pH effect on the retention of hydroquinone revealing photographic substances by nanofiltration
Photographic development is the process of transforming the latent images into visible images thanks to a reducer. The principal components of picture development are agents of development, additives, an alkaline substance and an antiscreen. Hydroquinone and its derivatives are the most commonly used agents of development and revealing. Our study aimed to investigate the influence of pH on the retention of hydroquinone, hydroquinone monosulphonate and hydroquinone disulphonate solutions, which are present in the photographic revealing baths after nanofiltration. The results showed that the retention depends on the dissociation state of hydroquinone. In the absence of any ionized form, the retention is weak. As ionization increases, the retention increases considerably. The retention increases further if the A2- form is present. When pH < pKa1, hydroquinone is in molecular form and the retention is found to be related to the size of the molecule. The retentions are weak. When pH > pKa1 and pH > pKa2, the molecules become ionized, and the retention is affected by repulsion of the negative charges of the membrane. The retention becomes considerable. The retention of hydroquinone, hydroquinone monosulphonate and hydroquinone disulphonate generally follow the same evolution curves when the compound is alone or in a mixture. However a small shift in the curve for compounds in mixtures indicated more retention. We attribute this shift to the chemical properties of the hydroquinone compounds in solution, particularly their mobility and concentration. A further study of influence of these parameters is proposed to confirm these assumptions.