Triphenylmethane, titration indicator

Juaristi, E. Martinez-Richa, A. Garcia-Rivera, A. Cruz-Sanchez, J. S. Use of 4-biphenylyl-mefhanol, 4-biphenylylacetic acid, and 4-biphenylcarboxylic add/triphenylmethane as indicators in the titration of lithium alkyls. Study of the dianion of 4-biphenylylmethanol. /. Org. Chem. 1983, 48, 2603-2606. 

The use of the dimsyl anion2 (Corey and Chaykovsky, 1962) to titrate weak acids (using triphenylmethane as indicator) was first... 

Roberts and Whiting developed a method for effecting the anhydrous hydrolysis of esters and nitriles in DMSO. A solution of sodium methylsulfinylmethide is prepared from sodium hydride and DMSO and titrated with a solution of an appropriate amount of water in DMSO, using triphenylmethane as indicator. This produces a fine suspension of sodium hydroxide which hydrolyzes ethyl benzoate very rapidly at room temperature. As compared with reactions in hydroxylic solvents, rates are enhanced by a factor of 10 -10 . Benzonitrile is hydrolyzed to benzamide. Methyl and ethyl mesitoates are hydrolyzed readily at 25°. 

Materials. The preparation, purification, and analyses of 9-phenylflu-orene, fluorene, triphenylmethane, [9-14C]fluorene, triphenyl[14C]methane and [13,14C]H3I have been described previously (2). Tetrabutylammonium hydroxide (Fluka, 1.5 M), n-Bu4NOH, was titrated against standard HCl prior to use. n-Butyllithium (n-BuLi, Aldrich, 1.6 M) was standardized periodically against di-phenylacetic acid according to published procedures (5). Tetrahydrofuran (THF) was freshly distilled from LiAlH4 by using triphenylmethane as an indicator. 

Hydrolysis would then give alkali arising both from Bu°Li and EtOU, so the titration with acid would overestimate the amoimt of Bu Li present. Several methods have been devised for dealing with this problem. In one of these a weighed amount of benzoic acid in a mixture of dimethyl-sulphoxide, 1,2-dimethoxyethane and ether or hydrocarbon is titrated with the organolithimn solution. The indicator is triphenylmethane, which develops a red colour when all the benzoic acid is used up ... 

There are two main types of indicator. One involves a complex of a metal ion, which changes colour when the oxidation state of the metal ion changes. The iron-1,10-phenanthroline system is a typical example, the iron(//) chelate (ferroin) being red, the iron(//7) chelate (ferrion) being essentially colourless (actually very pale blue). The transition potential is about 1,1 V, so the indicator is very suitable for use in titrations with cerium(Ty) in sulphuric acid. The second type of indicator comprises various types of organic compound (aromatic amines, triphenylmethane dyestuffs, for example) which can be oxidised and reduced reversibly, and change colour on doing so. 7V,iV -Diphenylbenzidine is a typical example. 

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