A Text-book of Assaying: For the Use of Those Connected with Mines Part 53

Brush the precipitate into a weighed dish, and weigh it. It is pota.s.sium platino-chloride (K_{2}PtCl_{6}), and contains 16.03 per cent. of pota.s.sium, or 30.56 per cent. of pota.s.sium chloride (KCl), which is equivalent to 19.3 per cent. of potash (K_{2}O).

If the filter-paper is not free from precipitate, burn it and weigh separately. The excess of weight over that of the ash will be due to platinum and pota.s.sic chloride (Pt and 2KCl). This multiplied by 1.413 will give the weight of the pota.s.sic platino-chloride from which it was formed. It must be added to the weight of the main precipitate.

The mixed alkaline chlorides obtained in the usual course of a.n.a.lysis are treated in this manner; the quant.i.ty of platinum added must be about three times as much as the mixed chlorides weigh.

VOLUMETRIC METHODS.

These are the same as with soda.

~Examination of Commercial Carbonate of Potash.~--The impurities to be determined are moisture, silica, and insoluble matter, chlorine, sulphuric oxide, and oxide of iron. These determinations are made in the ways described under the examination of common salt.

The ~pota.s.sium~ is determined by converting it into chloride and precipitating with platinum chloride, &c., as just described.

~Available Alkali.~--Weigh up 23.5 grams of the sample, dissolve in water, and make up to 500 c.c. Take 50 c.c., tint with methyl orange, and t.i.trate with the normal solution of acid. The c.c. of acid used multiplied by 2 gives the percentage of available alkali calculated as potash (K_{2}O).

~Soda.~--This is calculated indirectly in the following way:--Deduct from the pota.s.sium found the quant.i.ty required for combination with the chlorine and sulphuric oxide present, and calculate the remainder to potash (K_{2}O). The apparent surplus excess of available alkali is the measure of the soda present.

~Carbon Dioxide.~--The c.c. of acid used in the available alkali determination, multiplied by 2.2 and divided by 2.35, gives the percentage of carbon dioxide.

LITHIUM.

Lithia, the oxide of lithium (Li_{2}O), occurs in quant.i.ties of 3 or 4 per cent. in various silicates, such as lepidolite (or lithia-mica), spodumene, and petalite. It also occurs as phosphate in triphyline. It is a const.i.tuent of the water of certain mineral springs. A spring at Wheal Clifford contained as much as 0.372 gram of lithium chloride per litre. In small quant.i.ties, lithia is very widely diffused.

The ~Detection~ of lithia is rendered easy by the spectroscope; its spectrum shows a red line lying about midway between the yellow sodium line and the red one of pota.s.sium. It also shows a faint yellow line.

The colour of the flame (a crimson) is characteristic.

The reactions of the lithium compounds lie between those of the alkalies and of the alkaline earths. Solutions are not precipitated by tartaric acid nor by platinic chloride. The oxide is slowly soluble in water.

The carbonate is not freely soluble. Lithia is completely precipitated by sodic phosphate, especially in hot alkaline solutions.

In its determination the mixed alkaline chlorides obtained in the separation of the alkalies are dissolved in water, a solution of soda is added in slight excess, and the lithia precipitated with _sodic_ phosphate. Before filtering, it is evaporated to dryness and extracted with hot water rendered slightly ammoniacal. The residue is transferred to a filter, dried, ignited, and weighed. The precipitate is lithium phosphate (3Li_{2}O, P_{2}O_{5}), and contains 38.8 per cent. of lithia.

The separation of lithia from magnesia is not given by the usual authorities. Wohler recommends evaporating the solution to dryness with carbonate of soda. On extracting the residue with water, the lithia dissolves out and is determined in the filtrate. One hundred parts of water dissolve, at the ordinary temperature, 0.769 parts of lithium carbonate (Li_{2}CO_{3}); the basic magnesia compound is almost insoluble in the absence of carbon dioxide and ammonium salts.

CAESIUM.

The oxide of caesium, caesia (Cs_{2}O), is found a.s.sociated with lithia in lepidolite, &c., and, together with rubidium, in many mineral waters.

The mineral pollux is essentially a silicate of alumina and caesia; it contains 34.0 per cent. of the latter oxide.

Caesium is best detected by the spectroscope, its spectrum being characterised by two lines in the blue and one in the red; the latter is about midway between the lithium and sodium lines.

If not detected by the spectroscope, or specially looked for, caesia would, in the ordinary course of work, be separated with the potash and weighed as pota.s.sium platino-chloride.

Caesia is separated from all the other alkalies by adding to the acid solution of the mixed chlorides a strongly acid cold solution of antimonious chloride. The acid used must be hydrochloric. The caesium is precipitated as a white crystalline precipitate (CsCl.SbCl_{3}), which is filtered off, and washed, when cold, with strong hydrochloric acid; since it is decomposed by water or on warming. The precipitate is washed into a beaker, and treated with sulphuretted hydrogen; after filtering off the sulphide of antimony, the solution leaves, on evaporation, the caesium as chloride.

RUBIDIUM.

Rubidium occurs widely diffused in nature, but in very small quant.i.ties.

It is generally a.s.sociated with caesium.

It is detected by the spectroscope, which shows two violet lines and two dark red ones. Like caesium, it is precipitated with platinic chloride, and in the ordinary course of work would be weighed as pota.s.sium. It is separated from pota.s.sium by fractional precipitation with platinic chloride. Rubidium platino-chloride is much less soluble than the pota.s.sium salt.

AMMONIUM.

It is usual to look upon the salts of ammonia as containing a compound radical (NH_{4} = Am), which resembles in many respects the metals of the alkalies. Ammonium occurs in nature as chloride in sal ammoniac (AmCl), as sulphate in mascagnine (Am_{2}SO_{4}), as phosphate in struvite (AmMgPO_{4}.12H_{2}O). Minerals containing ammonium are rare, and are chiefly found either in volcanic districts or a.s.sociated with guano. Ammonia and ammonium sulphide occur in the waters of certain Tuscan lagoons, which are largely worked for the boracic acid they contain. The crude boracic acid from this source contains from 5 to 10 per cent. of ammonium salts. It is from these that the purer forms of ammonium compounds of commerce known as "from volcanic ammonia" are derived. But the bulk of the ammonia of commerce is prepared from the ammoniacal liquors obtained as bye-products in the working of certain forms of blast furnaces and c.o.ke ovens, and more especially in gas-making.

Ammonia hardly comes within the objects of a.s.saying; but it is largely used in the laboratory, and the a.s.sayer is not unfrequently called on to determine it. Ammonium salts are mostly soluble in water. In strong solutions they give a yellow precipitate of ammonium platino-chloride on the addition of chloride of platinum; and with the acid tartrate of soda yield a white precipitate of hydric ammonic tartrate. These reactions are similar to those produced with pota.s.sium compounds.

Heated with a base, such as lime or sodic hydrate, ammonium salts are decomposed, yielding ammonia gas (NH_{3}), which is readily soluble in water. The solution of this substance is known as ammonic hydrate or "ammonia."

They are volatilised on ignition; either with, or without, decomposition according to the acid present. This fact is of importance in a.n.a.lytical work; since it allows of the use of alkaline solutions and reagents which leave nothing behind on heating. It must be remembered, however, that, although ammonic chloride is volatile, it cannot be volatilised in the presence of substances which form volatile chlorides without loss of the latter. For example: ferric oxide and alumina are thus lost, volatilising as chlorides; and there are some other compounds (notably ammonic magnesic a.r.s.enate) which on heating to redness suffer reduction.

The presence of ammonic chloride in such cases must be avoided.

~Detection.~--Compounds of ammonium are detected by their evolving ammonia when mixed or heated with any of the stronger bases. The ammonia is recognised by its odour, by its alkaline reaction with litmus paper, and by yielding white fumes, when brought in contact with fuming acid.

In consequence of the use of ammonium salts and ammonia as reagents, it is necessary to make a special test for and determination of ammonium.[94] In the ordinary course of work it will be "lost on ignition." The determination presents little difficulty, and is based on the method used for its detection.

[Ill.u.s.tration: FIG. 61.]

~Solution and Separation.~--Although ammonium salts are soluble in water, there is no necessity for dissolving them. The compound containing the ammonia is boiled with an alkaline solution; and the liberated ammonia condensed and collected. The substance is weighed out into a flask of about 200 c.c. capacity. The flask is closed with a rubber cork perforated to carry a 20 c.c. pipette and a bulb exit tube.

The latter is connected with a receiver, which is a small flask containing dilute hydrochloric acid (fig. 61). The flask containing the substance is corked, and the greater part of the soda solution is run in from the pipette. The solution is then boiled. The ammonia volatilises, and is carried over into the hydrochloric acid, with which it combines to form ammonic chloride. The distillation is carried on gently until the bulk of the liquid is driven over. The ammonia in the receiver will be mixed only with the excess of hydrochloric acid. This separation is used in all determinations.

GRAVIMETRIC DETERMINATION.

The contents of the flask are transferred to a weighed platinum dish, and evaporated on the water-bath. It is dried until the weight is constant. The chloride of ammonium remains as a white ma.s.s which, after cooling in a desiccator, is weighed. It contains 33.72 per cent. of ammonium (NH_{4}), or 31.85 per cent. of ammonia (NH_{3}). On heating over the Bunsen burner it is completely volatilised, leaving no residue.

VOLUMETRIC DETERMINATION.

Weigh up 1.7 gram of the substance and place it in the flask. Measure off 50 c.c. of the normal solution of acid, place them in the receiver, and dilute with an equal volume of water. Run in through the pipette (by opening the clip) 20 c.c. of a strong solution of soda, boil until the ammonia has pa.s.sed over, and then aspirate a current of air through the apparatus. Disconnect the receiver, and tint its contents with methyl orange. t.i.trate the residual acid with a semi-normal solution of alkali.

Divide the c.c. of the "alkali" solution used by 2, and deduct from the 50 c.c. The difference will give the number of c.c. of the normal acid solution neutralised by the ammonia distilled over. Each c.c. of "acid"

so neutralised, represents 1 per cent. of ammonia in the sample. If the results are to be reported as ammonium, 1.8 gram of the sample is taken instead of 1.7 gram.

COLORIMETRIC DETERMINATION.

This is effected by means of "Nessler's" reagent, which strikes a brown colour with traces of ammonia, even with a few hundredths of a milligram in 100 c.c. of liquid. With larger quant.i.ties of ammonia the reagent gives a precipitate. This reagent is a strongly alkaline solution of pota.s.sic mercuric iodide; and is thus made:--

_Nessler's solution_: Dissolve 17 grams of mercuric chloride in 300 c.c.

of water; and add the solution to one of 35 grams of pota.s.sium iodide in 100 c.c. of water until a permanent precipitate is produced. Both solutions must be cold. Then make up to a litre by adding a 20 per cent.

solution of potash. Add more of the mercuric chloride (a little at a time) until a permanent precipitate is again formed. Allow to settle, decant, and use the clear liquor. Four or five c.c. are used for each 100 c.c. of liquid to be tested.

_A Standard Solution of Ammonia_ is made by dissolving 0.315 gram of ammonic chloride in water, and diluting to 100 c.c. Ten c.c. of this are taken and diluted to 1 litre. One c.c. contains 0.01 milligram of ammonia (NH_{3}).

In working, the solution containing the ammonia is diluted to a definite volume, and to such an extent that 50 c.c. of it shall not contain more than 0.02 or 0.03 milligram of ammonia. Fifty c.c. of it are transferred to a Nessler gla.s.s and mixed with 2 c.c. of Nessler's reagent. The colour is noted, and an estimate made as to the amount of ammonia it indicates. A measured quant.i.ty of the standard ammonia, judged to contain about as much ammonia as that in the a.s.say, is then put into another Nessler gla.s.s. It is diluted to 50 c.c. with water, and mixed with 2 c.c. of "Nessler." After standing a minute or two, the colours in the two gla.s.ses are compared. If the tints are equal, the a.s.say is finished; but if the standard is weaker or stronger than the a.s.say, another standard, containing more or less ammonia, as the case may be, must be prepared and compared with the a.s.say. Two such experiments will generally be sufficient; but, if not, a third must be made. The addition of more standard ammonia to the solution to which the "Nessler" has already been added does not give a satisfactory result.