The Determination of Sodium in the Presence of Phosphates

The interference of phosphate with the estimation of sodium by the zinc uranyl acetate method has been pointed out by several investigators (l-3). Recently the question as to whether the amount of phosphorus present in serum is great enough to influence the sodium values has been raised again. Hald (2) points out the necessity of removing phosphate before sodium is determined, while Consolazio and Dill, in a recent paper (4), disagree with her. They state that the amount of phosphate present in plasma is not sufficient to cause an increase in the sodium values. However, the highest phosphate value which they tested was 14.7 mg. of P per 100 ml. of solution, while Hald tested values as high as 100 mg. of P per 100 ml. of solution. It has been observed in this laboratory from time to time that, when sodium estimations were carried out in conjunction with a study of the other bases (Ca, K, Mg) and compared against a direct estimation of the total base, the sum of the individual cation values was greater than that of the total base value. The validity of these determinations was carefully controlled by running known solutions and recoveries (of known added amounts of cation to the serum) simultaneously with the determinations mentioned above. Most of these sera were from patients in whom the total serum phosphate content was increased; i.e., patients with severe nephritis and various hyperlipemic conditions such as diabetes and xanthomatosis. In some of these cases the inorganic phosphate alone was above 15 mg. per 100 ml. of serum. Upon investigating the problem we have found that the addition of phosphate does cause an increase in the serum sodium values.