Protein does not interfere with the ion-selective electrode determination of calcium, sodium or potassium ions.

The protein interference with the determination of ionised Ca2+, Na+ and K+ reported in several studies could be due to effects on liquid/liquid junction potentials or on the ion-selective electrodes, but could also be due to Donnan effects during sample preparation, e.g. ultrafiltration or dialysis. The possible interference of protein was studied by subjecting a bovine serum albumin solution, 100 g/l, to gel-filtration in Sephadex G-25 columns equilibrated with 150 mmol/l NaCl, 5 mmol/l KCl, 20 mmol/l MOPS, pH 7.4 and varying concentrations of Ca2+; 0.75 and 1.25 mmol/l. The albumin was dissolved in the basic electrolyte solution and pH adjusted before the gel-filtration. Samples were taken for measurements before, during and after the elution of the protein peak. In this way it was ensured that the activities were not changed by the presence of albumin. Also the temperature, the ionic strength of the electrolyte and the bridge solution were varied. Ca2+, Na+ and K+ activities were measured with the aid of a Kone Microlyte instrument and Ca2+ in addition with the Radiometer ICA-1 instrument or a measuring system consisting of a Philips IS 561-Ca electrode, a Beckman KCl reference electrode connected to the sample chamber via a 2% agarose bridge containing either 0.15 or 2 mol/l KCl, a pH/voltmeter and a voltage-bucking device. No interference by protein was found.