298. CaCl2 added to NaF solution* gives a gelatinous almost transparent precipitate (CaF2), which becomes more visible on heating or adding AmHO: slightly soluble in HCl, almost insoluble in HA.

Hydrogen fluoride, or hydrofluoric acid (HF), is a colourless gas which fumes in the air, etches glass and dissolves easily in water. The solution of HF is acid in reaction and differs from all other acids in dissolving SiO2 and insoluble silicates.

FLUOSILICATES ("SiF).—Wse H¿SiF ̧·

299. BaCl2: white precipitate (BaSiF), which usually appears only on gently warming the solution, and thus differs from BaSO4: it is somewhat soluble in boiling HCl, but insoluble in alcohol. Put by some of this precipitate for reaction (301).

This precipitate differs from the precipitate of BaSO4, which it somewhat resembles in being rather insoluble in HCl, by being more or less transparent and crystalline: but fluosilicates differ from sulphates also in giving no precipitates with Sr(NO3)2 or Pb(NO。), and in giving, when heated with excess of AmHO, a flocculent precipitate of HSiO and AmF in solution (297); fluosilicates differ also by the following very distinctive reactions:

300. KCl, on warming, gives a very transparent gelatinous precipitate (K,SiF), which is visible only after being allowed to settle for a time; it is insoluble in alcohol.

301. By evaporating H2SiF solution, or heating a fluosilicate, with strong H2SO4, HF is given off and may be detected by its etching action on glass (294).

Evaporate a little H2SiF [or BaSiF, from (299)], after adding strong H2SO4, upon a watch-glass to dryness; on washing off the residue the glass will be found to be etched.

6

301 a. A solid substance (e.g. the precipitate of BaSiF or K2SiF) may also be examined by any of the methods given above for HF, the most delicate being (296). This

* Prepared by boiling in water the mass obtained by fusing for several minutes on platinum-foil some powdered CaF, and fusion-mixture, filtering the solution and acidifying the filtrate with HA,

method is particularly useful for proving a precipitate produced by BaCl, to consist of BaSiF.*

302. The above reactions readily distinguish between sulphates and fluosilicates, both of which give precipitates with BaCl, which are ininsoluble in boiling HCl. There is only one other class of salts-the selenates-which give with BaCl, a white precipitate insoluble in HC1; and this precipitate (BaSeO4), if boiled with strong HCl for some time, evolves Cl and gradually dissolves as BaSeO3, thus differing from BaSO4 and BaSiF; further, this HCl solution, if warmed with H2SO, yields a reddish precipitate of Se.

Hydrogen fluosilicate, or hydrofluosilicic acid (HSiF6), is a strongly acid liquid, volatilising entirely when heated in platinum as 2HF+SiF: by evaporating it in glass, the glass is etched. It is precipitated by KCl and by BaCl2.

REACTIONS OF ORGANIC ACID-RADICLES.

The following acid-radicles are included here (although the first four are frequently, and the other three always, classed amongst organic compounds), because they are of such frequent occurrence, and their detection is usually a matter of so little difficulty, that they may reasonably be appended even to an elementary treatise on inorganic analysis.

CYANIDES ('Cy).—Use KCy.

HCy smells of bitter almonds, the alkaline cyanides smell only faintly of the same.

303. AgNO: white precipitate (AgCy): show with three separate portions that this precipitate is soluble in KCy solution added in excess, and in AmHO, but insoluble in dilute HNO3.

* The readiest means of producing an etch with this precipitate is to pour it off into a watch-glass, decant the water, and warm the precipitate for some time with strong H2SO4; on washing the glass and drying it, it will be found to be corroded. When the more delicate process (296) has to be employed, the precipitate should be shaken up with the liquid, poured off into a platinum crucible or leaden cup, the liquid decanted, and the precipitate warmed with strong H2SO, as described in (296).

If some of the precipitate be filtered off and well washed with hot water until free from AgNO, (shown by HCl causing no milkiness with a few drops of the washing water), then dried, scraped from the filter into a porcelain crucible, and heated strongly, Ag is left: if this residue on cooling be warmed with a little dilute HNO3, it dissolves, giving off red fumes; and a drop of HCl added to this solution gives a curdy precipitate of AgCl. This reaction distinguishes AgCy from AgCl, AgBr and AgI, which like it are precipitated by AgNO, and are insoluble in HNO3, since these three precipitates when dried and strongly heated melt without decomposing; hence, after cooling, if HNO, be warmed with them it dissolves out no Ag, and HCl added to the HNO, gives either no precipitate or a mere milkiness due to imperfect washing of the precipitate.

This method is somewhat tedious and need seldom be used, cyanides being readily detected by the following reactions.

304. If some FeSO4 solution, mixed with several drops of Fe,Cl, be added to KCy solution and the liquid be then. made strongly alkaline with KHO and boiled, then cooled and HCl added to acid reaction, "Prussian Blue" (Fe""Cfy,*) remains as a deep blue precipitate; or, if in small quantity, it dissolves giving a bluish-green solution. If no cyanide had been present, the addition of HCl would have given a nearly colourless liquid.

305. If to some KCy solution, contained in a test-tube fitted as shown in fig. 37 (p. 132), dilute H2SO4 is added, then a small piece of marble dropped in and the liquid boiled, allowing the steam to pass off into some Am2S solution, AmCyS is formed. This substance gives with FeCl a bloodred coloration, which may be obtained by pouring the Am,S solution out into a porcelain dish, boiling it, allowing it to cool, making acid with HCl, and adding several drops of FeCl a blood-red colour, only faintly seen when but little KCy is used, shows that HCy has passed into the Am,S producing AmCyS, and therefore proves the presence of a cyanide.

Note. The marble is used to cause evolution of CO, gas, which lessens the risk of the Am,S being sucked back during the reaction.

A little HgCl solution should be added to a few drops of the red liquid, if the red colour is destroyed it was certainly due to the presence of AmCyS.

* The symbol Cfy conveniently represents the group (FeCy); it is used here to avoid brackets.

306. The former test can often be more simply performed by boiling the solution (of KCy or HgCy) with Am2S, which must be added until the yellow colour remains after boiling for a short time. [If a dark-coloured precipitate forms, this must be filtered off before the colour of the filtrate can be seen.] Then filter if necessary, acidify with HCl in a porcelain dish, and add FeCl in small quantity; a blood-red colour is produced in the liquid, which is not removed by heat or addition of HCl, but may be destroyed by adding to a few drops of the liquid some HgCl2.

Note 1.-On adding HCl to the Am,S (305, 306), white S separates, unless the yellow liquid has been previously boiled in a porcelain dish until it becomes colourless (filling in distilled water as it evaporates); but this separation of S is advantageous, since it helps to show by its whiteness even a faint reddish tint in the liquid. Methods (304, 305, 306) are exceedingly delicate tests for a cyanide. Methods (305) and (306), if less readily performed than (304), exceed it in delicacy. Method (306) is the more easy of the two to perform, and (305) is only used in cases where after boiling with excess of Am,S, the filtrate is darkcoloured, and therefore the red coloration would be invisible.

Note 2.-HgCy, does not show the reactions for a cyanide by methods (303, 304, and 305); but by boiling with Am,S, according to method (306), AmCyS is obtained in the filtrate from the black HgS precipitate, and will give the red coloration with FeCl.

Solid HgCy, heated in a small tube closed at one end also gives off cyanogen gas, which burns with a peach-blossom coloured flame.

Hydrogen cyanide, or hydrocyanic ("Prussic") acid, is a colourless, volatile, inflammable liquid, smelling like bitter almonds it is highly poisonous, both when inhaled as vapour and swallowed as liquid: it readily mixes with water. The acid is easily detected by AgNO, (303), also by (304), or by formation of AmCyS when the liquid is boiled with Am,S (306).

* The latter part of this formula is conveniently written Cfy to avoid

using brackets.

L

(FeCfy): insoluble in HCl, soluble in H2C2O4 to a dark blue liquid: changed by KHO into brown Fe,Ho.

Note.-The solubility in H2Ō and insolubility in HCl are shown by pouring off portions, heating them with HCl and H2O respectively, filtering, and noting whether the filtrate is blue : third portion is mixed with KHO, and becomes brown.

308. FeSO4 light-blue precipitate (Fe"K,Cfy), which quickly becomes darker on standing in the air, or immediately on addition of Cl-water or Br-water, or on warming with HNO. It is insoluble in HCl.

309. CuSO4: a chocolate-coloured precipitate (Cu",Cfy), insoluble in HA.

310. AgNO: white precipitate (Ag,Cfy), insoluble in HNO, and in AmHO. If heated with HNO, this precipitate changes to orange-red AgCfy2, soluble in AmHO.

vi

FERRICYANIDES [ – (FeCy¿)2, or – Cƒy2].-Use K(FeCу6)2, (K&Cfy2).

311. FeCl dark-greenish or brown coloration; no precipitate is formed, as may be seen by adding much water to the liquid.

312. FeSO4 dark-blue precipitate, "Turnbull's Blue," (Fe",Cfy2): insoluble in acids: blue colour destroyed by KHO. 313. CuSO4: yellowish precipitate.

314. AgNO: orange-coloured precipitate (AgCfy2), insoluble in HNO3, soluble in AmHO. Frequently a white residue is left on adding AmHO, consisting of Ag,Cfy, and showing that KCfy was present in the K.Cfy, solution.

SULPHOCYANIDES ('CyS).-Use KCyS.

315. FeCl blood-red coloration, but no precipitate: the colour is destroyed by HgCl, solution, but not by HCl.

316. Cyanides are usually detected in analysis by methods (304, 305, 306). Ferro- and ferri-cyanides can be found and distinguished by their first three tests, or by the precipitate given by AgNO3 in the solution acidified with HNO3; this precipitate, if a ferricyanide is present, will have a more or less decided orange-red colour; on filtering, or decanting, and warming the precipitate with AmHO, white AgCfy