- Definition of pH: pH is defined as the hydrogen ion concentration in a fluid, indicating whether it is acidic or basic.
- Components of a pH Meter: A pH meter consists of a glass electrode and a reference electrode that measure the potential difference to determine pH.
- Digital pH Meters: These devices can measure pH at various temperatures, providing more versatile and accurate readings.
- Importance of pH Measurement: pH measurement helps monitor the body’s acid-base balance, which is vital for diagnosing respiratory and metabolic conditions.
- pO2 and pCO2 Measurement: Partial pressures of oxygen (pO2) and carbon dioxide (pCO2) are crucial for assessing the function of the respiratory and cardiovascular systems.
Blood-gas analysers directly measure blood pH and the partial pressures of carbon dioxide and oxygen. An arterial pH below 7.35 indicates acidemia, while a value above 7.45 indicates alkalemia. The pH result alone does not show whether the cause is respiratory or metabolic. Interpretation also needs pCO2, bicarbonate, oxygen results and the clinical context, so it cannot by itself determine treatment such as ventilation.
pH Meter
pH is the negative base-10 logarithm of relative hydrogen-ion activity in an aqueous solution. A pH meter measures a voltage that changes with this activity and temperature. In dilute water near room temperature, pH 7 is approximately neutral; lower values are more acidic and higher values are more alkaline. The sensing glass does not act as a simple filter that lets hydrogen ions pass through. Ion exchange at its hydrated surface creates a potential that responds mainly to hydrogen-ion activity.
A combination pH probe places the glass indicator electrode and a stable reference electrode in one body. The glass side commonly contains an internal buffer and Ag/AgCl element. The reference side may use Ag/AgCl or calomel with a potassium-chloride electrolyte and a liquid junction to the sample. The meter reads the potential difference and converts it to pH after calibration with standard buffers. Glass electrodes cover a wide pH range, but temperature, sodium-ion error, junction potentials and ageing can affect the result.
A digital meter can apply temperature compensation to the electrode slope, but it must use a suitable probe and calibration for the sample temperature. To measure pH accurately, rinse the probe, calibrate it with traceable buffers, allow the reading to stabilise and follow the manufacturer’s storage limits.
pO2 and pCO2 Measurement
Blood-gas pO2 is the partial pressure of oxygen dissolved in the sample, while pCO2 is the partial pressure of dissolved carbon dioxide. Arterial pO2 helps assess oxygen transfer into blood. Arterial pCO2 reflects ventilation and supplies the respiratory component of acid-base interpretation. Results depend on sample type, temperature, inspired oxygen and handling.
pO2 Measurement
A conventional pO2 sensor uses a polarised platinum cathode and an Ag/AgCl reference electrode in electrolyte behind an oxygen-permeable membrane. Oxygen diffuses from the sample through the membrane and is reduced at the platinum surface. The resulting current is proportional to oxygen partial pressure under the sensor’s calibrated operating conditions. The instrument controls temperature and applies the polarising voltage specified for its electrode system; a fixed voltage alone does not define every analyser.
