1.4: Measurement of pH

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Methods

The hydrogen gas platinum electrode was originally used for measuring [H⁺] but is not useful for clinical pH analysis. The sample had to be fully saturated with hydrogen gas and all the oxygen eliminated. The method is not suitable for rapid automated analysis of blood samples. Current methods of pH measurement include:

Colorimetric methods. Litmus paper is used to decide between acid or base but papers incorporating pH-sensitive dyes have been been designed to measure finer gradations of pH (eg urine pH is estimated by use of indicator dyes in dipsticks). Progress in colorimetric pH methods using indicator dyes (incl. fluorescent dyes) has lead to the development of accurate intravascular methods of pH measurement. The Paratrend 7+ is a commercially available system for measuring intra-arterial pH and blood gases.
Glass electrodes. These are widely used in medical applications eg blood-gas machines.
ISFET electrodes - using 'Ion-selective field effect transistors'. These are used mostly in industry but have beendeveloped for intravascular use.

The Glass pH Electrode

Cremer in 1906 discovered that a electrical potential developed across a glass membrane which was proportional to the pH difference across the membrane. Kerridge in 1925 developed the first glass electrode for analysis of blood samples.MacInnes & Dole in 1929 experimented with different types of glass to find the one which was most sensitive. This MacInnes-Dole glass (known as Corning 015 glass) consists of 72% silicon dioxide, 6% calcium oxide and 22% disodium oxide (Na₂O).

The pH electrode consists of 2 half cells: the glass electrode and a reference electrode (eg calomel electrode). This unit develops an electrical potential across the glass which is dependent on the difference in aH⁺ across the glass membrane. This effectively allows measurement of the pH of the test solution because the pH in the solution on the other side of the membrane is constant. Other potentials develop in the pH electrode (eg liquid junction potential, asymmetry potential & diffusion potentials) and these are usually not quantified in a particular electrode. The problem is overcome by standardisation and calibration. Standardisation refers to the process of requiring that these potentials are the same when measuring the sample solution and when measuring the calibrating solutions. In particular, the liquid junction potential must remain unchanged. The calibrating solutions are chemical standard buffer solutions with a known pH. Many of the components of the electrode (eg the calomel reference cell) are very temperature sensitive. The temperature of the measurement must be precisely controlled: usually at 37°C.

Temperature Correction

If required, modern blood gas machines will report the pH value for actual patient temperature but this corrected value is calculated mathematically from the pH measured at 37°C in the machine. The change in pH with temperature is almost linear and 'anaerobic cooling' of a blood sample (ie cooling in a closed system) causes the pH to rise. The Rosenthal correction factor is recommended for clinical use.

Rosenthal Correction Factor

Change in pH = 0.015 pH units per degree C change in temperature

Example

If the measured pH is 7.360 at a blood gas electrode temperature of 37°C, then the pH at a patient temperature of 34°C is calculated as follows:

\( pH = 7.360 + (37-34) \cdot (0.015) = 7.405 \)

The potential generated in the pH electrode is about 61.5 mV/pH unit. The electrode has a high internal resistance so the measuring apparatus has to have a very high (10¹¹ Ohms) impedance to avoid drawing current from the cell and changing the potential.