An investigation of methods for the determination of oxalic acid in urine

Abstract

The urinary oxalate excretion on a twenty-four hour basis has been investigated by three methods. The range of oxalate concentration in aqueous solutions studies was 0 to 100mg/1.That range defines the expected levels of oxalic acid in urine. Each of those methods investigated involves direct precipitation of oxalic acid and a specific method of detection.

The first, a colorimetric method, involves the precipitation of oxalic acid from urine with calcium sulfate and absolute ethanol. The precipitated oxalate anion is reduced to glycolic acid by boiling with sulfuric acid in the presence of a zinc pellet. The glycolic acid from that reaction step is reacted with chromotropic acid, and the absorbance of the resulting chromophore is determined at 570 nm. Thirteen subjects were analyzed by this method, and the N.M.V. was 28.8 +- 3.2mg/24 hr. That N.M.V. is comparable to other reported values. The second, a spectrofluorimetric method, involves the precipitation of urine oxalate with calcium sulfate in the presence of absolute ethanol. The precipitated oxalate is extracted with tributyl phosphate to remove the interfering metabolites. The oxalate is reacted with Ce (IV), and the fluorescence of the Ce (III) produced is determined. Seven subjects were analyzed by this method; the normal range of oxalic acid per twenty-four hours was 15.6 to 30.1 mg with a N.M.V. of 21.7 +- 1.4 mg/24 hr.

The third, a colorimetric method, involves the direct precipitation of urine oxalate with calcium chloride. The calcium oxalate precipitate is reacted with Ce (IV), and the decrease in absorbance is determined. This method was applied to urines containing known amounts of oxalic acid. The percent recovery of oxalate from urine compared to that of the recovery from aqueous oxalic acid solutions was ninety percent. The relative merits of each method are discussed, and conclusions about the applicability to a clinical laboratory are presented.