NOTES TO TABLE VIII.

1. Substances insoluble in water and acids are of comparatively rare occurrence, especially among artificial products.

The following are most frequently met with: silica, and the silicates of various metallic oxides; the sulphates of baryta and strontia; the fluorides of barium, strontium, and calcium; chloride, bromide, and iodide of silver; the insoluble modifications of sesquioxide of chromium, alumina, binoxide of tin, and antimoniate of teroxide of antimony.

2. In order to test specially for alumina and silica, the finely-powdered substance may be fused, in a silver crucible, with pure hydrate of potassa, the fused mass dissolved in water, the solution acidulated with hydrochloric acid and evaporated to dryness; the residue is then boiled with dilute hydrochloric acid, when the silica is left undissolved; the solution is mixed with excess of ammonia, which precipitates the alumina.

We may also test for silica by mixing the substance with fluoride of calcium, and heating with concentrated sulphuric acid, in a platinum crucible, when terfluoride of silicon will be evolved (see p. 536).

3. Should there be any portion undissolved by the acid, it will probably consist of silica, of charcoal, or undecomposed substance, and may generally be disregarded.

4. In general, the only bases which need be sought for in this solution are the oxides of tin and antimony, but alumina and even small quantities of lime and oxide of iron may sometimes be found in it.

5. It need hardly be observed that the organic acids, oxalic acid, &c., need not be sought for in the solution, since they would have been decomposed by the high temperature employed in the fusion.

QUALITATIVE ANALYSIS OF ALLOYS AND AMALGAMS.

§ 358. Since compounds belonging to this class always contain metals in an unoxidized state, they exhibit such a difference in their general behavior as to require a special method for their analysis.

The preliminary experiment on charcoal before the blowpipe-flame is the only one which generally yields any very satisfactory result; though it is sometimes useful to heat the alloy gently in a tube closed at one end, when arsenic and mercury will sublime.

The method employed for reducing alloys to a proper state of division, varies according to the hardness of the substance operated on.

Some alloys may be cut into small fragments with the scissors or shears, others may be broken in an iron or steel mortar; hard alloys which are not attracted by the magnet may be reduced by filing, the particles of steel being afterwards carefully removed with a magnet.

It is unsafe to granulate an alloy by fusing and pouring it into water, since its composition may be materially altered by such an operation.

The analysis of alloys and amalgams is conducted according to the following table.

Boiled with CONCENTRATED HYDROCHLORIC ACID, the undissolved metal allowed to subside.

NOTES TO TABLE IX.

1. If the alloy contain a large proportion of tin and a little arsenic, the latter is generally left in the residue, and must be sought for in another portion of the alloy by a special method.

2. The more-easily oxidizable metals, as potassium, sodium, magnesium, &c., are not generally sought in alloys. This solution may also sometimes contain small quantities of platinum and antimony, which must not be disregarded if it be the object of the analyst to detect these metals in particular. Even a considerable amount of platinum may be dissolved by nitric acid from an alloy containing much silver.

QUANTITATIVE ANALYSIS.

APPARATUS USED IN QUANTITATIVE ANALYSIS.

§ 359. The following is a list of such apparatus, not mentioned in the introduction to qualitative analysis, as will be required by the student in commencing this branch of chemistry.

A platinum dish capable of containing about four ounces.1

A platinum crucible which will contain from four to six drachms. This crucible should be provided with a cover made in the form of a capsule.

Two or three flasks of German glass, containing about one pint. They should have a globular form, with flat bottoms, and lips slightly turned out, but without rim or spout; their necks should be rather narrow.

A washing-bottle for hot water.

One or two German beakers of about one quart capacity.

A pair of watch-glasses, to fit over each other.

One or two small stoppered weighing bottles, two or three inches long.

A wide test-tube, three or four inches long, and one inch in diameter, provided with a smooth, sound cork.

Quantitative filtering-paper.

Several glass plates of various diameters, to fit over funnels and beakers; some of these should have a notch in the side to admit the stem of a funnel, or a glass rod.

A water-oven for drying filters, &c.

An air-bath (Taylor's).-A water-bath.—A pipette.-A small glass siphon.— A balance and weights.

A few observations on the above articles may prove of service to beginners. The quantitative filtering paper should not only possess the good qualities requisite in that used in qualitative analysis, but should leave a very small amount of ash. The paper should be cut into filters of different sizes, the two most useful diameters being about 5 and 7 inches. In order to determine the ashes of these filters, six are to be completely incinerated in a platinum crucible (see p. 575), and the weight of the resulting ash divided by the number of filters, in order to obtain a fair average. Good filters of the smaller size should not leave more than about 0.04 grain of ash. The Swedish paper leaves even less ash, but is very expensive and generally filters very slowly.

The water-oven should be made of copper, and provided with apertures to allow of the passage of a current of air; it is conveniently supported upon a tripod, over a gas-burner, or may be placed upon the sand-bath. A somewhat imperfect substitute for a water-oven may be made by immersing a beaker in an appropriate vessel of water, which is then heated to the boiling point; filters, however, are dried very slowly in this manner, since the removal of the moisture has to be effected by the irregular current of air in the beaker.

This costly vessel is not absolutely indispensable; a Berlin dish or a platinum crucible may, with some sacrifice of time and convenience, be substituted for it in most cases.

Two air-baths will be found useful in the laboratory; one of these should be made after Taylor's pattern, with double bottom and sides, a tall chimney for promoting a current of air, which enters through apertures at the bottom of the bath, and a tube for introducing a thermometer. An apparatus of this kind serves for drying substances at regulated temperatures above the boiling point of water.

The other air-bath (to be used for evaporations, &c.) is a cylindrical vessel of tinned iron, 7 inches in height, and 9 in diameter, with a large hole close to the bottom, for the entrance of air. It may be covered with a sheet of porous paper, supported by a glass rod.

The water-bath is a pretty capacious vessel of copper or tin-plate, furnished with one or two sets of movable rings for various dishes.

The balance is, of course, an apparatus of the first importance to the quantitative analyst, and much care should be therefore bestowed upon its selection. For most practical purposes, a balance capable of turning with 0.01 grain, when loaded with 1,000 grains in each pan, will be found sufficiently accurate. For very accurate analyses, however, especially in organic chemistry, a balance is required which turns distinctly with 0.001 grain. A balance is also required which will carry about 5,000 grains in each pan, and yet turn with 0.1 grain. The best general balance will be found in one which is capable of bearing the above weight, and yet turns with 0.01 grain.

Our space will not permit us to enter into all the details necessary to be attended to in a good balance, but we think it advisable to lay particular stress upon the following points.1

The balance should be inclosed in a tight glass case, to protect it from the action of air and acid fumes. It should be provided with a handle external to the case, by which the knife-edge may be brought down on to the agate plane. The beam should be graduated, so that the smaller weights may be placed upon it, on the principle of the steelyard, a little rider of gold wire being used for all the smaller weights, and shifted along the beam by means of a lever worked from the outside of the case. The pans should be suspended from knife-edge supports.

A small dish containing lumps of quicklime should be kept in the balance, to absorb moisture, &c.

OPERATIONS IN QUANTITATIVE ANALYSIS.

§ 360. Before proceeding to consider the methods actually employed for the determination of various substances, it may be well to describe some of the more important manipulations, upon the execution of which the success of the quantitative analyst will entirely depend.

1

WEIGHING.In the process of weighing, the following cautions should be carefully attended to.

The knife-edge must not be allowed to fall suddenly on to the agate plane, but must be let down gradually and gently.

The fulcrum must not rest upon the planes during the removal or addition of a weight. The pans must not be allowed to swing to and fro. Accurate equilibrium is best indicated by a gentle oscillation of the beam, causing the index to deviate to the same extent on either side of the scale; in weighing by rest, an error may be caused by a particle of dirt upon the planes, &c. Of course the balance must be so placed that the beam is truly horizontal (indicated by a pendulum, or by spirit-levels), and should stand upon a firm table. The weights are always lifted with pincers, never touched with the fingers.

These remarks refer to accurate scientific balances, rather than to such as may be employed for the practically useful analysis of ores, &c.