Appendix IX E. Limit Test for Carbon Monoxide in Medicinal Gases

Apparatus The apparatus (see Figure 2.5.25.-1) consists of the following parts connected in series:

• — a U-tube (U₁) containing anhydrous silica gel R impregnated with chromium trioxide R;

• — a wash bottle (F₁) containing 100 ml of a 400 g/l solution of potassium hydroxide R;

• — a U-tube (U₂) containing pellets of potassium hydroxide R;

• — a U-tube (U₃) containing diphosphorus pentoxide R dispersed on previously granulated, fused pumice;

• — a U-tube (U₄) containing 30 g of recrystallised iodine pentoxide R in granules, previously dried at 200  °C and kept at a temperature of 120  °C (T) during the test. The iodine pentoxide is packed in the tube in 1 cm columns separated by 1 cm columns of glass wool to give an effective length of 5 cm;

• — a reaction tube (F₂) containing 2.0 ml of potassium iodide solution R and 0.15 ml of starch solution R.

Method Flush the apparatus with 5.0 litres of argon R and, if necessary, discharge the blue colour in the iodide solution by adding the smallest necessary quantity of freshly prepared 0.002 M sodium thiosulphate. Continue flushing until not more than 0.045 ml of 0.002 M sodium thiosulphate is required after passage of 5.0 litres of argon R. Pass the gas to be examined from the cylinder through the apparatus, using the prescribed volume and the flow rate. Flush the last traces of liberated iodine into the reaction tube by passing through the apparatus 1.0 litre of argon R. Titrate the liberated iodine with 0.002 M sodium thiosulphate. Carry out a blank test, using the prescribed volume of argon R. The difference between the volumes of 0.002 M sodium thiosulphate used in the titrations is not greater than the prescribed limit.

Gases absorb light at unique wavelengths. This property is widely used to allow highly selective measurement of the concentrations of gases.

Description and principle of measurement The concentration of carbon monoxide in other gases can be determined using an infrared analyser.

The infrared analyser generally consists of a light source emitting broadband infrared radiation, an optical device, a sample cell and a detector. The optical device may be positioned either before or after the sample cell; it consists of one or several optical filters, through which the broadband radiation is passed. The optical device in this case is selected for carbon monoxide. The measurement light beam passes through the sample cell and may also pass through a reference cell if the analyser integrates such a feature (some use an electronic system instead of a reference cell).

When carbon monoxide is present in the sample cell, absorption of energy in the measurement light beam will occur according to the Beer-Lambert law and this produces a change in the detector signal. This measurement signal is compared to a reference signal to generate an output related to the concentration of carbon monoxide. The generated signal is linearised in order to obtain the carbon monoxide concentration. To prevent the entry of particles into the sensors, which could cause stray-light phenomena, the apparatus is fitted with a suitable filter.

Required technical specifications When used for a limit test, the carbon monoxide infrared analyser meets the following technical specifications:

• — limit of detection: (generally defined as a signal-to-noise ratio of 2) maximum 20 per cent of the maximum admissible concentration;

• — repeatability: maximum RSD of 10 per cent of the maximum admissible concentration, determined on 6 measurements;

• — linearity: maximum 10 per cent of the maximum admissible concentration.

The technical specifications must be met in the presence of the other gas impurities in the sample.