Summary Factors affecting citric acid determination in the presence of casein were studied with the pyridine-acetic anhydride method. Although the opalescence in skimmilk tests affected results by only 2%, the turbidity introduced by solutions of isolated casein could cause gross overestimation. A pH above 12.3 in the test aliquot assured a clear reaction mixture with both skimmilk and casein solutions. It is, therefore, recommended that test aliquots containing casein should comprise sufficient alkali to insure a clear reaction. Acid casein required seven resuspensions in fresh washing solution at pH 4.5 for complete removal of occluded citrate, lactose, and calcium. Adsorption of citric acid by casein precipitated at the iso-electric point, or with trichloroacetic acid, was demonstrated and appears to account for the lower citrate concentration in acid sera, as compared to skimmilk. These results extend the conclusions of a previous paper
