Operation of blood: O₂ electrodes; CO₂ electrodes; pH electrodes

1. Blood O₂ electrodes

a) Also known as a Clark cell or polarographic electrode

b) A platinum cathode and a silver/silver chloride anode are immersed in an electrolyte solution of potassium chloride

c) A voltage of 0.7 v is applied between the electrodes and the current flow is measured

d) At the anode, electrons are provided by the oxidation reaction of the silver with the chloride ions of the potassium chloride electrolyte to give silver chloride and electrons

ⁱ⁾ 4Ag ® 4Ag⁺ + 4e^-

ii) 4Ag⁺ + 4Cl^- ® 4AgCl

e) At the cathode, oxygen combines (reduction) with the electrons and water giving rise to hydroxyl ions

i) O₂ + 4e^- ® 2O₂

ⁱⁱ⁾ 2O₂ + 2H₂O ® 4(OH)^-

f) The more oxygen available, the more electrons that can be taken up at the cathode and consequently the greater the current flow.

g) The current flow through the cell is dependent on the oxygen concentration at the platinum electrode

h) For every molecule of oxygen reduced, four electrons are supplied by the electrical circuit

i) Thus the current, having been determined by the availability of oxygen molecules, is directly proportional to the partial pressure of oxygen

j) Measurement of the current and conversion to mm Hg is accomplished electronically with the results displayed numerically or on a meter

k) Temperature control is important so the electrode is kept at 37°C

l) The platinum cathode cannot be inserted directly into the blood as protein deposits would develop. So a plastic membrane is used, separating the blood from electrolyte solution that bathes the cathode

2. pH electrode

a) The H⁺ electrode is an ion selective electrode dependent on the hydrogen ion sensitive glass at its tip

b) A potential develops across this glass that is dependent on the difference of H⁺ across it

c) The H⁺ within the H⁺ electrode is maintained at a constant value by the buffer solution so that the potential across the glass is dependent on the H⁺ in the blood sample channel

d) To measure the potential difference it is necessary to make an electrical contact with the blood and with the buffer solution; in the case of the blood a reference electrode is used which has a membrane at its tip to avoid contamination

e) In both the H⁺ electrode and the reference electrode, stability of the metal to solution is maintained through a special electrode of silver in contact with its chloride which is in turn in contact with a solution of chloride ions

f) The silver/silver chloride connection in the H⁺ electrode is then in contact with the buffer solution while the silver/silver chloride connection of the reference electrode is linked to the membrane and the blood via a saturated solution of potassium chloride which completes the electrical circuit

g) The potential difference between the electrodes is measured and converted into a direct reading of H⁺ or pH

h) Temperature control is important; the system is kept at 37°C

i) Each electrode consists of two conductors, a metal component which conducts electrons and a electrolytic component to conduct ions

j) A potential difference exists at the interface of the two conductors, and this is referred to as the electrode potential

k) The only variable in the circuit (assuming constant temperature) is the difference in pH between the inner buffer of the electrode and that of the sample

3. pCO₂ electrode

a) Severinghaus CO₂ electrode provides a direct method of pCO₂ measurement from the H⁺ change associated with the reaction of CO₂ with water

i) CO₂ + H₂O Ö H₂CO₃ Ö H⁺ + HCO₃^-

ii) The pCO₂ is directly related to H⁺ concentration

b) The electrode incorporates hydrogen ion sensitive glass with electrodes either side of it

c) The glass is in contact with a thin film of sodium bicarbonate solution in an nylon mesh which is fixed over the glass tip

d) The blood is separated from the nylon mesh and bicarbonate by a plastic membrane which is permeable to carbon dioxide

e) At the tip of the electrode, carbon dioxide diffuses through the plastic membrane into the mesh impregnated with the bicarbonate solution and combines with the water present producing hydrogen ions and bicarbonate

f) The resulting change of hydrogen ion concentration is measured by the glass electrode

g) The analyser then calculates the pCO₂