Chemistry
Testing for Gases
Testing for gases involves using various methods to identify the presence of specific gases in a sample. These methods include using indicators, such as litmus paper or universal indicator, and performing chemical reactions to produce visible changes, such as the formation of a precipitate or a change in color. Testing for gases is an important aspect of many scientific fields, including environmental science and industrial chemistry.
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eBook - PDFMeasurement of Temperature and Chemical Composition
Jones' Instrument Technology
- B E Noltingk(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
By choice of columns, operating conditions and detectors, a gas-chromatographic analysis system may be built up individually tailored to analyse several different preselected components in a single aliquot taken from a gas sample. Because of its importance in process analysis, gas chromatography is given particularly detailed treatment. In addition to the analysis techniques described in this chapter, a number of spectroscopic methods are given under that heading in Chapter 3, while some electrochemical methods are outlined in Chapter 4. 5.2 Separation of gaseous mixtures Although detectors have been developed which are specific to particular gases or groups of gases, for example flammable gases or total hydrocarbons there is often a need to separate the sample into its components, or to remove interfering species, before the sample is passed to the detector. A non-specific detector, such as a katharometer, may also be used to measure one component of a gas mixture by measuring the change in detector response which occurs when the component of interest is removed from the gas mixture. Methods for separating gaseous mixtures may be grouped under three main headings. Chemical reaction A simple example of chemical separation is the use of desiccants to remove water from a gas stream. The percentage of carbon dioxide in blast furnace gas may be determined by measuring the thermal conductivity of the gas before and after selective removal of the carbon dioxide by passing the gas through soda-lime. Similarly the percentage of ammonia gas, in a mixture of nitrogen, hydrogen and ammonia may be measured by absorbing the ammonia in dilute sulphuric acid or a suitable solid absorbent. Physical methods The most powerful physical technique for separation of gases is mass spectrometry, described in Chapter 3 - though only minute quantities can be handled in that way.
eBook - PDFProgress in Chemical Toxicology
Volume 3
- Abraham Stolman(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
Gas chromatography also provides the degree of sensitivity demanded by the toxicologist. Sensitivity has a special meaning for the chemical toxicologist, for not only must he be able to detect microgram quantities of a given agent, but these small quantities of materials may be the total amounts of the agents present in the samples to be analyzed. The use of gas chromatography in chemical toxicology is in its infancy. Whereas its greatest usefulness will ultimately be realized in the screening of specimens in the general unknown toxicological analysis, it is currently used largely in special procedures designed for the analysis of a single agent or class of agents, for the identification of a species within a class of com-pounds, or for further confirmation of the identity of an agent that has already been partially characterized. Most studies have dealt with pure, or nearly pure materials. The toxicologist frequently must work with very small quantities of foreign substances contained in relatively large amounts of tissue or other biological materials. Even after extensive purification pro-cedures, which in themselves must be curtailed due to the principle of limiting returns, the substance in question may represent but a small percentage of the final residue. This means that many interfering or spurious peaks may be recorded by the gas chomatograph. The toxicologist in his search for an unknown agent is thus challenged with the need to determine which of the observed peaks represent foreign substances and which represent naturally occurring substances or foreign materials of no toxicological importance. The individuality of the specimen, the state of decomposition of the specimen, microbial contamination, and even perhaps, the diet of the subject from which the specimen was obtained, can all be expected to alter the pattern of spurious and interfering peaks.
eBook - PDF- T.-W. Lee(Author)
- 2008(Publication Date)
- CRC Press(Publisher)
251 7 Gas Sampling Measurements 7.1 INTRODUCTION Gas sampling is still widely used for analyzing chemical composition. As the phrase gas sampling suggests, a volume of gas is sampled (usually continuously) from the flow and sent through analyzers to discern the various gas molecules present in the sample. Gas sampling analyzers represent a mature technology, can be calibrated using reference gases, reach sensitivities down to ppm levels, and are rugged and readily available from various vendors. There is a wide vari-ety of analyzers for detection of pollutant and toxic gases, fuel and oxidizers, and for identifying a complete spectrum of molecules (gas chromatography and mass spectroscopy). In many chemically reacting flows, including engine combustion, power plants, environmental monitoring and chemical processing, it is of interest to determine the constituents of gas going in and out of systems for efficiency, process control, and pollutant emissions. For these reasons, gas analyzers rep-resent an important diagnostic method for direct monitoring of chemical com-position or calibration of other sensor devices. On the other hand, the sampling probes do introduce fluid dynamic perturbations to the flow and also suffer from relatively poor spatial resolutions. The analyzers are often bulky and heavy, and sampling requires care to preserve the content of the gas from the sampling point to the analyzer. In this chapter, we will begin by discussing the considerations for accurate physical sampling of gas using probes and go on to present various analysis methods for detection and monitoring of gas species concentrations. Figure 7.1 shows a general schematic of a gas-sampling probe connected to a gas analyzer. A pump draws the sample from the test section through the sampling probe and the analyzer. To optimize the sampling condition at the probe, pres-sures are monitored and controlled in the sampling line.
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