Anal. Chem. 1985, 57, 1027-1032
1027
Determination of Arsenic(I I I) and Arsenic(V) in Ferric Chloride-Hydrochloric Acid Leaching Media by Ion Chromatography Liang K. Tan and John E. Dutrizac* Mineral Sciences Laboratories, CANMET, Energy, Mines and Resources Canada, 555 Booth Street, Ottawa, Ontario K1A OG1,Canada
An analytlcal method has been developed to determlne arsenlc( V) in ferrlc chlorlde-hydrochlorlc acld leaching media uslng Ion chromatography wlth conductlvliy detection. 0x1dation of As(II1) by aqua regia allows arsenlc(II1) to be determlned by dlfference. The method Involves a preseparation of trace quantltles of arsenlc from the relatlvely large concentratlons of ferrlc chloride and hydrochlorlc acld prior to the Ion chromatography measurement. Iron(II1) Is separated by passlng the sample through a hydrogen-form cation exchange column, and arsenlc( I I I ) and arsenlc(V) are then eluted with water. The effect of the concentratlon of acld In this separation Is discussed. The effluent collected from the catlon exchange column Is evaporated to remove the hydrochlorlc acld. The accuracy and precision of the method were determlned from the analysls of various synthetic solutlons and are discussed; an accuracy of f4 % was obtalned even at arsenlc(V) concentratlons as low as 10 ppm. The extent of oxldatlon of arsenic(I I I ) In acldlc ferrlc chlorlde solution and the reduction of arsenlc(V) In acldlc ferrous chloride solutlon were measured. The results obtalned by lon chromatography are compared to the values reallzed uslng colorimetry after the preseparatlon step.
The ferric chloride leaching of base metal sulfide concentrates is being advocated as a means of avoiding SO2 emissions during conventional smelting. These concentrates often contain sulfarsenide minerals such as tennantite (CuI2As4S1J or arsenopyrite (FeAsS), and the behavior of arsenic during leaching is an important consideration from both the processing and environmental points of view. In particular, the oxidation state of arsenic is of concern as this impacts on the relative ease of precipitation of this element and its stability in storage ponds. The dissolution of niccolite (NiAs) and tennantite in FeCl,-HCl media was reported to yield a formal +3 oxidation state of arsenic although thermodynamic considerations predict that the reaction between ferric ion and arsenide minerals should yield arsenic(V) (1). The oxidation of As(II1) to As(V) is fairly slow, however, and is under kinetic rather than thermodynamic control even in the presence of strong oxidizing agents such as Ce(1V) (2,3). A recent report has demonstrated the metastability of arsenic(II1) in FeC13-HCl media ( 4 ) . Because of the complexity of the As(II1) As(V) reaction and its importance in hydrometallurgy, it is clear that an accurate analytical method is required to measure arsenic(II1) and arsenic(V) in FeC1,-HCI leaching solutions. Some aspects of the analytical techniques employed for arsenic have been discussed (5). In brief, classical flame atomic absorption spectrometry is not a sensitive technique for arsenic and does not speciate between arsenic(II1) and arsenic(V). Electrochemical techniques, such as anodic stripping voltammetry and differential pulse polarography, are sensitive techniques which allow detection at the parts-per-billion level, but arsenic is electroactive only in its +3 oxidation state. -+
0003-2700/85/0357-1027$01.50/0
Despite the sensitivity of the electrochemical methods, the electrode surfaces are subject to poisoning, and this imposes severe matrix constraints. In differential pulse polarography, hydroxylamine hydrochloride has been introduced as a reducing electrolyte to overcome the oxidation of the mercury electrode by ferric ion ( 4 ) , but there are limits to the Fe(III)/As(III) ratio which can be handled by this method. The development of analytical techniques for the complete speciation of arsenite, arsenate, and organoarsenic species continues. Recent emphasis has centered on chromatographic techniques such as ion chromatography (IC). IC with conductivity detection is able to identify the arsenate anion in the presence of other common anions ( 6 ) . In this paper the term “arsenate” includes all anions having arsenic in the formal +5 oxidation state; Le., H,AsO;, HAsOt-, and As02- as only one elution peak is obtained for all these anions. A similar argument also applies for the “arsenite anion” which has arsenic in the formal +3 oxidation state. The application of IC interfaced with arsine generation and atomic absorption has also been used for the analysis of trace levels of arsenite, arsenate, and organoarsenic species (7). Other recent work has described the application of IC with UV detection to speciate arsenite from arsenate (8). These determinations have dealt only with solutions which are free from matrix ions, whereas the analytical techniques for determining low levels of the different arsenic species in metallurgical leaching solutions must contend with relatively overwhelming concentrations of iron and acid. It is the purpose of this work to describe an analytical method using IC with conductivity detection for the determination of trace quantities of arsenite and arsenate in leaching solutions containing ferric chloride (0.1-2.0 M) and hydrochloric acid (0.1-1.0 M). Ferrous chloride arises in the leaching solutions because of the reduction of ferric ion by the sulfarsenide minerals. Accordingly, the effect of ferrous chloride on the stability of the arsenic species is also reported.
EXPERIMENTAL SECTION Apparatus. A Dionex Model 2020i ion chromatograph equipped with Dionex AG-3 guard column, AS-3 separator column, AFS-1 anion hollow fiber suppressor, 0.1-mL sample loop, and conductivity detector was used to obtain the chromatograms. Most experiments were run at 2.7 mL/min eluant flow rate using 3.5 mM Na2C03-1.0 mM NaOH stored in a collapsible plastic container. The suppressor was continuously regenerated at 3.5 mL/qin regenerant flow rate using 0.025 N H2SO4. Output data were recorded on a Dionex Model 4270 integrator. Cation Exchange Column. Standard Econo borosilicate glass columns of 1 cm X 20 cm were used for separating iron(I1) and iron(II1) from sample solutions. The bottom end was sealed with a glass frit and the upper end was connected to a sample receiver of about 10 mL volume. The column was prepared for use by pouring an aqueous slurry of BIO RAD AG50W-X8 hydrogenform, 1OC-200 mesh resin, to a bed height of 18 cm. Regeneration of the resin was done by passing four 10-mL portions of 3 M HCl through the column, followed by four 10-mL portions of water. All samples and eluants were passed through the columns dropwise at a rate of about 1.3 mL/min.
Published 1985 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
Table I. Retention Times of Anions Using Various Eluants” eluant (A) 3.0 mM NaHCO8-2.4 mM Na2C03
(B) 1.9 mM Na2C03-2.4mM NaHC03-1.0 mM NazB40Tb (C) 3.5 mM Na2C03 (D)3.5 mM Na2C03-1.0mM NaOH (E) 3.5 mM NazC03-2.6mM NaOH‘
flow rate,
retention time, min NO; SO-: arsenate
eluant pH
mL/min
C1-
11.0
2.8
2.0 1.9 1.9 1.9
8.0 7.2
1.9
5.4
9.5 10.3 11.2 11.2
2.7 2.2 2.7 2.7
3.5 5.8
7.8 19.1 9.1 6.5 9.1
5.1 10.1 11.2
12.5 18.0
“Retention times vary with individual columns, the concentration of the ions, age, and history of the columns. Data presented are approximate and are intended to illustrate the order of elution of the ions. bEluant B has been used in ref 7. CEluantE has been used in ref 6.
Reagents. All solutions were prepared in doubly distilled water with reagent grade chemicals. The arsenic(II1) standard (1000 (certified ACS, mg/L) was prepared by dissolving 1.3200 g of k z 0 3 Fisher Scientific) in 25 mL of 1.0 M NaOH. The solution was diluted to about 100 mL with water, and 2 drops of 0.2% phenolphthalein were added. It was then neutralized with 1.0 M HCl and further diluted to 1 L. Arsenic(V) standard solution (1000 mg/L) was prepared by mixing 25 mL of 1000 mg/L As(II1) standard with 25 mL of freshly prepared aqua regia (3:l HC1:HNO3mixture) in a 150-mL beaker. After evaporation to dryness on a hot water bath, the arsenic(V) product was dissolved and diluted with water to 25 mL. Intermediate standard solutions were prepared from this solution by a series of dilutions using water. Procedure. A lo-, 5-, 2.5-, LO-, or 0.5-mL aliquot of sample in 1.0 M HC1 with iron chloride levels of 0.1-0.3,0.3-0.6,0.6,1.0, and 2.0 M, respectively, was passed through the cation exchange column and followed by five 10-mL portions of water. A 5-mL aliquot of sample with an HCl concentration less than 1.0 M required the addition of 0.5 mL of concentrated HCl in the sample receiver on top of the column. After the solution was stirred with a glass rod, it was passed through the column by releasing the stopper at the bottom. When all the sample had passed through the column, the glass rod was rinsed with 10 mL of water in the sample receiver. This rinsing water was passed through the column, followed by four 10-mL aliquota of water. The collected aqueous effluent was then evaporated to dryness on a hot water bath to remove the hydrochloric acid. The remaining arsenic was dissolved and diluted quantitatively with water to 25 mL or 10 mL depending on its concentration. This solution was then injected into the ion chromatograph or used for comparative colorimetry measurements. Although both the arsenite and arsenate anions are separated by the columns, the former anion yields undissociated arsenous acid (H3As03)which has a low conductivity (6). Arsenite can be determined by the difference of the arsenate concentration of the initial solution and the totalarsenate concentration of the solution after an oxidant is added to convert arsenite to arsenate. For the measurement of total arsenic, the aqueous effluent collected after the separation of iron was mixed with an equal volume of freshly prepared aqua regia. The mixture was then evaporated to dryness on a hot water bath. The arsenic(V) product was dissolved and diluted to 25 mL with water. Concentrations were determined from the peak heights using the linear calibration routine of the instrument. Standard solutions of three different concentrations were used for this Calibration and these bracketed the concentrations of the test solutions studied. Colorimetry measurements were done by using a method described in the literature (9): An aliquot of test solution was placed in a 50-mL volumetric flask, and 5 mL of ammonium molybdate reagent (1% ammonium molybdate tetrahydrate in 2.3 M sulfuric acid) and 1mL of freshly prepared 0.5% hydrazine sulfate were added. The solution was diluted to 45 mL and was heated for 30 min on a hot water bath for color development. It was then cooled, diluted to volume and measured at 845 nm.
RESULTS AND DISCUSSION Selection of Eluant. In ion chromatography, selectivity of the resin denotes the relative preference of ions in the sample compared to ions in the eluant; both the nature of the
ions and the concentration of the eluant affect the separation. As shown in Table I, several eluants were used to obtain chromatograms of a solution containing arsenate, chloride, nitrate, and sulfate which were also considered because of their potential presence as an impurity in the leaching solutions. As will be discussed below, nitrate also resulted because of the oxidation of arsenic(II1) in the standard and sample by aqua regia. Although eluant A gives a short analysis time and high sensitivity, the arsenate and nitrate peaks are too close for accurate measurement. A similar proximity of peaks was also obtained with eluant B. In addition, at high attenuation, a negative peak just before the arsenate peak is observed with eluant B. This “dip”, which possibly originated from a cationic impurity or an anionic impurity of very low conductivity, distorts the arsenate peak and affects the accuracy of integration. Eluants C , D, and E give a good separation of the peaks. Eluant C was not chosen because its buffering capacity is lower than that of the paired eluants; a 13% increase in the arsenate peak area or peak height was noted as the sample pH increased from 2.8 to 12.1. Eluant E gave a long analysis time and a broader arsenate peak. Eluant D was therefore selected for the present studies. The detection limit of arsenate with this eluant, defined as the concentration which gives a peak intensity twice that of the base line noise, is 0.5 mg/L As. This value compares favorably with the detection limits reported previously using eluant E (6), Le., 1.8 mg/L As and 0.67 mg/L As at flow rates somewhat lower than the 2.7 mL/min used in this study. Arsenic(V) Standard. Because a pure and stable standard compound for arsenic(V) is not available, the standard for arsenic(V) was prepared by the oxidation of As203. Previous work has suggested oxidation with hydrogen peroxide and MnOz under basic conditions (6). In the present work, the leaching solutions contain large amounts of hydrochloric acid. Adding base would result in the formation of NaCl which is more difficult to eliminate than hydrochloric acid which can be evaporated. Consequently, arsenic(V) was prepared by oxidation of As203 with aqua regia. The validity of the standard was assessed by colorimetry based on the molybdenum blue complex of arsenic(V) (9). The molar absorptivity determined at 845 nm using five different concentrations ranging from 0.2 to 2.0 mg/L is (24.7 f 0.4) X lo3L mol-’ cm-l, in agreement with the literature value of 25.5 X lo3L mol-’ cm-l where arsenic(V) was produced by oxidation using bromine (9). The reproducibility of the As(V) standard prepared by oxidation with aqua regia was determined from four separate preparations and the standard deviation of the mean value was 1.3%. Figure 1A illustrates a chromatogram of a 10 mg/L standard As(V) solution. The elution peak a t 1.84 min is due to the trace quantity of chloride left after the evaporation of the acid. The nitrate peak, eluted at 5.9 min, is surprisingly large and is difficult to remove by repetitive evaporation, especially when arsenic(V) is present in large quantities. There is a possibility that the arsenic(V) product complexes with nitrate, whereas
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985 arsenate
T
0.2 umho
1 A
B
C
I ' 0
1
4
I 8
I
I
12
16
I 20
Time,min
Figure 1. Typical chromatograms obtained at an eluant flow rate of 2.7 mL/min using 3.5 mM Na,C03-1.0 mM NaOH: (A) standard solution of 10 mg/L As(V); (B) leach solution after dilution by a factor of 10 that contains 10 mg/L As(V), 0.03 M FeCI,, and 0.03 M HCI; (C) leach solution as in (B) after preseparatlon from matrices and dilution to give 10 mg/L As(V).
chloride is almost all displaced. For the analytical work, nitrate does not interfere with the arsenate peak which is eluted at 12.3 min. Dilution and Injection. The concentration of FeCl, is one of the important control parameters in ferric chloride leaching as increasing FeCl, concentrations generally accelerate the leaching rate ( I ) . For this reason leaching studies have employed large FeC1, concentration ranges (0.1-1.5 M). A total arsenic concentration of about 100 mg/L (composed of As(1II) and As(V)) could be found in a typical 0.3 M FeC1,+.3 M HCl leaching medium. Assuming that all the iron is present as FeCl,, the total C1- ion in the solution approaches 1.2 M. When two separate solutions of 1.2 M chloride and 100 mg/L As(V) are run individually on the ion chromatograph, examination of the two chromatograms indicates there is no serious overlapping of the peaks due to chloride and arsenate. When 100 mg/L As(V) solution in 1.2 M chloride is injected into the ion chromatograph, however, only 9% of the arsenate anion is detected as a tiny broad peak. This phenomenon is known as column overloading, and it occurs when trace quantities of an anion are accompanied by overwhelming amounts of other anion. This overloading effect has also been reported in a previous study in which the determination of chloride was prevented by the presence of overwhelming
1029
concentrations of fluoride, nitrate, and sulfate ions of various metal salts (IO). In another study, the IC response to 100 mg/L of sulfate was reduced by the presence of more than 0.4% chloride although the peaks were well separated (11). When a typical leaching solution consisting of 100 mg/L As(V) in 0.3 M FeC1,-0.3 M HCl was diluted by a factor of 5 and then by a factor of 10,65% and 30% deviations from the true As(V) concentration were observed, respectively. Figure 1B shows the chromatogram of this solution after dilution by a factor of 10. This diluted solution contains 10 mg/L As(V), 0.03 M Fe(III), and about 0.12 M Cl-. The large peak at early elution is due to chloride which is followed by an elution peak labeled a. Peak a is assigned either to an impurity present in the iron chloride used for leaching or to an impurity leached from the concentrate. On the basis of its retention time, it is tentatively identified as sulfate. The arsenate peak is eluted at a slightly greater retention time (13.2 min) than the standard (12.3 min, Figure lA), and this discrepancy is due to the difference in ionic strength of the sample solution and the standard solution. Aside from being broader, the peak height is also reduced by 30% relative to the standard (Figure 1A) which has an equal concentration of As(V). Further dilution of the above leach solution by factors of 25 and 50 diminished the overloading effect, and true concentrations of As(V) were obtained. Although dilution helps to overcome the overloading effect, it also, of course, decreases the concentration of the analyte. Since arsenic(II1) is often the major constituent in the sample relative to arsenic(V) and because the detection is based on arsenate only, dilution alone is not a desirable way to overcome the problem of column overloading. Although the standard addition method could be employed, it should be applied with caution because of the possible presence of ferrous chloride in the leaching medium. A fraction of the As(V) spiked into the dilute sample might be reduced to As(III),depending on the concentration of FeC1,. As a consequence of the high dilution factor, the ionic strength of the solution is greatly reduced. Depending on the nature of the equilibria, there is a danger of altering the ratio of As(II1)to As(V) in the diluted sample. Moreover, it is possible that dilution and direct injection of the sample would poison the guard and separator columns by precipitation of ferric hydroxide on the resin bed. Rejuvenation of these columns is a time-consuming practice which is not always totally effective in restoring column efficiency. For these reasons, it was decided to develop a convenient means to separate the arsenic species from the major interferences prior to measurement using IC. Preseparation of Arsenic from Major Matrices. The removal of metal ions with hydrogen-form cation resins has been studied extensively (12). The literature indicates that adsorption of iron(II1) on Dowex 50-X4 resin from HC1 solutions first decreases with increasing acid concentration to a minimum near 4 M HC1 and then increases rapidly as the HC1 concentration is raised. The arsenic(II1) and arsenic(V) are negligibly adsorbed from 19 M HCI. In the present study, iron is adsorbed on BIO RAD AG50W-X8 resin from sample solutions containing 1.0 M HCl, and arsenic(II1) and arsenic(V) are eluted with water. Elution with 0.1 M HC1 resulted in incomplete arsenic recovery. Sample solutions in C1.0 M HCl gave lower arsenic recovery after being eluted with water and, therefore, must be acidified before passage through the column (Experimental Section). As the concentration of iron chloride in the sample increases, the H+ liberated from the resin, after the exchange of the H+ of the resin and Fe3+ or Fe2+,is sufficiently large to desorb some of the previously adsorbed iron. The desorption of iron(II1)in nitric acid and hydrofluoric acid has been reported (13). With a smaller volume of sample,
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
Table 11. Recovery of Arsenic(V) from Various Matrix Synthetic Solutions concn in original solution Y
mg L-'
[FeC131,M
[HClI,M
aliquot/ final volume
50.0 5.00 100 200 100 100 100 100 100 100
0.100 0.300 0.300 0.300 0.600 1.000 2.000 0.200 0.300 0.300
1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.3 0.3 0.1
10125 5/25 10125 10125 25/25 1.0/25 0.5/10 5/25b 5/25b 5/25b
[As(V)I
9
[As(V)] in final
% recovery
solution, mg L-l
ion chromatography
colorimetry
20.0 1.00 40.0 80.0 10.0 4.00 5.00 20.0 20.0 20.0
99.9 f 1.2 112.5 f 1.9 98.8 f 4.7 99.9 f 1.2 94.9 f 2.0 90.8 f 2.4 92.9 f 3.2 100.4 f 6.8 96.2 f 5.4 97.3 f 0.6
94.7 f 1.7 99.1 f 4.1 100.5 f 0.3 102.2 f 0.3 102.7 f 2.5 96.4 f 3.5 94.7 f 1.7 105.3 f 2.0 98.3 f 1.5 98.1 f 0.7
Average values from four independent determinations carried through all stages of the analytical procedure. 0.5 mL of concentrated HCl was added into the 5-mL aliquot before the solution was passed through the cation exchange column (see text). Table 111. Recovery of Total Arsenic from Various Compositions of As(II1)-As(V) and Various Matrix Synthetic Solutions concn in original solution
[As(III)I, mg L-' 500 100 100 50.0 50.0 90.0 10.0
[As(V)I, mg L-' 0 0 0 50.0 50.0 10.0 90.0
% recovery
[FeC1,1, M
[FeChI, M
[HCll, M
0.100 0.300 2.000 0.300 0.300 0.300 0.300
0 0 0 0.050 0.100 0.100 0.100
1.0 1.0 1.0 1.0 1.0 1.0 1.0
ion chromatography 100.7 98.8 95.1 96.9 98.3 99.8 99.3 98.4 f 1.9
the desorbed iron can be readsorbed on the lower part of the column as elution with water is performed. This limits the volume of the aliquot which can be introduced into the column for samples with high levels of iron chloride (Experimental Section). This preseparation method ensures that no dilution is carried out before the removal of iron(II1) and iron(I1) that might cause the oxidation and reduction of arsenic(II1) and arsenic(V). Therefore, the ratio of arsenic(II1) to arsenic(V) is not altered. The latter part of the preseparation step removes hydrochloric acid by evaporation to dryness. The fact that arsenic(II1) is not oxidized by air upon heating on a hot water bath is proved by the following experiments. A freshly prepared synthetic solution containing 500 mg/L As(II1) in 0.1 M FeCl,-1.0 M HC1 was carried through the preseparation step. The resulting solution gave no detectable As(V). A freshly prepared synthetic sample containing 500 mg/L As(II1) in 0.1 M FeC13-1.0 M HC1 was also carried through the preseparation step. Only 0.3% of the As(II1) was oxidized to As(V). The amount of air oxidation is clearly negligible when the presence of FeC13,which is thermodynamically able to oxidize As(III), is taken into account. Arsenic(II1) is, however, volatilized nearly totally (99.5%) during the HCI evaporation stage due to the low boiling point (63 OC) of AsC13. Thus the separation of arsenic from the matrix also results in the separation of As(II1) from the As(V) species. Figure 1C illustrates a typical chromatogram obtained from a solution containing 100 mg/L As(V) in 0.3 M FeC13-0.3 M HC1 after the above preseparation procedures were carried out and the sample was diluted after evaporation to result in 10 mg/L As(V). The sharp peak a t retention time 1.85 min is due to trace quantities of chloride. The elution peaks labeled a and b (6.51 and 5.82 min) are impurities; their retention times suggest sulfate and nitrate, respectively. The arsenate peak eluted at retention time 12.5 min is in good agreement with that of the standard (12.3 min, Fig. 1A). The width at the half height as well as the peak height itself are
colorimetry 99.6 100.9 96.9 101.8 101.5 101.8 102.1 100.6 f 1.8
equal to those of the 10 mg/L As(V) standard. Evaluation of Analytical Curve Linearity. Ten synthetic solutions containing As(V) concentrations ranging from 1.56 to 250 mg/L in 0.3 M FeC13-1.0 M HCl were carried through the preseparation procedures individually. The final concentrations of As(V) injected into the ion chromatograph ranged from 0.625 to 100 mg/L. The relationship of concentration to conductivity is linear with a slope of 0.0545 mg-' L fiW and a correlation coefficient, r = 0.999. This slope is comparable to that obtained by using a series of standard As(V) aqueous solutions in the absence of FeC1, and HC1 (slope = 0.0540 mg-' L fiW,r = 0.999). These results indicate that the use of the linear calibration routine of the instrument is acceptable. Accuracy and Precision of Analysis. Synthetic samples were prepared with various As(V) concentrations in FeC13-HCl media and were analyzed four times with each analysis being carried through all the analytical procedures. This medium was chosen over the FeCl,-HCl system to prevent any reduction of As(V) by FeC12. The accuracy of the overall method is reflected by the percent As(V) recovery values. The data presented in Table I1 demonstrate a 12.5% accuracy a t the 1 mg/L concentration level, 5 to 10% at the 4 to 10 mg/L level, and 0.1 to 4% a t higher As(V) concentrations. In general, the precision of analysis is better than 7% at all concentration levels. The accuracy and precision of the proposed aqua regia oxidation method for the determination of total arsenic can be determined from the data listed in Table 111. By use of various compositions of arsenic(II1) and arsenic(V) in synthetic FeC1,-HCI media, 0.2-5 % accuracies were realized and precisions of 2% were obtained. In a previous IC study with conductivity detection (6), repeated injections of solutions containing from 1.8 to 141.5 mg/L As(V) in water showed good reproducibility and direct proportionality between peak height and As(V) concentration. Oxidation a n d Reduction of As(II1) a n d As(V) i n Acidic Iron Chloride Media. Table IV presents analytical
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
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Table IV. Analytical Data for Freshly Prepared Synthetic Solutions concn in original solution [As(V)I, [FeCld, [FeCld, mg L-' M M
[As(III)l, mg L-'
0.100 0.300 0 0 0 0 0 0.300 0.300 0.300 0.300
0 0 100 5.00 100 100 100 50.0 50.0 10.0 90.0
500 100 0 0 0 0 0 50.0 50.0 90.0 10.0
0 0 0.100 0.300 0.300 1.00 2.000 0.050 0.100 0.100 0.100
[As(V)]measd," mg L-' ion chromatography colorimetry
[HCk M
1.7 f 0.1 3.3 f 0.1 98.9 f 0.6 98.7 1.9 f 0.05 1.2
1.7 f 0.3 3.3 f 0.2 100.2 f 0.6 1.8 f 0.2 95.8 f 1.6 85.3 f 1.7 63.3 f 3.3 61.0 f 1.7 58.9 f 2.1 11.3 f 0.7 96.9 f 3.0
1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
87.9 f 2.5 64.4 f 0.7 64.7 f 1.7 63.8 f 2.0 13.9 f 0.8 97.9 f 1.3
'Average of four determinations which were carried through all stages of the analytical procedure.
20
it
0' 0
I
4
I
8
12
I
I
16
20
I
24
I 28
T i m e , d a y s
"i ob
a
b
11 T ime
:1
,
2b
2'4
:3
d a y s
Figure 2. Decay of As(II1). Initial concentration of solution a is 100 mg/L As(III), 0.600 M FeCI,, and 1.0 M HCI. Initlal concentration of solution b is 100 mg/L As(III), 1.000 M FeCI,, and 1.0 M HCI.
Flgure 3. Decay of As(II1). Initial concentration of the solution is 50 mg/L As(III), 50 mg/L As(V), 0.300 M FeCI,, 1.00 M FeCI,, and 1.0 M HCI.
data for freshly prepared synthetic solutions with various concentrations of arsenic(II1) and arsenic(V) in acidic FeC13-FeC12 media. The data show that 0.1 M FeC1, does not reduce the arsenic(V) in solution. As the concentration of FeC1, increases, however, progressively more arsenic(V) is reduced to As(II1) with about 40% reduction occurring in 2 M FeC12 media. Similar behavior is also obtained for the oxidation of arsenic(II1) in the presence of FeC1,. Figure 2 illustrates the decay of As(II1) in a solution initially containing 100 mg/L of As(II1) in 0.6 M FeC1,-1.0 M HC1 (curve a) and in 1.0 M FeC1,-1.0 M HC1 (curve b). Figure 3 represents the decay of As(II1) in a solution initially containing 50 mg/L As(II1) and 50 mg/L As(V) in 0.3 M FeC13-0.1 M FeC1,-1.0 M HCl. In all solutions the arsenic(II1) is gradually oxidized, over a period of several days, by the available ferric ion. Such gradual decay has also been observed in a previous study using differential pulse polarography ( 4 ) . Comparison of t h e Data Resulting from Ion Chromatogiaphy and Colorimetry. After the preseparation of iron and hydrochloric acid, the solutions no longer contain significant amounts of As(II1) and can also be analyzed by colorimetry based on the molybdenum blue complex of arsenic(V). Although the data of Tables 11,111,and IV demonstrate
fairly good agreement with the results from the IC measurements, the results obtained by colorimetry are generally high by 2 to 5%. This is especially true at the higher iron chloride concentrations. This may be due to a trace quantity of phosphate present as an impurity in the reagent grade iron chlorides used for the preparation of the synthetic solutions (phosphate also reacts with molybdate). Since metallurgical leaching solutions usually contain many ionic impurities, analysis of arsenic(II1) and arsenic(V) by IC is likely to be a superior technique for these solutions. CONCLUSIONS The application of ion chromatography with conductivity detection to the determination of trace quantities of arsenic(V) in ferric chloride-hydrochloric acid leaching media is demonstrated. Oxidation of As(II1) by aqua regia allows As(II1) to be determined by difference. The analytical method, which involves a preseparation of the arsenic from the relatively high concentrations of iron(II1) and hydrochloric acid, is quantitative and effectively eliminates the difficulties encountered during direct analysis of the leach solution. The use of hydrogen-form cation exchange resins for the separation of iron has several advantages. It is a simple technique and several
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Anal. Chem. 1985, 57, 1032-1035
columns can be operated simultaneously; automation is also possible. Since no dilution occurs prior to the separation of iron, no changes occur in the ratio of arsenic(II1) to arsenic(V). The oxidation of As(II1) by FeCl, and the reduction of As(V) by FeCl, in the leaching solution effectively stop after iron removal on the resin. The analytical method is useful for the study of the oxidation of arsenic(II1) to arsenic(V) during the ferric chloride leaching of arsenic-bearing minerals at moderate temperatures.
Registry No. As, 7440-38-2;FeC13,7705-08-0;HCl, 7647-01-0. LITERATURE CITED (1) Dutrizac, J. E.; Morrison, R. M. I n “Hydrometallurgical Process Fundamentals”; Plenum: New York, 1984 pp 77-1 12. (2) Yates, J. S.; Thomas, H. C. J. Am. Chem. SOC. 1956, 78, 3950-3953. (3) de Smecht, L. M.; Berube, Y. “Chemlcal Stability of Arseniferous Waste”; Arctic Land Use Research Program, Publlcatlon No. 8018000-EE-A 1; Department of Indian Affarls and Northern Development Canada, Ottawa, 1975.
(4) Morrison, R. M. “The Analysis of Arsenic(II1) by Differential Pulse Polarography in Hydrochloric Acid - Hydroxylamine Hydrochloride Electrolyte”; Division Report MRP/MSL 84-56 (TR); CANMET, Energy, Mlnes and Resources Canada. Ottawa. 1984. (5) Morrison, R. M. “Application of Voltammetric Techniques to the Analysis of Arsenic-Containing Solutions”; Division Report MRP/MSL 83-59 (TR); CANMET, Energy, Mines and Resources Canada, Ottawa, 1983. (6) Hansen, L. D.; Richter, B. E.; Rollins, D. K.; Lamb, J. D.; Eatough, D. J. Anal. Chem. 1979, 51, 633-637. (7) Ricci, G. R.; Shepard, L. S.; Colovos, G.; Hester, N. E. Anal. Chem. 1981, 53, 810-613. (8) Williams, R. J. Anal. Chem. 1983, 55, 851-854. (9) Donaldson, E. M. Talanta 1977, 24, 105-110. (10) Siemer, D. D. Anal. Chem. 1880, 52, 1874-1877. (11) Bynum, M. A. 0.; Tyree, S. Y., Jr.; Weiser, W. E. Anal. Chem. 1981, 53, 1935-1936. (12) Nelson, F.; Murase, T.; Kraus. K. A. J. Chromatogr. 1964, 13, 503-535 - - - - - -. (13) Kireeva, G. N.; Nam, L. S.; Ryzhkova, V. N.;Savel’eva, V. I . ; Seieznev, V. P.; Sudarlkov, B. N. Chem. Abstr. 1976, 85, 9 9 7 0 3 ~ .
RECEIVED for review November 26,1984. Accepted January 9, 1985. The receipt of a Visiting Fellowship from the Natural SCknces and Engineering Research Council of Canada is greatly acknowledged (L.K.T.).
Determination of Sub-Part-per-Million Levels of Formaldehyde in Air Using Active or Passive Sampling on 2,4-Dinitrophenylhydrazine-Coated Glass Fiber Filters and High-Performance Liquid Chromatography Jan-Olof Levin,* Kurt Andersson, Roger Lindahl, and Carl-Axel Nilsson
National Board of Occupational Safety and Health, Research Department in Umeb, Box 6104, S-900 06 Umeb, Sweden
Formaldehyde Is sampled from air wlth the use of a standard mlnlature glass flber fllter Impregnated wlth 2,4-dlnltrophenylhydrazine and phosphorlc acid. The formaldehyde hydrazone Is desorbed from the fllter with acetonitrlle and determined by high-performance llquld chromatography using UV detection at 365 nm. Recovery of gas-phasegenerated formaldehyde as hydrazone from a 13-mm Impregnated fllter Is 80-100% In the range 0.3-30 kg of formaldehyde. Thls corresponds to 0.1-10 mg/m3 In a 3-L alr sample. When the fllter sampling system Is used In the active mode, air can be sampled at a rate of up to 1 L/mln, affording an overall sensttlvlty of about 1 pg/m3 based on a 60-L alr sample. Results are given from measurements of formaldehyde In Indoor alr. The DNP-coated fllters were also evaluated for passive sampling. I n thls case 37-mm standard glass fibers were used, and the sampling rate was 55-65 mL/mln in two types of dosimeters. The diffusion samplers are especially useful for personal exposure monitoring in the work environment.
Formaldehyde is an extremely important industrial chemical. Approximately 5.2 billion pounds of formaldehyde were consumed in 1983 in the manufacturing of more than 29 types of products and in the performance of medical services. About 50% of the formaldehyde produced is used to make resins for adhesives in the manufacture of particleboard, fiberboard, and plywood. More than 1.5 million workers are potentially exposed to formaldehyde (1). The threshold limit value for occupational exposure in the U.S.is 1 ppm (2). Also, large groups of the general public are exposed to low levels of
formaldehyde, since it is a ubiquitous contaminant of indoor air. A number of analytical methods for the determination of formaldehyde have been published. The most widely used methods are spectrophotometric (3, 4 ) . These have been reported to suffer from a number of negative interferences, however (5), and the sensitivity is rarely sufficient for measuring indoor air levels. More recently, chromatographic methods have been used for determining formaldehyde or formaldehyde derivatives, such as hydrazones. One of the most rapid and sensitive methods is high-performance liquid chromatographic determination of the 2,4-dinitrophenylhydrazone of formaldehyde. This method has been used by several investigators (6-8). Various devices have been developed for sampling formaldehyde from air. Most methods employ impinger collection ( 3 , 4 ,6,8), but bubblers or impingers are not convenient in field investigations, especially not in personal monitoring of worker exposure. In personal monitoring, solid adsorbents are preferable for sampling. The first method for sampling formaldehyde using a reagent-coated solid adsorbent was reported by us in 1979. The method utilized chemosorption of formaldehyde on 2,4dinitrophenylhydrazine (DNP)-coated Amberlite XAD-2 (9). The method was later evaluated for acrolein and glutaraldehyde (10) and for simultaneous sampling of formaldehyde, phenol, furfural, and furfuryl alcohol (7). Other workers have subsequently used DNP-coated solid sorbents with various solid supports, such as silica (11),glass beads (8, 121, and octadecylsilane-bonded silica (13). Other methods utilizing solid sorbent sampling of formaldehyde include the use of a 13X molecular sieve (14), impregnated active charcoal (151,
0 1985 American Chemical Society 0003-2700/85/0357-1032$01.50~0
