Standard calorimetric methods based on the initial reduction of formate to formaldehyde were found to yield erratic results when applied to the analysis of millimolar concentrations of formate in a microbial culture medium. The source of interference was identified as inorganic orthophosphate inhibition of the magnesium/hydrochloric acid reduction stage. Passivation of magnesium by milhmolar concentrations of phosphate is known to occur at low pH and it is proposed that this phenomenon is responsible for the inhibition of the reduction process. The presence of orthophosphate in biological extracts is almost universal and would lead to acceptance of spuriously low values for formate concentration if the previously unreported inhibitory effect is not recognized. The calorimetric method of Barker and Somers in which formate is reacted directly with 2-thiobarbituric acid to form the chromophore was evaluated and proved to be entirely free from interference by orthophosphate and other medium components. This method although less sensitive than the formate reduction methods is therefore suggested as the method of choice for the determination of formate in biological or other solutions containing phosphate.
MATERIALS
Reagents. Chromotropic acid (1,8-dihydroxynapthalene-3,6-disulfonic acid), acetyl