are not soluble in water, are dissolved in hydrochloric acid, or in aqua regia. Some natural arseniates of metallic oxides require fluxing with carbonate of soda, as an indispensable preliminary to their solution. Metallic arsenic, and likewise metallic arseniurets are dissolved in aqua regia; those of the metallic arseniurets which are insoluble in this menstruum, are fluxed with carbonate of soda and nitrate of potass; this process gives rise to the formation of a soluble arseniated alkali and an insoluble metallic oxide. All solutions of compounds of arsenic which have been effected by heating with aqua regia in excess, contain arsenic acid.

Determination.

Arsenic is weighed either as ARSENIATE OF LEAD, or as SULPHARSENIOUS ACID; and in some instances also as ARSENIATE OF PEROXIDE OF IRON.

We may convert into

1. ARSENIATE OF LEAD.

Arsenious and arsenic acid when present singly either in aqueous or nitric acid solution.

2. SULPHARSENIOUS ACID.

All compounds of arsenic without exception.

3. The method of determining arsenic in the form of arseniate of peroxide of iron (Berthier) is executed like that used for the determination of phosphoric acid, vide § 101. I omit describing this method here, since it is, in every respect, inferior to that of precipitating as sulpharsenious acid.

1. Determination as arseniate of lead.

a. Arsenic acid in aqueous solution.

A weighed portion of the solution is introduced into a platinum or porcelain dish, and a definite amount of recently ignited pure oxide of lead added; the mixture is cautiously evaporated to dryness, and the residue heated to gentle redness, and maintained some time at this temperature. The residue, which consists of arseniate of lead + oxide of lead, is then weighed. The amount of arsenic acid present in the analysed solution is now readily found by

subtracting the weight of the oxide of lead added, from that of the residuary mass, and calculating the result upon the whole solution. For the properties of arseniate of lead, vide § 66. The results are perfectly accurate, provided the residue be not heated beyond gentle redness.

b. Arsenious acid in solution.

The solution is mixed with nitric acid, a small portion of hydrochloric acid added, and the mixture evaporated to dryness in an obliquely-placed flask; the residuary arsenic acid is then heated somewhat more strongly, subsequently dissolved in water with the aid of a gentle heat, and the solution treated as in a.* 2. Determination as sulpharsenious acid.

a. The operater has in solution, arsenious acid, or an arsenite free from arsenic acid.

The solution is introduced into a bottle provided with a glass stopper; some hydrochloric acid is added, and the solution subsequently precipitated either with sulphuretted hydrogen water, or gas. The flask is then stopped, and allowed to stand for the space of an hour; after this, washed carbonic acid gas is transmitted through the contents of the flask until all odour of sulphuretted hydrogen has completely disappeared; the fluid is then filtered through a tared filter, and the precipitate collected upon the latter, washed, dried at 212°, and weighed. For the properties of the precipitate, vide § 66. The results are very accurate.

Should any other substance possessing the property of decomposing sulphuretted hydrogen, peroxide of iron, chromic acid, &c., be present in the analysed solution, free sulphur will precipitate in conjunction with the sulpharsenious acid. In this case, the free sulphur is to be determined as sulphate of barytes in the manner described § 95, 1. b., a. (determination of antimony as sulphuret,)

* If a solution containing nitric acid is evaporated directly with oxide of lead, it is difficult to avoid some loss of substance; since, in the first place, nitrate of lead decrepitates violently when heated, and, in the second place, the hyponitric acid disengaged during the decomposition of the nitrate of lead may readily carry off minute particles of the salt.

and the thus ascertained weight of free sulphur subtracted from the total weight of the precipitate. Another method has been recommended to separate the free sulphur from the sulpharsenious acid, viz. treating the precipitate with ammonia; by which means the sulpharsenious acid is dissolved, whilst the free sulphur remains undissolved;-however the results obtained by this method are only approximate, since the ammoniacal solution of the sulpharsenious acid dissolves a little free sulphur.

b. The operator has in solution arsenic acid, or an arseniate, or a mixture of the two degrees of oxidation of arsenic.

The solution is introduced into a flask, and mixed with a concentrated aqueous solution of sulphurous acid in considerable excess; the flask is placed in an oblique position over the flame of a spirit lamp, and its contents are heated slowly to incipient ebullition; the temperature is subsequently maintained a trifle below the boiling point, until the fluid ceases to smell of sulphurous acid. The solution, which now contains simply arsenious acid, is then treated as directed at a.

II. QUANTITATIVE DETERMINATION OF THE ACIDS IN COMPOUNDS CONTAINING BUT ONE BASE AND ONE ACID, OR BUT ONE METAL AND ONE METALLOID; AND SEPARATION OF THE ACIDS FROM THE BASES.

For the quantitative determination of these two acids, I refer to the preceding paragraph;-the methods of separating them from the bases will be found §§ 129 and 130.

§ 99.

2. CHROMIC ACID.

I. DETERMINATION.

Chromic acid is determined either in the form of OXIDE OF CHROMIUM, or in that of CHROMATE OF LEAD.

a. Determination as oxide of chromium.

The chromic acid is reduced to the state of oxide, and the amount of the latter determined, (§ 79). The reduction is effected either by heating the solution gently with hydrochloric acid and alcohol-or by mixing hydrochloric acid with the solution, and

transmitting subsequently sulphuretted hydrogen through it; or by adding to the solution a strong solution of sulphurous acid, and applying a gentle heat. Concentrated solutions are generally determined according to the first-dilute solutions according to one of the other two methods. With regard to the first of the three methods given, we have to remark here, that the alcohol which has been added in the process of reduction, must be expelled again previously to the precipitation of the oxide of chromium with ammonia ;-and with regard to the second, that the solution when supersaturated with sulphuretted hydrogen is to be kept standing in a moderately warm place until the separated sulphur has completely subsided.

b. Determination as Chromate of lead.

The solution is mixed with acetate of soda in excess, and should it be necessary, acetic acid added, until the fluid acquires a feebly acid reaction; the solution is then precipitated with neutral acetate of lead; the precipitate is collected upon a tared filter, washed, dried in the water-bath, and weighed. For its properties, vide § 67. The results are accurate.

II. SEPARATION OF CHROMIC ACID FROM THE BASES.

a. OF THE FIRST GROUP.

a. The ehromic acid is reduced to the state of oxide according to the directions given at I.; and the oxide separated from the alkalies, as directed § 118.

B. Chromate of ammonia is reduced to the state of chromic oxide by cautious ignition.

b. OF THE SECOND GROUP.

a. The compound under examination is fused together with four parts of a mixture of carbonate of soda and carbonate of potass. Upon extracting the fused mass subsequently with hot water, the chromic acid is dissolved as an alkaline chromate, whilst the alkaline earths remain as carbonates. The chromic acid in the former is finally determined as directed at I. The carbonates of the alkaline