- 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 analyzers measure the partial pressures of hydrogen, carbon dioxide, and oxygen to determine the body’s acid-base balance. If the pH value drops below 7.35, it indicates respiratory acidosis and potential respiratory failure, treatable with a ventilator. If the pH rises above 7.60, it signals respiratory alkalosis, which can also be treated with a ventilator.
pH Meter
According to the Goldman equation, the membrane’s electrolyte potential is proportional to the logarithm of ion concentration and temperature. The body’s chemical balance is determined by the pH of blood and other fluids. pH is defined as the hydrogen ion concentration in a fluid. A pH meter measures the acidity or alkalinity of a fluid. A neutral solution has a pH of 7; below 7 is acidic, and above 7 is basic. A pH meter has a thin glass membrane that only allows hydrogen ions to pass through, with a membrane interface inside the glass electrode for these ions.
A glass bulb is present at the bottom of the pH meter that contains highly acidic buffer solution. Glass tube has Ag/AgCl electrode and a calomel reference electrode. It is placed inside the solution for which pH is to be measured. Across two electrodes, potential is measured. Electrochemical measurement obtained between two electrodes is known as half-cell and potential of the electrode is half-cell potential. In this setup, glass electrode inside the glass tube acts as one, half – cell and reference electrode act as another half-cell. For easy pH measurement, electrode combination is used. Glass electrodes are used to measure pH values up to 7. Special type of pH electrodes are used when glass electrodes produces error.
Digital pH meters can measure pH at all temperatures. These meters have a glass (active) electrode terminal and an Ag/AgCl (reference) terminal. Potassium chloride is used as the electrolyte solution. A salt bridge dipped in the KCL solution has a fiber wick at the tip of the reference electrode. The active terminal is sealed with glass that has a hydrated layer. Both electrodes are enclosed in a single glass tube, as discussed above.
pO2 and pCO2 Measurement
Partial pressures of oxygen and carbon dioxide, denoted as pO2 and pCO2, are important physiological measurements. These measurements assess the function of the respiratory and cardiovascular systems. The partial pressure of a gas is directly related to the amount of that gas present in the blood.
pO2 Measurement
In this measurement, platinum wire acts as active electrode. They are combined with glass for insulation and only the tip is exposed. Oxygen is diffused into the electrolyte solution. Ag/AgCl acts as reference electrode. Between platinum wire and reference, electrode voltage of 0.7 V is applied. Active electrode is connected to negative terminal through micro ammeter and reference electrode is connected to the positive terminal. At the platinum electrode, oxygen reduction occurs due to the negative terminal connection. Amount of oxidation – reduction current is proportional to the partial pressure of oxygen present in the electrolyte. It is measured using micro ammeter.
