Page images
PDF
EPUB

and by its property of blackening PbA-solution by the formation of PbS. The gas may be made to act upon the PbA, solution by the methods given for CO2 (229), or better by placing a piece of filter-paper moistened with PbA-solution on the end of a glass rod so that one-half of the slip adheres to the glass rod and the other hangs free, and holding the rod in the gas as in the test for CO2 (229, 1).

Note. This test is made much more delicate if the paper is moistened with a solution prepared by adding KHO to boiling PbA-solution until the precipitate first formed dissolves: a test-paper thus prepared becomes intensely blackened by H,S.

231. Solutions of soluble sulphides if dropped upon a bright silver coin produce a black stain (Ag2S); this may be removed by rubbing the coin with moist lime.

232. Insoluble sulphides which give off HS with HCl (e.g. FeS) produce a black stain when placed on a silver coin and moistened with a drop of HCl.

233. PhД-solution gives with soluble sulphides a black precipitate (PbS). The solution produced by adding KHO in excess to PbA, solution is more delicate than PbA-alone.

234. Many sulphides (e.g. FeS2, "Iron Pyrites") and also free S, if strongly heated in a tube open at both ends and held obliquely in the flame, or in the tube shown in fig. 35 (p. 114), give off SO2 gas, recognised by its pungent smell and by turning paper moistened with K,Cr2O, green.

235. Reaction (306) given under HCy may be employed also as a very delicate test for soluble sulphides. Test (228) for a sulphate, also answers for a sulphide.

Hydrogen sulphide, or hydrosulphuric acid (H2S), in aqueous solution is easily recognised by (231) or (233), and, unless the solution is very dilute, also by its smell, and, by suspending lead-paper over the liquid: HS gas is detected by (230).

SULPHITES ("SO).-Use Na,SO,.

236. HCl (or H,SO) poured upon Na,SO, and warmed gives off SO2 gas, known by its pungent smell and by turning K,Cr2O, solution green: the K,Cr2O, solution may be

exposed to the gas by the methods described under (229), preferably by introducing a glass rod with a drop hanging upon its end into the gas; or by moistening a small strip of filter-paper with the K,Cr2O, solution and making it adhere by one-half of its length to the rod (230), the other half hanging freely, and introducing it into the gas as in (229, 1).

4

7

237. BaCl2: white precipitate (BaSO): entirely soluble in HCl, unless some Na2SO, is present, when BaSO4 remains undissolved on adding Cl- or Br-water to the HCl solution, BaSO, is formed and precipitated.

4

BaSO3+ 2HC1 = BaCl2 + H2SO3.

BaCl2 + H2SO3 + H2O + 2C1 = BaSO4 + 4HC1.

238. Na2SO3 added to a mixture of HCl and Zn which is giving off pure H, immediately causes an evolution of H2S with the H: the H2S is detected by its smell or by blackening a piece of filter paper moistened with solution of PbO in excess of KHO, see note (230).

Hydrogen sulphite, or sulphurous acid (H2SO), can be found in aqueous solution by adding a drop of K,Cr,O, solution, which will at once become green: SO, gas is found as directed under (236).

THEIOSULPHATES* (= "S2O3).—Use Na2S2O3.5H2O.

239. HCl (or H2SO4) causes no immediate change in the cold, but the liquid on standing or on being warmed deposits a precipitate of yellow S, and SO2 is given off; the SO, is recognised by its pungent smell and by changing the colour of K,Cr2O, solution (236).

240. FeCl gives a reddish-violet colour which vanishes after a short time or immediately when heated, the FeCl solution at the same time loses its yellow colour owing to the change of FeCl into FeCl2.

240 a. AgNO3; a white precipitate (Ag2S2O), becoming

Formerly known as Hyposulphites.

black after standing for a short time, or immediately upon being heated, owing to the formation of Ag2S :

Ag2S2O3 + H2O = Ag2S + H2SO4 ·

The above precipitate (Ag2S2O) dissolves very readily in excess of the Na2SO solution, hence it is most easily obtained by dropping the latter into some AgNO, solution.

Hydrogen theiosulphate, or theiosulphuric acid (H2S2O3), is extremely unstable, rapidly separating into S + H2SO ̧.

HYPOCHLORITES (-'CIO).-Use NaClO.NaCl (NaClO),

solution.

Hypochlorites give off a faint smell of Cl in the air if solid or in strong solution.

241. HCl (or H2SO or even H2Ō) in the cold, or when gently warmed if much water is present, sets free Cl-gas, which may be recognised by its smell, its yellowish colour, and by bleaching a piece of moistened litmus-paper, which is held for a short time in the tube without touching the liquid or the sides of the tube.

A piece of litmus shaken up with the solution, without addition of acid, is also bleached, the CO, in the air probably liberating the Cl: on addition of a drop of any acid the bleaching is very rapid :

Na2Cl2O + 2HCl = 2C1+2NaCl + H2O.

242. Pb(NO3)2 (or PbA) solution added in large quantity gives a white precipitate, becoming reddish, and then dark brown (PbO2) when boiled for a short time.

MnCl, likewise gives a dark brown precipitate on heating. Hydrogen hypochlorite, or hypochlorous acid (HClO), is a yellow liquid with sweetish smell, which if strong is very unstable it bleaches litmus, and evolves Cl when warmed with HCl. Cl2O gas is of a deep yellowish-green colour, and has an irritating smell, it dissolves easily in water to form the above acid.

NITRITES (NO2).-Use KNO2.

243. Warmed with dilute H2SO4, reddish-brown "nitrous" fumes with a characteristic smell, are given off; they are best seen by looking down the tube at some white object.

244. If cold FeSO4 solution be added to KNO, solution, it becomes brown; on adding cold dilute H2SO4 the colour becomes much more intense: it is produced by the combination of NO with FeSO4. This colour is destroyed by boiling, red fumes being given off (243).

245. If solution of KI and several drops of starch solution (made by boiling starch in water, and cooling it) be added to KNO2 solution and the liquid be then made acid with HA, an intense blue colour will be produced, owing to the combination of iodine (set free by the HNO2) with the starch the experiment is best done in a white procelain dish, the liquid, which often appears black at first, being diluted if necessary till its blue colour becomes visible. This is a most delicate test, and is rather characteristic if the colour is not produced till the HÃ is added.

Hydrogen nitrite, or nitrous acid (HNO2), is extremely unstable, decomposing into HNO, and NO.

3

**The student will have no difficulty in detecting any one of the above acid-radicles occurring singly in a liquid or solid warming with HCl will usually indicate which of them is present, and its presence may then be confirmed by a second test chosen from amongst its reactions as stated above.

246. (s.) Most of the acids formed by the salts of Group II. on addition of HCl, react upon one another; e.g. HCIO destroys HNO2, H2S and H2SO,, and HS destroys H2SO.. The analyst may often by skilful use of the above reactions detect these acids when mixed, but their detection in certain mixtures is a problem only soluble by careful consideration, and not unfrequently insoluble.

SO,, if evolved, may be detected by K,Cr2O, paper: HS by PbA, paper: Cl by bleaching moist litmus-paper: N2O, by its red colour. But CO2 can only be detected in presence of much SO, by passing the gases through K,Cr,O, solution which absorbs SO2, CO2 passing on and being detected by lime-water.

GROUP III.-NITRATE GROUP.

The two classes of salts which follow resemble one another

in many respects, more particularly in no member of them being insoluble in water; hence no method of precipitating them is known.

NITRATES (NO3).—Use KNO ̧ .

247. Add to some KNO, solution contained in a rather broad test-tube about an equal bulk of strong H2SO4; cool the liquid by letting a stream of cold water run upon the tube from the tap; then hold the tube in a slanting position, and pour some strong cold FeSO4 solution in a gentle stream down the inside of the tube. If this solution is poured in with proper care it will form a distinct layer resting on the surface of the acid liquid, and at the surface of contact of the two layers a dark-brown film or ring will appear, sometimes extending upwards towards the surface of the liquid. The colour is most easily seen by holding a piece of filter-paper between the tube and a bright flame or window, and looking through the tube at the light. A mere yellow colour is often produced in the absence of a nitrate and may be disregarded.

The test is made more delicate by letting the tube stand in a small beaker of cold water for a few minutes, since heat prevents or destroys the brown colour. If much nitric acid is present, on mixing the acid and FeSO4 solution by shaking, the brown colour extends upwards, until by the heat evolved by dilution of the acid the NO is expelled forming brown fumes in the test-tube, and the liquid loses its dark brown colour (244).

248. Place some small scraps of Cu in some KNO, solution, and add strong H,SO,; reddish nitrous fumes appear at once or on warming the tube: they are best seen, if small in quantity, by looking down the tube at a white surface.

« PreviousContinue »