PH Determination of body fluids with a micro glass electrode and a saturated KCl bridge in the cell

Abstract

The problems and practical performance of an accurately standardized pH determination in body fluids, particularly in blood, have been studied. It was shown that: 1. 1. For the standardization of the cell, a choice can be made between four standard phosphate buffers, of which the pH values, redetermined by us with a new type of hydrogen electrode cell, are on the pH(S) scale of the National Bureau of Standards. 2. 2. The Radiometer micro glass electrode in the physiological pH range deviates by a constant amount ( ± 0.008 pH) from the hydrogen electrode, and it has mostly a smaller electromotive force change per pH unit (0–2%) than the hydrogen electrode; for both differences a method for correction has been given. 3. 3. Under standardized circumstances the saturated KCl bridge gives a minimal residual liquid junction potential ( ± 0.01 pH) in the pH measurement; for the extra influence of blood cells at the boundary (normally ± 0.01 pH) a correction has been given depending upon the haematocrit. 4. 4. Using an isotonic (0.160 M NaCl) salt bridge in the cell, the pH value of blood is lower by more than 0.1 pH than that determined with a cell with saturated KCl bridge; this isotonic junction has the advantage that no cell effect occurs. 5. 5. When the pH determinations are performed as described, a reproducibility of better than 0.002 pH can be achieved; the pH value measured in this way is interpreted as a quantity that deviates by no more than ± 0.025 pH from the activity pH a scale.