Hydrogen sulphide measurement

Because of the toxicity of hydrogen sulphide (H2S), a variety of portable devices have been developed to monitor H2S levels in air. Many of these devices have alarms that alert workers when H2S levels are dangerously high. These instruments can be purchased from various suppliers. In addition, detector tubes attached to air pumps can be used to measure H2S levels in air. 

There are also a number of colorimetric tests for measuring H2S levels in water). Most of these tests are based on a reaction that generates blue methylene, which is subsequently detected by its color. Unfortunately, the color of water samples and the presence of interfering substances can interfere with this test. 

A semi-quantitative test for measuring H2S in water utilises Alka-Seltzer, and should be relatively unaffected by color or chemical interference. Keep in mind that H2S is easily oxidised. Therefore, unless the sulphide is precipitated, any measurement in water should proceed rapidly after sample collection with minimal aeration. 

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Lefebvre M, Yee D, Fritz D, et al. 1991. Objective measures of ocular irritation as a consequence of hydrogen sulphide exposure. Vet Hum Toxicol 33 564-566. 

Wells are purged until stable readings are obtained for field chemical parameters including pH, temperature, dissolved oxygen, redox and electrical conductivity. Samples are then collected for a wide variety of chemical parameters. Time sensitive parameters are analyzed within specific holding times. For example, alkalinity and hydrogen sulphide are measured at the time of sampling, iodide... 

Variation of the nature of the gate electrode results in the different types of FET. For example, in the metal oxide semiconductor FET (MOS-FET) palladium/palladium oxide is used as the gate electrode. This catalyti-cally decomposes gases such as hydrogen sulphide or ammonia with the production of hydrogen ions, which pass into the semiconductor layer. An enzyme may be coated on the palladium, e.g. urease, which catalyses the production of ammonia from urea and thus provides a device for the measurement of this substrate. 

The effect of biofilters has been studied by Kowalewsky (4). In these studies he used both Dragerrof as well as photometric and gas chromatographic measurements to study the reduction of certain odours such as ammonia and hydrogen sulphide. The measurements showed that the concentration of the gases studied decreased by, on average, 50%. When the moisture content of the filter was at an optimum level the reduction was 80%. 

With respect to smoke and fumes, measurement of SC in the mixing area showed no increase above background, and no hydrogen sulphide was detected. In the placement areas, there was some odor from the aromatics in the sulfur modifier. Pouring conditions were similar to those of an asphalt paving operation and included a similar odor. No special protective equipment was utilized or required. Operators at the mixer wore dust masks. 

The same principle can be applied to measuring sulphur that has been converted to hydrogen sulphide (H2S) then oxidised by ozone. 

In aqueous solution the acid has a metallic taste and only feebly affects litmus the latter characteristic is in accord with the evidence of its electrical conductivity, which is small and indicates relatively slight dissociation, comparable with that of hydrogen sulphide rather than with that of sulphuric acid. Measurements of the hydrogen-ion concentration of normal telluric acid solution gave [IT ] =4 x 10 5 gram-ion per litre. On the assumption that the acid is ionised in accordance with the equation (see p. 387)... 

The results of earlier workers on this subject were vitiated by experimental errors. These arose from the circumstance that union takes place quite rapidly at temperatures below the boiling-point of sulphur, and is not conveniently measurable at higher temperatures. Consequently experiments were always made in presence of liquid sulphur, which possesses the property of dissolving hydrogen sulphide, so that the products of reaction are removed to a greater or less extent depending on the amount of sulphur in the reaction bulb. 

An alternative approach, adopted by Albery et al. [59-61], is to determine the mechanism giving rise to the sensor response and to use this information together with the measured data at short times to calculate the final response. This was used for an electrochemical sensor system incorporating cytochrome oxidase where the steady-state responses of the measurement system were insufficiently fast for useful measurement of respiratory inhibitors such as cyanide, hydrogen sulphide, etc. By using mechanistic information, it was possible to successfully calculate the concentration in a test sample by real-time analysis of the sensor signals at short times after exposure to the test sample. The analysis could cope with the gradual loss of enzyme activity commonly found in these biosensor devices. 

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