Colorimetric reaction

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

An enzymatic assay can also be used for detecting anatoxin-a(s). " This toxin inhibits acetylcholinesterase, which can be measured by a colorimetric reaction, i.e. reaction of the acetyl group, liberated enzymatically from acetylcholine, with dithiobisnitrobenzoic acid. The assay is performed in microtitre plates, and the presence of toxin detected by a reduction in absorbance at 410 nm when read in a plate reader in kinetic mode over a 5 minute period. The assay is not specific for anatoxin-a(s) since it responds to other acetylcholinesterase inhibitors, e.g. organophosphoriis pesticides, and would need to be followed by confirmatory tests for the cyanobacterial toxin. 

In view of the selective character of many colorimetric reactions, it is important to control the operational procedure so that the colour is specific for the component being determined. This may be achieved by isolating the substance by the ordinary methods of inorganic analysis double precipitation is frequently necessary to avoid errors due to occlusion and co-precipitation. Such methods of chemical separation may be tedious and lengthy and if minute quantities are under consideration, appreciable loss may occur owing to solubility, supersaturation, and peptisation effects. Use may be made of any of the following processes in order to render colour reactions specific and/or to separate the individual substances. 

Although widely used, colorimetric methods for determining total ur-onic acid and hexosamine are now regarded as providing, at best, only semi-quantitative estimates of heparin.42 Colorimetric reactions in a strong acid, such as the carbazole reaction for the uronic acids48 and the Morgan-Elson reaction for the hexosamines,49 are not specific for hepa-... 

DFDT has been subjected to the usual analysis for its halogen content, but no specific method has been worked out for its determination, nor has an investigation been made of its response to the known colorimetric reactions of DDT. 

Matrix effects in the analysis of nutrients in seawater are caused by differences in background electrolyte composition and concentration (salinity) between the standard solutions and samples. This effect causes several methodological difficulties. First, the effect of ionic strength on the kinetics of colorimetric reactions results in color intensity changes with matrix composition and electrolyte concentration. In practice, analytical sensitivity depends upon the actual sample matrix. This effect is most serious in silicate analysis using the molybdenum blue method. Second, matrix differences can also cause refractive index interference in automated continuous flow analysis, the most popular technique for routine nutrient measurement. To deal with these matrix effects, seawater of... 

The colorimetric reaction of urea nitrate with p-DMAC [91] was also adopted as a field test, UN-1. The reagent (0.4% in ethanolic solution) is sprayed on the sample, or on the suspected area, and the appearance of a red color within 1 min indicates the presence of urea nitrate [103]. 

Fig. 4.1. Colorimetric reaction converting protein nitrogen to the indophenol blue colour.

The section Analysis starts with elemental composition of the compound. Thus the composition of any compound can be determined from its elemental analysis, particularly the metal content. For practically all metal salts, atomic absorption and emission spectrophotometric methods are favored in this text for measuring metal content. Also, some other instrumental techniques such as x-ray fluorescence, x-ray diffraction, and neutron activation analyses are suggested. Many refractory substances and also a number of salts can be characterized nondestructively by x-ray methods. Anions can be measured in aqueous solutions by ion chromatography, ion-selective electrodes, titration, and colorimetric reactions. Water of crystallization can be measured by simple gravimetry or thermogravimetric analysis. 

Assay methods for Procaine have been reported which make use either of its direct ultraviolet absorption, or which are based on colorimetric reactions of the drug entity. 

Add 100 pL of AETS and incubate for 6 min at room temperature for visualization by colorimetric reaction. 

A spectroelectrochemical sensor has been developed for the bromide monitoring in industrial brine samples [154]. The bromide is oxidized to bromine and a sensitive colorimetric reaction with chloramine-T is employed for detection with a fiber-optic device. 

In view of its rapidity we found thin layer chromatography convenient for identification of the amino acids liberated by the first 20—30 degradation cycles. For identification of PTH-derivatives from additional degradation steps we prefer gas-liquid chromatography because of its merits mentioned above, particularly its greater sensitivity. Several colorimetric reactions and chromatographic systems are available for the identification of those PTH-amino acids which remain in the aqueous phase when the PTH-derivatives are extracted with ethyl acetate 23.24,25,13) our hands, thin layer electrophoresis was found to be satisfactory 26,27)... 

Another sensor system that further expands the capabilities of the Chronotox is the Personal Tape Sampler (PTS). The PTS (Figure 5) is a miniaturized gas monitor designed primarily to be worn by a worker during his entire workshift. It operates on the principle of specific chemical colorimetric reactions occurring on a dry chemically impregnated paper tape and gives... 

The PTS uses the principle of a specific colorimetric reaction, each toxic substance being detected by a reaction system specific to that gas. This principle provides the capability for extreme sensitivity down to less than one part per billion (10 ) while maintaining excellent specificity. Reactive chemicals are impregnated onto a continuous paper tape by a special process producing a dry reaction system. The tape is put into cassettes for convenient use and replacement in the Personal Tape Sampler (PTS). 

Certain medications may induce discoloration of the urine sample, which may interfere with the results. The colorimetric reaction may be affected by riboflavin or drugs containing azo dyes, or by nitrofurantoin. Other interferences may include ... 

Several gas detector tubes are used in conjunction with common colorimetric reactions to detect butadiene. The reactions include the reduction of chromate or dichromate to chromous ion and the reduction of ammonium molybdate and palladium sulfate to molybdemun blue (Saltzman Harman, 1989). 

Note In Ref 4 are given colorimetric reactions with aq K nitrate—sulfuric acid for acardite l color of ring bm-red violet and color afrer mixing grn-brn turning into yel-hrn) and for acardite ///(color of ring blue-gm, and color after mixing blue-gm turning into violet)... 

Ethanol and a long list of carbonyl compounds and aliphatic acids occur in fresh milk (Table 1.5). Some of them have been detected in only a few of the samples in which they were sought. Techniques for detecting such compounds include derivatization with 2,4-dinitrophe-nylhydrazine and various methods of volatilization, extraction, and chromatography (Harper and Huber 1956 Morr et al. 1957 Harper et al. 1961 Wong and Patton 1962 Scanlan et al. 1968 Marsili et al. 1981). The sum of the concentrations of acids listed in Table 1.5 is only 1-3 mmol/liter, compared to the citrate concentration of 10 mmol/liter. Oxalate has been reported to occur in milk (Zarembski and Hodgkin-son 1962) on the basis of a certain colorimetric reaction, but positive identification has not been made. 

If a large excess of neutral sugars is present or browning is observed, determine the amount or neutral sugars present (unitej.2) in the sample. Record the absorbance spectrum (400 to 700 nm) of the corresponding amount of neutral sugars and subtract it from the spectrum of the colorimetric reaction. 

Whether eluted from columns or from thin-layer plates, the quantitative determination of sugars was traditionally based on colorimetric reactions involving the use of chemical reagents, e.g., anthrone. These detection methods have been largely replaced in modem HPLC by the refractive index detector, although ultraviolet detectors are also employed. Recently we have also seen the introduction of other types of detector (e.g., the mass detector), as will be discussed later. 

The presence of siderophores in a medium may be shown by adding an iron(III) salt, as complex formation will be demonstrated by the colour of the Feni-siderophore complex, due to charge-transfer bands in the visible region. Chemical and spectroscopic tests allow ready classification into catecholate and hydroxamate types, for example the use of the Arnow and Czaky colorimetric reactions, respectively.1172... 

Post-column derivatization has been applied to the determination of trace amounts of cyclohexanone in cyclohexanone oxime, and consists of the initial separation of the ketone from the oxime by liquid chromatography, followed by colorimetric reaction with 2,4-dinitrophenylhydrazine [64]. The product is determined at 430 nm under basic conditions. 

Detailed chemical analysis of blood (for example, in hospital analyzers) is usually performed upon blood serum, the clear or yellowish liquid in which the red and white blood cells reside. (One obvious and important exception for which whole blood is required is the measurement of hematocrit, or fraction of blood volume occupied by red blood cells.) Removing the cells makes the tests more straightforward experimentally, both by reducing chemical interferents and by making colorimetric reactions easier to detect optically. Further filtering to remove large proteins can improve detection of minor constituents still more. 

Although best known for simple serial assays in homogeneous solution, such as colorimetric reactions, FI methods have also been developed that perform separations or utilize solid phases.33 34,42 73 -76 The use of solid-phase separation columns in FI or SI systems for radiochemical analysis gathered momentum in the 1990s. 

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