المواد الحافظة .. كتاب التحميض والطبع والتكبير

Second - Preservatives:

Since the reducing agents are oxidized as a result of their union with the oxygen of the air, compounds whose function is to slow down the oxidation of the reducing agent are added to the group of salts inhabiting the presenting solution. These compounds are called preservatives, and in addition to their work, they interact with the unwanted products of the fermentation process and transform them into harmless substances. for the photo. It is required in the composition of the display solution that the percentage of preservatives in it not exceed the necessary limit, so that the silver salts do not dissolve in the solutions of the preservatives, and so as not to cause a chemical haze on the negative, or limit it. Distortion factor, the following are examples of such materials:

1- Sodium sulfite
2- Sodium bisulfite
3- Potassium metabisulfite

The following are the characteristics of this material:
1- Sodium sulfite:
It is the most used of these materials for this purpose, and it can be obtained in the form of crystals or anhydrous powder, and in both cases it performs the same function.
(a) Crystalline sodium sulfite is more susceptible to damage than powder, as it turns into sodium sulfate when exposed to air, and this last salt does not act as a preservative, and therefore it is taken into account to preserve the crystalline salt in tightly closed glass containers.
(b) The required amount of crystals is equivalent to twice the required quantity of powder, meaning that if it is necessary to put 50 grams of crystalline sodium sulfite as a preservative in the apparent solution, then a quantity of 25 grams of powder could be sufficient for this purpose.

See from behind

Sodium sulfite is a salt that dissolves quickly in water, but it is quickly damaged after dissolving it, as it turns into sodium sulfate.

Sodium sulfite, which is prepared for many commercial purposes, is mixed with clay and contains other salts such as bisulfites, sulfites, and carbonates. Therefore, when buying it, you must choose pure salts specially prepared for photography.
In addition to the use of sodium sulfite as a preservative for the reducing agent from spoilage, it is also included in the composition of some other solutions used in imaging operations, such as fixation solutions and solutions of increasing the density with mercuric salts. minutes, and then sodium sulfite acts as a silver solvent, and we will talk about that in the context of our research in
This type of solution.
When estimating the amount of sodium sulfite in the appearing solution, the following considerations are taken into account:
(a) That by increasing its quantity, the activity of manifestation decreases, which calls for an increase in the period of manifestation.
(b) One of its characteristics is the ability to dissolve silver salts (silver halides), whether those were exposed to light or those that were not. This ability increases if its quantity is greatly increased, unless other salts are added to the appearing solution that reduce its previous activity.

2- Sodium bisulfite
A white salt that may be in the form of prismatic crystals or in the form of a powder, and it emits a slight smell of sulfur dioxide gas, which is quickly soluble in water, and its solution is strongly acidic when exposed to air and turns into sulfurous acid and sodium sulfite. Sulphite.
It can be obtained commercially as saturated (its concentration is about 40) and is also saturated with sulfur dioxide gas. It can be synthesized by adding five parts of sulfuric acid to one hundred parts of. , a saturated solution of sodium sulfite.
In addition to what this salt does as a preservative for the reducing agent from spoilage, it is also used in the composition of the fixing solution. Sodium bisafite is often replaced by using another salt (of the same weight), potassium metabisulphite instead.

3- Potassium metabisulfite:

A crystalline salt with a distinctive smell of sulfur dioxide gas, which dissolves quickly in water. It is used as a preservative, and it is also included in the composition of fixing solutions. As we mentioned in the previous item, a weight of it can be replaced by a similar weight of sodium bisulfite. This salt - whether in the saline state or as a solution - has the ability to resist damage for a long time if it is kept in airtight bottles.

The difference between sulphite solutions and bisulphites solutions:
Sulfite solutions are weakly alkaline. As for bisulfite solutions, they are acidic, and this difference greatly affects the properties and conditions of use of either type, because since it is necessary to add an alkaline substance to the presenting solution to activate the reducing agent - as you will see below - then if we add bisulfite to the composition of the presenting solution, The amount of alkaline material should increase to a large enough extent to equalize the strong acidity that characterizes bisulfite.
Advantages of using bisulfite instead of sulfite:
(a) It is sometimes customary to prepare a bleaching solution such that it consists of two parts that contain the reducing agent and the preservative with it, and the second part contains activated alkaline substances, and due to the “acidic” characteristic of bisulfite, it is therefore better able to preserve the first solution. which carries the reducing agent - from spoilage.

(B) In the case of preparing highly concentrated show solutions, it is better to use bisulfite instead of sulfite due to the greater solubility of the first salt in water than the second salt.