Sodium Nitrate Sodium Phosphate
The first salt, AgNaS203, is almost insoluble in water, and soluble in hyposulphite of soda ; therefore excess of hypo should always be used for fixing: the incomplete elimination of this insoluble salt is one of the chief causes of fading in prints. To test whether the whole of the hyposulphites are eliminated either by washing or by the use of an eliminator, the following may be used : -
Potassium permanganate ... ...... 2 grs.
Potassium carbonate ...... ... 20 ,,
Distilled water ... ... ... ... 40 ozs.
A few drops of this pinkish purple liquid should be added to the last washing water, when, if hypo be present, the pink colour will be discharged ; or a few drops of the last wash-ing water may be added to a solution of mercuric chloride when a cloudiness will make its appearance if hypo be present Another test is by making a little starch paste by boiling a pinch of starch in distilled water, and adding a drop or two of solution of iodine in alcohol to it, when a deep blue coloration, due to iodide of starch, will make its appearance. A drop or two of this deep blue liquor may be added to the last washing water, when, if hypo be present, the blue colour will be destroyed. The addition of any acid to a solution of hyposulphite will cause evolution of sulphurous acid and deposition of sulphur; hence will be seen the necessity of making the fixing bath for prints distinctly alkaline, to prevent sulphuration.
Sodium Nitrate (Ger., Natriumnitrat, Salpetersaures Natron; Fr., Azolate de sonde; Ital., Azotato di soda). NaNOs = 85. Synonym : Chili Saltpetre. This occurs native in Chili, and has been recommended as an addition to developers to give a good chocolate colour to negatives. Solubility: 1 in 1*2 of water; soluble 1 in 37 parts of alcohol.
Sodium Phosphate (Ger., Phosphorsaures Natron, Natriwn-phosphat} Perlsalz; Fr., Phosphate de sonde; Ital., Fosfato di