Electrolytes are measured by a process known as potentiometry. This method measures the voltage that develops between the inner and outer surfaces of an ion selective electrode. The electrode (membrane) is made of a material that is selectively permeable to the ion being measured. For example, sodium electrodes are made from a special glass formula that selectively binds sodium ions. The inside of the electrode is filled with a fluid containing sodium ions, and the outside of the glass membrane is immersed in the sample. A potential difference develops across the glass membrane that is dependent upon the difference in sodium concentration (activity) on the inside and outside of the glass membrane. This potential is measured by comparing it to the potential of a reference electrode. Since the potential of the reference electrode is held constant, the difference in voltage between the two electrodes is attributed to the concentration of sodium in the sample. Ion selective membranes can be made from materials other than glass. For example, the antibiotic valinomycin is used to make potassium-measuring electrodes. Neutral carrier ionophores selective for lithium, calcium, and magnesium are also used for measurement of these substances in laboratory medicine. Ion selective electrodes can be used to measure whole blood, serum, or plasma since they respond to the electrolyte activity in the water phase of the sample only. One important aspect of electrolyte measurement is an artifact (erroneous result) called pseudohyponatremia that may occur when sodium is measured using a diluted blood sample. This happens when the plasma contains excessively high lipids or protein. These solids displace plasma water from the specimen, resulting in a low measurement of sodium that does not occur with an undiluted sample.
Total calcium and magnesium are usually measured by colorimetric procedures called dye binding assays. Calcium is displaced from protein by dilute acid or alkali and reacts with a dye (arsenazo III or cresolphthalein complexone) to form a colored product. When crosolphthalein complexone is used, 8-hydroxyquinoline is added to bind magnesium which also reacts with this dye. Magnesium is commonly measured by its reaction with a dye called Calmagite. A calcium chelator such as EGTA is added to prevent interference from calcium. Both calcium and magnesium may be measured by atomic absorption spectrophotometry. This procedure is more complex than colorimetric methods, but is also more accurate. Phosphorus is measured by reacting it with ammonium molybdate at an acid pH. The rate of ammonium phosphomolybdate formation is measured at 340 nm and is proportional to the inorganic phosphorus concentration (mono-and dihydrogen phosphate) of the sample.
Erika J. Norris, The Gale Group Inc., Gale, Detroit,