fig. 33. Pour enough concentrated sulphuric acid into the retort to completely cover the crystals of potassium nitrate. Place the receiver in cold water, and allow a stream of water to flow over its outer surface. On warming the contents of the retort, nitric acid is formed, and is condensed to a yellowish liquid in the receiver;-KNO, + H,SO, = KHSO + HNO2.

3

4

Part of the nitric acid is decomposed by the action of the hot sulphuric acid, giving water, oxygen, and brown fumes of nitrogen tetroxide, NO. Boil a little nitric acid in a small flask arranged with a cork and exit tube dipping under water, and prove that oxygen is evolved; notice also the brown gas which is produced. The change may be represented thus

Nitric acid then ought to act as an oxidising agent. To prove that it does thus act, perform the next Exp.

Exp. 14. Pour a little of the nitric acid you have prepared, drop by drop, on to some powdered antimony in a basin; the antimony is oxidised to white antimony oxide (Sb,O,).

We shall now take advantage of the oxidising power of nitric acid in order to prepare phosphoric acid from phosphorus.

Exp. 15. Place a few small pieces of phosphorus in a basin, add some concentrated nitric acid, and heat so long as a brown gas (chiefly NO) is evolved; then add a little more nitric acid; and repeat this treatment until the phosphorus has been wholly dissolved. Heat the basin over a low flame in the draught place until the liquid becomes thick and syrupy; add a little water, and heat again. The unchanged nitric acid is thus removed, and a concentrated solution of orthophosphoric acid (H.PO,) remains

2P+10HNO,Aq = 2H,PO1Aq + 2H2O +5N2O.

Neutralise a small quantity of this solution by ammonia, and then add silver nitrate solution; a pale yellow pp. of silver orthophosphate (Ag,PO) is produced.

Now dissolve a little phosphorus pentoxide (PO) in water; boil for some time, and prove, by the foregoing test, that the solution contains orthophosphoric acid. The change is

P2O5 + 3H2O + Aq = 2H,PO1Aq.

Boil the rest of the solution of orthophosphoric acid which you prepared to dryness in a platinum dish, and continue to heat the residue over a Bunsen-lamp for some little time. On cooling, you obtain a solid glass-like mass. This is metaphosphoric acid HPO, (H,PO – H2O = HPO2). Dissolve this solid in water, and test the solution; (1) by neutralising by ammonia and adding silver nitrate solution, a white pp. of silver metaphosphate (AgPO) is formed; (2) by adding an albumencontaining liquid (a very little white of egg shaken up with much water), the albumen is coagulated and precipitated.

Now dissolve a little phosphorus pentoxide in a very little ice-cold water, and prove that this solution contains metaphosphoric acid. The change is

P2O + H2O + a little Aq= 2HPO„Aq.

There is another phosphoric acid, called pyrophosphoric, HP2O,; this may be obtained by heating orthophosphoric acid to about 250° until water is no longer removed (2H,PO̟ ̧ – H„O = HP,O,), but it is more easily prepared from lead pyrophosphate, which may be readily obtained from sodium pyrophosphate, which salt is itself produced by heating ordinary sodium orthophosphate. This process is representative of a commonly employed method for preparing acids from their salts.

Exp. 16. Prove, by the silver nitrate test (Exp. 15), that ordinary sodium phosphate is a salt of orthophosphoric acid. Now heat some sodium phosphate to redness in a platinum dish as long as there is any apparent change. Allow the solid residue to cool, dissolve in cold water and apply the following tests to small quantities of the solution :

(1) silver nitrate; a white pp. is produced.

(2) albumen is not coagulated, after the solution has been acidified by acetic acid.

The salt you have prepared is sodium pyrophosphate; the change from orthophosphate may be thus represented; 2Na, HPO - H2O = Na1P2O7.

Prepare lead pyrophosphate, Pb,P,O,, by adding an aqueous solution of lead nitrate to the remainder of the solution of sodium pyrophosphate; collecting the white pp. which forms, and washing it with cold water until the washings are free from nitrates;

(Na,P2O,Aq + 2Pb2NO2Aq = Pb2P2O, + 4NaNO„Aq).

Now suspend the greater part of the lead pyrophosphate in cold water, pass a stream of washed sulphuretted hydrogen (HS) through the liquid until it smells decidedly of the gas; then add the remainder of the lead pyrophosphate, in order to remove the excess of HS which is present, and filter off the pp. of PbS and excess of Pb,P,O,. The changes which have occurred may be represented thus:

Pb,P2O, + Aq + xH ̧S = 2PbS + H ̧Ð ̧О„Aq + (x − 2) H2S; then the remaining HS is decomposed by the Pb,PO, added, and the excess of Pb.PO, remains unchanged.

Neutralise by ammonia the aqueous solution of pyrophosphoric acid which you have thus prepared; and prove that it then gives the same reactions as the solution of sodium pyrophosphate already tested. Boil a portion of the solution and prove that it now contains orthophosphoric acid (H,P,O,Aq + H2O = 2H2PO1Aq).

The foregoing exps. (13 to 16) have taught us that the oxide NO, interacts with water to form one acid only; and that the oxide P2O, interacts with water to form three distinct acids: of these, orthophosphoric (H,PO) is the most stable, as regards the action of heat, in aqueous solution, but metaphosphoric (HPO3) is the most stable, as regards the action of heat, when in the solid state. (For some account of the acids obtained from the oxides of arsenic and antimony, see ELEMENTARY CHEMISTRY, Chap. XI. pars. 214-217.)

We shall now perform a few experiments illustrative of the preparation and properties of some of the sulphides of the elements we are considering.

Exp. 17. Through solutions (i) of arsenious oxide in very dilute hydrochloric acid, (ii) of tartar emetic in very dilute hydrochloric acid, (iii) of bismuth chloride in very dilute hydrochloric acid, pass sulphuretted hydrogen gas; collect the pps. which form and wash each two or three times with water. The composition of these pps. is represented by the formula

M2S, where M = As, Sb, or Bi.

To separate small quantities of each pp. add (i) ammonium sulphide solution, (ii) potash solution, and warm; the sulphides of arsenic and antimony dissolve in each case, the sulphide of bismuth remains unchanged.

To the solutions in ammonium sulphide add a good deal of alcohol; white crystals are pptd. These crystals are ammon

M. P. C.

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ium thioarsenite, (NH),ASS,, and thioantimonite (NH) SbS2, respectively. The changes which occur when As,S, and Sb,S, severally interact with ammonium sulphide may be represented as follows:

The sulphides of arsenic and antimony, M,S,, are therefore acidic, i.e. they interact with sulphides of very positive. elements, or with sulphides of the positive compound radicle NH, to form salts. The corresponding sulphide of bismuth, BiS, is not acidic. Compare the properties of the oxides MO, with those of the sulphides M,S, where M = As, Sb, or Bi.

The foregoing experiments shew that the elements nitrogen, phosphorus, arsenic, antimony, and bismuth are analogous in their chemical properties; and that as the combining weights of the elements increase the elements become more metallic and less negative.

Reference to ELEMENTARY CHEMISTRY, Chap. XI. pars. 206 -225.

CHAPTER XV.

CLASSIFICATION OF ELEMENTS (continued).

THE elements chromium, manganese, and iron, are placed in the same class.

These elements form hydrated oxides M ̧ ̧. 3H ̧O where M= Cr, Mn, or Fe.

Exp. 1. To solutions of (i) ferric chloride (Fe,Cl ̧), and (ii) chromic chloride (CrCl), add a slight excess of ammonia solution. Collect the pps. which form; wash them repeatedly with hot water; and prove that a small quantity of each dissolves readily in hydrochloric acid; set these solutions evaporating in the draught cupboard, (see next Exp.). Now place the rest of each pp. in a small basin and heat these at 100° so long as water comes off. The solids have now the composition MO.3H0 where M = Fe or Cr. Heat these solids strongly over a Bunsen-lamp; water is removed and the oxides M,O, remain. Prove that the oxides thus prepared are nearly insoluble in concentrated hot hydrochloric acid.

From these reactions we would expect that a solution of a manganic salt (say manganic chloride) would interact with ammonia to produce a pp. of the composition Mn,O,.3H0. But the salts of manganese corresponding to Fe,Cl, and CrCl are extremely unstable and can hardly be obtained even approximately pure.

Exp. 2. Add some ammonium chloride and then a slight excess of ammonia to an aqueous solution of manganous chloride (MnCl); an almost white pp. of manganous hydrate, MnO.H2O, forms. Pour the contents of the tube into a large stoppered bottle, and shake up the pp. repeatedly, removing the stopper at intervals and blowing in fresh supplies of air. The pp. slowly turns brown; the brown solid eventually pro