Color indicator application, determination

Prior to the development of the glass electrode and pH meter, the color indicator application found a wide use for the direct determination of hydrogen-ion concentration. William Mansfield Clark (De Lamur Professor Emeritus of Physiological Chemistry at Johns Hopkins University) spent most of his lifetime investigating organic substances suitable for acid-base indicators. He never accepted the pH meter. In his classic book Oxidation-Reduction Potential of Organic Systems he stated ( ) ... 

CDs also can be used as stabihzersfor color indicators to increase the stability of indicators used for the spectrophotometric determination of hydrogen peroxide in body fluids. In addition, CDs and their derivatives also have applications in enzyme assays and enzyme activity measurement. For example, glucosyl- or maltosyl-a-CD have been used to increase the accuracy and sensitivity of the assay of amylase. 

As in any titration, a suitable method of detection of the equivalence point is necessary. In general, it can be said that any of the techniques that are useful in classical volumetric titrimetry are also applicable to coulometric titrations. As noted above, Szebelledy and Somogyi used traditional color-change indicators to determine their end points. 

Quantitative and qualitative anthocyanin composition must be known in order to determine the feasibility of application of new plant materials as anthocyanin-based colorant sources and to understand the relationships of structures and functions of anthocyanins. In addition, anthocyanin compositions of fruits and vegetables have also been used to detect adulteration of anthocyanin-based products - and as indicators of product quality. - ... 

The second method for measuring pH, the optical indicator method, has more limited applications. The success ol this procedure depends on matching the color lhai is produced by the addition ol a suitable indicator dye to a portion of the unknown solution with the color produced by adding the same quantity of the same dye to a scries of standard solutions of known pH. The indicator dyes can also be immobilized onto paper strips ie.g.. litmus papert or, more recently, have been placed onto the distal olid of liber-optic probes which, when combined with photometric readout, provide more quantitative indicator-dye pH determinations... 

This same experimental approach can be used to determine the applicability of the aDAS—AP to a competitive assay for DAS. As shown in Figure 6, increasing amounts of free DAS were used to define the 50% inhibition level (ID50) of DAS for binding of two aDAS—AP conjugates to immobilized DAS. This approach was also used to determine the sensitivity of an ELA, as well as the specificity of the assay, as shown in Table 2. Increasing amounts of trichothecene mycotoxins closely related to DAS were added to microtiter plate wells containing a constant amount of prereacted DAS—aDAS—AP. After 30 min, excess toxin and any free toxin—aDAS—AP were washed out, and substrate was added. Quantification of the color produced was directly related to the ability of the added toxin to displace aDAS—AP from the immobilized DAS, which is an indication that the aDAS also has an avidity for that toxin. 

The Henderson-Hasselbalch equation is very useful in determining the pH at which an indicator changes color. For example, application of Equation (8.2) to the fCa expression for the general indicator HIn yields... 

Determination of the acid strength of solid catalysts using Hammett indicators, however, has been criticized frequently because of the heterogeneity of the solid surface [81, 104, 110, 114—116]. The principle of the Hammett acidity function is based on the equilibrium equation in a homogeneous solution, and its application to the heterogeneous condition is subject to severe criticism. In addition, the color change of the adsorbed indicators on solids as determined by the naked eye is subjective. The effects of interactions between the solvent and the solid surface has also been raised [9]. 

The interest in the application of indicators in oxidation-reduction titrations has followed on the discovery that the familiar color change undergone by diphenylamine on oxidation could be used to determine the end-point of the titration of ferrous ion by dichromate in acid solution. Diphenylamine, preferably in the form of its soluble sulfonic acid, at first undergoes irreversible oxidation to diphenylbenzidine, and it is this substance, with its oxidation product diphenylamine violet, that constitutes the real indicator. ... 

Some materials are strongly colored or discolor the skin at the application site. This makes the traditional scoring process difficult or impossible. One approach is to try to remove the pigmentation with a solvent if successful, the erythema can then be evaluated. If use of a solvent fails or is unacceptable, another possibility is to (wearing thin latex gloves) feel the skin to determine if there is warmth, swelling, and/or rigidity - all secondary indicators of the irritation response. 

Micro determination of pH. The same principle applies to the determination of the pH in small quantities of liquid except that capillary tubes are used instead of the usual test tubes. F. Yuks has described a microcolorimeter designed for the measurement. L. D. Felton adds several drops of indicator to several drops of unknown solution on a porcelain spotplate and compares the color so obtained with the color of buffer mixtures similarly treated. Great care must be exercised in the application of the micro determination, especially when the solution being studied is poorly buffered. Large errors may be introduced because the ratio of indicator concentration to buffer... 

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