Surface acidity and colour reactions on clay minerals and oxides.

Abstract

A reliable method was devised for the determination of surface acidity of clay minerals and oxides, by the adsorption of m-nitroaniline from aqueous solution, which accounted for the energy of adsorption of the protonated and unprotonated forms. The Hammett acidity function (Ho) of a sample of H+ -Kaolinite was 2.21, at bulk pH 3.0 The reaction of crystal violet (CV) and crystal violet lactone (CVL) were studied in solution and on clay minerals. CV and CVL decompose in methanolic and ethanolic solution if exposed to light, to yield demethylated derivatives. The products of CVL differ from CV and this suggests, the colourless lactone is more photolabile than CV. A colourless solution of CVL will undergo lactone cleavage on clay minerals and oxides to form a violet zwitterion. This zwitterion can be displaced from the surface by water vapour. Minerals with a high surface acidity resist the displacement of the zwitterion to a greater extent then do those with a low surface acidity, the counterion being an important factor in determining the acid strength of the surface. CVL undergoes demethylation on silica and Silton to yield the same products as in solution. Silton is a brand name of an acid- treated dioctchedral mo montmorillonite. The reactions of methylene blue (MB) and benzoyl leuco methylene blue (BLMB) were studied on clay minerals and in solution. The first order rate constant for the production of methylene blue from BLMB at 39 and 50°C were 1.54 x 10-6 and 3.47 x 10-6 sec-1 , respectively. If sodium cobalti - nitrite or eerie ammonium nitrate were added to an acidic solution of BLMB, the rate of MB production was greatly enhanced. The overall reaction was 2nd order in the case of NaCo(NO2)6 and 3rd order in the case of (NH4)2 Co(NO3)6. The rate constants for the reactions were 8.95 1M-1 sec-1 and 6.25 x 108 12 M-2 sec -1. This suggested that two electrons were transferred from BLMB to Na3Co(NO2) 6 simultaneously, and singly to (NH4)2Ce(NO3)6. MB and BLMB demethylated on silica gel and Silton, to yield Azure A, B, C, sym-dimethylthionine, and thionine. Production of MB from BLMB requires light and operates via a photon induced free radical mechanism. The presence of Co3+ or Ce4+, electron acceptors, on silica or Silton accelerates the reaction rate and does not require light.

Date of Award	1981
Original language	English
Awarding Institution	University of Bath