Temperature15°30°50°100°"Chromate" required19.8 c.c.19.5 c.c.19.3 c.c.19.2 c.c.
The first two of these filtered badly, the precipitate coming through the filter; the last was very satisfactory in the working.
Effect of Varying Bulk.—Using 20 c.c. of lead nitrate, and 10 grams of sodium acetate as before, diluting to the required bulk, heating to boiling, and titrating, the results were:—
Bulk100.0c.c.200.0c.c.500.0c.c.1000.0c.c."Chromate" required19.6"19.3"19.4"19.4"
Effect of Varying Acetic Acid.—Since the experiments are carried out in the presence of sodic acetate, acetic acid is the only acid whose effect need be considered. Working as before, but with 200 c.c. bulk and varying amounts of the acid, the results were:—
Acid present—10.0c.c.20.0c.c.40.0c.c."Chromate" required19.7 c.c.19.1"18.5"17.3"
These experiments show that only slight quantities of acid are admissible.
Effect of Varying Sodium Acetate.—With the same conditions as before, but with varying weights of sodium acetate, the results were:—
Sodium acetate present—5 grams10 grams25 grams50 grams"Chromate" required19.7 c.c.19.6 c.c.19.6 c.c.18.8 c.c.17.8 c.c.
These experiments show that excessive quantities of sodium acetate must be avoided. Ammonium acetate interferes to agreater extent, and if both acetic acid and this salt are present, each exerts its disturbing influence. With 10 grams of ammonium acetate, 19.4 c.c. of the chromate solution were required instead of 19.7 c.c. in the absence of this salt; with 10 grams of the acetate and 10 c.c. of acetic acid, only 18.6 c.c. were required.
Effect of Foreign Salts.—As already stated, sulphates interfere. Twenty c.c. of the lead nitrate solution were taken, precipitated with sulphate of soda, and the precipitate dissolved in 10 grams of sodium acetate and titrated as before. Duplicate experiments required 18.6 c.c. and 18.7 c.c. of the chromate solution. A similar experiment with 40 c.c. of lead nitrate required 37.4 c.c. of chromate. If the sulphate had not been present, the results would have been 19.7 c.c. and 39.4 c.c. respectively.
Effect of Varying Lead.—In these experiments the conditions were as before, but with varying amounts of lead.
Lead nitrate solution present10.0c.c.20.0c.c.50.0c.c.100.0c.c.Chromate solution required.9.4"19.7"48.8"98.2"
Determination of Lead in Galena.—Weigh up 1 gram of the powdered and dried ore, and boil in an evaporating dish with 10 c.c. of dilute hydrochloric acid. When the action becomes sluggish, dilute with an equal bulk of water, and add a weighed piece of zinc rod about 1 inch long and quarter-inch across. Keep up a moderate action by warming till the ore is seen to be completely attacked and the lead precipitated. Decant off the solution, wash once, strip off the lead, wash and weigh the remaining zinc. Dissolve the lead in 5 c.c. of dilute nitric acid, and 5 c.c. of water with the aid of heat. Dilute and transfer to a pint flask; add a slight excess of dilute ammonia, and render faintly acid with acetic acid. Dilute to 150 c.c., heat to boiling, and run in the standard chromate in slight excess, noting the amount required, and make up to 250 c.c. with water. Boil the solution, allow to settle for a minute or so, filter off 50 c.c., and determine the excess of chromate colorimetrically. As an example, 1 gram of an impure galena was precipitated with 75 c.c. of standard chromate (100 c.c. = 1.020 gram lead). The excess found in 50 c.c. was 0.3 c.c., which, multiplied by 5, gives 1.5 c.c. as the excess in the whole solution. The remaining 73.5 c.c. of "chromate" required by the assay, are equivalent to 0.7497 gram of lead. The zinc used up weighed 1.5 grams, and contained 0.0165 gram of lead. Thus we get—
Lead in the assay0.7497gramLead from the zinc0.0165"———∴ Lead in the galena0.7332"
Equivalent to 73.3 per cent.
Another sample, in which the galena was accompanied with a large quantity of pyrites, gave the following results:—Three grams were treated with 30 c.c. of dilute hydrochloric acid and a rod of zinc. The zinc and lead were carefully transferred to another vessel, the zinc cleaned, and the lead (dissolved in 5 c.c. of dilute nitric acid and 20 c.c. of water) treated as before.
14.5 c.c. of the chromate were required= 0.1479gram leadLead in 2 grams of zinc= 0.0220"———∴ Lead in 3 grams of the ore= 0.1259"
Equivalent to 4.20 per cent.
The same ore gave by separation of the lead with sulphuretted hydrogen, and conversion into sulphate, 4.16 per cent.
With fairly pure ores, free from sulphate, the assay may be made more quickly as follows: Dissolve 1 gram of the finely powdered ore by boiling gently with 40 c.c. of dilute hydrochloric acid for 15 minutes; cool; add a few drops of permanganate; neutralise with ammonia, add acetic acid and a little sodium acetate. Titrate with standard chromate.
This is based upon the brown coloration produced in very dilute solutions of lead by the action of a solution of sulphuretted hydrogen. The quantity of lead in the 50 c.c. of the assay solution must not much exceed 0.5 milligram, nor be less than 0.01. The sulphuretted hydrogen is used in the form of a solution, and is not bubbled through the assay. The principle of working is the same as previously described.
Standard Lead Solution.—Each c.c. of this should contain 0.1 milligram of lead. It is made by diluting 10 c.c. of the solution of lead nitrate, described under the volumetric process, to 1 litre.
Sulphuretted hydrogen wateris made by passing 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 assay 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 assay 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 abrown 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.
1c.c.at 15° C.showed thecolour of0.5c.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 assay. 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 assay the sample gave 1.10 per cent.
1. Thirty grams of galena gave on dry assay 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 assay?
3. Having given you a sample of white lead freed from oil by washing with ether, how would you proceed to determine the percentage of lead in it?
Thallium is a rare metal, found in small quantities 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 potassic 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, finishing off with alcohol; dried at 100° C., and weighed. The precipitate is thallous iodide TlI, and contains 61.6 per cent. of thallium.
Bismuth is nearly always found in nature in the metallic state; but occasionally it is met with as sulphide in bismuthine and as carbonate in bismutite. It is also found in some comparatively rare minerals, such as tetradymite, combined with tellurium, and associated with gold. In minute quantities it is widely distributed: it is a common constituent 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 quantity to interfere with the working qualities of that metal.
Bismuth compounds are used in medicine and in the manufacture of alloys. Bismuth 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. Bismuth 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 bismuth separating out. The nitrate, carbonate, and hydrate yield the oxide (Bi2O3) 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, bismuth may take the place of lead in cupellation. The metal itself is easily fusible, and may be separated from its ores by liquation.
The assay of bismuth 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 bismuth 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 assay can only be considered as having a qualitative value.
By Liquation.—This is adapted to ores containing the bismuth 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 bismuth 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 potassium 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 bismuth is lost in these assays; it is stated as being nearly 8 per cent. of the metal present.
Detection.—Bismuth 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 washing, 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 bismuth. Add a solution of potassium iodide in excess, and boil; a yellow or dark brown solution proves that bismuth is present. Another good test for small quantities of bismuth 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 bismuth in 200,000 of solution. The test is not applicable in the presence of mercury, copper, or manganese.
Compounds of bismuth fused with cyanide of potassium in a Berlin crucible readily give a globule of bismuth which is recognised by its appearance and fracture.
Solution and Separation.—The solution of bismuth 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 bismuth 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 bismuth will be contained in the precipitate with perhaps traces of lead, antimony, tin, or sometimes iron from incomplete separation or washing. When only traces of a precipitate are got it must be tested. The bismuth precipitate is readily soluble in dilute nitric acid.
The bismuth 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 bismuthic oxide (Bi2O3) and contains 89.65 per cent. of bismuth. Fusion with potassium 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 button 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).
Bismuth compounds in a nitric acid solution are readily decomposed by the electric current, but the deposited bismuth is not coherent. It comes down in shaggy tufts which are difficult to wash and easy to oxidise.
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 titration with permanganate of potash. These offer little advantage over the easy gravimetric determination.
Bismuth iodide dissolves in excess of potassium 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 bismuth.
It requires asolution of bismuth, made by dissolving 0.1 gram of bismuth 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 bismuth. And asolution 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 bismuth, are placed in a Nessler tube and the colour compared with that observed in a similar tube containing water and potassium iodide on adding the standard solution of bismuth.
The assay solution is prepared by separating the bismuth 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 potassium 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 potassium iodide solution as is contained in the assay tube, with sulphurous acid and water to within a few c.c. of the bulk. Then add the standard bismuth solution till the tints are equal.
The student must be careful not to confuse the colour of the bismuth 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 potassium iodide.
The following experiments illustrate the effect of variation in the conditions of the assay:—
Effect of Varying Temperature.—At a higher temperature the colour is somewhat lessened.
1.0c.c.at 15° C.showed the colour of0.8 c.c.at 70° C.2.5"""2.0"5.0"""5.0"
Effect of Free Acid.—
2.5c.c. with5 c.c. ofnitric acidequalled2.5c.c.5.0""sulphuric acid"5.0"
Hydrochloric acid almost completely removes the colour, which, however, is restored by the addition of a few crystals of potassium iodide.
Effect of Alkalies.—Ammonia, soda, or potash destroys the colour, but it is restored on acidifying with nitric or sulphuric acid.
Effect of Ammonic Salts.—The following table shows the results after addition of ammonic salts:—
C.c. present.With 10 grams Ammonic Nitrate.With 10 grams Ammonic Sulphate.With 10 grams Ammonic Chloride.1.0 c.c.0.9 c.c.1.1 c.c.—2.5  "2.5  "2.7   "—5.0   "5.0  "5.5  "—
Ammonic chloride, like hydrochloric acid, removes the colour, which may be restored on the addition of more potassium iodide. Nitrates and sulphates do not thus interfere.
Effect of Foreign Salts.—Sodic hyposulphite almost completely removes the colour. Copper salts liberate iodine; but when this has been removed by boiling and the cuprous iodide has been filtered off there is no further interference. Dilute solutions of lead salts give no colour.
1. A fusible alloy is made up of 8 parts of bismuth, 5 of lead, and 3 of tin. What weight of oxide of bismuth, Bi2O3, would you get on the analysis of 1 gram of it?
2. What weight of bismuth can be got from 2 grams of the subnitrate BiONO3.H2O?
3. How would you detect and separate arsenic, lead, and copper in a sample of bismuth?
Antimony occurs in the native state, but is rare; its common ore is antimonite, the sulphide (Sb2S3). Jamesonite and other sulphides of lead and antimony are frequently met with. Sulphide of antimony is also a constituent of fahlerz and of many silver ores.
Antimonite occurs generally in fibrous masses, has a lead-like metallic lustre, is easily cut with a knife, and melts in the flame of a candle.
Antimony itself has a very crystalline fracture, is brittle, and has a bluish-white colour. It is used in the preparation of alloys with lead and tin for the manufacture of type-metal. It is readily fusible, and imparts hardness and the property of taking a sharp cast to its alloys. It is practically insoluble in hydrochloric acid. On boiling with strong nitric acid it is converted into antimonic oxide (Sb2O5), which is a powder almost insoluble in this acid or in water, but which may be got into solution with difficulty by the prolonged action of hydrochloric and tartaric acids. Antimonic oxide is converted on ignition into the tetroxide (Sb2O4) with loss of oxygen. Antimony forms two series of salts, antimonious and antimonic; and advantage is taken of this in its determination volumetrically. Either sulphide of antimony yields antimonious chloride on boiling with hydrochloric acid, sulphuretted hydrogen being given off; and, in the case of antimonic sulphide, sulphur is deposited. Antimonious is converted into antimonic chloride by treatment with permanganate of potash in an acid solution. Antimonic chloride and potassium iodide react, forming antimonious chloride and free iodine. This latter may be got rid of by boiling. Sulphide of antimony is separated from the ore by liquation; this regulus is met with in commerce as "crude antimony."
An approximate determination of the amount of sulphide of antimony in an ore may be made by fusing and liquating in a luted double crucible in the manner described under bismuth.This is unsatisfactory. The determination of metallic antimony in an ore is made either by fusion with potassium cyanide or by fusion with iron, as in the galena assay. Both methods yield poor results; and, where iron is used, it must be added in quantity only sufficient for desulphurising; this amounts to about 40 per cent. in pure ores. If the iron is in excess it alloys with the reduced antimony. If, on the other hand, it is insufficient, the metal will contain sulphur; or sulphide of antimony will be lost in the slag.
The following note, for which we are indebted to Mr. Bedford McNeill, A.R.S.M., gives a description of the method adopted in the commercial valuation of a parcel of antimony ore:—
The antimony smelter, when he wishes to determine the value of any parcel of ore—usually the sulphide—that may be offered for sale, practically has recourse to the smelting operation. That is, a quantity of 2 or 3 cwts. taken by his sampler having been obtained, he treats it under the immediate supervision of the foreman smelter as if it formed part of the ore in process of daily reduction at his works. He thus determines by actual trial the output which it may fairly be anticipated will be yielded by the bulk, and upon the result of this trial or assay, and the knowledge gained of the actual behaviour of the ore under treatment, he bases his tender, knowing that, should he secure the parcel, he may confidently expect a similar return.
Briefly, the process consists of the three ordinary operations of—
(a) Singling or removing most of the antimony from the ore;(b) Doubling;(c) Refining or "starring."
But in the assay sufficient information is generally given by the first two of these.
A new pot having been taken and made hot in the furnace, 40 or 45 lbs. of the ore is weighed in (the mineral from the necessities of sampling not exceeding walnut size); 1 to 3 lbs. of salt cake is now added to render the separation of the resulting sulphide of iron more easy, as also to assist in the fusion of the gangue; 20 to 25 lbs. of tin-plate scrap, beaten more or less into ball shape, is weighed, placed on the top of the ore and salt cake, and the whole brought to a state of fusion. The foreman from time to time takes notice of the behaviour of the ore under the working conditions. Ores that manifest a tendency to "boil" or "froth " require the admixture of other more sluggish mineral in order to render their reduction economically practicable.
After 1-1/4 to 1-1/2 hours (the time depending mainly on the temperature), the contents of the crucible are usually in a state of tranquil fusion. The pot is now lifted from the fire, and itscontents transferred to a conical iron mould, the empty pot being immediately put back into the fire, and the latter "mended" with sufficient coke for another run. The conical mould (when dealing with a "strange" ore, and the possibility of insufficient iron being present to satisfy the sulphur contents) is wiped inside with clay previous to pouring in the molten charge. Otherwise the mould itself will be attacked, and the contents after solidifying will require to be chiselled out piecemeal.
A further 40 lbs. of the ore is now charged into the crucible with iron as above; but before this second charge is ready to be drawn an inspection of the first may suggest the addition of either 3 or 5 lbs. more iron, or 5 or 10 lbs. more ore.
It is a good fault rather to aim at an excess of iron as tending to clean the ore from antimony, any of the latter that (from an insufficiency of iron) may be left in the slag from the first process being irretrievably lost; whereas, if the iron be in excess, that which is combined with the crude antimony resulting from the first process is easily got rid of by adding 3 to 5 lbs. or so of ore in the second process.
This latter, as practised for the determination of the value of a parcel of ore, consists in selecting two of the best quality singles, resulting from perhaps four or five trials as above, and running them down with a few pounds of salt cake, or a mixture of salt cake with American potash, and (as is generally necessary) a small addition of ore.
Upon the final result (confirmed perhaps on another pair of singles, and, judging from the total weight or output of the metal as calculated from the ore used in "singling," plus any added in the "doubling," the crystalline fracture and face of the metal, its colour, etc.) the price to be offered for the parcel of ore is fixed.
Detection.—The antimony, if any, being got into solution by treating the ore with hydrochloric acid or aqua regia may be detected by evaporating with hydrochloric acid, diluting, and filtering into the cover of a platinum crucible or (better) a platinum dish. A small lump of zinc is then added, and, if antimony is present,the dishwill in a minute or so be stained black with a deposit of metallic antimony. This stain is removed by nitric, but not by hydrochloric, acid. The reaction is delicate and characteristic; arsenic under like conditions is evolved as arseniuretted hydrogen, and tin is deposited as metalon the zinc.
Solution.—Ores, &c., containing antimony are best opened up by boiling with hydrochloric acid or aqua regia; treatment withnitric acid should be avoided wherever possible, since it forms antimonic acid, which is subsequently dissolved only with difficulty. Salts of antimony in solution have a tendency to form insoluble basic salts; so that care must be exercised in diluting. Compounds such as antimonite which are soluble in hydrochloric should be dissolved at once in that acid.
Separation.—To the solution add potash in excess and a little free sulphur, and pass a current of sulphuretted hydrogen for some minutes; allow to digest for an hour or so on a hot plate; filter; and wash the residue. Acidulate the filtrate with hydrochloric acid: the precipitate will contain the antimony (as Sb2S5), and possibly arsenic or tin. The precipitate is transferred to a beaker and boiled with hydrochloric acid; the solution is filtered off and diluted. Add a few crystals of tartaric acid, and pass a current of sulphuretted hydrogen for some time. The first flocculent precipitate will become denser, and render the filtering more easy. Transfer the precipitate (after washing free from chlorides) to a Berlin dish, and treat cautiously with fuming nitric acid. The action of this acid on the sulphide is very violent. Evaporate and ignite, transfer to a silver dish, and fuse with four or five times its weight of caustic soda, cool and extract with a little water, then add an equal volume of alcohol, and allow to stand overnight. Filter, wash with dilute alcohol. (The filtrate will contain the tin.) The residue contains the antimony as antimonate of soda, and is dissolved off the filter with hot dilute hydrochloric, with the help of a little tartaric, acid. The filtrate is now ready for the gravimetric determination.
Pass a current of sulphuretted hydrogen through the solution containing the antimony to which a little tartaric acid has been previously added. Pass the gas till the precipitate becomes dense, and the antimony is all down. The solution must not be too strongly acid. Filter off the precipitate, wash with hot water, dry in the water oven, transfer to a weighed porcelain dish, and cautiously treat with fuming nitric acid. Continue the action on the water bath till the sulphur and antimony are completely oxidised. Evaporate; ignite, gently at first, then strongly over the blast; cool, and weigh. The residue is a white infusible powder, and consists of antimony tetroxide, Sb2O4, containing 78.94 per cent. of the metal.
Determination of Antimony as Bigallate.—What appears to be a very good method has been worked out by M.A. Guyard, and is described in Crookes'Select Methods, p. 398.
The antimony must be in solution as antimonious chloride, and must not be accompanied by an excess of hydrochloric acid. To ensure these conditions, the solution is treated with potassium iodide until no more iodine is evolved, and is then evaporated to remove the excess of hydrochloric acid. To the concentrated, and nearly neutral, solution a freshly-prepared solution of gallic acid is added in slight excess. A bulky white precipitate is formed that settles rapidly. The solution is diluted with hot water and washed by decantation. Then the precipitate is collected on a weighed double filter, washed once or twice with hot water, and dried at 100° C. The dried substance is antimony bigallate, and contains 40.85 per cent. of antimony. It should be completely soluble in ammonium sulphide. The solution in which the antimony is precipitated need not be quite free from other metals.
This is based on the reduction of antimonic chloride (SbCl5) to antimonious (SbCl3) by the action of potassium iodide in strong hydrochloric acid solution.[63]Iodine is at the same time liberated, and the amount of antimony reduced is got at by titrating with sodium hyposulphite, which measures the iodine set free.
The standard solution of sodium hyposulphite is made by dissolving 41.32 grams of the salt (Na2S2O3.5H2O) in water, and diluting to 1 litre. One hundred c.c. will be equivalent to about 1 gram of antimony.
It is standardised with the help of a solution of antimony made as follows:—Weigh up 5 grams of powdered antimony, transfer to a flask, and cover with 50 c.c. of hydrochloric acid; boil, and add nitric acid (5 or 10 drops at a time) until the metal is dissolved. Allow the action of the nitric acid to cease before adding more. Boil down to a small bulk, add 250 c.c. of hydrochloric acid, and dilute to nearly 1 litre. Warm until any precipitate which has formed is redissolved; allow to cool slowly, and run in from a pipette a weak solution of permanganate until a faint brown colour is produced. Dilute to exactly 1 litre; 100 c.c. contain 0.5 gram of antimony as antimonic chloride.
In standardising, take 50 c.c. of the antimony solution, and transfer to a flask; add 2 grams of potassium iodide crystals, and when dissolved, after standing a few minutes, run in the solution of "hypo" from an ordinary burette until the greater part of the iodine has been reduced. Add a few drops of starch solution, and continue the addition of the "hypo" until the muddy-green colourchanges to a clear brownish-yellow. The solution must be shaken after each addition of the "hypo."
In determining antimony in ore, weigh up 0.5 to 1 gram, and dissolve in hydrochloric acid with, if necessary, the help of chlorate of potash. The antimony is separated as sulphide, redissolved in hydrochloric acid, and oxidised with a crystal of chlorate of potash. Chlorine is boiled off, and the solution diluted with an equal bulk of water. To the clear cold solution potassium iodide is added, and after a few minutes the liberated iodine is titrated with "hypo," as already described. The method only yields satisfactory results when the standard and assay are carried out alike.
FOOTNOTES:[50]"Modern American Methods of Copper Smelting" (Dr. Peters).[51]"Journal of the Society of Chemical Industry," vol. v. No. 2.[52]Lead when present is precipitated on thespiralin the form of a dark powder of dioxide (PbO2). Manganese is also thrown down on the spiral as dioxide (MnO2), the solution at the same time becomes violet from the formation of permanganic acid.[53]See the method given underExamination of Commercial Copper.[54]CuSO4+ 4KCy = 2KCy.CuCy2+ K2SO4.[55]2CuSO4+ 3KCy + Am2O = Cu2Cy2+ Am2SO4+ K2SO4+ KCyO.[56]2CuSO4+ 4KI = Cn2I2+ 2I + 2K2SO4.[57]2Na2S2O3+ 2I = 2NaI + Na2S4O6.[58]For further information, see Appendix B., and a paper by J.W. Westmoreland,Journal of the Society of Chemical Industry, vol. v. p. 48.[59]3Cu2O + 6AgNO3+ 3H2O= 2Cu2H3O3NO3+ 2Cu(NO3)2+ 6Ag.(Insoluble basic salt.)[60]K2CrO4+ Pb(NO3)2= PbCrO4+ 2KNO3[61]Made by dissolving 12 grams of tartaric acid and 4 grams of stannous chloride in water, and adding potash solution till it is alkaline. The solution should remain clear on heating to 60° or 70° C.[62]It must be remembered that arsenate of bismuth is completely insoluble in this acid.[63]SbCl5+ 2KI = I2+ SbCl3+ 2KCl.
[50]"Modern American Methods of Copper Smelting" (Dr. Peters).
[50]"Modern American Methods of Copper Smelting" (Dr. Peters).
[51]"Journal of the Society of Chemical Industry," vol. v. No. 2.
[51]"Journal of the Society of Chemical Industry," vol. v. No. 2.
[52]Lead when present is precipitated on thespiralin the form of a dark powder of dioxide (PbO2). Manganese is also thrown down on the spiral as dioxide (MnO2), the solution at the same time becomes violet from the formation of permanganic acid.
[52]Lead when present is precipitated on thespiralin the form of a dark powder of dioxide (PbO2). Manganese is also thrown down on the spiral as dioxide (MnO2), the solution at the same time becomes violet from the formation of permanganic acid.
[53]See the method given underExamination of Commercial Copper.
[53]See the method given underExamination of Commercial Copper.
[54]CuSO4+ 4KCy = 2KCy.CuCy2+ K2SO4.
[54]CuSO4+ 4KCy = 2KCy.CuCy2+ K2SO4.
[55]2CuSO4+ 3KCy + Am2O = Cu2Cy2+ Am2SO4+ K2SO4+ KCyO.
[55]2CuSO4+ 3KCy + Am2O = Cu2Cy2+ Am2SO4+ K2SO4+ KCyO.
[56]2CuSO4+ 4KI = Cn2I2+ 2I + 2K2SO4.
[56]2CuSO4+ 4KI = Cn2I2+ 2I + 2K2SO4.
[57]2Na2S2O3+ 2I = 2NaI + Na2S4O6.
[57]2Na2S2O3+ 2I = 2NaI + Na2S4O6.
[58]For further information, see Appendix B., and a paper by J.W. Westmoreland,Journal of the Society of Chemical Industry, vol. v. p. 48.
[58]For further information, see Appendix B., and a paper by J.W. Westmoreland,Journal of the Society of Chemical Industry, vol. v. p. 48.
[59]3Cu2O + 6AgNO3+ 3H2O= 2Cu2H3O3NO3+ 2Cu(NO3)2+ 6Ag.(Insoluble basic salt.)
[59]3Cu2O + 6AgNO3+ 3H2O= 2Cu2H3O3NO3+ 2Cu(NO3)2+ 6Ag.(Insoluble basic salt.)
[60]K2CrO4+ Pb(NO3)2= PbCrO4+ 2KNO3
[60]K2CrO4+ Pb(NO3)2= PbCrO4+ 2KNO3
[61]Made by dissolving 12 grams of tartaric acid and 4 grams of stannous chloride in water, and adding potash solution till it is alkaline. The solution should remain clear on heating to 60° or 70° C.
[61]Made by dissolving 12 grams of tartaric acid and 4 grams of stannous chloride in water, and adding potash solution till it is alkaline. The solution should remain clear on heating to 60° or 70° C.
[62]It must be remembered that arsenate of bismuth is completely insoluble in this acid.
[62]It must be remembered that arsenate of bismuth is completely insoluble in this acid.
[63]SbCl5+ 2KI = I2+ SbCl3+ 2KCl.
[63]SbCl5+ 2KI = I2+ SbCl3+ 2KCl.
Iron rusts or oxidises very readily, and, consequently, is rarely found in the metallic state in nature; such native iron as is found being generally of meteoric origin or imbedded in basalt and other igneous rocks. It chiefly occurs as oxide, as in magnetite, hæmatite, and in the brown iron ores and ochres. Chalybite, which is carbonate of iron, is an ore of great importance. Iron is found combined with sulphur in pyrrhotine and pyrites, and together with arsenic in mispickel. It is a common constituent of most rocks, imparting to them a green, black, or brown colour; and is present, either as an essential part or as an impurity, in most substances.
The chemistry of iron is somewhat complicated by the existence of two oxides, each of which gives rise to a well-marked series of compounds. Those derived from the lower oxide, known as ferrous salts, are generally pale and greenish. Ferric salts are derived from the higher oxide, and are generally red, brown, or yellow. The existence of these two well-marked families of salts renders the assay of iron comparatively easy, for the quantity of iron present in a solution can be readily measured by the amount of oxidising or reducing agent required to convert it from the one state into the other—that is, from ferrous to ferric, or from ferric to ferrous, as the case may be.
In the red and brown iron ores and ochres ferric iron is present; in chalybite the iron is in the ferrous state; and in magnetite it is present in both forms. Traces of iron in the ferrous state may be found (even in the presence of much ferric iron) by either of the following tests:—