How Estimation of Oxygen and Carbon dioxide is Carried out in Biological Solutions

Estimating Oxygen in Biological Solutions

Oxygen concentration in solution is usually estimated electrolytically. The basic principle is that, if a bright platinum electrode is placed in a solution, and +0.7 V is applied to it with respect to a reversible (silver-silver chloride in biological systems), oxygen will be reduced at it, and the current flowing is a measure of the rate at which oxygen is being reduced.

By enclosing the platinum electrode in a film of collodion and keeping the solution in contact with its outer surface well stirred, a constant flow of oxygen is set up through the film, proportional to the external concentration, and so a current flow proportional to oxygen concentration is obtained. There are commercially produced oxygen electrodes available that can be used in this process.

Estimating Carbon dioxide in Biological solutions

An estimation of Carbon dioxide is required since the ability of blood to transport Carbon dioxide from the body tissues to the lungs is of vital importance. The estimation of Carbon dioxide is carried out on a single drop of blood. The blood sample is held at a constant temperature, and its hydrogen ion concentration measured electrometrically by a pH electrode. It is then equilibrated in turn with two known concentrations of carbon dioxide gas, and its pH  determined  for each; calculation using the Henderson-Hasselbalch equation then gives the original blood bicarbonate level, and hence its ability to transport Carbon dioxide.

Related: How ionic composition of Tissue fluids is determined