USE OF THE DECIMAL SOLUTION OF SALT.

797

quantity which is purposely rather less than the assay is expected to contain; 10 grains of alloy, if of correct composition, containing 925 grains of silver. When each bottle in succession has received from the pipette a charge of exactly the same value, the bottles are transferred to the agitator, shown at fig. 363, which is suspended from an iron arm, d, between two strong springs, e, e, made of vulcanized caoutchouc. This agitator is usually constructed to contain 10 bottles, which are lodged in the compartments, a, a; the stoppers are secured in their places by the rims, b, b, one of which is represented in the figure as thrown back for the admission of the bottles; the rims when closed are confined by the springs shown at c, c. On agitating the apparatus briskly for 60 or 80 seconds, the solutions become clear, and the bottles are removed from the agitator, and transferred to a stand, behind which is a black board divided into 10 numbered compartments, each bottle being placed opposite the compartment which corresponds with its number.

FIG. 364.

The adjustment of the remaining portion of the assay is made by means of the decimal solution. This is contained in a small bottle of 10 or 12 ounces in capacity, fig. 364, provided with a tube or small pipette, b, open at both ends, but drawn out to a narrow aperture below. On this small pipette a mark, c, is made at a height corresponding exactly to 10 grains of the liquid, 10 grains of this solution containing sufficient chlorine to precipitate oor grain of silver.

The assayer now plunges this small pipette into the decimal solution, and closing the upper opening of the tube with his fore

A

finger, partially withdraws it from the bottle, and allows the liquid to escape until it stands exactly at the line of the graduation, c; he then transfers the pipette to the first bottle, and allows the solution to flow into it. The same operation is repeated with each assay bottle in succession. A mark is next made with a piece of chalk, opposite to each bottle in which a precipitate is occasioned. These bottles are then replaced in the agitator and shaken a second time; the solutions having thus again been rendered clear, are replaced upon the table, and a second pipette of the decimal solution is added to each of the bottles in which a precipitate was first produced. This operation is repeated until in each bottle no further precipitate is occasioned. The contents of the pipette, g,

798

ASSAY OF SILVER-DECIMAL SOLUTION OF SILVER.

of the standard solution, which have been added to each assay, occasion a precipitate out of the 10 grains equal to 9:22, or of 922 parts out of 1000 parts of alloy. Each pipette of decimal solution is equivalent to T of fine silver in the alloy, and by counting the number of marks against each bottle, reckoning the last only as equal to half a thousandth, since a portion of it probably remains in the liquid in excess, the assayer ascertains the value of each bar. If, for instance, two marks stand opposite to any bottle, the fineness of the bar will be more than 923, but less than 924, and may be reported as 923*5.

Each of

But suppose that there be some bottles in which the addition of the first pipette of the decimal solution produces no precipitate ; these samples must be either exactly of the fineness 922, or below that point. The following method is adopted for completing the assay of these samples: a decimal solution of silver is prepared by dissolving 10 grains of pure silver in nitric acid, and diluting it with distilled water till the solution occupies the bulk of 10,000 grain-measures of water; each 10 grains of this liquid will then contain exactly o'01 grain of silver. A bottle of this solution is provided with a pipette similar to that shown in fig. 364, but graduated to deliver 50 grains of the liquid from the mark d. the assay bottles, which indicates a fineness below 922, is supplied with 50 grains of this decimal silver solution, or with 005 grain of silver; a mark of -5 is made upon the board against each of these bottles. The bottles are then agitated as before, and a fresh dose of 10 grains of the decimal salt solution is now added to each: if a cloud be thus produced, a mark is chalked against each bottle in which a precipitate is observed, and the bottles are again agitated, and another dose of decimal salt liquid is added, and so on, until a precipitate ceases to be formed. Suppose that the first two pipettes of the solution produce a cloud, but the third does not; each bottle, it will be remembered, received a dose of salt solution in the first instance, as usual, in addition to the quantity received after the decimal silver solution was added; the quantity of salt which has produced a precipitate is therefore equivalent to 922 +1+1, or 924'5; but since 5 of silver have also been added beyond that which the alloy originally contained, the amount to be reported becomes 9245-5, or the fineness of the bar is 9195. It is preferable, in cases where the bars are below the standard, to add an excess of silver solution at once, and then to estimate the excess of silver in the manner above described; because if, instead of acting thus, successive doses of o'01 of silver be added until no

PREPARATION OF FINE SILVER.

799

further precipitate is formed, it becomes very difficult to render the solution clear by agitation.

The standard solution of salt is prepared at a temperature, say, of 60°, consequently the pipette, g, will only deliver a volume of liquid rigorously equal to 9:22 grains of silver, at that temperature. At a higher temperature the liquid will expand, and a given volume will therefore contain a smaller amount of chloride of sodium, whilst at a lower temperature it will contract, and will contain a larger amount. A correction for this variation in

the strength of liquid is therefore required. This is made very simply in the following manner :-Each time that a number of assays is made, a piece of fine silver, equal to 9:25 grains, is weighed off, dissolved in nitric acid, and assayed as above directed. The number of pipettes of the decimal solution of salt which is required to complete the precipitation is noted, and the value of the contents of the large pipette, g, is thus verified upon each occasion. If, for example, 24 pipettes were required for completing this precipitation, the large pipette would deliver a quantity of the solution sufficient to precipitate 9225 grains on that day, instead of 9:22. Any deviation from the calculated value is allowed for, and a correction is made upon the assays by means of a table constructed for the purpose.

It is easy to apply this apparatus to the assay of silver of other degrees of fineness; but it is necessary to know approximatively the value of the alloy, in order that a suitable weight of it may be dissolved in nitric acid. Suppose, for instance, a number of bars approximatively of the value of 900 (the French standard) are to be assayed; a piece of the alloy, which contains approximatively 925 grains of fine silver, must be taken; the quantity required is easily calculated, since the weight of the alloy needed will be inversely as its fineness; for 900: 925:: 10 grs.: 10'277 grs. The weight required in this case will consequently be 10*277 grains.

Mercury is the only metal the presence of which interferes with the accuracy of the assay by the humid method; but the process may be modified so as to give correct results even in this case.

(940) Preparation of Fine Silver.-In order that the foregoing process shall be accurately performed, it is necessary to be provided with silver of absolute purity. The following is the best method of procuring the metal in this condition. Standard silver is dissolved in nitric acid: the liquid is diluted, and decanted or filtered from undissolved particles of gold or of sulphide of silver, and the solution is precipitated by the addition of a solution of

chloride of sodium in slight excess. The precipitate is washed in a large jar by subsidence, until the washings are tasteless. The chloride is then mixed with oil of vitriol, in the proportion of 3 ounces to each pound of chloride, and several bars of zinc are placed in the mass; the zinc speedily becomes converted into chloride of zinc, which is dissolved, whilst the silver is reduced to the metallic state, and by a voltaic action the reduction gradually extends through the mass; Zn+2 AgCl=Ag2+ZnCl ̧. The mixture is not to be agitated. In the course of a day or two the decomposition is usually completed. If a portion of the reduced silver, after being thoroughly washed, is entirely soluble in nitric acid, the reduction is complete. The bars of zinc, with the crust which adheres to them, are then carefully removed, and the reduced metal is digested for two days with diluted sulphuric acid, to remove any portions of the basic salts of zinc which are occasionally formed, and is washed in a large vessel by subsidence, until the washings cease to precipitate nitrate of silver.* The reduced silver is now redissolved in nitric acid, and a second time precipitated as chloride, pure hydrochloric acid being employed for this purpose: the precipitated chloride is again washed by subsidence until the washings no longer redden litmus. The chloride of silver is next dried until it ceases to lose weight, 100 parts of the chloride are mixed with 704 of chalk, and 42 of powdered charcoal, and the mixture is heated in a deep clay crucible. The temperature is kept at a dull red heat for half an hour, after which it is gradually raised to full redness: a considerable disengagement of gas takes place, owing to the evolution of carbonic anhydride and carbonic oxide, and oxychloride of calcium is formed, constituting a fusible slag, beneath which the pure silver collects; 2 AgCl + 2 Ea¤O ̧+E=E0+2 €Ð ̧ +¤aÐ, CaCl2 + Ag2. The silver may be poured into an ingot mould, remelted in order to free it from slag, and afterwards rolled into sheets. Silver sufficiently pure for all ordinary purposes may also be obtained in a crystalline form by boiling a slightly acid solution of nitrate or other salt of silver with sheet copper: the precipitated silver is well washed, digested in a solution of ammonia, to remove any traces of adhering oxide of copper, and again washed.

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*The reduced silver may be dried, and cast into ingots if desired. The metal is refined in large quantities for commercial purposes in this manner. It is not absolutely pure, and therefore, for delicate chemical operations, it undergoes the further process of purification described in the text.

The washed chloride may also be reduced without difficulty by fusion with about half its weight of dried carbonate of sodium.

OXIDES OF SILVER-FULMINATING SILVER.

801

(941) OXIDES OF SILVER.-Silver forms three oxides; a suboxide, Ag,; argentic oxide, Ag,, which is the basis of the salts of the metal; and a peroxide, probably Ag2, which does not combine with acids.

Argentous oxide, or Suboxide of silver (Ag, or Ag,O).– According to Wöhler, if the citrate of silver be heated to 212° in a current of hydrogen, the salt loses half an equivalent of oxygen, and a compound is produced which is sparingly soluble in water, forming with it a brown solution, from which, on the addition of hydrate of potash, a suboxide of silver is precipitated. This compound is very unstable; hydrochloric acid converts it partially into subchloride, but it is decomposed by other acids, and by ammonia, into argentic oxide and metallic silver. A mixture of metallic silver and argentous oxide is also obtained by boiling the yellow arsenite of silver with a strong solution of caustic soda, arseniate of sodium being formed in the liquid, 2 Ag,As ̧ +6 NaHQ= Ag10+Ag2+2 Na ̧As→4+3 H2→.

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Argentic oxide, or Protoxide of silver (Ag20=232, or AgO =116) Comp. in 100 parts, Ag, 93'1; 0, 6·9.-This oxide may be procured by adding a solution of potash or of soda to a solution of the nitrate or any soluble salt of silver. A brown hydrated oxide falls, which readily parts with its water, and if dried at a temperature above 140°, becomes anhydrous; it gives off oxygen below a red heat, and is reduced to the metallic state. Light also reduces it, and hydrogen, even at 212°, has a similar effect; contact under water with metallic tin or copper also deprives it of oxygen. Oxide of silver is a powerful base; it combines easily with acids, yielding salts which in some cases are isomorphous with the corresponding salts of sodium. It forms with nitric acid a salt which is not acid in its reaction upon litmus. It is slightly soluble in pure water, to which it communicates a feebly alkaline reaction. Oxide of silver combines with the fusible silicates, and is sometimes employed for producing a yellow glass. Hydrates of potash and soda do not dissolve the oxide, but it is freely soluble in ammonia, and the solution, by exposure to the air, deposits a black micaceous powder, which is powerfully explosive, and which has received the name of fulminating silver.

(942) Fulminating silver is also produced if a concentrated solution of ammonia be digested for some hours upon freshly precipitated oxide of silver; a black powder is formed which is allowed to dry in minute quantities on separate pieces of filtering paper. The same compound is formed on precipitating an ammoniacal solution of nitrate or chloride of silver by the addition

11.

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