Tuesday, 11 July 2006

A Comparison Between a Laboratory Chromium Reducible Sulfur Technique and a Reduced Inorganic Sulfur Analyzer for the Determination of Reduced Inorganic Sulfur in Acid Sulfate Soils.

Crystal A. Maher, Leigh A. Sullivan, and Kerr Geoff. Centre for Acid Sulfate Soil Research, Southern Cross University, P.O. Box 157, Lismore, 2480, Australia

Effective management of Acid Sulfate Soils (ASS) relies on an accurate quantitative assessment of the Reduced Inorganic Sulfur (RIS) content of the soil. The RIS content is used to identify ASS, to determine the potential acidity risk and to calculate the liming requirement. RIS analysis must be capable of delivering a suitably high degree of accuracy and precision. The method currently approved by the Acid Sulfate Soil Management Advisory Committee for the determination of RIS is the Chromium Reducible Sulfur (CRS) method. This method relies on the reduction of inorganic sulfur in a hot acidic chromous chloride solution. Evolved H2S is then carried in nitrogen gas and trapped in a zinc acetate trapping solution as zinc sulfide. This solution is then titrated with iodine. Recently another method for determining RIS has been developed. The Reduced Inorganic Sulfur Analyzer (RISA) operates on a similar principal to the CRS method in that inorganic sulfur is reduced in an acidic chromous chloride solution. The method differs, however, in that evolved H2S is measured by a gas detector rather than being fixed in trapping solution. The RISA has several advantages over conventional CRS techniques including field portability, real time analysis, digital graphic display, lower reagent requirements and an internal quality control procedure. The aim of this study was to compare the accuracy and precision of the RISA to the currently accepted CRS method under controlled laboratory conditions. Sodium thiosulfate, crushed pyrite in talcum powder and an ASS material were compared as reference materials. In addition, the accuracy of the internal quality control procedure was examined. This involved trapping the evolved H2S gas in zinc acetate trapping solution after it had passed though the gas detector. The trapping solution was then titrated according to the CRS method. Samples were collected from 6 ASS environments on the North Coast of NSW using a Russian D-Section corer. Replicate cores were placed in thick plastic bags and combined into a composite. Samples were then dried and ground in a ring mill grinder for 1 minute. Each sample was analyzed 5 times by the CRS method and 5 times by the RISA method. The results indicate that the RISA is both accurate and precise if an appropriate reference standard is used. Use of sodium thiosulfate, resulted in the RISA only achieving an average of 66% of the RIS recovered by the CRS technique. This meant a correction factor of 1.509 was necessary to improve the accuracy of the RISA results. When the crushed pyrite in talcum powder was used as the reference material the RISA recovered an average of 91.1% of the RIS detected by CRS. This reduced the correction factor to 1.1. The use of an ASS material as a standard, however allowed the RISA to achieve a recovery equal to the CRS without the need for a correction factor. The titration of the RISA trapping solution as an internal quality control method also produced consistent results with an average recovery of 94.8%. This study concluded the RISA is capable of delivering the degree of accuracy and precision required in RIS determination, providing an appropriate reference material is used. It also found that titrating the trapping solution provided an accurate internal quality assurance check, a feature unique to the RISA.

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