Appendix IX H. Determination of Oxygen in Medicinal Gases

Oxygen in gases is determined using a paramagnetic analyser.

The principle of the method is based on the high paramagnetic sensitivity of the oxygen molecule. Oxygen exerts a strong interaction on magnetic fields, which is measured electronically, amplified and converted to a reading of oxygen concentration. The measurement of oxygen concentration is dependent upon the pressure and temperature and, if the analyser is not automatically compensated for variations in temperature and pressure, it must be calibrated immediately prior to use. As the paramagnetic effect of oxygen is linear, the instrument must have a suitable range with a readability of 0.1 per cent or better.

Calibration of the instrument Make the setting in the following manner:

• — set the zero by passing nitrogen R1 through the instrument until a constant reading is obtained;

• — set the scale to 100 per cent by passing oxygen R through the instrument at the same flow rate as for nitrogen R1 until a constant reading is obtained.

Assay Pass the gas to be examined through the instrument at a constant flow rate until a constant reading is obtained. Record the concentration of oxygen in the gas to be examined.