Effect of water content on sensitivity and stability of the m-phenylphenol uronic acid assay

Abstract The influence of climate and hence analytical environment on the uronic acid assay of Blumenkrantz and Asboe-Hansen ( Anal. Biochem. , 54 (1973) 484–9) is described. Instability of diagnostic chromophore is attributed to variations in the water content of the sulfuric acid-tetraborate reagent used. Water, when added to the sulfuric acid-tetraborate reagent to a final concentration of 1–10% (v/v), caused a significant reduction in color intensity developed by the m -phenylphenol reagent. The acid-tetraborate reagent absorbed water from the atmosphere under high relative humidity (RH), owing to its high (85% v/v) sulfuric acid contents. Absorption of water by the reagent increased as RH and temperature increased, and color intensity correspondingly decreased. Exposure of the acid-tetraborate reagent to 25°C and 99% RH for 24 h caused a 30% decrease in chromophore intensity. This phenomenon was prevented by decreasing the aqueous sample size in the assay from 200 to 100 μl. To maximize stability of the reaction, the reagent should be stored under N 2 in a tightly sealed bottle. Also, in particularly hot and humid geographic locations, the aqueous sample size should be reduced by one-half.