GRAVIMETRIC ANALYSIS.

ESTIMATION OF METALS.

POTASSIUM.

This element is usually estimated in the form of double chloride of potassium and platinum. Qualitative analysis having proved the presence of potassium and other elements in a substance, a small quantity of the material is accurately weighed, dissolved, and the other elements removed by appropriate reagents; the precipitates are well washed, in order that no trace of the potassium salt shall be lost, the resulting liquid concentrated over a water-bath to avoid loss that would occur mechanically during ebullition, hydrochloric acid added if necessary, solution of perchloride of platinum poured in, and evaporation continued to dryness; excess of the perchloride is then dissolved by adding spirit of wine containing half its bulk of ether (a liquid in which the double chloride is insoluble), the mixture carefully poured on to a tared and dried filter, washed with the spirit till every trace of free perchloride of platinum is removed, the whole dried and weighed; from the resulting amount the proportion of potassium, or equivalent quantity of a salt of potassium, is ascertained by calculation.

From this short description it will be seen, first, that the chemistry of quantitative is the same as that of qualitative analysis; and second, that the principle of gravimetric is the same as that of volumetric quantitative analysis; namely, the combining proportions of substances being known, unknown quantities of elements may be ascertained by calculation from known quantities of their compounds.

Filtering-paper should be of the kind known as Swedish, the texture of which is of the requisite degree of closeness, and its ash small in amount. A large number of circular pieces of one size, six to eight centimetres in diameter, should be cut ready for use. In delicate experiments, where a precipitate on a filter has to be heated and the paper consequently burnt, the weight of the ash of the filter must be deducted from the weight of the residue. The ash is estimated by burning ten or twenty of the cut filters. These are folded into a small compass, a portion of a piece of platinum wire twisted a few times round the packet, so as to form a cage, the whole held by the free end of the wire over a weighed porcelain crucible placed in the centre of a sheet of glazed paper, the bundle ignited by a spirit-lamp or smokeless gas-flame, the flame allowed to impinge against the charred mass till it falls into the crucible below, any stray fragments on the sheet carefully shaken into the crucible, the latter placed over a flame till carbon has all burnt off and nothing but ash remains, the whole cooled, weighed, and the weight of the crucible deducted; the weight of the residue divided by the number of pieces used gives the average amount of ash in each filter.

A pair of Weighing-tubes, for holding dried filters during operations at the balance, may be made from two test-tubes, one fitting closely within the other. About five centimetres of the closed end of the outer and seven of the inner are cut off, by leading a crack round the tube with a pencil of incandescent charcoal, and the sharp edges fused in the blowpipe-flame. A filter, or filter and precipitate, after drying, is quickly folded and placed in the narrower tube, the mouth of which is then closed by the wider tube.

The Washing-bottle, holding the alcohol and ether, is simply a common bottle, through the cork of which a small tube passes. The outer end of the tube should be sufficiently narrowed to enable it to deliver a very fine stream of the liquid. The bottle

being inverted, the warmth of the hand expands the air and vapour to a sufficient extent to force out the liquid.

The ordinary washing-bottle for quantitative operations should be formed of a flask in which water may be boiled. Two tubes pass through holes in the cork, the one reaching to the bottom of the flask within, externally bent to a slightly acute angle, and its outer extremity drawn to a capillary opening, the other bent to a slightly obtuse angle, the inner arm terminating just within the bottle. Air blown into the bottle through the short tube by the lungs forces water out at the capillary orifice.

A Water-oven is the best form of drying apparatus. It is a small square copper vessel, jacketed on five sides and having a door on the sixth; water is poured into the space between the inner and outer casing, and the whole placed over a gas-lamp or other source of heat, moist air and steam escaping by appropriate apertures. Higher temperatures than the boiling-point of water may be obtained by using oil or paraffin instead of water, inserting a thermometer in the fat. The apparatus may be purchased of any maker of chemical instruments.

Pure distilled water must be used in all quantitative determinations.

Note. In practising the operations of quantitative analysis, experiments should at first be conducted on definite salts of known composition. The accuracy of results may then be tested by calculation.

Select two or three crystals of pure nitrate of potassium, powder them in a clean mortar, dry the powder by gently heating in a porcelain crucible over a flame for a few seconds, place about a couple of decigrammes (0·2 gr.) of the powder in a counterpoised watch-glass, accurately weigh the selected quantity, transfer to a small dish, letting water from a wash-bottle flow over the watch-glass and run into the dish, warm the dish till the nitrate is dissolved, acidulate with hydrochloric acid, add

excess of aqueous solution of perchloride of platinum (a quantity containing about 0.4 of solid salt), evaporate to dryness over a water-bath. While evaporation is going on place a filter and the weighing tubes in the water-oven, exposing them to a temperature of 212° F. for about half an hour; fold the filter and insert it in the tubes, place them on a plate under a glass shade, and when cold accurately note their weight. Arrange the weighed filter in a funnel over a beaker. Transfer the dried and cooled platinum salt from the dish to the filter by moistening the residue with the mixture of alcohol and ether and, when the salt is loosened, pouring the contents of the dish into the paper cone. Any salt still adhering may be freed by the finger, which, together with the dish, should be washed in the stream of spirit, the rinsings at once flowing into the filter. The filtrate should have a yellowish-brown colour, due to the excess of perchloride of platinum. If it is colourless, an insufficient amount of perchloride has been added, and the whole operation must be repeated. The washed precipitate and filter are finally dried in the water-oven, folded and placed in the weighing-tubes, the drying continued until the whole, when cold, ceases to lose `weight, and the weight noted.

Analytical memoranda in the note-book may have the following form:

:

Watch-glass and substance

Watch-glass

Substance..

Weighing-tubes, filter, and pt. salt....

Weighing-tubes and filter

PtCl, 2KC1..

The results of calculation and experiment should be identical. A table of atomic weights, from which to find molecular weights, is given at the end of the volume.

A Water-bath for the evaporation of liquids at temperatures below 212° F. is an iron, tin, or earthenware pan, the mouth of which can be narrowed by iron or tin diaphragms of various sizes, and having orifices adapted to the diameters of the rating-dishes.

evapoPlatinum residues &c. should be preserved, and the metal recovered from them from time to time (vide p. 158).

Hot alcohol decomposes perchloride of platinum, the metal being thrown out of solution in a finely divided form, known as platinum black; hence only aqueous solutions of the salt must be used where heat is employed. Hence, also, in washing out excess of perchloride of platinum from the double chloride of platinum and potassium by spirit, the application of heat must be avoided.

SODIUM.

Sodium is usually estimated as sulphate. Accurately counterpoise, and take the weight of a porcelain crucible and lid, place within about 3. of pure rock-salt, again noting the weight. Add rather more strong sulphuric acid than may be considered sufficient to convert the chloride into acid sulphate of sodium. Heat the crucible gradually, the flame being first directed against the side of the crucible to avoid violent ebullition, until fumes of sulphuric acid cease to be evolved, towards the end of the operation dropping in one or two fragments of carbonate of ammonium to facilitate complete decomposition. When cold, weigh