AMIDOL, OR DIAMIDOPHENOL (Fr. and Ger., Amidol) A developer having the formula OH(NH2)2 ,and introduced in the year 1892. It is sold in the form of a white or greyish crystal line powder, which keeps well in a dry state but in solution rapidly loses its developing powers. The dry amidol is therefore best added to the solution immediately before use. It forms a developer when mixed with a solution of sodium sulphite, no alkali being needed, and it works very rapidly, the detail appearing almost imme diately the developer is applied to the plate. This sudden appearance is apt to deceive those unacquainted with it, but the negative must not be taken from the developer until it has attained the required density. It has been stated that amidol will develop when in an acid condition, and appropriate formulae have been published, but it is better to regard amidol as an alkaline developer, because, even if it does work when slightly acid, it works better when slightly alkaline. The addition of acetone sulphite in quantity equal to that of the amidol preserves the developer for a considerable time, but acts as a restrainer ; other preservatives have been advocated, but it is better to add the dry amidol when required for use. A normal developer consists of : Sodium sulphite . 600 grs. 6o g.
Amidol . . . 4o-6o 4-6 Water to . . . 20 OZ. 1,000 ccs.
The mixed developer will keep well for four or five days. It should be made up with a new solution of fresh and pure sulphite. It is usual to make up a stock solution of sodium sulphite and to add 2 to 3 grs. of dry amidol to each ounce of solution.
Potassium bromide has but little restraining effect in an amidol developer, except when used in large quantities, but when added in small quantities it has a clearing effect. In cases of
over-exposure, about 15 drops of a io per cent. solution of potassium bromide may be added for every ounce of developing solution. Acetic, citric, and tartaric acids have been recommended as restrainers and sodium hyposulphite (" hypo " ) as an accelerator, but with the latter the image loses density beyond a certain point. Two- and three - solution amidol developers are rarely resorted to, as they have but little advantage over the one-solution, which is in wide use, par ticularly for bromide paper, for which most paper-makers give a special formula. The two solution form is not given here because it is of doubtful value.
Amidol has the property of staining the finger-nails. Sometimes it stains bromide prints a rosy pink colour, which may be removed by the use of eau-de-javelle followed by citric acid.
Another formula with bromide, and specially recommended by Abney for the development of " instantaneously " exposed plates, is : Amidol . . . . 5 grs. 5 g.
Sodium sulphite . 40 ,, Potass. bromide . . i i PP Water to . . . 2 OZ. 1,000 ccs.
This solution will keep for a few days, but is most energetic when fresh.
Another form of a one-solution amidol de veloper is that known as Balagny's acid-amidol, which has found great favour upon the Continent, both for negatives and bromide paper.
One of its advantages is its slowness. The original formula is : Water .to+ oz. 30o ccs. -