The Relation of Kinetics of Coupling of Developer Soluble Colour Couplers to Structure and Photographic Activity

Paper presented al the “Congres Internationel de Science Photographique” held in Paris, in September 1965

Abstract

The ratio of Coupling Rate Constant: Deamination Rale Constant (k₂, k₁) for a considerable number of magenta, yellow and cyan developer soluble colour couplers has been determined by competition between deamination and coupling. The determinations were made in a medium approximating to the colour developer using 5-diethylamino-2-aminotoluene as the developing agent. pK measurements showed that all the colour couplers were fully ionised in this medium. The photographic data have been shown to fit the empirical equation, γ = A log t + B where γ = Contrast, t = Time of Development in minutes, A = Activity Constant and B = Inducticn Constant, k₂, k₁ and A values have been shown to be related by the equations, A=3.55 e^{–5.05×102(K₁/K₂) for oxidation by K₃Fe(CN)₆} A=3.55 e^{–5.05×102(K₁/K₂}) for oxidation by AgCl.

These equations show that the photographic activity of a colour coupler is related to its rate of coupling with oxidised developer and increases with increase in k₂/k₁. Further they also show that there is a limiting value for A of 3·55. The results also show a broad indication that log k₂/k₁, is proportional to the pK of the colour couplers. This indicates that electron releasing substituents would increase the coupling rate and hence the activity of the colour coupler.

Of 21 colour couplers studied approximately one-third did not obey the above equations. Experiments have shown that the anomalies arise from azomethine dye destruction in the film due to attack by (a) Na₂SO₃, and NH₂OH under alkaline conditions and (b) excess pyrazolone colour coupler in the case of magenta colour couplers.

Notes

Paper presented al the “Congres Internationel de Science Photographique” held in Paris, in September 1965