عملية التثبيت .. كتاب أسس التصوير الضوئي

(fixing process)

After the end of the process of showing that the salt of the story was reduced to metallic silver, we find in reality that only 25% of the silver salt is reduced, but the rest is not. It is still in its condition, and the sensitive plate or film is liable to change its color if it is exposed to light after the display process: therefore, it is necessary to remove excess silver salts so that the image becomes stable and not affected by light, and these silver salts are removed (or in other words the stabilization process) by the compound known commercially as Hypo and chemically as Sodium thiosulfate. Thiosulphate is the main compound in the fixation bath. The fixation process takes place in three steps, as follows:

(1) Converting the silver salt into a salt that is insoluble in water.

(2) Conversion of the latter compound into a salt that is more soluble in water.

(3) Converting the second compound into a salt that is easily soluble in water and can be easily removed by washing. We explain these processes as follows:

(1) Silver bromide - sodium thiosulfate = silver thiosulfate + sodium bromide and silver thiosulfate. This is a salt that is insoluble in water and its color is white. Therefore, the color of the film at this temperature is white. Likewise, the film is not stable, that is, it is still liable to change its color, so that if the process of Fixation (either deliberately for experimentation or by mistake due to lack of sufficient amount of hypo).. The silver thiosulfate salt turns into silver sulphide and the color of the film changes thereby to black. If the fixation reaction continues, the resulting silver thiosulfate reacts with another part of sodium hypo-thiosulfate, according to The following equation:

(2) Silver thiosulfate + Sodium thiosulfate = compound salt is
Silver-Sodium thiosulphate is a slightly soluble salt in water. But as the reaction continues with the rest of the hepo quantity, the following third reaction takes place.

​​​​​​(3) Sod. thiosulphate + Silver-Sod, thiosulphate = a compound salt
Silver Bisodium thiosulphate, and this salt is easily soluble in water and dissolves with washing, and thus we find that we have finally got rid of the rest of the silver salts.. We conclude from the foregoing that the necessary conditions for the stabilization process are:-

(1) The fixation bath must contain an ample amount of hypo-Sod-thiosulphate so that it is sufficient to complete these three reactions that we mentioned.

(2) The time required for the image stabilization process must be sufficient to complete the three operations. Accordingly, it is not permissible for the negative image to come out of the stabilized acid, even if the film appears transparent. The film may appear as well, although it has not finished in the last process: to guarantee the result, the film must be kept in the fixing solution even after it appears transparent. The time required to complete the three operations is approximately a quarter of an hour in order to obtain a negative image that is guaranteed to remain in good condition. As for the positive image, the period required to fix it must not be less than ten years.
Minutes in acid, a strong stabilizer