Appendix IX J. Determination of Water in Medicinal Gases

Water in gases is determined using an electrolytic hygrometer, described below.

The measuring cell consists of a thin film of diphosphorus pentoxide, between 2 coiled platinum wires which act as electrodes. The water vapour in the gas to be examined is absorbed by the diphosphorus pentoxide, which is transformed to phosphoric acid, an electrical conductor. A continuous voltage applied across the electrodes produces electrolysis of the water and the regeneration of the diphosphorus pentoxide. The resulting electric current, which is proportional to the water content in the gas to be examined, is measured. This system is self-calibrating since it obeys Faraday′s law.

Take a sample of the gas to be examined. Allow the gas to stabilise at room temperature. Purge the cell continuously until a stable reading is obtained. Measure the water content in the gas to be examined, making sure that the temperature is constant throughout the device used to introduce the gas into the apparatus.

The electrolytic hygrometer achieves accurate sample flows by using a mass flow controller to deliver a constant volumetric flow rate to ensure that the water content is determined accurately. The calibration of the mass flow controller is normally performed using nitrogen. When using gases other than nitrogen for calibration, consult the manufacturer′s instructions for the appropriate conversion factors and ensure that the correct cell is used for the type of gas to be examined.