Colorimetric Determinations

M.p. 296 C. Accepts an electron from suitable donors forming a radical anion. Used for colorimetric determination of free radical precursors, replacement of Mn02 in aluminium solid electrolytic capacitors, construction of heat-sensitive resistors and ion-specific electrodes and for inducing radical polymerizations. The charge transfer complexes it forms with certain donors behave electrically like metals with anisotropic conductivity. Like tetracyanoethylene it belongs to a class of compounds called rr-acids. tetracyclines An important group of antibiotics isolated from Streptomyces spp., having structures based on a naphthacene skeleton. Tetracycline, the parent compound, has the structure ... 

E. B. SandeU, Colorimetric determination of Trace Metals, 3rd ed.. Interscience PubUshers, New York, 1956, p. 976. 

Z. Marc2enko, Separation and Spectrophotometric Determination of Elements, Ellis Horwood Ltd., West Sussex, UK, 1986 E. B. SandeU, Colorimetric Determination of Traces of Metals, Interscience Publishers, New York, 1959. 

By means of colorimetric determination in the laboratory you measured the concentration of FeSCN+i, which we shall designate [FeSCN+2], in solutions containing ferric and thiocyanate ions, Fe+3 and SCN. The reaction is... 

Table 10.1 contains a list of indicators suitable for titrimetric analysis and for the colorimetric determination of pH. The colour-change intervals of most of the various indicators listed in the table are represented graphically in Fig. 10.1. 

SOME GENERAL REMARKS UPON COLORIMETRIC DETERMINATIONS... 

Visual methods have been virtually displaced for most determinations by methods depending upon the use of photoelectric cells (filter photometers or absorptiometers, and spectrophotometers), thus leading to reduction of the experimental errors of colorimetric determinations. The so-called photoelectric colorimeter is a comparatively inexpensive instrument, and should be available in every laboratory. The use of spectrophotometers has enabled determinations to be extended into the ultraviolet region of the spectrum, whilst the use of chart recorders means that the analyst is not limited to working at a single fixed wavelength. 

In any colorimetric determination the exact procedure will be governed partly by the particular instrument to be employed and partly by the nature of the substance which is being determined. There are nevertheless certain general principles which are universally applicable and these are also relevant to spectrophotometric determinations in general. 

A large number of reagents for metals are available and in some instances a reagent will produce a reaction suitable for colorimetric determination with several metals. The table reproduced in Appendix 2 gives a clearer idea of the wide range of reagents available. 

Discussion. J. Nessler in 1856 first proposed an alkaline solution of mercury(II) iodide in potassium iodide as a reagent for the colorimetric determination of ammonia. Various modifications of the reagent have since been made. When Nessler s reagent is added to a dilute ammonium salt solution, the liberated ammonia reacts with the reagent fairly rapidly but not instantaneously to form an orange-brown product, which remains in colloidal solution, but flocculates on long standing. The colorimetric comparison must be made before flocculation occurs. 

Discussion. An excellent method for the colorimetric determination of minute amounts of cobalt is based upon the soluble red complex salt formed when cobalt ions react with an aqueous solution of nitroso-R-salt (sodium 1-nitroso-2-hydroxynaphthalene-3,6-disulphonate). Three moles of the reagent combine with 1 mole of cobalt. 

Toluene-3,4-dithiol, usually called dithioP, yields a slightly soluble, dark-green complex, (CH3.C6H3.S2)3Mo(VI), with molybdenum(VI) in a mineral acid medium, which can be extracted by organic solvents. The resulting green solution is used for the colorimetric determination of molybdenum. 

Two procedures are commonly employed for the colorimetric determination of chloride. 

Discussion. Small quantities of dissolved silicic acid react with a solution of a molybdate in an acid medium to give an intense yellow coloration, due probably to the complex molybdosilicic acid H4[SiMo12O40]. The latter may be employed as a basis for the colorimetric determination of silicate (absorbance measurements at 400 nm). It is usually better to reduce the complex acid to molybdenum blue (the composition is uncertain) a solution of a mixture of l-amino-2-naphthol-4-sulphonic acid and sodium hydrogensulphite solution is a satisfactory reducing agent. 

Discussion. The barium salt of chloranilic acid (2,5-dichloro-3,6-dihydroxy-p-benzoquinone) illustrates the principle of a method which may find wide application in the colorimetric determination of various anions. In the reaction... 

Discussion. The formation of coloured compounds by coupling phenols with diazotised primary aromatic amines has long been recognised as a method of determining phenols, and procedures have been evolved whereby the phenol solution is titrated with a diazonium solution which has been calibrated against known concentrations of the phenol. The resultant reaction products are coloured, but many are only sparingly soluble in water and organic solvents and do not therefore lend themselves to colorimetric determination. 

Some general remarks upon colorimetric determinations 672... 

Vol. 8. Colorimetric Determination of Nonmetals. Second Edition. Edited by the late David F. Boltz and James A. Howell... 

Colorimetric procedures used In steroid assays are often subject to drug Interference. In the determination of 17-Ketosterolds by the Zimmerman reaction, drugs with the 17-Keto basic structure such as ascorbic acid, morphine and reserplne will cause Increased values. In the determination of 17,21 -dlhydroxysterolds by the Porter-Sllber reaction the dlhydroxy-acetone chain Is the reactive unit. Drugs like meprobamate, chloral hydrate, chloropromazlne and potassium Iodide will Interfere with this reaction and cause elevated values. In the colorimetric determination of vanlllylmandellc acid (VMA) by a dlazo reaction, drugs like methocarbamol and methyl dopa cause... 

Pyridylazo-ligands (Scheme 2) have widely been used in colorimetric analyses of various metal ions. For example, l-(2-pyridylazo)-2-naphthol (Hpan) is one of the most well-known reagents for the colorimetric determination and complexometric titration of... 

Colorimetric Determination of Small Quantities of 1,1,1-Trichloro-2,2-bis(p-methoxyphenyl)-ethane... 

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