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tartrate of lime), upon the precipitation of other oxides or salts by ammonia or some other reagent. This application of chloride of ammonium is based upon the tendency of the ammonia salts to form double compounds with other salts. Chloride of ammonium serves also to distinguish between precipitates possessed of similar properties; for instance, to distinguish the basic phosphate of magnesia and ammonia, which is insoluble in chloride of ammonium, from other precipitates of magnesia. It is used also to precipitate from their solutions in potassa various substances which are soluble in that alkali, but insoluble in ammonia; e. g., alumina, sesquioxide of chromium, &c. In this process the elements of the chloride of ammonium transpose with those of the potassa, and chloride of potassium, water, and ammonia are formed. Chloride of ammonium is applied also as a special reagent to effect the precipitation of platinum as ammonio-bichloride of platinum.

$ 52.

13. CYANIDE OF POTASSIUM (K Cy).

Preparation. Heat ferrocyanide of potassium of commerce (perfectly free from sulphate of potassa) gently, with stirring, until the crystallization water is completely expelled; triturate the anhydrous mass, and mix 8 parts of the dry powder with 3 parts of perfectly dry carbonate of potassa; fuse the mixture in a covered Hessian or, better still, in a covered iron crucible until the mass is in a faint glow, and appears clear, and a sample of it, taken out with an iron spatula, looks perfectly white. Remove the crucible now from the fire, tap it gently, and let it cool a little until the evolution of gas has ceased; pour the fused cyanide of potassium into a heated crucible-shaped vessel of clean scoured iron or silver, or into a moderately hot Hessian crucible, with proper care, to prevent the running out of any of the minute particles of iron which have separated in the process of fusion and have subsided to the bottom of the crucible. Let the mass now slowly cool in a somewhat warm place. The cyanide of potassium so prepared is exceedingly well adapted for analytical purposes, although it contains carbonate and cyanate of potassa ; which latter is upon solution in water transformed into carbonate of ammonia and carbonate of potassa (KO, C, NO+ 4 HO KO, CO, NH, O, C O). The proper formula for the cyanide of potassium is therefore 5 K Cy + KO, Cy OKO, CO,. Keep it in the solid form in a well-stoppered bottle, and for use dissolve 1 part in 4 parts of distilled water, without application of heat.

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Tests.-Cyanide of potassium must be of a milk-white color, and quite free from particles of iron or charcoal. It must completely dissolve in water to a clear fluid. It must contain neither silicic acid nor sulphide of potassium; the precipitate which salts of lead produce in its solution must accordingly be of a white color, and the residue which its solution leaves upon evaporation, after previous supersaturation with hydrochloric acid, must completely dissolve in water to a clear fluid.

Uses.-Cyanide of potassium prepared in the manner described produces in the solutions of most of the salts with metallic oxides, precipitates of cyanides of metals or of oxides or carbonates, which are insoluble in water. The precipitated cyanides are soluble in cyanide of potassium, may therefore be separated from the oxides or carbonates which are

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insoluble in cyanide of potassium, by further addition of the reagent. Some of the metallic cyanides redissolve in the cyanide of potassium, even in presence of free hydrocyanic acid and upon boiling, invariably as double cyanides; whilst others combine with cyanogen, forming new radicals, which remain in solution in combination with the potassium. The most common compounds of this nature are cobalticyanide of potassium, and ferro- and ferricyanide of potassium. These differ from the double cyanides of the other descriptions particularly in this, that dilute acids fail to precipitate the metallic cyanides which they contain. Cyanide of potassium may accordingly serve also to separate the metals which form compounds of the latter description from others, the cyanides of which are precipitated by acids from their solution in cyanide of potassium. In the course of analysis, this reagent serves to effect the separation of cobalt from nickel, and also of copper from cadmium.

$ 53.

14. FERROCYANIDE OF POTASSIUM (2 K, C, N, Fe + 3 aq. = 2 K,

Cfy + 3 aq.).

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Preparation. The ferrocyanide of potassium is found in commerce sufficiently pure for the purposes of chemical analysis. 1 part of the salt is dissolved in 12 parts of water for use.

Uses.—Ferrocyanogen forms with most metals compounds insoluble in water, and which frequently exhibit highly characteristic colors. These ferrocyanides are formed when ferrocyanide of potassium is brought into contact with soluble salts of metallic oxides, with chlorides, &c., the potassium changing places with the metals. Ferrocyanide of copper and ferrosesquicyanide of iron exhibit the most characteristic colors of all; ferrocyanide of potassium serves therefore particularly as a test for oxide of copper and sesquioxide of iron.

$ 54.

15. FERRICYANIDE OF POTASSIUM (3 K, C1, N, Fe, = 3 K Cfdy).

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Preparation.-Conduct chlorine gas slowly into a solution of 1 part of ferrocyanide of potassium in 10 parts of water, with frequent stirring, until the solution exhibits a fine deep red colour by transmitted light (the light of a candle answers best), and a portion of the fluid produces no longer a blue precipitate, in a solution of sesquichloride of iron, but imparts a brownish tint to it; evaporate the fluid now in a dish to of its weight, and crystallize. The mother liquor will upon further evaporation yield a second crop of crystals equally fit for use as the first. Dissolve the whole of the crystals obtained in 3 parts of water, filter, if necessary; evaporate the solution briskly to half its volume, and crystallize again. Dissolve 1 part of the crystals, which are of a splendid red color, in 10 parts of water for use. The solution, as already remarked, must produce neither a blue precipitate nor a blue color in a solution of sesquichloride of iron.

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Uses. Ferricyanide of potassium decomposes with solutions of metallic oxides in the same manner as ferrocyanide of potassium. the metallic ferricyanides, the ferriprotocyanide of iron is more particularly characterized by its color, and we apply ferricyanide of potassium therefore principally as a test for protoxide of iron.

§ 55.

16. SULPHOCYANIDE OF POTASSIUM (K, C, N S, or K, Cy S1).

Preparation. Mix together 46 parts of anhydrous ferrocyanide of potassium, 17 parts of carbonate of potassa, and 32 parts of sulphur; introduce the mixture into an iron pan provided with a lid, and fuse at a gentle heat; maintain the same temperature until the swelling of the mass which ensues at first has completely subsided and given place to a state of tranquil and clear fusion; increase the temperature now, towards the end of the operation, to dull redness, in order to decompose the hyposulphite of potassa which has been formed in this process. Remove the half refrigerated and still soft mass from the pan, pulverize it, and boil with alcohol. Let the alcoholic solution cool, when part of the sulphocyanide of potassium will separate in colorless crystals; to obtain the remainder, distil the alcohol from the mother-liquor. Dissolve 1 part of the salt in 10 parts of water for use.

Uses.-Sulphocyanide of potassium serves for the detection of sesquioxide of iron; it is for that substance at once the most characteristic and delicate test.

b. SALTS OF THE ALKALINE EARTHS.

$56.

1. SULPHATE OF BARYTA, CHLORIDE OF BARIUM (Ba Cl + 2 aq.).

Preparation.-a. From heavy spar. Triturate crude sulphide of barium (§ 32), boil about of the powder with 4 times the quantity of water, and add hydrochloric acid until all effervescence of sulphuretted hydrogen has ceased, and the fluid manifests a feeble acid reaction; add now the remaining part of the sulphide of barium, boil for some time longer, then filter, and let the alkaline fluid crystallize. Dry the crystals, redissolve them in water, and crystallize again.

b. From Witherite. Pour 10 parts of water upon 1 part of Witherite, and gradually add crude hydrochloric acid until the Witherite is almost completely dissolved. Add now a little more finely pulverized Witherite, and heat, with frequent stirring, until the fluid has entirely or very nearly lost its acid reaction; add some baryta water or solution of sulphide of barium, as long as a precipitate forms; filter, evaporate to crystallization; dry the crystals, redissolve them in water, and crystallize again. For use, dissolve 1 part of the chloride of barium obtained in 10 parts of water.

Tests.—Pure chloride of barium must not alter vegetable colors; its solution must not be colored or precipitated by hydrosulphuric acid, nor by sulphide of ammonium. Pure sulphuric acid must precipitate every fixed particle from it, so that the fluid filtered from the precipitate formed upon the addition of that reagent leaves not the slightest residue when evaporated on platinum foil.

Uses.-Baryta forms with many acids soluble, with others insoluble compounds. This property of baryta affords us, therefore, a means of distinguishing the former acids which are not precipitated by chloride of barium from the latter, in the solution of the salts of which this reagent

produces a precipitate. The precipitated salts of baryta severally show with other bodies (acids) a different deportment. By subjecting these salts to the action of such bodies, we are therefore enabled to subdivide the group of precipitable acids, and even to detect certain individual acids. This makes chloride of barium one of our most important reagents to distinguish between certain groups of acids, and more especially also for the detection of sulphuric acid.

$ 57.

2. NITRATE OF BARYTA (Ba O, N 0.).

Preparation.-Pour 12 parts of water upon 1 part of carbonate of baryta, no matter whether Witherite or precipitated by carbonate of soda from solution of sulphide of barium, gradually add dilute nitric acid free from chlorine, and proceed exactly as directed in the preparation of chloride of barium from Witherite. For use, dissolve 1 part of the salt in 15 parts of water.

Tests. Solution of nitrate of baryta must not be made turbid by solution of nitrate of silver. Other tests the same as for chloride of barium.

Uses.-Nitrate of baryta is used instead of chloride of barium in cases where it is necessary to avoid the presence of a metallic chloride in the fluid.

$58.

3. CARBONATE OF BARYTA (Ba O, C 0,).

Preparation.-Dissolve crystallized chloride of barium in water, heat to boiling, and add a solution of carbonate of ammonia mixed with some caustic ammonia, or of pure carbonate of soda, as long as a precipitate forms; let it subside, decant five or six times, transfer the precipitate to a filter, and wash until the washing water is no longer rendered turbid by solution of nitrate of silver. Stir the precipitate with water to the consistence of thick milk, and keep this mixture in a stoppered bottle. It must of course be shaken every time it is required for use.

Tests.-Pure sulphuric acid must precipitate every fixed particle from a solution of carbonate of baryta in hydrochloric acid (compare caustic baryta).

Uses.-Carbonate of baryta completely decomposes the solutions of many metallic oxides, e. g., sesquioxide of iron, alumina; precipitating from them the whole of the oxide as hydrate and basic salt, whilst some other metallic salts are not precipitated by it. It serves therefore to separate the former from the latter, and is an excellent means to effect the separation of sesquioxide of iron and alumina from protoxide of manganese, oxide of zinc, &c., and also from lime and magnesia.

$ 59.

4. SULPHATE OF LIME (Ca O, SO,, crystallized Ca O, SO, +2 aq.).

Preparation.-Digest and shake powdered crystallized gypsum for some time with water; let the undissolved portion subside, decant, and keep the clear fluid for use.

Uses.-Sulphate of lime being a difficultly soluble salt, is a convenient agent in cases where it is wished to apply a solution of a lime salt or of a sulphate of a definite degree of dilution. As dilute solution of a lime salt, it is used for the detection of oxalic acid; whilst as dilute solution of a sulphate it affords an excellent means of distinguishing between baryta, strontia, and lime.

$ 60.

5. CHLORIDE OF CALCIUM (Ca Cl, crystallized Ca Cl + 6 aq.).

Preparation.-Dilute 1 part of crude hydrochloric acid with 6 parts of water, and add to the fluid marble or chalk until the last portion added remains undissolved; add some hydrate of lime, and let the mixture stand for several days in a gentle heat; filter, exactly neutralize the filtrate, concentrate by evaporation, and crystallize. Let the crystals drain, and dissolve 1 part of the salt in 5 parts of water for use.

Tests.-Solution of chloride of calcium must be perfectly neutral, and neither be colored nor precipitated by sulphide of ammonium; nor ought it to evolve ammonia when mixed with hydrate of potassa or hydrate of lime.

Uses.-Chloride of calcium is, in its action and application, analogous to chloride of barium. For, as the latter reagent is used to divide the inorganic acids into groups, so chloride of calcium serves in the same manner to effect the separation of the organic acids into groups, since it precipitates some of them, whilst it forms soluble compounds with others. And, as is the case with the baryta precipitates, the different conditions under which the various insoluble lime salts are thrown down, enable us to subdivide the group of precipitable acids, and even to detect certain individual acids.

$61.

6. SULPHATE OF MAGNESIA (Mg O, S O,, crystallized Mg O, S O,

HO+6 aq.).

Preparation.-Dissolve 1 part of sulphate of magnesia of commerce in 10 parts of water; if the salt is not perfectly pure, subject it to recrystallization.

Uses.-Sulphate of magnesia serves almost exclusively for the detection of phosphoric acid, which it precipitates from aqueous solutions of phosphates in presence of ammonia and chloride of ammonium, in the form of a double salt (basic phosphate of magnesia and ammonia), which is nearly insoluble, and has highly characteristic properties. Sulphate of magnesia is, moreover, employed to test the purity of sulphide of ammonium (see § 38).

c. SALTS OF THE OXIDES OF THE HEAVY METALS.

§ 62.

1. SULPHATE OF PROTOXIDE OF IRON (Fe O, S O,, crystallized Fe O, SO, HO+6 aq.).

Preparation.-Heat an excess of iron nails free from rust, or of clean iron wire, with dilute sulphuric acid, until the evolution of hydrogen ceases; filter the sufficiently concentrated solution, add a few drops of

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