Gas analysis
The gas analysis is a branch of analytical chemistry . With the help of chemical and physical methods, the qualitative and quantitative composition of gases and gas mixtures is investigated. Simple preliminary examinations can be carried out using special reagents in test tube experiments as well as with test papers.
Simple qualitative evidence
Acid and basic gases
Acidic or basic reacting gases like hydrogen chloride , sulfur dioxide , ammonia or volatile amines change the pH value of aqueous solutions. This can be checked with color indicators . To do this, the gas to be tested is passed through the indicator solution or a moistened indicator paper is used.
Precipitation reactions
Lead acetate solution is used for the detection of hydrogen sulfide , with black lead sulfide precipitating. Moist lead acetate paper can also be used for this. Carbon dioxide can be detected with lime water or barium hydroxide solution . The white alkaline earth carbonates precipitate. With barium hydroxide solution, the effect is more pronounced because the substance dissolves much better in water than calcium hydroxide. Acetylene can be detected with a silver nitrate solution. Gray silver acetylide precipitates .
Semi-quantitative evidence
A gas can be examined semi-quantitatively with test tubes which are filled with a carrier substance and a reagent. To do this, a precisely defined amount of gas is pumped through the test tube. A visible reaction front strides through the filling of the tube. The approximate analyte content is read off using a (usually logarithmic) scale (for example in ppm ). The traditional alcohol test tubes also belong in this category. They are equipped with potassium dichromate / sulfuric acid were charged and allow yes / no statement as to whether the blood alcohol limit of blood alcohol exceeded, because the alcohol content of the breath depends on the blood alcohol concentration. During the reaction in the test tube, the alcohol reduces the orange dichromate to deep green chromium (III).
There is a wide range of detector tubes for almost all practically relevant gases and vapors. They are mainly used by fire brigades and hazardous substances services to obtain quick information about dangerous gases in the ambient air.
Quantitative determination of gaseous components
The gaseous analytes are mostly quantitatively absorbed by liquids, bound to adsorbents (such as activated carbon ) or frozen in cold traps . Then, for example, the liquids are examined further. The classic methods here include the investigation of gas mixtures with the gas burette , gravimetry and volumetry . Today, many gas analyzes can also be performed with automated instruments. Many of these methods play a central role in organic elemental analysis. Gas chromatography is ideal for gas mixtures with a more complex composition, especially in the trace range . Other methods that are often used in industrial processes are IR spectroscopy , mass spectrometry and thermal conductivity measurements .
carbon dioxide
Carbon dioxide can be determined by passing the gas through a strong solution of potassium hydroxide . The container is weighed before and after. The weight difference corresponds to the amount of carbon dioxide absorbed. A suitable device was invented by Justus von Liebig : the five-ball device .
oxygen
Elemental oxygen is reactive, but it only reacts spontaneously with a few substances. It is eagerly and quantitatively absorbed by a strongly basic pyrogallol solution .
Hydrogen halide
The hydrogen halides can be absorbed in a lye. It is then neutralized and titrated with silver nitrate . With potentiometric indication, chloride , bromide and iodide can be determined side by side. The same is possible with ion chromatography .
See also
literature
- K. Cammann: Instrumental Analytical Chemistry . Spectrum Academic Publishing House, Heidelberg-Berlin, 2001.
Individual evidence
- ↑ Gas analysis. www.infratec.de, accessed on April 30, 2018 .
- ↑ Determine concentration using thermal conductivity. In: Messkonzept GmbH. Retrieved October 11, 2019 .