Colorimetric indicator tubes

Direct reading samplers include simple devices such as colorimetric indicating tubes in which a color change indicates the presence of the contaminant in air passed through the tube, or instruments which are more or less specific for a particular substance. In the latter category are carbon monoxide indicators, combustible gas indicators (explosimeters) and mercury vapor meters, as well as a number of other instruments. 

Since no device is completely specific for the substances of interest, care must be taken that interferences do not invalidate the sampling results. Many common gases and vapors react with the same chemicals, or have similar physical properties, so that the instrument may give falsely high or low readings for the substance being sampled. The manufacturer s data for colorimetric indicating tubes lists those substances which may interfere with the desired determination. 

Colorimetric indicator tubes required 5. Flammable vapor detection apparatus ... 

Lungs, brain, blood Gases (carbon monoxide, cyanide) Direct-reading colorimetric indicator tubes Head-space Microdiffusion GC... 

Dichromate test for ethanol, aldehydes, and ketones colorimetric indicator tube (e.g Alcotest) Fujiwaratest fortibhloro-com pounds... 

Direct Reading Colorimetric Indicator Tubes Manual, Akron, OH, American Industrial Hygiene Assoc. 1976... 

Carbon Monoxide Testers — Carbon monoxide is one of the most commonly encountered toxic gases and may result from many industrial processes as well as from automobile or truck exhaust. Since pure CO is colorless, odorless, and tasteless, the senses cannot be relied on to give warning of its presence. Carbon monoxide may be sampled in several ways, including the use of the colorimetric indicating tubes, but for making repeated measurements over a period of time, a direct reading meter is trequently used. Such a meter is similar to a combustible... 

H2S exposure may be monitored by diffusion type colorimetric dosimeters (such as Vapor Gard) color changes from white to brown-black dosage exposure measured from the length of stain in the indicator tube. 

Relatively inexpensive portable colorimetric indicators are now commonly used to detect the presence of a variety of contaminant gases in the atmosphere. These are normally specific for one or perhaps a series of gases. For example, the NBS colorimetric detector uses an indicator tube that contains a 15-mm section of yellow palladous silico-molybdate gel the gel changes color when exposed to carbon monoxide at concentrations as low as 0.001 vol. % in air. Figure 52 shows one version of a commercial unit used for carbon monoxide as well as carbon dioxide, the unsaturated hydrocarbons, and a variety of other gases this unit can also be used to determine the approximate concentrations of contaminants in liquid oxygen. Precise determinations can be made with a variety of analytical tools, including infrared... 

Colorimetric testing tubes may be used if a specific chemical agent is suspected (Figure 2.17). These tubes with their indicator will usually reveal the nature of the chemical agent and its level. It should be remembered that cross-reactions are common among chemical agents measured in this manner (see Practical Skills 5 for use of these detection devices). 

As in the case of any flammable gas, never use a flame in trying to detect carbon monoxide leaks. Portable detection equipment or the formation of bubbles by a soapy solution applied to a suspected area will indicate leaks. Carbon monoxide alarm detectors must be installed in all indoor areas in which the gas is regularly used in more than small laboratory amounts. Colorimetric sniffer tubes are also utilized for detection of carbon monoxide. Samples of atmospheric air containing carbon monoxide are aspirated through such tubes. 

Indicator (Colorimetric) Tubes — The use of solid chemical detectors (indicating tubes) is common practice. They are simple devices to operate which tends to cause many users to ignore their limitations which must be recognized if they are to be useful in evaluating potential hazards due to air contaminants. This apparent simplicity increases the number of people who attempt to use them, which in turn results in wide variations in both individual competence and the accuracy of the data obtained. 

Detector Tubes/Pumps Detector tube pumps are portable equipment which, when used with a variety of commercially available detector tubes, are capable of measuring the concentrations of a wide variety of compounds in industrial atmospheres. Operation consists of using the pump to draw a known volume of air through a detector tube designed to measure the concentration of the substance of interest. The concentration is determined by a colorimetric change of an indicator which is present in the tube contents. 

Colorimetric studies in sealed glass tubes indicate that ammonia combines with water, above the critical temperature of NH3, to form ammonium hydroxide. The curves of Fig. 8-1 show that the pH of 28 per cent aqueous ammonia decreases, however, with rising temperature, from 11.8 at room temperature to only 8.2 at 175°C, under approximately 500 psi pressure. The pH drops off more rapidly when hydrogen chloride or its ammonium salt is introduced, corresponding to conditions prevailing during ammonolysis. At 175°C, the pH of neutrality is 5.7. 

Tubes for colorimetric systems are available that are capable of qualitatively identifying nerve agents, blister agents, vomiting agents. Tubes are also available that can identify as well as provide semi-quantitative indication of vapor concentration for blood and choking agents. Many of these tubes have cross sensitivities and care must be taken to correctly interpret the results. 

An aliquot (20 ml) of a sample is added in the glass digestion tube, followed by the addition of 0.5 g of mercuric sulfate, 10 ml of dichromate solution, and 5 ml of sulfuric acid. The tube is placed into the microwave and connected to a condenser. An additional volume (25 ml) of sulfuric acid is added to the top of the condenser. The mixture is digested in the microwave at 150°C for 8 min. Excess dichromate in the digestion tube is determined with the standard titrimetric or colorimetric procedure. COD determination of real water and wastewater samples indicates that the results obtained from both microwave digestion and closed reflux digestion are consistent. 

Colorimetric or colour indicator (detector) tubes should be used as in the manufacturer s directions and thrown out on the use-by date. Some tubes give a colour with another chemical (called cross sensitivity ), so the manufacturer s notes should be checked. A new type as noted earher still uses detector chemicals but measures changes electronically, not by colour. 

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