Abstract: A method for determining the concentrations of total calcium and at least one monovalent ion in a sample includes the steps of mixing the sample with a diluent that has a pH within the range pH 6.5 to 7.0 and includes 2-amino-2-hydroxymethyl-1,3-propanediol phosphate and is free of the monovalent ion. An aliquot of the diluted sample is concurrently contacted with a calcium-specific ion selective electrode and an ion selective electrode specifically responsive to the monovalent ion, the response of the calcium-specific ion selective electrode is measured as an indication of the concentration of total calcium in the sample, and the response of the monovalent ion specific ion selective electrode is measured as an indication of the concentration of the monovalent ion in the sample.

Abstract: A method is disclosed for determining calcium ion content of serum which comprises calibrating a potentiometric analytical system with calcium standards having a solution containing both TRIS and BisTRIS at a pH of 7.5. The serum sample is diluted with a silicone oil/water emulsion which may further contain a protein complexing agent such as iron, zinc, cadmium, nickel or copper. Measurement is then carried out on the diluted serum sample.