Home >> Encyclopedic Dictionary Of Photography >> Photo to So Co >> Silver_2

Silver

solution, cubic, nitrate, grains, ounce and oz

SILVER TESTER.—An apparatus for testing the quantity of silver nitrate in any solution pure or impure. For ordinary purposes the argentometer or hydrometer are usually used, the specific gravity of the liquid determining the amount of silver nitrate contained in it. When working with the silver bath in photographic processes, however, it gradually gets contaminated with other soluble matter, and the hydrometer test is no longer accurate for the amount of silver nitrate alone. Another method must therefore be employed. One much used abroad is Vogel's silver tester. (See fig. 412.) It consists of a stand S, a burette a, two pipettes p and F, and a beaker glass G. A solu tion of iodide of potassium is prepared, containing 1023.4 cubic centimeters of water and exactly io grammes of pure dry iodide of potassium. zoo cubic centimeters of this solution precipitate x gramme of nitrate of silver, so that if one cubic centimeter of a silver solution is measured off and tested, every cubic centimeter of the test solution used gives z per cent. of nitrate of silver (x,000 cubic centimeters-2.1x pints and z gramme= 15.4 grains). This prepared solution is placed in the burette a, which is divided off into cubic centimeters, and furnished with a pinch-cock k. The pipette p is then dipped in the silver solution to be tested, filled, by drawing with the mouth at the upper end, to the mark i, which is an exact cubic centi meter, and the solution allowed to run into the glass G. Into the same glass G are placed z or 2 cubic centimeters of pre pared nitric acid, using the pipette F. (This nitric acid con tains x grain of photosulphate of iron to every 2 ounces of pure acid). And finally lo to 14 drops of a prepared starch solution are added. (This solution is made by rubbing up oz. of starch to a thin paste with distilled water, pouring it into 124 ounces of boiling distilled water, and stirring for several minutes ; after settling for a few hours, the clear solution is poured off, and 24 ounces of pure pulverized nitrate of potassium added, when it is ready for use, and will keep undecomposed for about six weeks.) The solution in the

burette a is then allowed, by pressing open the pinch-cock, to run into the glass G, until the blue color which is produced does not disappear by shaking, but remains permanent. With a little care at the close of the testing, a single drop will be found sufficient to produce this permanent blue color. A simple reading of the number of cubic centimeters of solution used contains 71 per cent., that is, zoo c. c. of solution contains 71 grammes silver nitrate, which is equivalent to about 35 grains to the ounce.

Another method which, although not so accurate, is much simpler, is the following, the only articles required being an 8 oz. narrow-mouthed bottle, a graduated measuring glass, and some pure salt. The test solution is made by dissolving 55 grains of dried salt in a pint of dis tilled water. Pour into the 8 oz. bottle half an ounce of the silver solution to be tested, and add the testing solution gradually from a clean graduate measure, so that the quantity used can be accurately determined. After each addition shake the mixture well and add until no cloudiness is produced. If it requires one ounce of this solution, the silver solution contains io grains of silver nitrate to the ounce, if 2 OZ. it contains 20 grains, and so also for fractional parts. 3i oz. shows 311 grains, and so 4i oz. 45 grains, etc. As every one ounce of solution used shows io grains of silver nitrate to the ounce, it is only necessary to multiply the number of ounces and parts of an ounce to give the required result. If the silver solution contains ammonia after measuring off half an ounce of it into the 8 oz. bottle, it should be made acid with a little pure nitric acid, and then the operation can be proceeded with in the same way as before.