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Dräger sampling tubes and systems - 20351 A

2.2. Chemical Basis – Reaction Mechanisms

Since chlorinated hydrocarbons are not indicated by direct colorimetric reaction, an oxidative cleavage of the molecule is necessary as a first step. This reaction is either done with potassium permanganate or chromium (VI) compounds, which forms elementary chlorine. The chlorine then reacts with the reagent preparation in the indicating layer to produce the colorimetric reaction product.The measurement of carbon dioxide is done by oxidation of hydrazine hydrate in the presence of crystal violet as an oxidation-reduction (redox) reaction:CO
2 + N
2 H
4 NH
2 –NH–COOHTypically carbon dioxide will be present at a substantially higher concentration than anypotentially cross sensitive substances, therefore this reaction is very selective. Possibleinterferences by hydrogen sulphide and sulfur dioxide are not expected since these interferences can only occur with unusually high concentrations. Another large group of Dräger-Tube reactions is based on pH indicators, for example: NH
3 + bromophenol blue blue reaction productThis type of reaction is valid for basic as well as acid gases.Compounds containing the C N-group are measured using multiple stage reactions. In thecase of acrylonitrile, the first step is a reaction with a chromium (VI) compound. In the nextstep the cyanide ion reacts with mercury chloride to form hydrochloric acid and undissociated mercury cyanide. The hydrochloric acid is indicated in the last partial step ofthis complex reaction system by means of a pH indicator. Suitable prelayers are used toensure a selective measurement. A similar reaction principle is also used in the most sensitive hydrogen phosphide (i. e. phosphine) tubes, Phosphine 0.01/a. The hydrogenphosphide also reacts with mercury chloride, but in this case yields mercury phosphide andhydrochloric acid. Again, the hydrochloric acid is indicated by means of pH-indicator.Most hydrides of the elements from group III or V of the periodic table (e.g. borane orarsine), react because of their reducing characteristics with gold salts by forming elemental gold.Aromatics condense under strongly acidic conditions with formaldehyde to form intenselycolored quinoid compounds with different molecular structures. Each of these reaction