exhibit the most characteristic colors of all; ferrocyanide of potas sium serves therefore particularly as a test for oxide of copper and sesquioxide of iron.

§ 57.

12

6

=

15. FERRICYANIDE OF POTASSIUM (3 K, C2 N, Fe, 3 K Cfdy). 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 color 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. [The commercial salt is suited for all analytical purposes. This salt undergoes decomposition when long kept in solution. It is therefore best applied in the state of a fine powder.]

Uses.-Ferricyanide of potassium decomposes with solutions of metallic oxides in the same manner as ferrocyanide of potassium. Of 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.

§ 58.

16. SULPHOCYANIDE OF POTASSIUM (K, C, N S2 or K, Cy S). 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 refrige rated 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 char acteristic and delicate test. Its solution must remain colorless when heated with pure and dilute hydrochloric acid.

b. SALTS OF THE ALKALINE EARTHS.

§ 59.

CHLORIDE OF BARIUM (Ba C1+2 aq.).

Preparation.-a. From heavy spar. Triturate crude sulphide of barium (§ 34), 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.

§ 60.

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

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.

§ 61.

3. CARBONATE OF BARYTA (Ba O, C O2).

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. For this purpose it must be remembered that the sulphates cannot be employed, as from them it throws down all the bases above mentioned.

8 62.

4. SULPHATE OF LIME (Ca O, S O, crystallized Ca O, S O,+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.

§ 63.

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 then solution of hydrosulphuric acid until a filtered sample is not affected by sulphide of ammonium. Let it stand 12 hours at a gentle heat in a closed vessel; 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.

§ 64.

6. SULPHATE OF MAGNESIA (Mg O, S O3, crystallized Mg O, S O3, HO+6 aq.).

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

Tests. The solution of sulphate of magnesia must have a neutral reaction. When mixed with sufficient chloride of ammonium it must remain unaffected by ammonia, carbonate and oxalate of ammonia, and sulphide of ammonium for the space of half an hour.

Uses. Sulphate of magnesia serves almost exclusively for the detection of phosphoric and arsenic acids, which it precipitates from aqueous solutions of their salts in presence of ammonia and

chloride of ammonium, in the form of double salts (basic phosphate or arsenate of magnesia and ammonia), which are nearly insoluble, and have highly characteristic properties. Sulphate of magnesia is, moreover, employed to test the purity of sulphide of ammonium (see § 41).

c. SALTS OF THE OXIDES OF THE HEAVY METALS.

$ 65.

1. SULPHATE OF PROTOXIDE OF IRON (Fe O, S O, crystallized Fe 0, SO, HO+6 aq.).

39

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 dilute sulphuric acid to the filtrate, and allow it to cool. Wash the crystals with water very slightly acidulated with sulphuric acid, dry, and keep for use. The sulphate of prot oxide of iron may also be prepared from the solution of sulphide of iron in dilute sulphuric acid, which is obtained in the process of evolving hydrosulphuric acid.

Tests. Sulphate of protoxide of iron must appear as fine, pale, green crystals. Those which by action of the air have become yellow, or which give with water a brownish-yellow solution, are to be rejected. The solution, after addition of a little hydrochloric acid, must not be precipitated black by hydrosulphuric acid.

Uses.-Sulphate of protoxide of iron has a great disposition to absorb oxygen, and to be converted into the sulphate of the sesquioxide. It acts, therefore, as a powerful reducing agent. We employ it principally for the reduction of nitric acid, from which it separates nitric oxide by withdrawing three atoms of oxygen from it. The decomposition of the nitric acid being attended, in this case, with the formation of a very peculiar brownish-black compound of nitric oxide with an undecomposed portion of the salt of the protoxide of iron, this reaction affords a particularly characteristic and delicate test for the detection of nitric acid. Sulphate of protoxide of iron serves, also, for the detection of hydroferricyanic acid, with which it produces a kind of Prussian blue, and also to effect the precipitation of metallic gold from solutions of the salts of that metal.

§ 66.

2. SESQUICHLORIDE OF IRON (Fe, Cl).

Preparation. Heat in a flask a mixture of 10 parts of water and 1 part of pure hydrochloric acid with small iron nails until no further evolution of hydrogen is observed, even after adding the nails in excess; filter the solution into another flask, and conduct