The investigation of conditions for liberation of sulfate from biofogical samples and two modifications (0 to 40-nmol micromethod and 0 to 200-nmol macromethod) for its calorimetric determination by the barium-rhodizonate method are reported. A brief pyrolysis of the sample in ignition tubes was found to be superior to acid hydrolysis for liberation of sulfate. It is more rapid and obviates errors due to the presence of protein and the failure to completely remove acid from hydrolyzates. The rhodizonate reagent was found to be stable for at least 60 days at -20ยฐC. A slight excess of rhodizonate over barium was found to be critical and the range of the assay could be increased twofold by increasing the ratio of barium to rhodizonate from the previously used value of 0.4 to 0.8. Neither inorganic phosphate nor nucleic acid phosphate interfered at concenttations in the sample below 100 FM, but at higher concentrations the apparent sulfate equivalent varied in pro~rtion to phosphate concentration. No interference from calcium was observed at concentrations in the sample of less than 5 mM.
The coefficient of variation of replicate analyses carried simultaneously was 3% while that of analyses of a quality control sample repeated over the course of a year was 6%.