As one of studies on the application of microbiological assay method for sulfate using E. coli B (H), determination of sulfur content in orange leaves was examined. Preparation of sample solution was as follows : Dried orange leaves were subjected to combustion in the presence of Mg (NO
3)
2·6H
2O as an oxidizing agent at 800°for 1 hr. The residue was dissolved in 3 N HCl, neutralized with 3 N KOH, and the solution was passed through a column of Amberlite IR-120 (H
+form). The effluent was used for the microbiological assay, and the results obtained were as follows : 1) It was possible to determine sulfur content in orange leaves by microbiological assay method using Mg (NO
3)
2·6H
2O as an oxidizing agent. 2) This microbiological method gave small standard deviation (σ=3.8%) and accurate recovery (98.1-101.5%). 3) This proposed microbiological method will make it possible to determine sulfur content in orange leaves with the amount of a sample less than that in the gravimetric method.
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