The role of sodium sulphite in the processing of photosensitive materials

Sodium sulphite is used to prepare developer:

First, protect the developer from oxidation

The formulated developer is easily oxidized by oxygen in the air. If it is added in an alkaline solution with an accelerator, its oxidation will be more rapid. As a result, the developing ability will be greatly reduced, causing the photosensitive material to produce fog, resulting in a decline in the developing ability until the loss of the developing effect.

Therefore, in order to prevent the main developing agent in the developer from being oxidized by air, it is necessary to add the most representative anti-oxidation protective agent sodium sulphite when preparing the developer. Because the presence of sodium sulphite terminates the process of autocatalytic chain reaction into conjugate oxidation,

It greatly slows down the oxidation rate of the developer and prolongs the life of the developer. It forms a colorless sulfonate with the oxidation product of the developer, which not only improves the activity and capacity of the developer, but also prevents the pollution of the oxidation product.

Second, promote development

The activity of the developer is related to the pH of the solution. The higher the pH, the stronger the developing ability and activity. Adding a large amount of sodium sulphite to the developer, the alkalinity of the developer can be increased to pH> 8, so that the activity of the developer is enhanced, the developing ability is improved, the developing speed is accelerated, and the development is promoted.

Third, protect the fixer

Sodium sulphite can be added to the fixing solution for protection. The fixer is based on sodium thiosulfate or ammonium thiosulfate. It is unstable in an acidic aqueous solution. As the pH value decreases and the temperature of the liquid increases, its stability weakens, and it decomposes to produce sulfur. Precipitation,

To invalidate the fixing effect. If sodium sulphite is added to the fixing solution, the sulphite ions can combine with hydrogen ions to form bisulfite ions, that is, SO 2- 3 + H + HSO-3 due to the increase of bisulfite ions, the reaction proceeds to the left , Inhibit the decomposition of sodium thiosulfate,

Maintain the stability of the fixing solution, prevent the precipitation of sulfur, and ensure the normal fixing of the photosensitive material. Therefore, when formulating a shadow solution, a certain amount of sodium sulphite must be added as a protective agent. For example, if you prepare 1000 ml of fixing solution, if you dissolve 200～350 g of sodium thiosulfate, you should dissolve 10～15 g of sodium sulphite accordingly

Fourth, improve the washing quality of photosensitive materials

After the filming, the photosensitive material usually needs to be cleaned sufficiently with a large amount of running water or replacement water in order to wash away the residual thiosulfate and soluble silver thiosulfate complex on the photosensitive material. Especially when cleaning thicker paper-based photos, because silver thiosulfate ions must pass through the cell walls of paper fibers that are often protected by rubbers and other substances, it takes several hours to clean. Sometimes due to conditional restrictions, or in order to save water, the effect and quality of the washing cannot be guaranteed. If some sodium sulphite is added to the washing container, the washing can be made more adequate, the quality of the washing can be improved, and the washing time can be shortened.

Finally, prevent the color developer from being oxidized

For color developing agents, since its oxidation products are the same as those of most developing agents, when combined with sodium sulphite, sulfonates of color developing agents are formed. The oxidation product loses its coupling ability due to the combination of sodium sulphite and can no longer generate dyes from it. However, sodium sulphite competes with the color forming agent for the oxidation products of the color developer, and ultimately prevents the color developer from being oxidized.

 When there are 2 moles of sodium sulphite per mole of color developing agent, its inhibitory effect on oxidation reaches its maximum. Therefore, sodium sulphite can not only be used for the processing of black and white photosensitive materials, but also can be used as a protective agent for the developer of color photosensitive materials, with the same anti-oxidation effect.

It can be seen that sodium sulphite can be regarded as a panacea in the washing of light-sensitive materials.