An automated turbidimetric method for total sulfur in plant tissue and sulfate sulfur in soils

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Abstract
An automated turbidimetric method was developed for the rapid and accurate determination of sulfate. The method is practical and useful for accurately measuring total S in plant tissues, and extractable sulfate in soils. The principle of intermittent reagent addition is used which eliminates drift and sensitivity changes caused by coating of BaSO4 on tubing and cell walls. Also, the appropriate chemistry is used to minimize interactions of the wash with the sample at a sampling rate of 30/h. The sensitivity of the method is excellent with a working range of 0 to 15 ppm S for soils. For plant digests the sample solutions are diluted to 0-35 ppm S. The precision as determined by repeated analysis of a soil sample extract was 0.58% RSD [relative standard deviation] with a mean of 9.26 .mu.g/g extractable SO4=-S. On another soil using a different extractant and extraction procedure, the RSD was 0.64%, mean of 9.26 .mu.g/g. Multiple automated S analyses on a plant tissue digest resulted in an RSD of 0.41% for a sample containing 0.21% S. The automated turbidimetric method for sulfate has excellent precision and sensitivity in plant tissue and soil analyses where gravimetric BaSO4 assays are not practical.