A Text-book of Assaying: For the Use of Those Connected with Mines Part 56

Potash 0.38 Soda 2.01 Magnesia 1.44 Lime 10.55 Ferric oxide 0.01 Silica 0.30 Sulphuric oxide 3.69 Nitrogen pentoxide 0.11 Carbon dioxide 8.38 Chlorine 1.70 Volatile and organic matter 0.66 ----- 29.23 Less oxygen equivalent to chlorine found 0.39 ----- 28.84

For the preparation of distilled water, the apparatus shown in fig. 64 is convenient for laboratory use. It consists of a copper retort heated by a ring gas-burner, and connected with a worm-condenser.

[Ill.u.s.tration: FIG. 64]

PRACTICAL EXERCISE.

A mineral, on a.n.a.lysis, gave the following results:--Water, 44.94 per cent.; sulphuric oxide, 28.72 per cent.; ferrous iron, 13.92 per cent.; ferric iron, 0.35 per cent.; copper, 6.1 per cent. The mineral was soluble in water, and showed nothing else on testing. How would you report the a.n.a.lysis? Calculate the formula for the salt.

THE HALOGENS.

There is a group of closely allied elements to which the name halogen (salt-producer) has been given. It comprises chlorine, bromine, iodine, and fluorine. These elements combine directly with metals, forming as many series of salts (chlorides, bromides, iodides, and fluorides), corresponding to the respective oxides, but differing in their formulae by having two atoms of the halogen in the place of one atom of oxygen.

For example, ferrous oxide is FeO and ferrous chloride is FeCl_{2}, and, again, ferric oxide is Fe_{2}O_{3}, whilst ferric chloride is Fe_{2}Cl_{6}. These salts differ from the carbonates, nitrates, &c., in containing no oxygen. Consequently, it is incorrect to speak of such compounds as chloride of potash, fluoride of lime, &c., since potash and lime are oxides. It is important to bear this in mind in reporting a.n.a.lyses in which determinations have been made, say, of chlorine, magnesia, and potash, or of fluorine, silica, and alumina. It is necessary in all such cases to deduct from the total an amount of oxygen equivalent to the halogen found, except, of course, where the base has been determined and recorded as metal. Compounds containing oxides and fluorides, &c., do not lend themselves to the method of determining the halogen by difference. For example, topaz, which, according to Dana, has the formula Al_{2}SiO_{4}F_{2}, would yield in the ordinary course of a.n.a.lysis--

Alumina 55.4% Silica 32.6 Fluorine 20.6 ----- 108.6

The oxygen equivalent to 20.6 per cent. fluorine may be found by multiplying the percentage of fluorine by 0.421; it is 8.7 per cent., and must be deducted. The a.n.a.lysis would then be reported thus:--

Alumina 55.4% Silica 32.6 Fluorine 20.6 ----- 108.6 Less oxygen equivalent to fluorine 8.7 ----- 99.9

Take as an ill.u.s.tration the following actual a.n.a.lysis by F.W. Clarke and J.S. Diller:--

Alumina 57.38% Silica 31.92 Fluorine 16.99 Potash 0.15 Soda 1.33 Water 0.20 ------ 107.97 Deduct oxygen equivalent 7.16 ------ 100.81

In calculating the factor for the "oxygen equivalent," divide the weight of one atom of oxygen (16) by the weight of two atoms of the halogen; for example, with chlorine it would be 16/71 or 0.2253; with bromine, 16/160 or 0.1000; with iodine, 16/254 or 0.063; and with fluorine, 16/38 or 0.421.

CHLORINE AND CHLORIDES.

Chlorine occurs in nature chiefly combined with sodium, as halite or rock salt (NaCl). With pota.s.sium it forms sylvine (KCl), and, together with magnesium, carnallite (KCl.MgCl_{2}.6H_{2}O). Of the metalliferous minerals containing chlorine, kerargyrite, or horn silver (AgCl), and atacamite, an oxychloride of copper (CuCl_{2}.3Cu(HO)_{2}.) are the most important. Apat.i.te (phosphate of lime) and pyromorphite (phosphate of lead) contain a considerable amount of it. Chlorine is a gas of a greenish colour, possessing a characteristic odour, and moderately soluble in water. It does not occur native, and is generally prepared by the action of an oxidising agent on hydrochloric acid. It combines directly with metals at the ordinary temperature (even with platinum and gold), forming chlorides, which (except in the case of silver) are soluble.

It is important in metallurgy, because of the extensive use of it in extracting gold by "chloridising" processes. It is also used in refining gold.

~Detection.~--Compounds containing the oxides of chlorine are not found in nature, because of the readiness with which they lose oxygen. By reduction they yield a chloride; the form in which chlorine is met with in minerals. In testing, the compound supposed to contain a chloride is boiled with water, or, in some cases, dilute nitric acid. To the clear solution containing nitric acid a few drops of nitrate of silver solution are added. If, on shaking, a white curdy precipitate, soluble in ammonia, separates out, it is sufficiently satisfactory evidence of the presence of chlorides.

~Solution and Separation.~--The chlorides are generally soluble in water, and are got into solution by extracting with warm dilute nitric acid. Or, if insoluble, the substance is fused with carbonate of soda, extracted with water, and the filtrate acidified with nitric acid. For the determination, it is not necessary to obtain the solution of the chloride free from other acids or metals. If tin, antimony, mercury, or platinum is present, it is best to separate by means of sulphuretted hydrogen. The chloride is determined in the solution after removal of the excess of the gas. Where traces of chlorides are being looked for, a blank experiment is made to determine the quant.i.ty introduced with the reagents. One hundred c.c. of ordinary water contains from 1 to 3 milligrams of chlorine. On the addition of nitrate of silver to the nitric acid solution, chloride of silver separates out. This is free from other substances, except, perhaps, bromide and iodide.

GRAVIMETRIC DETERMINATION.

Freely mix the solution containing the chloride with dilute nitric acid, filter (if necessary), and treat with nitrate of silver. Heat nearly to boiling, and, when the precipitate has settled, filter, and wash with hot distilled water. Dry, and transfer to a weighed Berlin crucible.

Burn the filter-paper separately, and convert any reduced silver into chloride by alternate treatment with drops of nitric and of hydrochloric acid. Add the main portion to this, and heat cautiously till the edges of the ma.s.s show signs of fusing (about 260). Cool in the desiccator and weigh. The substance is chloride of silver (AgCl), and contains 24.73 per cent. of chlorine.

The precipitated chloride is filtered and washed as soon as possible after settling, since on exposure to light it becomes purple, and loses a small amount of chlorine.

VOLUMETRIC METHOD.

There are several volumetric methods; but that based on the precipitation of silver chloride in neutral solution, by means of a standard solution of silver nitrate (using pota.s.sium chromate as indicator), is preferred. Silver chromate is a red-coloured salt; and, when silver nitrate is added to a solution containing both chloride and chromate, the development of the red colour marks off sharply the point at which the chloride is used up. Silver chromate is decomposed and consequently decolorised by solution of any chloride. The solution for this method must be neutral, since free acid prevents the formation of the red silver chromate. If not already neutral, it is neutralised by t.i.trating cautiously with a solution of soda. In a neutral solution, other substances (such as phosphates and a.r.s.enates) also yield a precipitate with a solution of nitrate of silver; and will count as chloride if they are not removed.

_The Standard Solution of Nitrate of Silver_ is made by dissolving 23.94 grams of the salt (AgNO_{3}) in distilled water, and diluting to 1 litre; 100 c.c. are equal to 0.5 gram of chlorine.

The _indicator_ is made by adding silver nitrate to a strong neutral solution of yellow chromate of potash (K_{2}CrO_{4}), till a permanent red precipitate is formed. The solution is allowed to settle, and the clear liquid decanted into a stoppered bottle labelled "chromate indicator for chlorine."

Standardise the silver nitrate by weighing up 0.5 gram of pure sodium chloride (or pota.s.sium chloride). Transfer to a flask and dissolve in distilled water; dilute to 100 c.c. Fill an ordinary burette with the standard silver solution, and (after adjusting) run into the flask a quant.i.ty sufficient to throw down the greater part of the chlorine. Add a few drops of the chromate indicator and continue the addition of the silver nitrate until the yellow colour of the solution becomes permanently tinted red, after shaking. This shows that the chlorine is all precipitated, and that the chromate is beginning to come down. The further addition of a couple of drops of the silver solution will cause a marked difference in the tint. Read off the quant.i.ty run in, and calculate the standard. One gram of sodium chloride contains 0.6062 gram of chlorine; and 1 gram of pota.s.sium chloride contains 0.4754 gram.

For the determination of small quant.i.ties of chloride (a few milligrams), the same method is used; but the standard solution is diluted so that each c.c. is equal to 1 milligram of chlorine; and the chromate indicator is added before t.i.trating. The standard solution is made by measuring off 200 c.c. of the solution described above, and diluting with distilled water to 1 litre.

BROMINE AND BROMIDES.

Bromine closely resembles chlorine in the nature of its compounds. It does not occur free in nature, but is occasionally found in combination with silver as bromargyrite (AgBr) and, together with chloride, in embolite. It mainly occurs as alkaline bromides in certain natural waters. Nearly all the bromine of commerce is derived from the mother liquors of salt-works--_i.e._, the liquors from which the common salt has been crystallised out. Bromine combines directly with the metals, forming a series of salts--the bromides. In ordinary work they are separated with, and (except when specially tested for) counted as, chlorides. They are detected by adding chlorine water to the suspected solution and shaking up with carbon bisulphide. Bromine colours the latter brown.

IODINE AND IODIDES.

Iodine does not occur in nature in the free state; and iodides are rare, iodargyrite or iodide of silver (AgI) being the only one which ranks as a mineral species. Iodates are found a.s.sociated with Chili saltpetre, which is an important source of the element.

Iodine and Iodides are largely used in the laboratory, and have already been frequently referred to. It is used as an oxidising agent in a similar manner as permanganate and b.i.+.c.hromate of potash, especially in the determinations of copper, a.r.s.enic, antimony, and manganese.

Iodine is not readily soluble in water; but dissolves easily in a concentrated solution of pota.s.sium iodide. Its solutions are strongly coloured; a drop of a dilute solution colours a large volume of water decidedly yellow; on the addition of starch paste, this becomes blue.

The delicacy of this reaction is taken advantage of in t.i.trations to determine when free iodine is present. The blue colour may be alternately developed and removed by the addition of iodine (or an oxidising agent) and hyposulphite of soda (or some other reducing agent). In decolorising, the solution changes from blue or black to colourless or pale yellow according to circ.u.mstances. Sometimes the solution, instead of remaining colourless, gradually develops a blue which recurs in spite of the further addition of the reducing agent. In these cases the conditions of the a.s.say have been departed from, or (and this is more often the case) there is some substance present capable of liberating iodine.

Iodine forms a series of salts--the iodides--resembling in many respects the chlorides. These can be obtained by direct combination of the metals with iodine.

~Detection.~--Free iodine is best recognised by the violet vapours evolved from the solution on heating, and by the blue or black colour which it strikes on the addition of starch paste. Iodides are detected by boiling with strong solutions of ferric sulphate or chloride. Iodine is liberated, distilled over, and collected. Chlorine also liberates iodine from iodides; and this reaction is frequently made use of in a.s.saying. A process based on this is described under _Manganese_. All substances which liberate chlorine on boiling with hydrochloric acid (dioxides, b.i.+.c.hromates, permanganates, &c.) are determined in a similar way.

~Solution and Separation.~--Most iodides are soluble in water or dilute acids. The separation is effected by distilling the substance with solution of ferric sulphate, and collecting the vapour in a dilute solution of sulphurous acid or a.r.s.enite of soda. On the completion of the distillation, the iodine will be in the distillate as iodide; and the gravimetric determination is made on this.

GRAVIMETRIC DETERMINATION.

To the solution containing the iodine, as iodide, and which is free from chlorides (and bromides), add a little dilute nitric acid and nitrate of silver till no further precipitate is produced. Filter off, wash with hot water, and dry. Clean the filter-paper as much as possible, and burn it. Collect the ash in a weighed porcelain crucible, add the main portion, and heat to incipient fusion; cool, and weigh. The substance is silver iodide, and contains 54.03 per cent. of iodine.

VOLUMETRIC METHOD.

This is for the t.i.tration of free iodine, and is practically that which is described under _Manganese_. The substance to be determined is distilled with ferric sulphate, and the iodine is collected in a solution of pota.s.sium iodide, in which it readily dissolves. If flaky crystals separate out in the receiver, more pota.s.sium iodide crystals are added. When the distillation is finished, the receiver is disconnected, and its contents washed out into a beaker and t.i.trated with "hypo." The standard solution of "hypo" is made by dissolving 19.58 grams of hyposulphite of soda (Na_{2}S_{2}O_{3}.5H_{2}O) in water and diluting to 1 litre; 100 c.c. are equal to 1 gram of iodine. To standardise the solution, weigh up 0.25 gram of pure iodine in a small beaker. Add 2 or 3 crystals of pota.s.sium iodide; cover with water; and, when dissolved, dilute to 50 or 100 c.c. t.i.trate, and calculate the standard.

FLUORINE AND FLUORIDES.

Fluorine is frequently met with as calcium fluoride or fluor-spar (CaF_{2}). It occurs less abundantly as cryolite (Na_{3}AlF_{6}), a fluoride of aluminium and sodium, which is used in gla.s.s-making. Certain other rarer fluorides are occasionally met with. Fluorine is also found in apat.i.te, and in some silicates, such as topaz, tourmaline, micas, &c.

Hydrofluoric acid is used for etching gla.s.s and opening up silicates. It attacks silica, forming fluoride of silicon (SiF_{4}), which is volatile. Silica is by this means eliminated from other oxides, which, in the presence of sulphuric acid, are fixed. The commercial acid is seldom pure, and generally weak; and the acid itself is dangerously obnoxious. The use of ammonium fluoride (or sodium fluoride) and a mineral acid is more convenient. Determinations of this kind are made in platinum dishes enclosed in lead or copper vessels in a well-ventilated place. Fluor-spar is useful as a flux in dry a.s.saying; it renders slags, which would otherwise be pasty, quite fluid. Fluorides generally are fusible, and impart fusibility to substances with which they form weak compounds. Their fluxing action does not depend on the removal of silicon as fluoride.

~Detection.~--Fluorides in small quant.i.ty are easily overlooked unless specially sought for. In larger amounts they are recognised by the property hydrofluoric acid has of etching gla.s.s. A watch-gla.s.s is warmed, and a layer of wax is melted over the convex side. When cold, some lines are engraved on the waxed surface with any sharp-pointed instrument. The substance to be tested is powdered; and moistened, in a platinum dish, with sulphuric acid. The watch-gla.s.s is filled with cold water and supported over the dish. The dish is then carefully warmed, but not sufficiently to melt the wax. After a minute or two, the gla.s.s is taken off, and the wax removed. If the substance contained fluorine, the characters will be found permanently etched on the gla.s.s. An equally good, but more rapid, test is to mix the powdered substance with some silica, and to heat the mixture in a test tube with sulphuric acid.