367. So much of the precious metals is used in photography that it i» important to guard against waste as much as possible. This waste occurs in two ways: First, by attempting to sensitize both collodion plates and paper without I knowledge of the strength of the solution used, thus wasting raw material: and, secondly, by allowing used solutions to run down the sink, and by throwing away scraps of silvered paper which could be burned, the ashes saved, and the silver obtained from them in
867. The method known as " volumetric analysis," applied to testing silver solutions in the usual way, requires the use of a burette graduated into cubic centimetres - a piece of apparatus easily broken, and which is hardly adapted to the wants of the ordinary darkroom, but, as the principle upon which it depends is extremely simple, the following modi-flcation of the process is offered, which it is thought will come within the practical working of utmost every photographic laboratory. Before describing the method, it will be well to briefly explain to those not familiar with volumetric analysis the principle involved. When a solution of salt (sodium chloride) is added to a nitrate of silver solution, chloride of silver is formed, which, being insoluble, is precipitated at the same time the sodium nitrate (the other prod-duct of the decomposition) goes into solution. Sixty grains of sodium chloride are required to precipitate one hundred and seventy grains of nitrate of silver. If, therefore, one ounce of water held in solution one hundred and seventy grains of nitrate of silver, it would require just sixty grains of salt to precipitate all the silver as chloride; or, should the one ounce of water contain one - half the quantity if nitrate of silver (eighty-five grains dissolved), then thirty grains of salt would be required to effect the complete precipitation. From this it will be seen that the quantity of salt required to precipitate a silver solution depends upon the quantity of silver dissolved; and, therefore, to learn how much silver is held in solution, it is only necessary to find out how much salt is required to completely pre-For this purpose a standard solution of salt is made, a given quantity of which will represent so many grains of nitrate of silver. The simple process suggested, based upon this well-known principle, is as follows:
First procure a one - ounce graduate, marked off in drachms; also one sixteen-ounce narrow-mouth, corked bottle, and one eight - ounce narrow-mouth, glass - stoppered bottle; this ia all the apparatus required. Next take an ounce or more of common salt, and dry it in a clean dish over a gentle heat for an hour or more, until the little moisture it contains is driven off. When the salt is thus well dried and cool, weigh out exactly ninety grains on a piece of paper, and pour it into the sixteen - ounce bottle, taking care not to spill any; then add exactly fifteen ounces of water, common water will answer, but distilled or clean rainrub a little soft wax or tallow just under the lip, up to the pouring edge; by this meant the liquid may be dropped without any danger of spilling. 8. In order to read the graduate correctly place it upon a level table, and have the sight upon a line with the markings. Owing to the adhesion of the liquid to the glass, it rises on the sides a little above the true level, and the readings should be made between the two liquid surfaces. - George Brinton Phillips.
( 339) the crucible. It is therefore important that the photographer should know how to test the strength of his solutions, and how to save himself from loss by wastage. There are several ways of coming at the first, but the volumetric method is the best, and a simple means is given by a diswater is much better. Cork the bottle and shake it well until all the salt is dissolved. Label the bottle, "Standard salt solution for testing silver baths. Formula: Ninety grains of salt; fifteen ounces of water. Each drachm of solution represents two grains of nitrate of silver." Now, in order to make the test, proceed as follows: Measure out exactly half an ounce of the silver solution (the strength of which you wish to ascertain) and pour it into the eight - ounce bottle; rinse out the graduate twice, with about half an ounce of water each time, and pour these washings into the bottle with the half ounce of silver solution. Now add a few drops of pure nitric acid to the bottle, in order to render it decidedly acid to litmus-paper. Next pour into the graduate exactly one ounce of the standard salt solution, and add it gradually, a drachm at a time, to the bottle containing the silver solution. Put the stopper in, and shake violently for a few moments; the salt solution will precipitate the silver as chloride, and the shaking will cause the liquid to settle clear; add another drachm of salt solution, and shake as before; go on doing this until the silver solution begins to look milky after the addition of a drachm of standard solution, and does not settle out clear. This is the critical point, as nearly all the silver has been precipitated, and a very small quantity of the salt solution will now effect the complete precipitation. Add the solution now, a few drops at a time, shaking well after each addition; and when the test-liquid in the bottle no longer shows a cloudiness after the last addition, the operation is completed. To ascertain now the strength of the silver bath, it is only necessary to bear in mind how many drachms of the standard solution have been used in the operation. Suppose, for instance, it took exactly seven drachms of the salt solution to effect the complete precipitation, as each drachm represents two grains of nitrate of silver, seven drachms equal fourteen grains to the half ounce, or twenty-eight grains to the ounce. If the test is always made with half an ounce of bath, the number of drachms of standard solution multiplied by four will give the number of grains of nitrate of silver to the fluid ounce. Should it be found, in another case, eight drachms (one ounce) were not sufficient to throw down all the silver as chloride, then another ounce of standard solution is measured out and added, in fractions of a drachm at a time, until the desired result is attained. If in this case nine and a half drachms precipitate all the silver, then nine and a half multiplied by four equals thirty-eight, or thirty - eight grains of nitrate of silver to the ounce of bath. In order to obtain accurate results, two tests should be made - the first will give an idea about how much standard solution will be required for the operation; then the second test must be made with care at the end of the trial, adding the salt solution in portions of a few drops only at a time. As it is quite possible to read down to a quarter of a drachm on the graduate, the strength of the bath may be ascertained, at least within one or two grains to the ounce. There are a few points which, if observed, will add to the accuracy of the results: 1. In the first test, add the salt solution, half a drachm at a time, until the number of drachms required is known. In the second test, the standard solution may be added in greater quantity, until within a few drachm - of the critical point; after that is reached add it only by drops, shaking well after each addition. 2. In order to drop the salt solution neatly, so that none of it runs down under the lip of the graduate (which would occasion loss and vitiate the results of the experiment), tinguished chemist in the note appeded. As to the saving of wastes, only a few practical hints will be given, since your dealer or chemist will supply you in pamphlet form, free, with much more elaborate instructions than comes within the reach of this work to give.
To convert chloride of silver into a metallic state, procure a porous earthenware tube, such at is used in galvanic batteries; make a zinc cylinder to enter it, leaving a small space around it; to the zinc cylinder must be soldered a piece of silver wire; the chloride to be converted is introduced into a large jar, the porous vase is made to stand upon the surface of the chloride, and the silver wire is bent in such a manner that it lies at the bottom of the jar in the middle of the chloride. The jar is then filled with acidulated water, which passing through the pores of the tube attacks the zinc and creates an electric current; the silver in a pure metallic state deposits itself round the silver wire. The jar must be kept in a warm place, and not touched until every atom of chloride is converted. a, jar; b, chloride of silver; c, porous vase; D, zinc cylinder; e, silver wire. When terminated, it is not even necessary to melt the product in a crucible; it can be dissolved in nitric acid and crystallized. - E. Stebbing.
Almost every photographer has some special way of his own for saving silver from the developing solution, but I doubt if a more primitive effectual plan has ever been hit upon than one adopted by a photographer, of no mean reputation, that I met during my travels this summer. The whole apparatus was nothing more than an old felt hat without holes, arranged upon a frame of four sticks over the developing tray, similar to a funnel. Plates were developed into the hat, which, from the porous condition of the felt, allowed all the solution to filter slowly through, but not before all of the silver had been precipitated by the continued action of the iron. After a season's work, the hat was burned and the silver recovered. Simple as well as effective appliances are what photographers desire, and this certainly deserves a trial, even at the sacrifice of an old felt hat. - J. C. Browne.
The greater part of photographers' residues are in the form of chloride of silver, which should be put dry into a large bottle, and a certain quantity of ordinary ammonia be poured upon them, and well shaken up every now and then. The ammonia will dissolve all the chloride. When the ammonia is saturated, decant it, and even filter it, and set it apart. Pour fresh ammonia upon the residues until all the chloride is dissolved out. It can easily be seen if the ammonia contains chloride by the following test: Take a test-tube, pour a few drops into it, then a little distilled water; pour into it a few drops of hydrochloric acid, and a heavy precipitate will be the result if it contains chloride of silver. When all the chloride has been dissolved out, put the ammonia into a large - mouth jar containing plates of sheet - copper. In a very short time the chloride will be reduced to a metallic state. It is then put into a crucible and melted. It can now be converted into nitrate of silver, which will be very pure and give excellent results. - E. Stebbing.