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The possibility of replacing alkaline salt with another alkaline one as well:

In our previous discussion about the properties of some alkaline salts, we mentioned that it is possible to replace a certain salt with another type: such as replacing sodium carbonate with potassium carbonate, or replacing caustic soda with caustic potash, provided that the aforementioned ratios are taken into account in relation to each of the previous salts, and here we say that, even if Some references used to say that a certain weight of an alkaline salt (such as sodium carbonate) could be replaced by another strong salt of another type (such as caustic soda), but it is not recommended that these substitutions be made except between salts that have one virtue: such as replacing sodium carbonate Potassium carbonate salt, or the replacement of caustic soda with caustic hotasa, because the replacement of an alkali salt from a certain species with another salt from another species results in a change in the appearance of the solution properties.

replacement of caustic soda by adding formalin;

It may appear in the composition of some highlighting solutions that it is possible to exclude caustic alkali from caustic soda or caustic potash by adding formalin instead, and here it says that such a substitution is not a false substitution, as formalin reacts with sodium sulfite (which is the preservative that is included in the composition of show solution) and their interaction together results in caustic soda (sodium hydroxide), and the reaction then takes place according to the following equation:
Formalin + Sodium Sulfite + Water = Four Musulphites (3) + Sodium Hydroxide. It is clear from the foregoing that adding formalin to a developer solution contains sodium sulfite and is free of alkali, which is completely equal to adding caustic soda directly to the composition of the appearing solution.

Here you may wonder about the reason for following this crooked method, instead of adding caustic soda directly, and in response to that we say that formalin has another function in this process, as it increases the hardness of gelatin (Gilatine Hardner) and is often used in solutions to increase hardness ( which we will talk about later). Perhaps it has now become clear that placing formalin in the composition of the appearing solution achieves the following two purposes:
(a) It reacts with sodium sulfite to form caustic soda, which is the activated alkali in the apparent solution.

(b) It contributes to increasing the hardness of the gelatin layer in the film. As an example, Kodak No. 85 display solution, which will be installed in connection with display solutions that increase contrast.

Fourth - antifog or damper

We knew before that the reduction factors that we use for photographic purposes require that they be able to distinguish between areas that have been exposed to light and those that have not been exposed to it, but the effect of the reduction factor often exceeds these limits, and affects a small percentage in areas that were not exposed to light as well. Reducing the silver salt in it, and this results in what can be likened to fog. Fog, which is called fog of display sometimes, and it is in fact an increase in the density of the negative image that is not caused by exposure to light.

Therefore, a small amount of potassium bromide salt (1 Pot) is added to the appearing solution. Bromide reduces the activity of the exposure agent and its effect on areas that have not been exposed to light, therefore it is called a Fog Preventer. It also helps in soft clarity, preventing the addition of shadow details. It is a mistake to increase the amount of potassium bromide in the presenting solution by more than the necessary limit, as this results in the image appearing at that time as if it was under developed and thus reducing the degree of contrast of the image. Potassium bromide is a salt with white crystals, almost linear in shape. It dissolves easily in warm water, and its solubility decreases in cold water. It does not deteriorate easily, whether it is in the form of salt or in solution, and it is usually free of other salt impurities.

Potassium bromide salt is called the reducing agent or the inhibitor in the presenting solution due to its effect on reducing the activity of the displaying process. Its effect on the slow release depends on the type of reducing agent in the solution. And we show in the following table (1) the amount of increase in the revealing period when an equal amount of potassium bromide is added to a group of different revealing solutions.
In addition to the use of potassium bromide in the previous purposes, it is often used in the manufacture of sensitive layers in films.

Potassium bromide may be replaced by other materials such as citric acid or potassium metabisulfite in the composition of the presenting solution. Metabisulphite.

The degree of purity of the chemicals used in the imaging:

It is difficult to guarantee the accuracy of the results of the demonstration operations unless pure chemicals are used to prepare the solutions needed for these operations. In this regard, we mention that chemicals - in general - are estimated to be pure at internationally agreed upon degrees, which we mention as follows:
There is no doubt that the "A.R" grade is ideal for performing the required purposes, but it is very expensive, so that its use is unjustified extravagance, so each of the two grades ".B.P.", "B.P.C" are suitable for performing most photographic purposes, while the last three grades are It may contain impurities that may be harmful to work.