Determination of oxalic acid in urine by atomic absorption spectrophotometry

A method is described for determination of oxalic acid in urine using atomic absorption spectrophotometry. The concentration of urinary oxalic acid is calculated by using the difference between two determinations of calcium: first, the excess of calcium in the supernatant after precipitation as calcium oxalate at pH 5, and second, the total calcium determined at pH less than 1 (endogenous and added). Analytical parameters (pH of precipitation, temperature, calcium and oxalate added, precipitation time, interfering substances) were studied with the aid of [14C]oxalic acid. A constant recovery of 95% of total oxalic acid allows the use of a correction factor. The accuracy and reproducibility of the method make it useful for routine determination of urinary oxalic acid.