HYDROQUINONE (Fr., Hydroquinone ; Ger., Hydrochinon) A developer, known also as hydrokinone, hydrochinone, and quinol. The use of hydro quinone was suggested by Sir William Abney in 1880, but it was not possible to bring it into general use at that date owing to its expense. Since then, however, manufacturers have been able to produce it very cheaply. It occurs in greyish-white or yellow prismatic needles, which darken on exposure to light, and in chemical composition it is allied to pyrogallol, which is trihydroxybenzene, whereas hydro quinone is dihydroxybenzene, Its solubility is 5.8 per cent. in cold water, and about ro per cent. in hot water. When used alone, particularly if bromide is added as a restrainer, hydroquinone has a tendency to give excessive contrasts, and although an excellent developer, it is more generally used with metol (see " Developers, Mixed"). A solution of hydro quinone is affected by temperature more than is any other developer, and works very slowly —sometimes not at all—when very cold. Hydro quinone may be used in one or two solutions, and the best results are obtained with a caustic alkali.
One-solution Developer (Concentrated) Hydroquinone . . 310 grs. 35'5 g.
Sodium sulphite . 5 oz. ,, Potassium carbonate 8 „ ,, Hot water to . . 20 „ 1,000 CCs.
The above is a concentrated developer, and for use requires to be diluted with from four to six times the quantity of water. Bromide will be required only in cases of over-exposure. Another one-solution form, in which sodium carbonate is used, is : One-solution Developer (Ready for Use) Hydroquinone . . 90 grs. 21 g.
Sodium sulphite . 2 OZ. 220 „Sodium carbonate . 2 „ 220 „ Water to . . . 10 „ 1,000 ccs.
Hydroquinone works more quickly with sod ium carbonate than with potassium, but the latter is thought to give better gradation.
Messrs. Lumiere have recommended the follow ing one-solution (ready for use) formula. It gives absolutely clean negatives of great con trasts, which makes it specially suitable for the reproduction of black-and-white and line draw ings, or for obtaining more contrasty positives from thin and weak negatives, care being taken in all cases to keep the exposure somewhat short : One-solution Developer (Ready for Use) Hydroquinone . . 4o grs. g.
Sodium sulphite • 183 „ Formaline . . 5o drops 20 ccs.
Water . . . 5 oz. 1,000 „ No bromide or alkali is required. Dozens of other formulw have been given, all more or less based on the above.
Probably the most popular of the two-solution forms—and some scores have been published—is the following : Two-solution Developer A. Hydroquinone . 16o grs. 18 g.
Sodium sulphite . 2 oz. TOO ccs.
Citric acid . . 6o grs. 7 g.
Potassium bromide 40 y, 4'5 , Water . . . 20 oz. 1,000 ccs.
B. Sodium hydrate . 16o grs. 18 g.
Water . . . 20 oz. 1,000 ccs.
For use, mix together i oz of A, i oz. of B, and 2 oz. of water. More or less water may be added as desired, and the more water used the softer will be the result. By using A and B in equal parts without water very hard negatives will be obtained, such as are wanted when copying black-and-white work.
Two-solution formulw embodying the use of sodium and potassium carbonates are as below : Two-solution Developer A. Hydroquinone . f oz. 27•5 g.
Sodium sulphite . 2 „ ITO „Water to . . 20 „ 1,000 CCs.
B. Sodium carbonate if „ 69 g.
Water . . . 20 „ 1,000 ccs.
Take equal parts of each. I oz. of potassium carbonate may replace the if oz. of sodium car bonate in the B solution.
Prof. Lainer has published many useful con centrated two-solution formulw, five of which are given on the next page, and in order to economise space the quantities are given in " parts," but to those unaccustomed to the " parts " system it may be useful to know that if the solids are weighed in grains and the liquids in minims (48o to the ounce), the pro portions will be about right.
For use, A and B are mixed together in pro portions as follow : No. i—r oz. of A, with 24 drops of B ; very rapid in action, but inclined to fog. No. 2-1 oz. of A, with 48 drops of B ; a rapid developer, and gives strong contrasts. No. 3—I oz. of A, with 96 drops of B ; gives softer negatives. No. 4—I oz. of A, with so to 7o drops of B ; slow working. No. 5—Mix in equal parts ; a good, normal developer, which works slowly and gives clean negatives.
One drawback to hydroquinone has been understood to be that it cannot be made up in a highly concentrated form, as, for example, a Jo per cent, solution. J. B. B. Wellington, however, as long ago as March, 1889, published the following formula for a io per cent. solution :— A. Hydroquinone 1 oz. r Io g.
Methylated spirit 3i „ 35o ccs Sulphurous acid . 31- „ » Water to . . io „ I,000 „ B. Sodium hydrate . 1 oz. IIo g.
Sodium sulphite . I „ I So „ Water to . . io „ i,000 ccs.
For use, add 1 drm. each of A and B to 21 oz. of water. In the above formula the methylated spirit is used to dissolve the hydroquinone and the sulphurous acid to prevent oxidation.
Hydroquinone may be used again and again for several negatives without fear of staining ; but ultimately it gets into a condition in which it stains badly. It keeps well in solution. It should always be dissolved after the sodium sulphite.
In process work, hydroquinone is regarded as the most useful developer for dry plates and collodion emulsion. For the former it is used either alone or in conjunction with metol.