Relative Rates of Reaction of Silver Bromide with Adsorbed Monolayers of Derivatives of Thiourea^*

* Read to the International Conference on the Science and Applications of Photography, organized by the Royal Photographic Society, London. September 1953.

Abstract

Thiourea and many of its derivatives are strongly adsorbed by silver bromide to form a monolayer. The adsorbed thiourea prevents adsorption of certain dye ions, but these dyes are rapidly adsorbed after reaction between the thiourea and silver bromide occurs. Use is made of this fact to measure the rate of reaction in the adsorbed layer.

Adsorption and reaction-rate data were obtained for thiourea and the following derivatives: methyl, 1.1-dimethyl, 1,1,3-trimethyl, 1,1,3.3-tetramethyl, allyl, acetyl, phenyl, 1,3-diethyl, 1,3-di-n-butyl. l-ethanol-3-allyl, and l-ethanol-3-phenyl. Only the 1,3-dibutylthiourea showed evidence of significant multilayer adsorption. Electron donor groups (methyl, ethyl) decrease the rate of reaction of the adsorbed thiourea; electron acceptor groups (ethanol, acetyl, phenyl, allyl) increase the rate. Tetramethylthiourea is substantially inert. Sulphide formed by reaction of thiourea and silver bromide catalyzes the reaction of thiourea and of its derivatives. The mechanism of the reaction is discussed on the basis of the experimental results.

Notes

* Read to the International Conference on the Science and Applications of Photography, organized by the Royal Photographic Society, London. September 1953.