Color-forming reagents 4-

Experimental A photometric method was found in the literature which seemed to suit the particular circumstances. Two cyanide stock solutions were prepared, and an electromechanical dispenser was used to precisely prepare solutions of 20, 40,. .., 240 respectively 10, 30, 50,. .., 250 fig CN /100 ml. 10 ml of each calibration solution were added to 90 ml of the color-forming reagent solution and the absorbance was measured using 1-cm cuvettes. (See Table 4.17 (left and middle panels) and data file CYANIDE.dat.)... 

The total free chlorine in wastewaters as measured by colorimetric techniques constitutes both the dissolved molecular chlorine, hypochlorite ion, OCl, and hypochlorous acid. An equilibrium exists between these species, the concentrations of which depend on the temperature and pH of the waste-water. Concentration of the hypochlorous acid may be estimated from the K value or from the ratio (33% of the measured concentration of free chlorine). The free chlorine may be measured by amperometric titration after the addition of a phosphate buffer solution to produce a pH between 6.5 and 7.5. The sample is titrated against a standard solution of phenylarsine oxide. Alternatively, the syringaldazine (3,5-dimethoxy-4-hydroxybenzaldazine) colorimetric test may be performed. This color-forming reagent in 2-propanol yields a colored product with free chlorine, the absorbance of which may be... 

The blank described here contains all sources of absorbance other than analyte. An alternative blank for some analyses contains analyte but no color-forming reagent. The choice of blank depends on which species interfere at the analytical wavelength. 

In colorimetric analysis, a reagent is selected that would form a colored complex or derivative with the analyte. Often, the analyte is extracted from the aqueous sample into an organic solvent before adding color-forming reagent. Such extractions become necessary, especially for organic analytes such as phenols, lignin, and tannin. 

The first step in any colorimetric analysis is to prepare a standard calibration curve (i.e., a series of standard solutions of the analyte is made at a specific concentration range), which are then treated with the color-forming reagent, the absorbance or transmittance of which is then measured. The lowest calibration... 

Color forming reagent, Rhodamine B soln. should have a strength of 0.01% in 0.5 ATHC1. 

Some of these metal indicators are used as reagents in photometric or visual test kits for the detection of metals. Here, these molecules act like color-forming reagents but are still indicators. 

Extraction. A system has been devised which allows for performing successive liquid-liquid extractions followed by reagent addition and color formation. In one application the test substance in water, after being extracted into another solvent, is backwashed with color-forming reagent, and the solution is then passed into the colorimeter for reading and recording. 

The procedures described below comprise a representative sampling of test methods in which use of the principal types of color-forming reagents is illustrated. Prior to performing the color reaction, the lignified material should be extracted in a Soxhlet apparatus with ethanol-benzene (1 2 v/v) and then with water to remove extractives that otherwise might interfere with the test. 

The title, "The Determination of Phosphorus, for example, would interest the present author, but at once questions arise. In what kind of material is the phosphorus determined Is a separation of the element necessary If so, what kind of method is applicable Is a colorimetric method of measurement used If so, what is the color-forming reagent If it is ammonium molybdate, is the system measured molybdophosphoric acid, molybdovanadophosphoric acid, or a heteropoly blue If the latter, what are the reductant and the conditions of reduction To the extent that the title does not answer these and other similar questions, the summary should. 

If a title is self-explanatory and adequate for indexing purposes, there is no abstract or annotation in Chemical Abstracts. Such cases are rare for analytical methods. In general one wants to know what was done, how, and the results obtainable for given substances or conditions. The information desired may be much the same as that suggested for a satisfactory summary. For example, the method may be for small amounts of silica. The reader wants to know if the procedure is colorimetric. If so, was the colored species developed a heteropoly acid If so, what was the color-forming reagent Too often these and similar questions are not answered. 

Table 4.4. Reactions of color-forming reagents and metals. X denotes a positive reaction [41].

Absorbance detection has been applied to ion analysis through two different approaches direct detection of the sample ion and indirect detection. In some cases, a post-column, color-forming reagent can be added to the column eluate to detect sample ions. 

Figure 4.43. (a) Automated FIA manifold used for the determination of nitrate by reduction to nitrite with spectrophometric detection (cf. Fig. 4.5). The sample (30 p.L), aspirated from the sample changer 5, is injected into a 0.25 M acetate solution of pH 6.0 (A) and then carried to the reduction column, which is filled with small zinc chips. The nitrite produced is then mixed with the color-forming reagent solution R. (b) Long-term stability test of the nitrate reduction column used in the system shown in (a). A series of samples (1.0-10 ppm N-NO3), each injected in triplicate, were placed in the sample changer and the system was operated continuously for 6 h every day over a period of 3 days (sampling rate 180 samples/ h). Not until the third day did the column show signs of gradually reduced efficiency.

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