Chemical indicator

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Industrial sterilization cycles tend to vary considerably, not only from manufacturer to manufacturer, but often from product type to product type, depending on the bioburden present on a given load. Chemical indicators have historically been used only to differentiate between sterilized and nonsterilized packages. More recent developments have resulted in the availability of chemical dosimeters of sufficient accuracy to permit their appHcation either as total monitors or as critical detectors of specific parameters. 

Chemical indicators for dry-heat steriliza tion are available either in the form of pellets enclosed in glass ampuls, or in the form of paper strips containing a heat-sensitive ink. The former displays its end point by melting, the latter by a color change (see Chromogenic materials). 

Indicators can determine if uniform steam penetration has been achieved during a Bowie Dick-type test. Produced in the form of sheets (23 X 30 cm), chemical indicators are capable of uniform color change over their entire surface when exposed to pure saturated steam under test conditions. Nonuniform color development is an indication of failure of the test. U.S. and international stands for the performance and accuracy of chemical indicators have been pubHshed (13,14). 

Sterilisation of Health Care Products—Chemical Indicators, Part 1 General Requirements, AAMI 1114001-D, 1995-11-30, Association for the Advancement of Medical Instmmentation, Arlington, Va., 1995, proposed new American National Standard. 

Chemicals. Indicator solutions ( 0.1 per cent, aq.). Bromophenol blue Congo red phenol red. 

Empirical evidence seems also to support the preference for AG. Ingold based all of the structure-reactivity discussions in his famous book on AG values and believed that this choice is in accord with overall experience, at least as long as very small differences are disregarded (22). Values of AG usually correlate better with quantum chemical indices (199) and with empirical reactivity... 

I) Chemspor which is a combined chemical and biological indicator the ampoule contains a spore suspension in culture medium together with a second, smaller ampoule which contains a chemical indicator. 

Table 23.1 Examples of chemical indicators for monitoring sterilization processes...

Gaseous sterilization Ethylene oxide (EO) Reactive chemical Indicator paper impregnated with a reactive chemical which undergoes a distinct colour change on reaction with EO in the presence of heat and moisture. With some devices rate of colour development varies with temperature and EO concentration Gas concentration, temperature, time (selected devices) NB a minimum relative humidity (rh) is required for device to function... 

Fournier, R.O. and Truesdell, A.H. (1970) Chemical indicators of subsurface temperatures applied to hot spring waters of Yellowstone National Park, Wyoming, U.S.A. Geothermics, 2, 529-535. 

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

Table VIII shows a sensitivity analysis on the EXAMS model. Changing the input load dramatically changes the concentration of chemical in both water and sediment. Photolysis rates appear to effect the model less than input loads. Changing the soil type effects the purification time of the system and not so much the water concentrations of the chemical indicating the influence of chemical adsorption to degradation.

Rosen JF, Chesney RW, Hamstra AJ, et al. 1981. Reduction in 1,25-dihydroxyvitamin D in children with increased lead absorption. In Brown SS, Davis DS, eds. Organ-directed toxicity Chemical indices and mechanisms. New York, NY Pergamon Press, 91-95. 

In all the above processes, the optimised quantities of the chemicals indicated will be specific to the substrate quality evaluated. They would require further re-optimisation for each substrate to take account of the type and concentration of size, the presence of other impurities and the degree of natural yellowness. In particular, the amount of oxidant will need to be adjusted to give the optimum balance between oxidative desizing and the degree of bleaching required. 

Interaction with a chemical indicator Can be highly specific, if suitable indicator. Can measure total exposure over time (dosimetry), if a non-reversible reaction is used. Can allow operation at a convenient wavelength, when gas has no convenient absorption in that spectral range. Poisoning can occur, and is easily fouled. Sensitive to groups of chemicals, e.g. acid gases, rather than to a specific gas. May exhibit non-reversible behaviour, which, in many cases, may be undesirable. May need water vapour present, to act as a catalyst, if dry reaction is too slow. 

Evershed R.P., Heron C., Goad L.J., Epicuticular wax components preserved in potsherds as chemical indicators of leafy vegetables in ancient diets, Antiquity, 1991, 65, 540 544. 

Chemical ignition sources, 23 116-117 Chemical imidization, 20 271 Chemical indicator pH determination,... 

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