A Text-book of Assaying: For the Use of Those Connected with Mines Part 32

_Sulphuretted hydrogen water_ is made by pa.s.sing a current of the washed gas into water till the latter is saturated.

Five c.c. of the sulphuretted hydrogen water are put into a Nessler tube, the measured portion of the a.s.say solution added, and the whole diluted with water to the 50 c.c. mark. Into the standard Nessler tube the same amount of the sulphuretted hydrogen water is put, and diluted to nearly 50 c.c. The standard lead solution is then run in till the tints are equal. The a.s.say solution must not contain much free acid, and if the conditions will allow it, may with advantage be rendered alkaline with ammonia. The chief cause of disturbance is the precipitation of lead sulphide forming a black turbid solution instead of a brown clear one. This may be caused by using hot solutions or an excess of acid.

Other metals precipitable by sulphuretted hydrogen must be absent as well as strong oxidising agents.

~Effect of Varying Temperature.~--The effect of increased temperature is to change the colour from brown to black, and to render the estimation difficult.

1 c.c. at 15 C. showed the colour of 0.5 c.c. at 60 C.

2 " " " " 1.5 " at 60 C.

3 " " " " 5.0 " at 50 C.

~Effect of Varying Time.~--The colour becomes lighter on standing: 2 c.c. on standing 10, 20, and 40 minutes became equal in colour to 1.7 c.c.

~Effect of Acids and Ammonia.~--Two c.c. of the solution with 2 c.c. of dilute hydrochloric acid became cloudy and equivalent to about 2.5 c.c.; and a similar result was got with 2 c.c. of dilute sulphuric acid. With 2 c.c. of dilute ammonia the solution became somewhat darker, or equal to 2.3 c.c.; but gave a very clear solution easy to compare.

~Determination of Lead in Commercial Zinc.~--Dissolve 0.1 gram of the metal in 1 c.c. of dilute nitric acid evaporates till a solid separates out, dilute to 100 c.c. with water, and take 20 c.c. for a.s.say. A sample treated in this way required 2.4 c.c.; this multiplied by 5 gives 12.0 c.c., equivalent to 1.2 milligram of lead, or 1.2 per cent. By gravimetric a.s.say the sample gave 1.10 per cent.

PRACTICAL EXERCISES.

1. Thirty grams of galena gave on dry a.s.say 21 grams of lead; and this, on cupellation, gave 15 milligrams of silver. Calculate the results in per cents. of lead and in ounces of silver to the ton of ore.

2. How many ounces of silver to the ton would be contained in the lead got from this ore if the loss in smelting is equal to that of the a.s.say?

3. Having given you a sample of white lead freed from oil by was.h.i.+ng with ether, how would you proceed to determine the percentage of lead in it?

THALLIUM.

Thallium is a rare metal, found in small quant.i.ties in some varieties of iron and copper pyrites, and in some lithia micas. It resembles lead in appearance. Its compounds resemble the salts of the alkalies in some respects; and, in others, those of the heavy metals.

It is detected by the green colour which its salts impart to the flame.

This, when examined with the spectroscope, shows only one bright green line.

It is separated and estimated by dissolving in aqua regia; converting into sulphate by evaporation with sulphuric acid; separating the second group of metals with sulphuretted hydrogen in the acid solution, boiling off the excess of the gas; nearly neutralising with carbonate of soda; and precipitating the thallium with an excess of pota.s.sic iodide. On allowing the liquid to stand for some time a bright yellow precipitate of thallous iodide separates out. This is collected on a weighed filter; washed with cold water, finis.h.i.+ng off with alcohol; dried at 100 C., and weighed. The precipitate is thallous iodide TlI, and contains 61.6 per cent. of thallium.

BIs.m.u.tH.

Bis.m.u.th is nearly always found in nature in the metallic state; but occasionally it is met with as sulphide in bis.m.u.thine and as carbonate in bis.m.u.t.i.te. It is also found in some comparatively rare minerals, such as tetradymite, combined with tellurium, and a.s.sociated with gold. In minute quant.i.ties it is widely distributed: it is a common const.i.tuent of most copper ores; hence it finds its way into refined copper, which is seldom free from it. It is occasionally met with in silver in sufficient quant.i.ty to interfere with the working qualities of that metal.

Bis.m.u.th compounds are used in medicine and in the manufacture of alloys.

Bis.m.u.th possesses many useful properties. It has considerable commercial value, and sells at a high price.

The metal is brittle, breaks with a highly crystalline fracture, and has a characteristic reddish-yellow colour. It is almost insoluble in hydrochloric, but readily dissolves in nitric, acid; and gives, if the acid is in excess, a clear solution. Bis.m.u.th salts have a strong tendency to separate out as insoluble basic compounds; this is more especially true of the chloride which, on diluting with a large volume of water, becomes milky; the whole of the bis.m.u.th separating out. The nitrate, carbonate, and hydrate yield the oxide (Bi_{2}O_{3}) on ignition. This oxide closely resembles litharge. It combines with silica, forming fluid slags; and at a red heat is liquid enough to be absorbed by a cupel; in fact, bis.m.u.th may take the place of lead in cupellation. The metal itself is easily fusible, and may be separated from its ores by liquation.

The a.s.say of bis.m.u.th by wet methods presents little difficulty, and is fairly accurate. The price of the metal is such that only methods which yield good results should be adopted; and, since bis.m.u.th is volatile at the temperature of the furnace, and is found mixed with ores not easy to flux, as also with metals which are not easily separated by the dry method, the dry a.s.say can only be considered as having a qualitative value.

DRY a.s.sAY.

~By Liquation.~--This is adapted to ores containing the bis.m.u.th as metal. Take 20 grams of the powdered ore and place in a crucible with a perforated bottom, put this crucible into another of about the same size and lute the joint. Lute on a cover, place in the furnace and heat to redness. The bis.m.u.th melts readily and drains into the lower crucible from which, when cold, it is taken and weighed.

~By Fusion.~--For fairly pure ores the process is as follows:--Take 20 grams of the ore and mix with 20 grams of fusion mixture, 10 grams of salt and 5 or 10 grams of pota.s.sium cyanide; place in a crucible, cover, and fuse at a moderate temperature for about fifteen minutes; pour; when cold detach the metal and weigh.

For coppery ores in which the metals are present as sulphides use the fluxes just given with 2 grams of charcoal (instead of the cyanide) and a little sulphur.

For coppery ores in which the metals are present as oxides, mix 20 grams of the ore with 10 grams of fusion mixture, 4 grams of salt, 4 grams of sulphur and 2 grams of charcoal; and fuse.

A considerable percentage of bis.m.u.th is lost in these a.s.says; it is stated as being nearly 8 per cent. of the metal present.

WET METHODS.

~Detection.~--Bis.m.u.th is detected by dissolving the substance in nitric or hydrochloric acid and precipitating the diluted solution with sulphuretted hydrogen. The precipitated sulphides, after digesting with soda and was.h.i.+ng, are dissolved in nitric acid and the solution boiled with ammonium carbonate. The precipitate is washed and then warmed with dilute sulphuric acid. The solution will contain the bis.m.u.th. Add a solution of pota.s.sium iodide in excess, and boil; a yellow or dark brown solution proves that bis.m.u.th is present. Another good test for small quant.i.ties of bis.m.u.th is to add tartaric acid to the solution to be tested, and then to make it alkaline with potash. Add a few c.c. of Schneider's liquid,[61] and heat. A brownish-black colour is produced by as little as one part of bis.m.u.th in 200,000 of solution. The test is not applicable in the presence of mercury, copper, or manganese.

Compounds of bis.m.u.th fused with cyanide of pota.s.sium in a Berlin crucible readily give a globule of bis.m.u.th which is recognised by its appearance and fracture.

~Solution and Separation.~--The solution of bis.m.u.th compounds presents no difficulty. They are soluble in nitric acid or aqua regia, and, provided the solution is sufficiently acid, they remain dissolved. In separating it from other metals the solution is made up to about 100 c.c. and treated with a current of sulphuretted hydrogen. The bis.m.u.th comes down in a tolerably strong acid solution. The sulphide is decanted on to a filter and washed. It is next digested with ammonic sulphide; or, better (especially when other metals are present), dissolved in nitric acid, and treated with an excess of ammonia and a current of sulphuretted hydrogen. The precipitate is filtered off and evaporated to dryness with nitric acid. It is taken up with a few drops of sulphuric acid and a little water; and warmed and filtered, if necessary. The filtrate is nearly neutralised with ammonia; ammonium carbonate added in slight excess; and the liquid heated to boiling and filtered. The bis.m.u.th will be contained in the precipitate with perhaps traces of lead, antimony, tin, or sometimes iron from incomplete separation or was.h.i.+ng. When only traces of a precipitate are got it must be tested.

The bis.m.u.th precipitate is readily soluble in dilute nitric acid.

GRAVIMETRIC DETERMINATION.

The bis.m.u.th having been separated and dissolved in nitric acid[62] is precipitated (after dilution) by the addition of carbonate of ammonium in slight excess, and boiling. The precipitate is filtered off, washed with hot water, dried, ignited, and weighed. The ignition should be performed carefully at not above a low red heat. The oxide which is formed has, at this temperature, a dark yellow or brown colour, and becomes yellow on cooling. It is bis.m.u.thic oxide (Bi_{2}O_{3}) and contains 89.65 per cent. of bis.m.u.th. Fusion with pota.s.sium cyanide at a temperature just sufficient to melt the salt reduces it to the metal which falls to the bottom and runs into a globule. The b.u.t.ton of metal may be weighed, but it often sticks tenaciously to the bottom of the crucible. The precipitation with ammonic carbonate must not be made in a sulphate or chloride solution; since basic compounds would then be thrown down, and the result on weighing would either be too low (because of the volatilisation of the chloride), or too high (because of the retention of sulphuric acid).

Bis.m.u.th compounds in a nitric acid solution are readily decomposed by the electric current, but the deposited bis.m.u.th is not coherent. It comes down in s.h.a.ggy tufts which are difficult to wash and easy to oxidise.

VOLUMETRIC a.s.sAY.

There are two methods which have been proposed; one based on the precipitation as chromate and the estimation of the chromic acid; and the other on the precipitation as oxalate and subsequent t.i.tration with permanganate of potash. These offer little advantage over the easy gravimetric determination.

COLORIMETRIC METHOD.

Bis.m.u.th iodide dissolves in excess of pota.s.sium iodide, forming a yellow-coloured solution, indistinguishable in colour from that given by iodine. The colour, however, is not removed by boiling or by sulphurous acid. Since none of the commoner metals give such a colour, and free iodine is easily separated by boiling, this method is specially suited for small determinations of bis.m.u.th.

It requires a _solution of bis.m.u.th_, made by dissolving 0.1 gram of bis.m.u.th in a drop or so of nitric acid, evaporating with a little sulphuric acid and diluting with water to 1 litre. 1 c.c. will contain 0.1 milligram of bis.m.u.th. And a _solution of sulphurous acid_, made by diluting 10 c.c. of the commercial acid to 1 litre with water.

The determination is made in the usual way: 50 c.c. of the prepared solution, which should not carry more than 0.75 milligram nor less than 0.01 milligram of bis.m.u.th, are placed in a Nessler tube and the colour compared with that observed in a similar tube containing water and pota.s.sium iodide on adding the standard solution of bis.m.u.th.

The a.s.say solution is prepared by separating the bis.m.u.th with sulphuretted hydrogen, boiling the precipitate with nitric acid, and evaporating with sulphuric acid. Take up with water, add 10 or 20 c.c.

of solution of pota.s.sium iodide, boil off any iodine liberated, dilute, filter, and make up to 100 c.c. According to the depth of colour take 10, 20, or 50 c.c. and transfer to the Nessler tube. Add a few c.c. of the solution of sulphurous acid. Into the other Nessler tube put as much pota.s.sium iodide solution as is contained in the a.s.say tube, with sulphurous acid and water to within a few c.c. of the bulk. Then add the standard bis.m.u.th solution till the tints are equal.

The student must be careful not to confuse the colour of the bis.m.u.th iodide with that of free iodine. If the yellow colour is removed by boiling and returns on standing it is due altogether to iodine; if it is lessened by the addition of a few drops of the dilute sulphurous acid, it is in part due to it. Hence the necessity of having a little free sulphurous acid in each tube. A strong solution must not be used, since it liberates iodine from pota.s.sium iodide.

The following experiments ill.u.s.trate the effect of variation in the conditions of the a.s.say:--

~Effect of Varying Temperature.~--At a higher temperature the colour is somewhat lessened.