A method is outlined for the detection of anticholinesterase agents similar to sarin (isopropyl methylphosphonofluoridate) in aqueous solution at concentrations as low as 0.1 p.p.m. in 10 ml. of solution by the use of the Schoenemann reaction. A new series of indicators, 4,4′-(2-thenylidene)bis[N,N-dimethylaniline] variously substituted in the thiophene ring, was used in the investigation. The dependence of color formation on pH, chloride ion concentration, and other factors led to an explanation of the mechanism of the reaction involved. Chloride ion is necessary for the reaction and, in the absence of the indicator, free chlorine can be detected in acid solution. Presumably, then, during acidification perphosphonate ion reacts with chloride ion to give hypochlorous acid, which oxidizes the mono ion of the leuco compound to a dye.Ions such as cyanide ion, with a lower oxidation–reduction potential than the leuco compounds, interfere with the test. Oxidation of the leuco compound by hypochlorous acid is bypassed in the method described for the detection of sarin in the presence of hypochlorous acid.
The syntheses and properties of a series of 4,4′-thenylidene bis[N,N-dimethylaniline] compounds, variously substituted in the thiophene ring, are reported. These compounds were tested as redox indicators in the Schoenemann reaction for the detection of phosphonofluoridate esters in solution. The most effective indicator was 4,4′(5-methyl-4-propyl-2-thenylidene)bis[N,N-dimethylaniline]
A series of triaryl~nethane dyes were made by varying the substituents in the thiophene ring of the dye [4-(p-dimethylamino-a-2-thienylbenzylidene)-2,5-cyclohexadiene-l-ylide~~e]-dimethylamlnonium chloride. The spectra of the zinc salts of these dyes were shown to be similar t o that of the dye formed in the Schoenemann reaction when the parent triarylmethane compound was used as a n indicator. Neither of the absorption bands occurring in the 475 mp or 625 mp regions is affected by pH or buffer salts in the pH range 2.4 to 6.4 but both bands are hypsochro~nically shifted when the pH is-lowered to 0.5. The substitution of 5-nitro and 5-acetamido groups in the thiophcne ring results in the largest bathochromic and hypsochromic shifts respectively. Interpretation of the effect of substitution and pH was attempted employing existing theories on the relation between color and constitution. INTRODUCTIOXThe preceding papers in this series (4, 14) described the preparation and use of some triarylmethane compounds as indicators in the Schoenemann reaction with a view to improving the method for detecting the phosphonofluoridate type of anticholinesterase compounds. As the success of the method depended greatly on the color of the dye produced it was of interest to ascertain the effect of various substituents attached to the thiophene ring on the spectra of the dyes. The effect of the replacement of the benzene nucleus by thiophene and a number of 5-substituted thiophene nuclei on triarylmethane dyes has been reported by Ailason and Nord (11,12). However, the present investig a t' ion enlarges on the previous studies by including a number of new mono-and di-substituted thienyl derivatives. The effect of pH on the spectra is also reported.I t was of interest to checlc the assumption that the oxidized compound produced in the Schoenernann reaction was the same as that of an authentic dye cation. Therefore spectral conlparisons have been made of a number of zinc chloride salts of prepared dyes with the dyes formed in the Schoenemann reaction. E X P E R I M E N T A L Preparation of DyesThe preparation and p~lrification of the triarylmethane compouilds from which the dyes were made have been described previously (4).The zinc chloride salts of the dyes were prepared from the correspoilding triarylmethane compounds according to the procedure of Mason and Nord (11) except that identical molar concentratioils of the solutions were always used in an attempt to produce salts of the same composition. The method of Davies and Hodgsoil (2) was investigated but was not found t o be superior to the method outlined by Ailason and Nord (11) or Levi (7).Briefly, the leuco compouilds (0.04 mole) were dissolved in 2 N HC1 (80 ml.) and the solutions were diluted with 800 ml. of water. Freshly prepared lead dioxide (0.04 mole) was added with stirring and cooling. After 2 hours sodium sulphate (0.05 mole) was 1Manz~script received J u l y 24, 1957.
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