Determination of Arsenic in Sandy Soils 1.J. Forehand, A. E.~Dupuy,Jr.,* and H. Tai Pesticide Monitoring Laboratory, Technical Services Division, Office of Pesticide Programs, Environmental Protection Agency, Bay St. Louis, Miss. 39520
A sensitive, reproducible, and interference-free method is presented for analyzing large numbers of soil samples. The method utlllres the selectlve extraction of As( 111) by benzene and analysis by atomic absorption spectrophotometry.
The analysis of arsenic in soil requires a procedure which is capable of releasing microgram quantities from a complex matrix and eliminating interferences for a specific determination. Kingsley a n d Schaffert ( I ) have described microdetermination of arsenic in biological material, using concentrated hydrochloric acid for digesting the sample. Beard and Lyerly ( 2 )have applied the selective extraction of As(II1) by benzene to separate arsenic from antimony and bismuth. Furthermore, atomic absorption spectrophotometry has been applied as one of the specific methods of determining arsenic ( 3 ) .Combining these procedures as a basis for our study, we have developed a procedure which is sensitive, reproducible, a n d suitable for analyzing a large number of sandy soil samples.
EXPERIMENTAL Atomic Absorption Spectrophotometry. A Perkin-Elmer Model 303 Atomic Absorption Spectrophotometer was used for measuring
the concentration of arsenic. A procedure was followed employing argon-hydrogen-entrained air flame and a premix burner with a three-slot head, as described by Kahn and Schallis ( 4 ) .We have encountered difficulty in obtaining a stable arsenic hollow cathode lamp, but a Perkin-Elmer Electrodeless Discharge Lamp was used for most of the present work. Absorbance measurements were made at the arsenic line of 193.7 nm. The instrument settings are: Damping 2; slit width 0.7 nm; argon pressure 40 lb/in.2,flowmeter reading 9 (about 15 l./min); hydrogen pressure 20 lb/in.2, flowmeter reading 4.5 (about 6 l./min). Reagents. The following AR grade chemicals were used without further purification: Potassium iodide solution, 15% (wiw) aqueous
solution; Stannous chloride solution, 50% (w/w) in concd HC1, freshly prepared; hydrochloric acid, 3896, arsenic free; benzene; and the arsenic standard solutions. The arsenic stock solutions (lo00 ppm) were: Arsenic metal (Fisher Certified Atomic Absorption Standard); arsenic(III), freshly prepared aqueous solution of sodium metaarsenite (NaAsO2) (Fisher Scientific Company), and freshly prepared aqueous solution of arsenic trioxide (Asz03) [Baker Chemical Company (Primary Standard)]; and arsenic(V), freshly prepared aqueous solution of sodium metaarsenate (NaAsOs) (Fisher Scientific Company),and freshly prepared aqueous solution of arsenic pentoxide (As205) [Baker Chemical Company (Primary Standard)]. Calibration Standards. Standards of 1,5,10,25,50,and 100 ppm are made from dilutions of the stock solutions. Procedure. The procedure used is as described below. Acid Digestion. Weigh accurately 100 g of soil sample (adequately prepared by mixing, grinding, and sifting through fi,-in. mesh screen) into a 250-ml Erlenmeyer flask. Add 100 ml of 9.6 N HC1. (For very basic soils, add additional acid.) Swirl the flask to ensure thorough mixing, and allow the mixture to stand at room temperature for at least 12 h. Reductzon o f A s ( V ) to A s ( I I I ) .Decant 50 to 75 ml of the clear supernatant, depending upon the amount of arsenic present, into a 500-ml Erlenmeyer flask with a ground glass stopper. Add water, usually about 30 ml, to adjust the acid strength to about 6 N. (It was found that the subsequent reduction was the most efficient at acid concentrations between 5 and 7 N. See Table IV.) Then add 4 ml of the 50%SnC12 solution, and 5 ml of the 15%KI solution. Mix well by swirling, and allow the mixture to stand at room temperature for 15 min. Soluent Extraction. To the mixture add concentrated HC1to adjust the acid strength to 9 N. According to Beard and Lyerly ( Z ) , the hydrochloric acid concentration was critical to the extractability of As(II1). Add 50 ml of benzene, stopper, and shake the mixture for 1 h on a mechanical shaker. After shaking, decant the mixture into a 250-ml separatory funnel. Allow the layers to separate, and drain and discard the bottom (acid) layer. T o the benzene solution in the same 250-ml separatory funnel, add accurately 15.0 ml of water, and shake vigorously for about 1 min. Allow the layers to separate, drain the bottom (water) layer into a
Table I. Recovery of Arsenic from Sandy Soils Composition Soil sample no. Sand
ILg As Av re1 error, Av recovery,
Silt
Clay
Found
Added
Found after spiking
%
%
100 300 500 1000 2000 5000 100 300 500 1000 2000 5000 100 300 500 1000 2000 5000
184.0; 199.0; 196.5 346.5; 371.4; 369.0 526.5; 539.5; 559.0 974.1; 1017.7; 977.0 1844.8; 1781.8; 1888.8 4277.0; 4374.5; 4420.0 231.5; 219.3; 222.8 407.9; 385.1; 395.0 593.0; 566.5; 587.5 1018.1; 1042.7; 1061.2 2023.3; 1893.8; 1946.5 4547.2; 4692.1; 4481.9 190.3; 189.2; 181.0 374.6; 350.9; 363.2 513.4; 546.0; 539.6 961.0; 970.9; 996.6 1869.4; 1953.6; 1879.9 4721.4; 4491.8; 4676.5
5.0 3.5 2.3 1.9 2.5 1.1 4.6 2.6 2.1 1.5 2.3 1.6 3.9 2.7 2.6 1.4 1.8 1.8
86.5 85.8 87.3 88.5 90.0 85.0 87.5 86.3 89.1 90.4 90.9 88.7 85.8 87.3 86.4 87.5 90.0 90.6
I
98
1.9
0.1
105
I1
95.7
3.2
1.1
137
I11
91.3
4.7
4.0
101
F
e
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
999
Table 11. Recovery of Arsenic Compounds from Soil I Pg As Arsenic compound added
Added
Found after spiking
Av re1 error, %
Av recovery, %
100 300 500 1000 100 300 500 1000 100 300 500 1000 100 300 500 1000
198.1; 188.2; 190.0 373.1; 352.2; 364.0 542.2; 555.8; 545.6 974.5; 985.2; 1004.7 196.2; 190.5; 200.6 381.0; 373.5; 388.1 571.6; 566.6; 578.4 1005.8; 1025.1; 1016.4 200.1; 188.7; 192.7 370.3; 375.0; 379.3 525.6; 558.9; 538.2 978.4; 1020.8; 1030.7 184.8; 189.5; 198.5 348.1; 375.9; 356.8 530.0; 558.2; 545.6 995.3; 993.8; 1002.4
4.0 2.4 1.1
87.1 86.0 88.6 88.3 90.7 92.0 93.4 91.1 88.8 89.9 87.2 90.5 85.9 85.0 87.9 89.2
NaAsO3
AS203
NaAsOs
AS205
-
1.1
3.5 1.6 0.8 0.6 4.2 1.0 2.4 2.1 5.0 3.5 1.9 0.3
Table 111. Recovery of Arsenic from Soils (
