Reliability color determinations

To obtain reliable, accurate, and reproducible methods for quantitative estimation of deoxy sugars, certain conditions must be fulfilled. Thus, it is necessary that the chromogen be formed quantitatively from the sugar. The chromogen must then react quantitatively with the compound used for color formation, and lastly, the dye, once formed, should be stable and have a well defined molar extinction coefficient. In methods in which all of these conditions are not or cannot be fulfilled, recourse must be had to simultaneous determinations with suitable standard substances, a requirement not always easy to fulfil. 

A simple, reliable, and fast method of determining the pH of a solution and of monitoring a titration is with a pH meter, which uses a special electrode to measure H 0+ concentration. An automatic titrator monitors the pH of the analyte solution continuously. It detects the stoichiometric point by responding to the characteristic rapid change in pH (Fig. 11.9). Another common technique is to use an indicator to detect the stoichiometric point. An acid-base indicator is a water-soluble organic dye with a color that depends on the pH. The sudden change in pH... 

Food colorants are analyzed either by direct inspection (sensorial analyses) or by physical or physicochemical instrumental methods. Direct inspections determine the sensorial attribute of color, frequently combined with assessments of smells and flavors. Visual color assessment is subjective and may be used with reliable visual evaluations controlling multiple variables. 

There are several approaches to establish the DL. Visual evaluation may be used for noninstrumental (e.g., solution color) and instrumental methods. In this case, the DL is determined by the analysis of a series of samples with known concentrations and establishing the minimum level at which the analyte can be reliably detected. Presentation of relevant chromatograms or other relevant data is sufficient for justification of the DL. 

Nowadays, thermo-optical methods are considered the most reliable measurement techniques for OC/EC split in atmospheric aerosols. Nevertheless, methods for TC/EC/BC analysis in atmospheric particles are still open to debate and their different analytical approaches have been the main cause for performing intercomparison studies (Schmid et al., 2001 ten Brink et al., 2004). The TC measurements showed good agreement, whereas the results of EC/BC determinations were highly variable due to EC overestimation by thermal methods. Furthermore, caution must be taken when using BC as an estimative of the EC content in aerosols, and vice versa BC and EC measurements are associated to the carbon content of colored and refractory organic compounds, respectively, which can lead to substantially different results between methods (Poschl, 2005). 

Assay The specific gravity of a sample, determined by any reliable method (see General Provisions), is not greater than 0.8096 at 25725° (equivalent to 0.8161 at 15.56715.56°). Acidity Transfer 10 mL of sample to a glass-stoppered flask containing 25 mL of water, add 0.5 mL of phenolphthalein TS, and then add 0.02 N sodium hydroxide to the first appearance of a pink color that persists after shaking for 30 s. Add 25 mL of sample, mix, and titrate with 0.02 N sodium hydroxide until the pink color is restored. Not more than 0.5 mL of 0.02 N sodium hydroxide is required to restore the pink color. 

There are many available methods for mixing colorants into plastic materials. Choosing a laboratory method that will most closely match the production process is ideal, but not usually the norm. Colorant dispersions in plastic materials determine the perceived color. Thus, choosing a consistent, reliable mixing method that closely mimics production dispersions is critical to a good visual color match. 

A color technician tests 3-10 batch samples on each piece of equipment two to three times each. The AV now becomes appraisal reproducibility for each EV piece of plastics dispersion equipment. This technique is also useful for comparing supplier Certificate of analysis (COA) data against your own incoming quality control (QC) testing to determine reliability between the two sets of data. To accomplish this, you must request duplicate sets of color data for each lot over 3-10 consecutive lots. 

In the colorimetric determination of pH we always make use of a comparison solution of known acidity. The accuracy of such a comparison method evidently is limited by the reliability of the comparison solution. As has been mentioned in the preceding chapter, the pH of the comparison or buffer solution is measured by means of the hydrogen electrode. Hence the colorimetric procedure is based ultimately upon the potentiometric method. It is the hydrogen ion activity rather than the con-centration which is determined by the latter method. Thus when it is stated that an unknown and a buffer solution have the same pH on the basis of equal color intensities, it is really the hydrogen ion activity which is implied. 

Thymolphthalein is not especially suitable for the determination of pH. The acid form is insoluble in water and fading of the color is nearly always observed. When 0.1 c.c. of a 0.1% indicator solution is added to 10 c.c. of a carbonate buffer of pH 10, there appears a light blue color which fades rapidly on standing, due to the flocculation of the acid form. Indicator must be added simultaneously to the buffer and unknown solution and the colors compared immediately if thymolphthalein is used. Even so, results are not always reliable. 

Obviously colorimetric determinations of pH in colored or turbid solutions are not very reliable. The color of the indicator should never be the same as the color of the liquid. For example, one should employ phenolsulfonephthalein for yellow solutions, and not p-nitrophenol. 

Directions for measuring the pH of alcoholic solutions. These determinations are as reliable as the procedure described by Michaelis and Gyemant for aqueous solutions. The following precaution, however, must be observed. The alcoholic comparison solution must have the same alcohol content as the solution under examination. The alkalinity necessary to permit the maximum color intensity must always be produced by addition of sodium hydroxide. The hydroxide concentration best suited for alcohol concentrations from 0 to 70% is 0.01 N, and for higher alcohol concentrations, 0.1 N. The pH calculation is made just as for aqueous solutions with the difference that the pK value for the particular alcohol concentration must be employed. A special table is available for phenolphthalein. 

The writer also has investigated the reliability of the determination of pH by means of indicator papers. These results will be summarized briefly. Contrary to the directions of Haas, the drop should not be permitted to dry. The color of the dried paper is not so distinct, and small differences may be overlooked. Instead of a glass rod, it is better to use a capillary tube in transporting the liquid to the paper. By so doing, 10-20 mm. will be sufficient for a determination. Hardened paper is best as a rule. Schleicher and SchOll for capillary analysis filter paper is also satisfactory. 

---