Colorimetric technique

The most common colorimetric technique involves a reaction between ammonia and a reagent containing mercuric iodide in potassium iodide (Messier reagent) to form a reddish-brown complex. Turbidity, color, and hardness are possible interferences that can be removed by preliminary distiHation at pH 9.5. 

Pretreatment of the collected particulate matter may be required for chemical analysis. Pretreatment generally involves extraction of the particulate matter into a liquid. The solution may be further treated to transform the material into a form suitable for analysis. Trace metals may be determined by atomic absorption spectroscopy (AA), emission spectroscopy, polarogra-phy, and anodic stripping voltammetry. Analysis of anions is possible by colorimetric techniques and ion chromatography. Sulfate (S04 ), sulfite (SO-, ), nitrate (NO3 ), chloride Cl ), and fluoride (F ) may be determined by ion chromatography (15). 

Figure 14-9 also shows a flowchart for analysis of wet and dry precipitation. The process involves weight determinations, followed by pH and conductivity measurements, and finally chemical analysis for anions and cations. The pH measurements are made with a well-calibrated pH meter, with extreme care taken to avoid contaminating the sample. The metal ions Ca, Mg, Na, and are determined by flame photometry, which involves absorption of radiation by metal ions in a hot flame. Ammorda and the anions Cl, S04 , NO3 , and P04 are measured by automated colorimetric techniques. 

The analysis of free nitrogen in gas mixts can usually be calcd by difference after detg the concn of the other gaseous ingredients. The presence of free N2 can be detected by spectro-graphic (Refs 55 60) or colorimetric techniques (See Vol 1, A114-R Refs 9 17). 

The particles were then separated by centrifugation and the supernatant was analyzed for PVA using the colorimetric technique described previously (15). 

Eisenbrand and Preussman [21] have described a colorimetric technique in which nitrosamines are cleaved to nitrosyl bromide and secondary amines, and the liberated NO+ ion is measured colorimetrically after reacting with N-l-naphthalenyl-1,2-ethanediamine. 

Even if few systems are proposed for inorganic compounds (with regard to the number of potential pollutants), instruments or sensors for parameters used for treatment process control are available UV systems for residual chlorine in deodorization, electrochemical sensors for dissolved oxygen (with nowadays a luminescent dissolved-oxygen probe utilizing a sensor coated with a luminescent material) and a colorimetric technique for residual ozone. 

Colorimetric techniques, in platinum-group metal analysis, 19 618 Colorimetry, 7 311-319 of ascorbic acid, 25 760 color difference measurement, 7 319-323 in fine art examination/conservation, 11 400... 

Ion Chromatography has also been utilized for the determination of azide in water effluents and in particulates generated from the deployment of an air bag system. In a recent study (12) particulate material was collected and leached with deionized water. The solutions were then analyzed by Ion Chromatography for anions. A typical chromatogram showing acetate, chloride, nitrite, azide and sulfate is shown in Figure 6. The concentration of azide was also analyzed using a colorimetric technique. Results for equivalent samples correlated to within 5% (12). 

Elemental composition H 11.83%, N 41.11%, S 47.05%. It may be analyzed by measuring its decomposition gaseous products, ammonia and hydrogen sulfide, either by gas chromatography using an FID or a TCD or by selective ion electrode or colorimetric techniques. 

Chlorine gas may be identified readdy by its distinctive color and odor. Its odor is perceptible at 3 ppm concentration in air. Chlorine may be measured in water at low ppm by various titrimetry or colorimetric techniques (APHA, AWWA and WEF. 1999. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington DC American Pubhc Health Association). In iodometric titrations aqueous samples are acidified with acetic acid followed by addition of potassium iodide. Dissolved chlorine liberates iodine which is titrated with a standard solution of sodium thiosulfate using starch indicator. At the endpoint of titration, the blue color of the starch solution disappears. Alternatively, a standardized solution of a reducing agent, such as thiosulfate or phenylarsine oxide, is added in excess to chlorinated water and the unreacted reductant is then back titrated against a standard solution of iodine or potassium iodate. In amperometric titration, which has a lower detection limit, the free chlorine is titrated against phenyl arsine oxide at a pH between 6.5 and 7.5. 

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... 

For a number of years, phenolic substances were dosed by colorimetric techniques, based on redox reactions usually known as Folin Ciocalteau methods, even if a number of adjustments were developed to fit different matrix characteristics. The Folin Cioalteau reagent is a mixture of phosphomolybdic and phosphotingstic acids, with molybdenum in the 6+ oxidation state and, when the reaction takes place, it is reduced to form a complex called molybdenum blue and tungsten blue. In this complex, the mean oxidation state is between 5 and 6 and the formed complex is blue so it can be read spectrophotometrically at 750 nm. 

Colorimetry. A variety of colorimetric techniques have been used to measure ions such as NH4, SO4-, and NO7 in ambient particles. For example, nitrate can be measured by reduction to nitrite using hydrazine in the presence of a copper catalyst, followed by its conversion to a colored azo dye, which can be measured by its absorbance at 524 nm (Mullin and Riley, 1955). Sulfate has been determined using an exchange reaction between sulfate and a barium-nitrosulfo-nazo(III) chelate in aqueous acetonitrile the chelate has an absorbance peak at 642 nm and hence the decrease in this peak can be followed as a measure of the amount of sulfate present that has exchanged with the chelate (Hoffer et al., 1979). Similarly, NH4 can be measured by the indophenol blue method (Weather-burn, 1967). 

Although the radiolabeled nucleic acid precursor assays described above produce accurate and reproducible quantification of the number of viable cells in a sample, Mosmann [187] sought to develop a more rapid assay capable of handling large numbers of samples. A colorimetric technique was developed based upon the tetrazolium salt, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetra-zolium bromide, or MTT. Early studies of MTT by Slater et al. [188] determined that when the MTT tetrazolium salt interacts with the dehydrogenase enzymes... 

Below we report methodological studies based upon HPLC, GC/FID, GC-MS, LC-MS, matrix-assisted laser desorption ionisation coupled with time-of-flight mass spectrometry (MALDI-ToF/MS), CE, proton nuclear magnetic resonance ( I INMR), RIA and enzymatic colorimetric techniques. 

Measurements of alkali solubles in these coal samples—conventionally accepted as indices of humic acid concentrations—were initially performed by using Kreulen s method (7). However, even when the most stringent precautions were taken to exclude air, this method yielded markedly time-dependent results (presumably owing to oxidation of the coal by the relatively strong alkali solution), and a more satisfactory colorimetric technique (by J. F. Fryer) was therefore employed. This entailed extracting the coal sample with 0.1 N aqueous sodium hydroxide for 16-20 hours in an inert atmosphere and subsequent photoelectric scanning of the extract solutions. Actual humic acid concentrations were then obtained from specially constructed reference curves which related optical density (at an appropriate wavelength) to humic acid contents. The inherent error in this determination is estimated at less than 10%. 

Colorimetric methods are most common and widely employed in environmental wet analysis. Most anions, all metals, and many physical and aggregate properties can be determined by colorimetric technique, which is fast and cost-effective. The method may, however, be unreliable for dirty and colored samples. Often, the presence of certain substances in samples can interfere with the test. In addition, if the color formation involves a weak color such as yellow, additional confirmatory tests should be performed. Despite these drawbacks, colorimetry is often the method of choice for a number of wet analyses. 

Methods 1 and 2 are colorimetric techniques based on the reaction between fluoride and a dye. Methods 3 and 4 are discussed in Chapters 1.9 and 1.11, respectively. 

Many anionic surfactants can react with a cationic dye such as methylene blue to form strong ion pairs that can be extracted by a suitable organic solvent and can be determined using colorimetric techniques. The anionic surfactants that respond to the methylene blue test are primarily the sulfonate (RS03 Na+) and the sulfate ester (R0S03 Na+) type substances. On the other hand, soaps and the alkali salts of fatty acids (C-10 to C-20) used in certain detergents do not respond to the above test. The various anionic surfactants and their characteristic structural features are presented in Figure 2.32.1. 

Ethylene oxide in air may be measured directly in situ by a rapid colorimetric technique (Pritts et al., 1982). Air is drawn through a multipart detector tube consisting of three reactor tubes containing periodic acid, xylene, and cone. E12S04, respectively. Ethylene oxide is oxidized by periodic acid to formaldehyde... 

It is important to appreciate that the colorants in products other than commercial sugars have been successfully defined by the new colorimetric techniques. Application to vegetable oils was made by McNicholas,36 who showed existence of a spread of the chromaticity points over an appreciable area instead of along a single line, but there was a sufficient degree of correlation for the method to be useful. The grading of lubricating oils has also been successfully performed, as shown by a study by Judd, Plaza and Belknap.39... 

Table 6.1. Comparison of the high pressure (HP) and sealed pan differential scanning calorimetric techniques (SP-DSC/ HP-DSC) and the colorimetric technique for oil change requirements (Kauffman and Rhine, 1988a)...

J. S. Fleitman, I. W. Partridge, and D. A. Neu, Thimerosal analysis in ketorolac tromethamine ophthalmic solutions comparing HPLC and colorimetric techniques, Drug Dev. Ind. Pharm., 17 519(1991). 

In the present experiment, we use a colorimetric technique to analyze the S02 content of raisins. 

Classical potentiometric titrations have a major weakness—their inability to characterize weak acids with a pKa less than 3 and greater than 10. This is because water in those pH regions is capable of acting as weak acid or weak base, respectively (Fig. 1.9). In such cases colorimetric techniques are more suitable. 

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