SILVEK. —Humid Assay. Volumetric Method.
760
The amount of the correction may be as much as10 parts in 1000, and the experiments of Hamblyappear to show that the loss of silver increases withthe weight of lead present.
Other Methods of Estimating Silver in theDry Way :—The two following processes for deter-mining the amount of silver without first dissolvingit are described by Fresenius, and are applicable inthe eases specified:—
(1) Determination as Chloride. —Specially applicableto bromides and iodides of silver. The mass underexamination is placed in a bulb tube, and after fusionis cooled and weighed. Chlorine gas is then causedto pass over for some time, the bulb being weighedfrom time to time until no change of weight isobserved; care must be taken to remove any residualchlorine gas before weighing.
(2) Determination as Metallic Silver. —Applicableto oxide and carbonate of silver, its compounds withorganic acids, and its chloride, bromide, and sulphide.In the case of oxides, carbonates, and compoundswith organic acids, a simple ignition in a porcelaincrucible will suffice. The results can be taken to beabsolutely accurate, except when organic salts areexamined, when there is a chance of a minute quantityof carbon being retained in the silver. For theestimating of chlorides, bromides, and sulphides (butnot iodides), the apparatus used for determining aschloride may be used, hydrogen gas being substitutedfor chlorine.
Humid Methods of Assay. —Various methodshave been proposed from time to time for estimatingthe amount of silver contained in a solution of thismetal; in some of these the precipitate is dried andweighed, and the amount of silver present determinedby calculation, and in others the volumetric methodis resorted to. Fresenius classifies the first seriesas follows:—
(1) Chloride of Silver . —This method is applicableto all compounds of silver. It has been alreadybriefly referred to; but it may be well to describe ina few words the system now practised in the IndianMints, where it is exclusively employed for the assayof silver bullion.
After the samples of alloy for assay have beenaccurately weighed, so that each weighs 18'817 grains(for a reason that will presently appear), they aretransferred to 12-ounce stoppered bottles, and toeach is added 1-5 drachm of nitric acid of specificgravity 1-200 for ordinary standard metal (that is,containing 916’6 parts per 1000, which is the stan-dard of the Indian coinage), or 1-320 for fine metal,or when the nature of the alloy is uncertain ; thesolution is aided by a gentle heat. About 6 ozs. ofdistilled water and 1-5 drachm of hydrochloric acid,of specific gravity 1-060, are added to each bottle.They are well shaken, filled with water, and allowedto rest for four hours. The supernatant liquid isthen carefully decanted by means of a syphon, andthe chloride washed several times in distilled water.It is then transferred, under water, to a Wedgewoodcup, in which it is broken up by a glass rod anddried at a moderate temperature, when it assumes a
compact form, and should be transferred at once tothe balance to be weighed while warm.
The weight of silver present can be deduced fromthe weight of chloride obtained, for each 100 partsof the latter contain 75-27 parts of fine metal; andthe weight taken, 18-817 grains, is such that itwould, if pure silver, produce exactly 25 grains ofchloride. Thus, a decimal series of weights is pre-pared in which the 1000 weighs 25 grains; and ifthese be employed in weighing the chloride obtained,they will indicate the standard of the alloy withoutcalculation. Chloride of silver, although extremelyinsoluble in water and dilute nitric acid, is solubleperceptibly in strong nitric acid and in hydrochloricacid, especially if concentrated and boiling. It isalso soluble in most aqueous solutions of chlorides,and in solutions of the nitrates of the alkalies andthe alkaline earths. Hence the method is speciallyapplicable when none of these salts are liable to bepresent.
(2) Sulphide of Silver . —Sulphuretted hydrogen orsulphide of ammonium may be employed to separatesilver—the former from any solution whatever,and the latter only from neutral and alkalinesolutions. The precipitate is allowed to settle,filtered rapidly with the exclusion of air, and driedon a weighed filter at 100° C. There is a certainamount of risk, however, in'the process, in conse-quence of sulphur being rapidly liberated if thesulphide be exposed to air, or if the solution con-tains substances which reduce sulphuretted hydrogen.When this seems probable, Bose recommends thatit be converted into metallic silver by heating toredness in a porcelain crucible in a stream ofhydrogen.
(3) Cyanide of Silver . — A neutral solution ofsilver is mixed with cyanide of potassium until theprecipitate formed is dissolved; then nitric acid isadded, and heat applied. Any free acid presentmust be neutralized. The precipitate is collectedon a weighed filter, dried at 100° C., and weighed.
(4) Metallic Silver. — When nitric acid is presentsulphuric acid is added, and the liquid evaporatedtill all the former is driven off; the sulphate formedis dissolved in hot water, and placed in a weighedporcelain crucible, in which is also placed a rod ofcadmium. The silver is soon reduced; it is separatedfrom the cadmium, and warmed with the acid fluidtill no more hydrogen escapes, then washed, dried,ignited, and weighed.
Volumetric Methods of Assay. —The first ofthese introduced was that proposed by Gay-Lussac in 1832, and it is still largely practised, especiallyin mints, and wherever frequent assays have to bemade on metal of nearly uniform composition. Itwill be necessary to describe it in some detail.After this some account will be given of othermethods which have been proposed.
Gay-Lussac 's Method. —It is based on the fact thathydrochloric acid and soluble chlorides are capableof completely precipitating silver from its solutionin nitric acid, without acting on any other metalwith which it may be associated, except mercury;