Restrainers potassium bromide

Potassium bromide, usually referred to as bromide, is the primary restrainer found in most developers. Bromide has the effect of holding back the overall action of the developer, reducing the effective sensitivity of the film, and diminishing the amount of useful density created in the shadow areas. By inhibiting the reduction of silver halide, bromide also acts to increase contrast. This action varies with different developers. 

Although it is recommended by no less an authority than Geoffrey Crawley, farmer editor of the British Journal of Photography and formulator of FX and Paterson proprietary developers, potassium iodide has not been thoroughly investigated for its use as a restrainer. However, what little research has been done indicates that it is superior to potassium bromide. As a substitute, it is usually recommended to use 1/10 to 1/100 the weight of bromide. Iodide can also be used in combination with bromide. 

Restrainer, which is usually potassium bromide or benzotriazole. 

It has already been mentioned that when formulating PQ developers of high pH an organic restrainer should be used. Conversely, when formulating PQ developers of low to moderate pH potassium bromide should be used. This is because while Phenidone is not very sensitive to bromide at high pH, it is sensitive at low to moderate pH. 

In aqueous solution potassium bromide reacts with bromine, polybromides with the formulae KBrs and KBrs being formed in solution.12 In the neighbourhood of its melting-point the salt evolves bromine freely.13 Potassium bromide finds application in medicine as a soporific, and in photographic development as a restrainer. ... 

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