precipitate, however, which is formed by an arsenate with AgNO, differs from that given by a phosphate by being brown in color; also AmHM004 gives with an arsenate a precipitate only on boiling.

280. The following differences serve to detect and separate a phosphate and an arsenate :

1. The fact that an arsenate only yields a precipitate with AmHMOO, when the liquid is boiled, whereas the corresponding precipitate with a phosphate is produced by a gentle heat.

2. An arsenate solution, if boiled with strong HCl, gives when HS is passed into the hot liquid, first a white precipitate of S, then yellow As,S,; a phosphate solution, under the same circumstances, gives no precipitate.

3. In a perfectly neutral solution, AgNO, gives with an arsenate solution a brown precipitate, with a phosphate a canary-yellow precipitate.

281. Hence, if occurring separately, a phosphate and arsenate may be distinguished by one or more of these differences.

281 a. The simplest method of ascertaining which of these two acids is present is to throw the precipitate yielded by AmCl, AmHO, and MgSO, upon a filter, wash it with a little cold water, and drop AgNO, upon it; if the precipitate consisted of MgAmAsO, it will become brown, if of MgAmPO, canary-yellow (difference 3). Or the precipitate produced by AmCl, AmHO, and MgSO, may be examined for arsenate by dissolving a part of it in dilute HNO,, adding AgNO, as long as it causes any white precipitate, and then very dilute Am HO gradually drop by drop; if arsenate is present a brown precipitate will form just before the liquid becomes alkaline to test-paper; phosphate gives a yellow precipitate. Instead of mixing the dilute AmHO it may be poured upon the top of the acid liquid when the color appears at the surface of contact.

282. If both arsenate and phosphate have to be tested for, AmCl, AmHO, and MgSO, are added as long as any further precipitate is caused; the precipitate is

filtered off* and dissolved in a little boiling strong HCl, HS is passed for several minutes into this hot HCI solution, when the presence of arsenate is shown as above (par. 280, 2); the precipitate is filtered off if any has formed, and HS passed into the boiling filtrate; if any further precipitate is produced, the gas must be passed again until it no longer causes a precipitate; the clear filtrate from this precipitate is then evaporated just to dryness, dissolved in a little dilute HNO,, and tested for phosphate by adding a few drops of it to AmHMOO solution and gently warming. Arsenite, if present, may be detected by HS in the filtrate from the Mg precipitate, acidified with HCI; yellow As,S, is precipitated.

UNGROUPED ACID RADICLES (INORGANIC). The salts of the following acid radicles present no marked resemblances in their reactions to one another or to other salts, and are usually detected by special tests.

BORATES ('BO,, or - "BO.).—Use Na,B,O,.10H2O 283. If some Na,B,O, be stirred with a little dilute HCl on a watch-glass and a strip of turmeric paper be half immersed in the liquid, then dried on a watch-glass in the steam-oven or on a water-bath,† the part which was moistened appears reddish-brown, and becomes bluish-black when moistened with AmHO. This is a most useful test for a borate.

284. If some methylated spirit be poured upon a little solid Na,B,O, in a test-tube or porcelain dish, some strong H2SO, added, and the mixture be heated and set

* A small portion of the precipitate may be filtered off upon another filter and tested at once for arsenate by dropping upon it AgNO3 solution. If the precipitate turns brown, arsenate is present and must be separated in the other part of the Mg precipitate before testing for phosphate; if the precipitate turns yellow, phosphate alone is present.

†The paper may also be dried over a small flame, if it is not heated above steam heat (100° C.).

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fire to, the spirit burns with a green-edged flame, seen best by blowing the flame out after it has burnt for a time, and then relighting it. This test is not quite characteristic, since some chlorides yield a similar green flame.

285. A mixture of CaF2, Na,B,O,,* and KHSO4, if finely powdered, moistened, and heated on a loop of platinum wire in the inner blowpipe flame, colors the outer flame momentarily green after being heated for a short time.

This test is not always decisive, since many phosphates and Cu salts give the same coloration, and it is besides liable to be masked by the flame colorations of other substances.

Hydrogen borate, or boric acid (HBO), is a white porous mass; with 2H,O it forms a crystalline scaly hydrate; when strongly heated these yield a fusible glass consisting of BO,. The solution of the acid reddens litmus-paper and colors turmeric reddish-brown, especially on drying the paper. The acid is soluble both in H.O and in alcohol; on evaporating these solutions, the acid passes off with the vapor of the liquid. Reactions 283, 284, and 285 will serve to detect boric acid.

CHROMATES ("CrO,).-Use K,Cr,O,.

All chromates are more or less yellow or red in color. The yellow solution of an alkaline chromate becomes orange-red on addition of an acid, owing to formation of a dichromate

286. Various substances, which readily combine with oxygen, deoxidize acid K¿Cr2O, solution to a solution of a chromic salt, the color of the solution at the same time changing from orange-red to a bright green. To show this add HCI or H,SO, to some K,Cr,O, solution, divide the orange-red solution into five parts, and deoxidize each of these portions by one of the following methods:

a. Pass HS into the hot solution; white S separates. b. Add some NaHSO, or H,SO, and warm.

*Boric acid is better, since the Na flame coloration tends to conceal the green coloration of BF3.

c. Add alcohol and boil.

d. Boil for some time, after adding much strong HCl. e. Add some Zn to the acidified liquid and warm. The change of color is in this case slow.

In each case the change in composition is shown by the liquid becoming green: the following equations show the decomposition occurring with HS and with HCl, the others may be drawn out by the student:

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K,Cr2O, +3H,S+8HC1=Cr2Cl + 2KCl +7H2O+S2. K ̧Cr2O, +14HCl = Cr2Cl ̧ + 2KCl + 7H2O ÷ 3C12.

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This green solution gives all the reactions for Cr (102-105).

Note. Since chromic acid is thus reduced by boiling with HCI and by HS to a chromic salt, a chromate leads to precipitation of Cr2Ho in Group III in the ordinary course of analysis.

Hydrogen peroxide (H ̧O2), if added after some ether to an acid solution of a chromate, on well shaking the liquids causes the ether to assume a blue color; addition of a few drops of strong HNO, is useful. The H2O2 may be produced by adding BaO2 to the cold acid liquid. This reaction is extremely delicate and characteristic.

287. AgNO3: purple-red precipitate (Ag,CrO,): soluble in AmHO and in HNO.

288. PbA,: yellow precipitate (PbCrO,): soluble in KHO, insoluble in HĀ.

289. BaCl, yellowish-white precipitate (BaCrO1) : insoluble in HA, soluble in HCl.

Hydrogen chromate or chromic acid (HCrO,?). CrO, dissolves in water to a reddish-yellow liquid, with a strongly acid reaction. The liquid evolves Cl, and becomes green if it is boiled with HCl; when neutralized by an alkali it becomes yellow, the solution yielding the reactions in pars. 287, 288, 289.

SILICATES.

Use Na,SiO, solution for liquid;
for solid, finely ground sand
or solid Na,SiÓ ̧.

290. HCl added slowly drop by drop to a solution of Na,SiO, whilst stirring or shaking it, will usually, unless the solution is very dilute [see note 2 below], give a gelatinous precipitate of HSiO.; but this precipitate is very apt to remain dissolved, hence its nonappearance is no proof of the absence of H1SiO.

290 a. Small quantities of H,SiO,, or H,SiO, in dilute solution, may be detected by evaporating the solution, made acid with HCl, to dryness in a porcelain dish, and heating the residue to 100° as long as acid fumes come off; on stirring and warming the dry residue with strong HCl, SiO, remains undissolved as a white powder.

Note 1.-During the last part of the evaporation the silica often separates as a gelatinous mass, and unless the heat is lessened and the residue stirred with a glass rod, it will spirt; spirting is best avoided by finishing the evaporation, as soon as the liquid becomes thick, upon a water-bath. The undissolved SiO, left on treating with HCl is best seen by pouring out the liquid into a test-tube; it is soluble in hot KHO solution.

Note 2.—The solution of Na„SiO, used for these reactions is made of such a strength that acids do not precipitate HSiO from it, but by boiling some of it down to a quarter its volume before adding the acid the precipitate is usually obtained.

291. Am Cl (or Am, CO) causes a gelatinous precipitate of HSiO4, which usually forms only after a time.

292. Blowpipe Tests.-A little solid SiO, (or most silicates), if fused into a bead of Na,CO, causes frothing from CO escaping:

NaCO3 + SiO2= CO2 + Na,SiO,.

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293. Fused in a bead of NaAmHPO, the SiO, is not dissolved, but floats about in transparent particles and on cooling remains as little opaque masses in the bead. The particles of SiO, are usually best seen in the fused bead.

Hydrogen silicate, or silicic acid (HSiO), is a white flocculent substance with no reaction to litmus; it is insoluble in water, and but very slightly soluble in acids, but dissolves as an alkaline silicate in solutions of the alkalies or alkaline carbonates. On ignition it forms a white insoluble powder (SiO2) which yields reactions 292 and 293.