inactive As(V). Another polarogram is recorded to give the “blank“ response arising from the residual current and from reduction of Pb. Sn, and TI. This blank is subtracted from the response obtained in the previously recorded polarograms. Ce(1V) does not interfere as it is reduced by the elemental mercury from the DME. A single addition of 0.20 mL of 0.10 M Ce(1V) usually oxidized As(II1) completely. When dealing with samples having unfamiliar matrices or when high levels of As(II1) are present, two 0.20-mL additions of 0.10 M Ce(1V) are made, with polarograms recorded after each, to assure quantitative oxidation of As(II1). Caution must be exercised in the blank determination, especially when attempting to measure low concentrations of As, as large shifts in the base line may occur (28). Holak (27) recommended an ion-exchange procedure as an alternative to the Ce(1V) oxidation to reduce the background response from Pb, Sn, and other metals.
RESULTS AND DISCUSSION Bisulfite Reduction of As(V). Quantitative reduction of As(V) was observed between pH 1.0 and 3.8. Beyond these limits, less t h a n 90% of the As(V) was converted t o As(II1); if t h e total concentration of acid was increased t o 1.5 M, n o As(II1) was detected after t h e bisulfite reduction. T h e hydrogen ion concentration directly affects the formal potentials couples, so t h a t in of t h e H3As04/H3As03a n d H2S03/S042t h e overall reaction:
H3As04+ H2S03
H,As03
+ SO4’- + 2H+
(1)
a net decrease in production of As(II1) occurs as the acidity increases. Furthermore, boiling a highly acidic solution of bisulfite (i.e., aqueous SO2),to speed the red-ox process, results in rapid loss of SOz, thereby making t h e active form of the reductant less available for reaction with As(V). These factors are responsible for the decrease in conversion of As(V) a t pH’s less than 1. At pH’s greater than 3.8, deprotonation of H2S03 and H3As0, occurs, lowering t h e availability of these species
for participation in t h e red-ox reaction. A p H of 3 was optimum for t h e reduction of As(V) t o As(II1). At concentrations of H S O c from 0.075 to 1 M, the efficiency of reduction of As(V) averaged 96.8%; the efficiency dropped t o 90% for 0.05 M HS03-. T o ensure a n adequate supply of reductant for quantitative production of As(II1) and to allow for potential competitive reactions with oxidants other than As(V), 0.50 M solutions of bisulfite were used in subsequent studies. At room temperature t h e reduction reaction proceeded slowly and nonquantitatively; only 86’70 of the As(\’) in a 6.5 X lo-’ IvT solution was reduced after 30 min. Reduction efficiency was increased t o 95.8% f 3.6% by boiling for 30 min, and more than 9 0 7 ~of the As(V) was reduced if solutions were boiled for 25-35 min. Low and erratic (68-82%) conversion of As(V) t o As(II1) was observed if boiling time was extended t o 40 min or longer. This is thought t o result from loss of As(II1) through spattering a s the solution evaporates during extended heating. Air oxidation of t h e spattered analyte may also contribute t o t h e low recoveries. T h e final step of t h e procedure is t h e removal of excess reductant. If this is not carried out, polarographically active SO2 (Epa t -0.25 V t o 0.30 V vs. SCE) will interfere with or completely obscure t h e peak due t o As(II1). Boiling t h e reduction mixture for 15 min after t h e unreacted HS0,- has been neutralized and a 1 0 0 7 ~excess of acid added was effective for SO2 removal when used in conjunction with continuous bubbling of nitrogen through the solution. Shorter heating, lower temperatures, or omission of N 2 purging led t o erratic removal of SO2. Electrochemical Characteristics of Bisulfite Reduction Systems. In HC104 and HC1O4-HSOy (bisulfite blank acidified with HC104) media, the peak potential of As(II1) was -0.49 V vs. SCE and linear working curves ( Y = 1.42 (PA/ p p m ) X - 3.40 X IO-‘, correlation coefficient = 0.999) extended from 10 p p b t o greater t h a n 7.5 ppm. T h e detection limit, defined as t h e minimum concentration of As(II1) producing a response that was two times t h e signal found at -0.49 V vs. SCE when analyzing the supporting electrolyte alone, was 20 PPb. T h e As(II1) peak occurred a t -0.38 vs. SCE in the HC1 and HCl-HSO< electrolytes. Linear calibration curves extended from 1 p p b t o greater than 7.5 ppm. However, these curves differed depending on the supporting electrolyte; in 1 M HC1 the regression equation was Y = 5.98 (pA/ppm)X - 3.76 X 1 O F (correlation coefficient = 0.999), and in 1 M HC1-HSOY, the curve was described by Y = 4.92 ( b A / p p m ) X + 2.18 X lo-’ (correlation coefficient = 0.999). These differences indicate t h a t t h e D P P response t o As(II1) is more sensitive t o changes in composition of the 1 M HC1 supporting electrolyte t h a n t o alteration of t h e 1 M HC104 electrolyte. T h i s conclusion was confirmed in analyses of water reference standards. The detection limit was 7 ppb in HC1 and 4 ppb in HC1-HSOY. Blanks were consistently smaller in t h e HCI-HS03- system so t.hat a lower detection limit was observed even though t h e slope of the calibration curve in 1 M HC1 was approximately 30% larger t h a n for HC1-HS03 . Efficiency of As(V) Reduction by HS03-. The efficiency of As(V) reduction was examined in HC104-HS0< and HC1-HS03- media (Table I). Reductions were quantitative, with t h e average recoveries over t h e concentration ranges examined being 96% in HC10,-HS03- a n d 105% in HCIHS03-. Standard deviations were under 5.2%. Neither reoxidation of As(V) in t h e presence of HC104 nor volatilization losses of As(II1) as AsC1, in t h e presence of HC1 occurred. Accuracy of As(II1) and As(tot) Determinations. T h e accuracy of the analytical procedure was examined using U S .
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY
Table I. Efficiency of h ( V )Reduction perchloric acid-bisulfite media As(II1) found
recovery, %
4.93 ppm 4.66 ppm 1.09 ppm 1.02 ppm 1 0 6 ppb 1 0 2 ppb 1 0 6 ppb 1 0 9 ppb 4 8 ppb 45 PPb average recovery: 96 standard deviation: + 3 . 6 % relative standard deviation: 3.8%
94.5 93.6 95.8 102 93
hydrochloric acid-bisulfite media 247 ppb 265 ppb 1 6 9 ppb 1 7 8 ppb 53 PPb 58 PPb 51 ppb 49 ppb 24 ppb 26 PPb average recovery: 1 0 5 % standard deviation: 5.2% relative standard deviation: 5.0%
107 105 109 96 108
polarographic conditions electrodes: working: DME reference: SCE counter: Pt wire drop time: 2 s
initial potential: -0.20 V potential range: 0.75 V scan rate: 1 mV/s modulation amplitude: 100 mV
Environmental Protection Agency water reference standards, a commercial trace element reference material (Eastman and samples of "high Kodak, TEG-5O-B, Rochester, N.Y.), purity" (34, 35) dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) (Table 11). Agreement between the measured and reported values for As(tot) was good except for the MMA and the commercial reference material. In the case of MMA, reduction conditions caused the organoarsenical to break down into an electroactive species, probably As(III), which prevented D P P determinations of the total inorganic arsenic impurity present in the sample. T h e relatively poor accuracy found in analyses of the commerical reference material resulted from difficulties in reliably interpreting base lines in the polarograms of the analyte and blank. This is attributed to the interference of the gelatin matrix with electrode processes. Table 11. DPP Determination of Arsenic in Standard Samples
As(III), PPm found reported
sample U.S. E.P.A. Water reference standards E.P.A. =1 E.P.A. = 2 E.P.A. 7 3 high purity dimethylarsinic acid high purity monomethylarsonic acid commercial trace element reference material a
0.023 0.106 0.154 4.5 163 116
-
-
-.
-
