984 SHORT PAPERS Analyst, Vol. 105 Determination of Sulphide in Flooded Acid - Sulphate Soils by an Indirect Atomic-absorption Spectrophotometric Method Ramesh C. Ray, P. K. Nayar, A. K. Misra and N. Sethunathan Division of Soil Science and Microbiology, Central Rice Research Institute, Cuttack-7 53006, India Keywords: Flooded acid - sulphate soils; sulphide determination; atomic- absorption spectrophotometry Hydrogen sulphide, evolved during the anaerobic metabolism of sulphate, is readily converted into insoluble metal sulphides, chiefly iron(I1) sulphide, in flooded acid - sulphate soils that are especially rich in ir0n.l The widely used method for determining sulphide is based on the precipitation of the sulphide in hydrogen sulphide as zinc sulphide and subsequent determina- tion by methylene blue formation2 or t i t r i m e t r ~ .~ The simple, rapid and reproducible method described here essentially involves the precipitation of zinc sulphide by the action of zinc on the hydrogen sulphide liberated on acidification of metal sulphides in flooded acid - sulphate soils, and then indirect determination of sulphide by determining the zinc in the precipitate and also the zinc remaining in solution, after the precipitation, by atoniic-absorption spectro- photometry. Experimental The assembly used for the conversion of metal sulphides into hydrogen sulphide consisted in a 250-ml Erlenmeyer flask, which was closed with a three-hole rubber bung, for holding a dropping funnel (for adding hydrochloric acid) and two glass tubes (an inlet for oxygen-free nitrogen and an outlet).The outlet was connected to two 100-ml Erlenmeyer flasks, in succession, containing an ammoniacal solution of zinc acetate to trap the hydrogen sulphide evolved. For standardisation of the method, a known amount of sodium sulphide was placed in a 250-ml flask and then treated with 100 ml of 1 N hydrochloric acid. The hydrogen sulphide evolved was swept into the zinc acetate solution with oxygen-free nitrogen forOctober, 1980 SHORT PAPERS 985 30 min-1 h (until tests with lead acetate paper strips showed complete cessation of hydrogen sulphide evolution) in order to precipitate zinc sulphide. Zinc sulphide was accumulated in the first trap while the second trap was visibly free from any precipitate; this indicated that hydrogen sulphide was completely precipitated in the first trap.After filtration, the precipitated zinc and soluble zinc remaining in the filtrate were assayed using a Varian Techtron atomic-absorption spectrophotometer, Model AA-1100. Sulphide equivalent to the zinc precipitated or to the decrease in the zinc content of the solution was then calculated. Care was taken to ensure that the molar concentration of sulphide was far exceeded by the molar concentration of zinc in order to provide a measurable excess of zinc in solution after complete precipitation of the sulphide. This method was compared simul- taneously with the conventional iodimetric m e t h ~ d , ~ in which the zinc sulphide precipitated in the trap was reacted with excess of iodine plus 2.5 ml of concentrated hydrochloric acid and the unreacted iodine was titrated against standard thiosulphate solution.Sulphide formed in two acid - sulphate soils, Pokkali (pH, 5.0; organic carbon, 2.28%; sulphate-S, 0.056%; total sulphur, 0.1%) and Kari (pH, 3.9; organic carbon, 4.65%; sulphate- S, 0.039%; total sulphur, 0.13%) under flooded conditions was determined by atomic- absorption spectrophotometry and iodimetry. Soil samples (20 g) were flooded with 25 ml of distilled water in test-tubes (25 x 200 mm). After 40 d, the reduced soil samples were transferred into 250-ml flasks and treated with 100 ml of 1 N hydrochloric acid in order to liberate hydrogen sulphide from the metal sulphides, chiefly iron(I1) sulphide. Hydrogen sulphide was absorbed in an ammoniacal solution of zinc acetate with precipitation of zinc sulphide.The zinc in the precipitate and the filtrate was determined by atomic-absorption spectrophotometry to give the result for the indirect determination of sulphide. Care was taken to bubble oxygen-free nitrogen through the apparatus for 5 min prior to acidification of the soil samples to prevent the instantaneous oxidation of the hydrogen sulphide evolved in the flask. The sulphide was also determined by the conventional iodi- metric method3 by adding excess of iodine plus hydrochloric acid directly to the trap as described for the sulphide determination from sodium sulphide in the standardisation of the method. In a modification of this method, the precipitated zinc sulphide was first separated by filtration and then treated with excess of iodine plus hydrochloric acid to avoid any inter- ferences from iodine consuming substances, if any, from the complex soil system.Results and Discussion The data in Table I showed that about 85% of sulphide was recovered from sodium sulphide standards by both the iodimetric and atomic-absorption spectrophotometric methods. Also, in the latter method, the sulphide values, obtained by determining the zinc either in the zinc sulphide precipitated or that remaining in solution in the filtrate, were almost identical. The atomic-absorption spectrophotometric method was simple, rapid and reproducible with variations of less than 5% within replicates, while the iodimetric method, though equally sensitive, was somewhat tedious.TABLE I SULPHIDE RECOVERED FROM SODIUM SULPHIDE STANDARDS The results are for sulphide recovered. Atomic-absorption spectrophotometry Result from determination of Result from determination of zinc in filtrate zinc in precipitate Iodimetry r- 7 -7 Sulphide Replicate/ Mean/ Recovery, 'Replicate/ &lean/ Recovery, ' Replicate/ Mean/ Recover; added/mg mg mg % mg mg % mg mg % 82.8 2.86) 2.86 84.8 :%} 2.86 84.8 2.85 2.87, 2.84 3.37 2.70 4.49 ::ti} 3.71 82.6 E} 3.75 83.5 84.0 2.79 J 3.83 3.76 3.66 The determination of sulphide in two acid - sulphate soils after a 40-d flooding showed that the sulphide values from both soils were realistic and reproducible, ranging from 0.26 to986 SHORT PAPERS Analyst, VoZ. 105 0.33 mg g-l when determined by atomic-absorption spectrophotornetry (Table 11).As in the pure system when sodium sulphide was used, soil sulphide levels derived from the determination of zinc either in the precipitate or in the filtrate were identical. However, the iodimetric method in which iodine was added directly to the trap gave abnormally high sulphide values (> 1.36 mg g1) for both soils; these levels are above the theoretical values for sulphide that could be generated from Pokkali soil with 0.056% sulphate-S and 0.1% of total sulphur and from Kari soil with 0.039% sulphate-S and 0.13% of total sulphur following flooding. However, the sulphide content of the Kari soil, obtained by the modified iodimetric method, in which the zinc sulphide was treated with iodine only after filtration, was realistic, reproducible and identical with that obtained by atomic-absorption spectrophotometry.This would suggest that overestimation of soil sulphide, by the con- ventional method of adding iodine directly to the trap, was a result of interferences by some reduction products of the flooded soil system. For instance, reduced sulphur compounds TABLE I1 SULPHIDE FORMED IN ACID - SULPHATE SOILS ON 40-d FLOODING The results are for sulphide formed in mg per gram of soil. Atomic-absorption spectrophotometry r-----L------ _7 determination of determination of Iodim e t ry 7 Result from Result from _______ A _______-_ zinc in filtrate zinc in precipitate Conventional* Precipitate? 7 ---- 7 - r---dL ‘-+--- r---- Soil Replicate Mean Replicate Mean Replicate Mean Replicate Mean N.D.$ Pokkali .. ::ti\ 0.29 ::;;\ 0.29 ::!:\ 2.03 N.D. 0.28J 0.30) 1.85) N.D. E} 1.65 0.32 1.56 0.33 Kari . . :::!} 0.28 ::ti} 0.29 0.26 0.29 Iodine was added directly to the trap. t Zinc sulphide precipitated was first separated by filtration and then treated with iodine. N.D. not determined. such as sulphite, thiosulphate, tetrathionate and hydrosulphike rnay decompose during acidification leading to erratic results in the determination using i ~ d i m e t r y . ~ Likewise, the methylene blue method, widely used in determining sulphide, is not always reliable as not only hydrogen sulphide but other reduction products that commonly occur in anaerobic ecosystems can readily react with methylene blue to produce erroneous results. The new method of indirect determination of sulphide through the determination of zinc by atomic- absorption spectrophotometry is simple, rapid and free from interference and thus has a definite advantage over the iodimetric and methylene blue methods, especially in a complex system such as waterlogged soil. The authors are grateful to Dr. H. K. Pande (Director) for encouragement, Dr. S. Patnaik and Dr. C. C. Biddappa for suggestions and Dr. S. B. Lodh for facilities. This project was partially funded by the Department of Science and Technology, Government of India, and the International Atomic Energy Agency, Vienna. 1. 2. 3. References Ponnamperuma, F. N., Adv. Agron., 1972, 24, 29. Johnson, C. M., and Nishita, H., Anal. Chem., 1952, 24, 736. American Public Health Association and Water Pollution Control Federation, “Standard Methods for the Examination of Water and Waste Water,” Thirteenth Edition, American Public Health Association, New York, 1971, p. 551. Received April lst, 1980 Accepted May 8th, 1980