Ion-selective electrodes (Na, K, Cl)

Principle

• Potentiometry: an ion-selective membrane develops a potential that depends on the activity of one ion, measured against a stable reference electrode (Ag/AgCl or calomel) at zero current.
• Nernst equation: potential changes by about 59 mV per tenfold change in activity of a monovalent ion at 25 °C (about 61.5 mV at 37 °C).
• Sodium: ion-selective glass membrane. Potassium: valinomycin in a PVC membrane. Chloride: ion-exchange (quaternary ammonium) membrane or Ag/AgCl.
• Direct ISE (undiluted sample, e.g. blood gas analysers) and indirect ISE (pre-diluted sample, large chemistry analysers).

Specimen

• Serum, lithium heparin plasma or whole blood. Do not use sodium heparin for sodium, or potassium EDTA.

Results

• Typical adult values: Na 135–145 mmol/L; K 3.5–5.1 mmol/L; Cl 98–107 mmol/L. Local reference ranges apply.
• Serum potassium is slightly higher than plasma because platelets release K during clotting.

Quality control and pitfalls

• Electrolyte exclusion effect: in severe hyperlipidaemia or hyperproteinaemia, indirect ISE (and flame photometry) give falsely low sodium (pseudohyponatraemia); direct ISE is unaffected.
• Pseudohyperkalaemia: haemolysis, fist clenching, prolonged tourniquet, delayed separation, refrigeration of whole blood, marked thrombocytosis or leucocytosis, K-EDTA contamination.
• Bromide, iodide, salicylate and thiocyanate can cause falsely high chloride.
• Protein build-up on membranes causes drift and slow response; follow cleaning and conditioning schedules; replace electrodes as they age.
• Calibrate with two-point standards and run at least two control levels per shift.

Clinical use

• Assessment of fluid, electrolyte and acid–base disorders; anion gap = Na – (Cl + HCO3), typical 8–16 mmol/L without K.