Determination of Arsenic in Sulfide Samples Containing Antimony by Atomic Absorption Spectrometry Shawky Shafeek Michael Department of Geology, The University of Western Australia, Nedlands, 6009, Australia
An indirect method for arsenic analysis was adapted and modified to analyze geological samples, some of which contain high quantity of sulfur and/or antimony. Arsenlc was reduced to arsenic(Ill), then evolved as arsine AsH3. Any hydrogen sulfide evolved was removed by a specially modified trap. The generated stibine SbH3 together with the arsfne gas were oxidized using iodine solutlon into the oxidatlon state (V). Antlmony was masked at pH N 7 using EDTA, and the remaining arsenic was then complexed with ammonium molybdate to glve arsenomolybdous acid H3A~Mo12040. This was extracted into MIBK and followed by molybdenum determination at 313.2 nm using a fuel rich nitrous oxide-acetylene flame.
Since its introduction as an analytical tool, atomic absorption spectrophotometry has found a wide variety of applications, one of which is the indirect determination of “difficult” elements such as As, P, and Se using the atomic absorption of another detectable and more stable element by complexing it quantitatively with the desired ones ( 1 , 2 ) . Arsenic determinations have been confined almost exclusively to biological and industrial materials. Not until recently has the presence of microgram amounts of arsenic in geological samples been found to be of significance in ore genesis and geochemical exploration ( 3 , 4 ) .The main two methods used for arsenic analysis were summarized by Harms and Ward 1975 (5) and both were based on complexing the generated arsine gas with either 1) silver diethyldithiocarbamate in pyridine to form a red-violet compound followed by colorimetric measurements, or with 2) ammonium molybdate (after oxidizing to the arsenic V state) to form the arsenomolybdate compound which is then reduced to molybdenum blue and determined by colorimetry. The arsenomolybdate extract was first analyzed by atomic absorption by Danchik and Boltz (6) by extracting the complex into MIBK, then stripping the heteropoly complex into alkaline aqueous solution and determining the molybdenum. Yamamoto et al. (7) modified this technique by the direct nebulization of MIBK. The method was later applied by different authors using different pH and organic solvents ( 2 , 8 ) to analyze for arsenic, phosphorus silicon, germanium, and silicon. Antimony was found to interfere with the above two methods (7,9) and since geological samples could contain up to a thousandfold excess of antimony over arsenic, severe interference would be observed in analyzing geological samples. The arsenomolybdate method was preferred in this study over the arsenic-silver-diethyldithiocarbamate technique because of the extra step between arsine evolution and the complexing which could allow the separation of antimony. Therefore, a new method was developed to prevent the interference of antimony in the solution, based on masking the element with EDTA at p H E 7. This modified technique was applied here with a sensitivity of 10 ppb and relative standard
deviation of f0.64-10.7%. Twenty sulfide or ten silicate samples/day/operator could be analyzed using the method.
EXPERIMENTAL Apparatus. All measurements were made with a Techtron Model AA5 atomic absorption spectrophotometer with a molybdenum lamp. The apparatus used for the generation of arsine is shown in Figure 1. Part 1, Figure 1,is the digestion-generation conical flask (250 mi with B24 ground stopper), connected to part 2 which is the modified hydrogen sulfide scrubber containing 4 ml of buffered lead acetate solution. Part 3 is a replaceable bubbling tube immersed in the collecting tube (part 4) which is marked to 4 ml. Ten of the above apparatus were used in this research. Separation of antimony, complexing of arsenic, and extraction of the complex were carried out in a 25 cm long X 2.4 cm diameter test tube (approximately 100 ml) with B24 ground stopper. The tube was marked a t the 50-ml volume. Fifteen-milliliter platinum crucibles were used for fusion of the silicate samples. Reagents. Analytical grade reagents were used (except for MIBK, being a Laboratory Reagent) throughout the analysis. The arsenic stock solution of 1.00 mg/ml was prepared by dissolving 1,320 g of arsenic trioxide in the minimum volume of 1 M sodium hydroxide solution, then acidified by adding 5 ml concentrated hydrochloric acid and the solution was made up to 1 1. with distilleddeionized water. Sample digestion was carried out using perchloricnitric acid mixture (3:7). Low arsenic content hydrochloric acid was prepared by making up 1 1. of 6 M acid from the AR reagent, followed by adding 6 g SnC12. 2H20. This was heated between 50-60 “C for 1 h, cooled down, and shaken in a separating funnel with 20 ml of chloroform, and allowed to stand for 5 min; then the chloroform layer was discarded. The shaking was repeated three times; then the acid was distilled, discarding the remaining undistilled 20 ml. Also used were: iodine-potassium iodide solution (0.005 N)which was prepared by dissolving 0.254 g of iodine in 0.80 g of potassium iodide contained in 5 ml of water, then diluted to 400 ml with distilled deionized water containing 50 ml ethanol, and stored in dark glass bottle; stannous chloride solution (40%SnC12.2Hz0 in 6 M HCl), and potassium iodide solution (15% KI in distilled deionized water). The masking agent ethylenediaminetetraaceticacid disodium salt (EDTA Naz) was prepared to a strength of 0.04 M by dissolving 14.89 g of the reagent together with 25 g of hydroxylamine hydrochloride. This was made up to 1 1. with distilled deionized water. The complexing agent ammonium paramolybdate (NH4)6M07024. 4H20 was prepared by dissolving 100 g in 900 ml of distilled deionized water, filtered, and diluted to 1 1. Special low arsenic content (AsT) zinc granules was used for the arsine generation. Finally, silicate samples were fused by an AR mixture of sodium tetraborate and sodium carbonate (3:l). Procedure. 1)Sulfide samples were digested by adding 2 ml of the acid mixture to an accurately weighed portion of the sample (approximately 0.01-0.1 g sample depending on the type of sulfide) in the conical flask (part 1, Figure 1).After the digestion was completed, nitric acid was expelled by evaporation until the appearance of white fumes, and the solution was brought to near dryness. While hot, 5 ml of 6 N hydrochloric acid and 20 ml deionized-distilled water were added, then the procedure continued as from step 3. 2) For silicate samples, 0.2 g approximately was accurately weighed from the powdered sample, and mixed thoroughly with 2 g fusion mixture in the platinum crucible. The contents were gently fused a t 700-900 “C on a flame for about 5 min and, upon completion of the fusion, the crucible was allowed to cool. The cake was dissolved by placing the crucible in a 250-ml glass beaker and rinsing with two portions ( 5 ml each) of 6 M HCl. The solution was diluted to 100 ml in a volumetric flask with distilled deionized water, and a known alANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
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T a b l e I. Arsenic C o n t e n t in Some Common Geological Samples. All Determinations W e r e Done in T r i p l i c a t e Sample type
Sample No.
Arsenic content (mean) ppm
S t d dev, %
Std error, %
Galena PbSa Sphalerite ZnSa Pyrite FeSz Chalcopyrite CuFeSz Pyrrhotite Fe&-FeS Pentlandite(Fe,Ni)& Diabase (silicate rock)
81724 78548 81725 81726 73611A 73611B w-1
47.6 24.3 2.66 10.15 3.3 11.24 1.9
10.72 3.8 0.64 0.99 1.06 2.22 5.3
6.19 2.2 0.37 0.57 0.61 1.28 3.06
a Weights used 0.01 g or less. All sample numbers refer to the collection of The Department of Geology, The University of Western Australia, except No. W-1 which is a U.S. Geological Survey Standard Sample.
2 j g Leod acetate
\
Digested ,sample
I
Rubber joint
0.8 1.8 As o n l y .
2 . - A;+Sb (equol o m o u n t ) + E D T A . 3 - As+Sb( ,, )no E D T A 0.6 3
0 0
0.4 0
n 4
0.2
:4 .-
Iodine solution
1
2
3
4
5
6
7
8
9
1
0
As and (or) S b odded ( p p m )
Scale t
10 cm
Figure 2. Effect of EDTA addition of antimony I
Figure 1. Schematic diagram of arsine generation apparatus
iquot (according to the arsenic content in the sample) was transferred into the conical flask followed by 5 ml of 6 M HCl, and the volume was made up to approximately 25 ml. 3) To the conical flasks, 1ml of stannous chloride and 2 ml of potassium iodide were added. The solution was left for 15 rnin to allow complete reduction of arsenic (Equation 1);1g of zinc was added and the stopper containing the lead acetate trap (p,art 2, Figure 1) was inserted immediately. The evolved arsine gas (Equation 2) was allowed to bubble through 4 ml of the iodine solution (contained in part 4,Figure 1) for 45 min at room temperature, t o allow the oxidation by iodine (Equation 3). The contents of the absorption tubes were transferred to extraction test tubes and volumes were made to 50 ml by distilled deionized water. The solution had a slight yellow color to it, which disappeared immediately after adding 2 ml of the masking EDTA solution. Ten min were found adequate for the completion of the reaction between EDTA and antimony. This was followed by adding 5 ml of 6 M HC1, shaking for 2 s, then adding 4 ml of the ammonium molybdate solution. The extraction tube was shaken for 5 sand allowed to stand for 10 min to complete the formation of arsenomolybdous acid H3(AsMo1204). The ionization of ammonium molybdate at low pH is shown in Equation 4 ( I O ) , while the formation of the arsenomolybdate complex is illustrated in Equation 5 (11).The complex was extracted into 10 ml MIBK after shaking for 3 min to attain equilibrium and the organic layer was then transferred (using a 10-ml medical syringe) to a 50-ml polythene bottle and washed three times with 10 ml of 1 M HCl. Finally molybdenum was determined using a reducing nitrous oxide-acetylene flame using the 313.2-nm line. Calibration was carried out on standards having the same arsenic range as the samples, following the same digestion, distillation, and extraction procedures as for the samples. Linear calibration was observed from concentrations between 0-10 ppm As, and above this range a curvature could be observed. 452
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
Calibration 2 illustrates the high recovery of As after masking Sb, while calibration 3 shows the behavior of As results in the presence of unmasked Sb ions.
RESULTS AND DISCUSSION The above method could be illustrated chemically as fol-
-
lows: hod3-
+ 8H+ + 4Sn2+ As3+ + 3H+
As3+
+
+ 4Sn4+ + 4H20
(1)
ASH^ f
(4)
+
+
(M024078)~~- 2 A ~ 0 4 ~ -18H+ 2H3(A~M012040) 6H20 +
+
(5)
Figure 2 illustrates t h e use of EDTA for t h e masking of antimony a n d shows t h e antimony interference (without adding EDTA) which is a negative t y p e interference, d u e t o either competition for t h e complexing agent or t h e formation of an unknown non-extractable (As-Sb-Mo) complex. T h e effect of hydrochloric acid purification on t h e background can also be noticed from Figure 2, a n d consequently i t was found important to purify the HC1 when requiring a detection limit below 0.05 yg As. T h e conventional cotton wool plug for absorbing hydrogen sulfide was found to give unsatisfactory results (121, handling and changing i t after each sample determination is inconvenient. Figure 1 shows the modified hydrogen sulfide t r a p which is easy t o refill, has a much larger capacity ( 5 m l of lead
acetate) than cotton wool, and is capable of running up to 50 sulfide samples without replacement of the solution.. It is important to follow the sequence of adding the chemicals in this method. For example, adding KI before SnClz may precipitate the insoluble iodate of some of the heavy metals. On the other hand, adding ammonium molybdate solution immediately after EDTA and before HCl would result in the complexing of most of the molybdate ions and may prevent it from forming the arsenomolybdate extract, while adding HCl would result in weakening of the complexing ability of EDTA due to lowering the pH (13). The addition of a polar compound such as ethanol ( 1 4 ) to the iodine solution when bubbling AsH3 was found necessary when analyzing amounts of 50 pg As or higher. This is due to the low solubility in HzO of AsH3 at room temperature ( I 5 ) , and the addition of ethanol was found to increase the arsine solubility, and samples containing few thousands ppm As could be analyzed. Finally the addition of sodium bicarbonate solution (6, 8) to the iodine solution to neutralize any HI formed was found to be inadvisable because of the possible formation of the NaS(AsMo12040) type of complex ( 1 1 )which may not be extractable under the above conditions, and As losses could result during the determination. Masking and extraction were carried out in test tubes rather than the conventional separation funnels as they were easy to handle, clean, and store, and they occupy much less space (a rack 15 cm X 45 cm will carry 48 tubes). Table I includes a summary of the results of some common sulfide samples together with one U.S. Geological Survey standard silicate sample (16). Sulfides were found to have arsenic contents ranging from 3.3 to 47.1 ppm, while the standard silicate sample (W-1) has 1.9 ppm. The result on this silicate sample also suggests that silicon and phosphorous do not interfere using the above methods as the sample contains 52.64%Si02 and 1,400 ppm PzO5 (16). The presence of almost a million-fold excess of some metals such as Cu, Fe, and Ni in the sulfide samples did not interfere with the final arsenic results. On the other hand, lead and zinc sulfides have caused severe interferences similar to that reported in selenium analysis in lead and copper sulfides ( 1 7 ) , and weights exceeding 0.01 g of pure PbS or ZnS were found
to decrease the arsenic recovery in that sample. This could be explained by either P b and Zn catalyzing a reaction to precipitate elementary arsenic, or the formation of compounds such as PbS(AsO&, PbHAs04 (IO), and Zn3(As04)~,which would prevent the formation of As(II1) ions, and consequently arsine evolution will not occur. In conclusion, the indirect AAS method for arsenic determinations not only increases the sensitivity, but also prevents interference by antimony and gives an excellent reproducibility and recovery. ACKNOWLEDGMENT The author thanks S. B. Wild, R. F. Lee, and D. I. Groves, all of the University of Western Australia, for their helpful discussions. LITERATURE CITED (1) H. K. Y. Lau and P. F. Lott, Talanta, 18, 303 (1971). (2) S.J. Simon and D. F. B o k , Anal. Chem., 47, 1758 (1975). (3)H. V. Warren, R. E. Delanauit, and J. Barakso, €con. Geol. Bull. SOC.€con. Geol.. 59. 1381 (1964). (4)W. %'Burbank, R: G. Luedke, and F. N. Ward, U.S.Geol. Surv., Bull., 1364, 31 pp (1972). (5) T. F. Harms and F. N. Ward, U.S. Geol. Surv., Bull., 1406, 13 (1975). (6)R. S.Danchik and D. F. B o k , Anal. Left., 1, 901 (1968). (7)Y. Yamamoto, T. Kumaharu, Y. Hayashi, M. Kanke, and A. Matsui, Talanta, 19, 1633 (1972). (8) T. V. Ramakrishna, J. W. Robinson, and P. W. West, Anal. Chim. Acta, 45,
43 (1969). (9) A. I. Vogei "A Text Book of Quantitative inorganic Analysis", 3d ed., Longmans Publishers, London, 1975,p 798. (IO) W. M. Latimer and J. H. Hildebrand "Reference Book of Inorganic Chemistry", 3d ed., The Macmillan Company, New York, 1958. (1 1) F. A. Cotton and G. Wilkinson "Advanced Inorganic Chemistry", 2d ed., lnterscience Publishers, London, 1966,p 941. (12) G. M. Crawford and Oscar Tavares, Anal. Chem., 46, 1149 (1974). (13)C. N. Reilley, R. W. Schmid, and F. S. Sadek, J. Chem. Educ., 36, 555
(1959). (14)S.B. Wild, Chemistry Department, The University of Western Australia, personal communication, 1976. (15) Robert C. Weast, Ed., "Handbook of Chemistry and Physics", 55th ed.,
C.R.C. Press, Cleveland, Ohio, 1974-1975,p 869. (IS)F. J. Fianagan, Geochim. Cosmochim. Acta, 37, 1189 (1973). (17)S.S.Michael and C. L. White, Anal. Chem., 48, 1484 (1976).
RECEIVEDfor review August 16,1976. Accepted November 12, 1976. The research for this paper was supported by the University of Western Australia Research Studentship.
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