Analysis of Substances in the Gaseous Phase

Apparatus for the Analysis of Gas Mixtures

Apparatus for the complete analysis of gas mixtures consists of a burette and a system of absorption or combustion pipettes.

The Hempel apparatus consists of these burettes and absorption pipettes combined with combustion pipettes, which will be described in the chapter on combustion methods (Sect. 8.6). This classical apparatus can be used to determine all the components of ordinary industrial gases, i.e. CO₂, C_nH_m, O₂, CO, H₂, CH₄ and higher homologues of saturated hydrocarbons. When working with the Hempel apparatus, it is necessary to connect a separate pipette (with a suitable absorption reagent) for each components. This constant exchanging of pipettes greatly increases the analysis time. In spite of this disadvantage, the Hempel burette is widely used in a number of modifications. The portable Sleigh instrument is based on the Hempel principle and can be used to determine small amounts of gas (0.01–0.5%), especially carbon dioxide and methane in the air or mine atmosphere, or gases absorbed in the blood. The absorption can be carried out with a small amount of fresh absorption agent. The dead spaces between the burette and pipette are very small. All rubber connections that could be a source of error are removed. This instrument is manufactured in two versions. One is designed for the analysis of mixtures containing up to 24 vol.% of absorbable gases, while the other can be used for any gas mixture.

Various types of Orsat instruments are used more widely; the long working procedure necessary with the Hempel instrument is avoided by connecting all the necessary pipettes directly with the burette (Fig. 8.6). The instruments are usually portable, consisting of a volumetric burette with a water jacket (a), levelling vessel (b), various numbers of absorption pipettes (c₁ to c₅), and a combustion apparatus (quartz tube with pieces of CuO (d) or combustion pipette with a platinum spiral (e) that is resistance-heated). This instrument can be simplified or fitted with special accessories according to need. It is commonly used in various modifications for analysis of most industrial gases, for hygienic control and for medicinal purposes.

The Haldan instrument, used in medicine to analyze carbon dioxide, is a single-purpose type of Orsat instrument. A version of the Haldan instrument has broader application and can be used to determine CO₂, O₂, CO, H₂ and CH₄.

Automatic analyzers based on the Orsat instrument principle are used industrially (Sect. 8.1.7). The Gooderham instrument based on dynamic analysis of a gas mixture is an interesting modification of the Orsat apparatus.

Most types of Orsat instrument are based on step-wise removal of the individual gases from the total sample volume, measured in a gas burette. The relatively long analysis time (1–1.5 h) is a disadvantage. In the Gooderham modification, the gas mixture continuously passes through a system of soap flow-meters and absorption pipettes or combustion apparatuses (Fig. 8.7). The volume change after absorption in the individual adsorption agents is found from the change in the flow-rate. The initial flow-rate is measured at the input and maintained constant by a manostat (1). Then absorption vessels (5) are placed in series and the reagents continuously renewed by flow from an upper stock bottle (6) to a lower one (7). The gas passes counter-current through the absorption reagent. The volume required to fill the whole apparatus with the studied gas is about 1500 to 2000 ml. Several analyses can be carried out using this amount. When the instrument is running, one analysis takes about 3 min. The precision of the results is comparable with other methods of absorption gas analysis. This type of instrument has further advantages:

(1)

A confining fluid need not be used (if mercury is used as a confining fluid, then this advantage implies that substances that react with mercury can also be analyzed).(2)

All dead-spaces and rubber connections are eliminated so that errors are minimized.(3)

Correction for temperature and pressure becomes unnecessary.