Partial pressures of hydrogen, carbon dioxide and oxygen are measured with blood gas analyzers. They determine the acid base balance in the body. When pH value goes below 7.35 it indicates respiratory acidosis and respiratory failure. It can be corrected using ventilator. At the same time when pH increases above 7.60 respiratory alkalosis occur. Here also ventilators are used to treat the alkalosis.
As stated by Goldman equation, the electrolyte potential of the membrane is proportional to the logarithm of ion concentration and electrolyte temperature. The chemical balance in a human body is determined due to the pH of blood and other fluids. Hence, pH is defined as hydrogen ion concentration in fluid. pH meter measures acid and base in a fluid. When the solution is neutral, it has a pH value of 7, when lesser than 7 it is acidic and more than 7 indicates it is basic solution. A pH meter consists of thin glass membrane that allows only hydrogen ions to pass through it. Inside glass electrode there is a membrane interface for hydrogen 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 are also used. It measures pH at all temperatures. pH meter has glass (active) electrode terminal and Ag/AgCl (reference) terminal. Potassium chloride is used as electrolyte solution. A salt bridge dipped in the KCL solution consists of a fiber wick at the tip of reference electrode. Active terminal is sealed using glass that has hydrated glass layer. Both these electrodes are enclosed inside a single glass tube as discussed above.
pO2 and pCO2 Measurement
Partial pressure for oxygen and carbon dioxide are denoted as pO2 and pCO2 and they are important physiological chemical measurement. pO2 and pCO2 are analyzed for functioning of respiratory and cardiovascular system. The partial pressure of gas is directly related to the quantity of gas present in the blood.
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.