Sodium Phosphate (Tribasic), 380.— Recommended by Lumiere as a substitute for ordinary alkalies in development (see Developing Formula), and is said to prevent pyro stains. In substitution of salts, too parts of sodium carbonate should have the chemical equivalent value of 133 parts tribasic phosphate.
Sodium Sulphite, 126.—Large, clear crystals, efflorescent by absorption of oxygen when exposed to the air, or anhydrous as a white powder ; the latter need only be used in half the quantity prescribed for the more common crystals. Used as a preservative for developing agents, and also to prevent stains on the film in development. Solubility, I in 2 (crystals), I in 4 (dry).
Thiocarbamide, CS(N 76.—Small prismatic crystals. Solubility, i in i 1. A weak solution, acidified with citric acid, is recommended by some makers for brightening bromide prints, but must not be used until all traces of hypo have been removed from the paper ; it is also added to developers for lantern (bromide) plates, and gives blue and violet tones. As a developer it often causes reversal.
Sulphuric Acid, 98.—A heavy liquid. The com mercial qualities of oil of vitriol contain impurities. For most photographic purposes it is expedient to substitute the less dangerous nitric or hydrochloric acid. It is highly
corrosive, and great care must be taken in mixing it with water. The acid should be poured into the water drop by drop ; water must never be poured into the acid. Antidotes : Chalk or bicarbonates. Water must not be administered in the early stages of vitriol poisoning.
Uranium Nitrate, 504.—Yellowish green deliquescent crystals. Prepared from pitchblende. Solubility, about i in of water. Decomposed by light in contact with organic matter. As used for toning and intensifying, there seems some doubt as to the stability of the results. As a moderate reducer of overprinted P.O.P. paper, when added to the ordinary fixing bath it possesses a much higher value.
Sodium Borate, 382.—White crystals or powder, of sweet taste and slightly alkaline. Solubility, about in 12. Used as a restrainer in pyro developers, but acts as an accelerator with eikonogen and hydroquinone. A favourite alkali for gold toning formulx, but the bath does not keep well.
Sodium Hydrate, NaOH, 4o.—Caustic soda. Very soluble. Sometimes specified in developing formula, and for other purposes when a strong alkali is required.