SODIUM HYPOSULPHITE, TESTING FOR The water in which negatives, etc., are being washed after fixing is often tested to ascertain whether " hypo " is present ; that is to say, whether the negatives, etc., have been washed long enough to remove all the " hypo." The best method of so doing is to use the following solu tion : Potassium carbonate , 5 grs. i g.
Potassium permanganate + „ ,, Distilled water . . so oz. 1,00o ccs.
A few drops of this pinkish-purple liquid added to a sample of the washing water will turn green when " hypo " is present ; the smaller the pro portion of " hypo " in the water, the longer the change takes ; and if there are very slight traces present, the change will be to blue, not green. The water to be tested should be the last the negatives or prints have been washed in, and preferably that which has been standing still for about ten minutes with the negatives therein ; take the water as much as possible from the bottom of the tank.
For the starch iodide test, powder and boil a piece of starch about the size of a pea in two or three drams of water until a clear solution is obtained ; add one drop of a tincture of iodine (iodine dissolved in alcohol), which will produce a dark blue colour. Fill one test tube with distilled water, and another with the water to be tested, and add to each tube one drop of the solution. If any " hypo " be present, the blue colour will disappear. The tubes should be warmed slightly
and examined side by side against white paper, as the test is very delicate.
The Bannon or silver test is to let a negative or print drain into a test tube, then heat the drainings, and add a few drops of a silver nitrate solution. A black precipitate will be formed if part of " hypo " is present, while a smaller amount will give a yellowish precipitate.
A rough and ready test is to taste the drainings ; the sweeter the taste the more " hypo " present. For the zinc and sulphuric acid test, dilute the acid to twice its bulk with water ; put some zinc into a flask, with the washing water, and add the dilute acid. If " hypo " is present, mil phuretted hydrogen is produced, as demonstrated by holding over the flask a piece of blotting paper moistened with a solution of lead acetate, which will be blackened. Chromic acid also gives 4. test ; about 4 grs. are dissolved in 5 oz. of water, and a few drops of sulphuric acid added. If on adding the washing water a greenish cloud appears, " hypo " is present. Potassium ferri cyanide in a weak solution and ferric chloride in a stronger solution, mixed in equal propor tions and added to the washing water, gives a greenish colour if " hypo " is present.