Thiosulfate interference in the biodegradable dissolved organic carbon assay

Abstract

Reducing agents, namely sodium thiosulfate, are used to eliminate oxidant residuals in samples for biodegradable dissolved organic carbon (BDOC) analyses. For the particular BDOC assay used in this study, samples are seeded using a natural bacterial inoculum. These bacteria grow in response to dissolved organic carbon (DOC) and the decrease in DOC measured during the thirty day batch incubation period determines the BDOC of the sample. Na 2S 2O 3 is usually added in excess, leaving a residual thiosulfate concentration which has been found in this study to interfere with the BDOC measurement. The effect of Na 2S 2O 3 residuals between 0 and 34 mg/l on the measured value of BDOC was tested. The interference increased with the amount of Na 2S 2O 3 added and above 20 mg/l, sodium thiosulfate caused a substantial underestimation of the BDOC. The natural bacterial inoculum was indirectly found to contain bacteria capable of metabolizing thiosulfate. At concentrations above 20 mg Na 2S 2O 3 l , these bacteria produced sufficient acid that the heterotrophic activity necessary in the BDOC assay was effected. More tests are required to determine if the growth of these sulfur-oxidizers effects BDOC values in the absence of pH change. The use of a phosphate buffer should be considered in samples of limited buffering capacity. Given the observed effects of Na 2S 2O 3, residuals should be limited and, where possible, the use of Na 2S 2O 3 should be avoided.