FOOTNOTES:

FOOTNOTES:[95]This will require two or three hours to thoroughly complete. It is best to powder the oxide first produced, and recalcine.[96]No magnetic oxide was formed.[97]For example:—CaO + 2HCl = CaCl2+ H2O.PbO + H2SO4= PbSO4+ H2O.MgO + 2HNO3= Mg(NO3)2+ H2O.Al2O3+ 6HCl = Al2Cl6+ 3H2O.Fe2O3+ 3H2SO4= Fe2(SO4)3+ 3H2O.[98]Fe2O3: 2FeO:: 0.2877: 0.2589.[99]100 c.c. contain 1 gram of sulphuric acid.[100]3SiF4+ 4KCl + 2H2O = 2K2SiF6+ SiO2+ 4HCl.

[95]This will require two or three hours to thoroughly complete. It is best to powder the oxide first produced, and recalcine.

[96]No magnetic oxide was formed.

[97]For example:—CaO + 2HCl = CaCl2+ H2O.PbO + H2SO4= PbSO4+ H2O.MgO + 2HNO3= Mg(NO3)2+ H2O.Al2O3+ 6HCl = Al2Cl6+ 3H2O.Fe2O3+ 3H2SO4= Fe2(SO4)3+ 3H2O.

[97]For example:—

CaO + 2HCl = CaCl2+ H2O.

PbO + H2SO4= PbSO4+ H2O.

MgO + 2HNO3= Mg(NO3)2+ H2O.

Al2O3+ 6HCl = Al2Cl6+ 3H2O.

Fe2O3+ 3H2SO4= Fe2(SO4)3+ 3H2O.

[98]Fe2O3: 2FeO:: 0.2877: 0.2589.

[99]100 c.c. contain 1 gram of sulphuric acid.

[100]3SiF4+ 4KCl + 2H2O = 2K2SiF6+ SiO2+ 4HCl.

Sulphur occurs native in volcanic districts, and is mined in Sicily, Italy, and California in considerable quantities. Combined with metals (sulphides), it is common in all mineral districts. Iron pyrites (FeS2) is the most abundant source of this element. Sulphates, such as gypsum, are fairly common, but have no value so far as the sulphur in them is concerned. In coal it exists as an impurity, occurring partly as a constituent of organic compounds.

Sulphur, whether free or combined with metals, forms, on burning, sulphurous oxide (SO2), which by the action of oxidising agents and water is converted into sulphuric acid. It forms two oxides, sulphurous (SO2) and sulphuric (SO3), which combine with bases to form sulphites and sulphates. Sulphites are of little importance to the assayer, and are converted into sulphates by the action of nitric acid and other oxidising agents.

The native sulphides, when acted on with hydrochloric acid, give off sulphuretted hydrogen; with nitric acid or aqua regia, sulphates are formed, and more or less sulphur separated.

Sulphur is detected in sulphides by the irritating odour of sulphurous oxide given off on roasting, by the evolution of sulphuretted hydrogen when treated with hydrochloric acid, or by a white precipitate of barium sulphate formed when barium chloride is added to the aqua regia solution.

Dry Assay.—There is no method of general application. Free or native sulphur may be volatilised, condensed, and weighed, but pyrites only gives up a portion of its sulphur when heated in a closed vessel, while most sulphides, and all sulphates, give up none at all.

In the determination of sulphur in brimstone, 10 grams of the substance are taken, placed in a small porcelain dish, heated over a Bunsen burner in a well-ventilated place, and ignited. When the sulphur has been completely burnt off, the residue (which consists chiefly of sand) is collected and weighed. In a separate portion the moisture and arsenic are determined; the amounts ofthese are deducted from the loss in the first experiment. The difference, multiplied by 10, gives the percentage of sulphur.

Solution.—All sulphates, excepting those of lead, barium, strontium, and lime, are soluble in water or dilute acid. All sulphides, except cinnabar, are converted into sulphates by the action of nitric acid at a gentle heat; or, better, by the action of a mixture of three volumes of nitric acid and one volume of hydrochloric acid. This last attacks cinnabar as well. With most substances it is difficult to convert the whole of the sulphur into sulphuric acid. The sulphur separates out at first as a dark spongy mass, which (on continued treatment) changes to light-coloured flakes. When the solution becomes concentrated and the temperature rises sufficiently, the sulphur fuses into one or more honey-coloured globules which, owing to the small surface they oppose to the acid, are very slowly oxidised. It is not desirable to assist the formation of these globules; therefore, the temperature is kept as low as possible, and strong nitric acid is used. When such globules form, it is best to allow the solution to cool, when the globules will solidify. They can then be filtered off and picked out from the insoluble residue, dried, weighed, ignited, and again weighed, the loss being counted as sulphur. With iron pyrites this difficulty seldom occurs.

Metallic sulphides when fused with an excess of nitre are completely oxidised. If the ore is rich in sulphur, some inert body (such as sodium chloride, or, better, sodium carbonate) is added to dilute the action. With pure sulphur, the action is so energetic as to cause an explosion, so that care should be taken. With burnt ores (incompletely calcined pyrites), there is sufficient oxide of iron present to prevent too rapid action.

These fusions with nitre are best conducted in a platinum dish covered with a piece of platinum foil. The ore is ground with the nitre to ensure complete mixing. The heat need not be excessive, so that a single Bunsen burner placed beneath the dish will suffice; if the bottom of the dish is seen to be red-hot, it is sufficient. On cooling and extracting with water, the sulphur will pass into solution as potassium sulphate, which is then filtered off from the insoluble oxides of iron, copper, &c. The filtrate, after having been treated with a large excess of hydrochloric acid, evaporated to dryness, and re-dissolved in water, is ready for the determination.

Lead sulphate may be dissolved by boiling with ammonium acetate. The insoluble sulphates of barium, strontium, and lime,are decomposed by fusing with 4 or 5 times their weight of "fusion mixture." The alkaline sulphates are then dissolved out with water, and filtered off from the insoluble residue. The filtrate is rendered acid with hydrochloric acid.

Separation.—The determination of the sulphuric acid in these solutions by precipitation with barium chloride also serves as a separation; but in hot acid solutions containing copper, and more especially iron salts, the baric sulphate has a strong tendency to carry down amounts of those bodies, varying, no doubt, with the conditions of the precipitation. Boiling hydrochloric acid fails to completely extract them. Moreover, the use of hot concentrated hydrochloric acid causes a loss by dissolving barium sulphate. Nitric acid and nitrates must be decomposed by prolonged boiling and evaporation with hydrochloric acid. The iron may be removed by adding a slight excess of ammonia to the faintly acid solution, filtering off, and washing the precipitated ferric hydrate with hot water. By slightly acidulating the filtrate with hydrochloric acid, it will be rendered ready for the determination.

This assay is one of those which strikingly shows the necessity of getting the assay solution under proper conditions, in order to obtain satisfactory results. The method has been repeatedly investigated, and the conclusion arrived at, "that it can be correct only by accident." Yet there are many chemists who get good results, and place considerable faith in its accuracy. This can only be due to differences in the manner of working. It is generally understood that nitric acid or nitrates must be absent; and our experience fully confirms this. Precipitations in nitrate solutions are worthless, as the following experiments show. In each experiment the bulk of the solution was 150 c.c. The solutions contained 10 grams of nitre, were freely acid with hydrochloric acid, and were precipitated (while boiling) with slight excess of baric chloride.

Sulphuric acidtaken0.020gram0.050gram0.100gram"found0.019"0.047"0.098""taken0.500"1.004"1.000""found0.526"1.126"1.126"

All the precipitates were boiled with hydrochloric acid, and thoroughly washed before weighing. The results of some other experiments on this subject are given under "sulphur" in the "examination of commercial copper," page 207.

The solution having been obtained free from nitrates and chlorates(and containing but little free hydrochloric acid), is largely diluted, heated to boiling, and precipitated with a moderate excess of a solution of chloride of barium (8 parts of the crystallized barium chloride are sufficient for 1 of sulphur). It is allowed to settle for half-an-hour, and then decanted through a filter. The precipitate is shaken up with boiling water, rendered slightly acid, filtered, washed, dried, ignited, and weighed. The ignited precipitate, when pure, is white, and is not decomposed at a red heat; it is barium sulphate (BaSO4), and contains 13.73 per cent. of sulphur, or 34.33 per cent. of sulphuric oxide (SO3).

Determination of Sulphur in Pyrites.—Weigh up half a gram of the dried and powdered sample, and treat with 10 c.c. of a mixture of 3 volumes of nitric acid and 1 volume of hydrochloric acid, occasionally heating. Evaporate to dryness, treat with 5 c.c. of hydrochloric acid, and again evaporate; take up with 1 c.c. of hydrochloric acid and 100 c.c. of hot water, filter through a small filter, and wash. The residue may contain sulphates of lead, barium, or lime; it must be separately examined, if the total sulphur is wanted. The filtrate is heated, and rendered slightly alkaline with ammonia. Filter off the precipitated ferric hydrate through a quick filter, and wash with hot water. If necessary, evaporate the bulk to about 200 c.c., render faintly acid with hydrochloric acid, and add 20 c.c. of solution of barium chloride; allow to stand for half-an-hour, and decant through a filter. Wash with hot water, dry, ignite, and weigh. Pure pyrites contains 53.33 per cent. of sulphur.

This is based upon the easy conversion of all sulphur compounds into sulphates by fusion with nitre or by oxidation with nitric acid; and on the determination of the sulphate formed by titration in an acetic acid solution with baric chloride.[101]The finishing point is determined by filtering off portions of the assay solution, and testing with sulphuric acid. A slight excess of baric chloride will cause a precipitate.

The process may be divided into—(1) the preparation of the solution, and (2) the titration.

Preparation of the Solution.—Weigh up from 1 to 5 grams of the dried and powdered substance, and mix intimately with 4 grams of powdered nitre; clean out the mortar with another gram of nitre, and add this as a cover. Heat in a platinum crucible for fifteen minutes at a low temperature; cool, and extractwith water in an evaporating-dish about 9 inches across, and holding 700 or 800 c.c. Add 10 grams of sodium acetate and 10 c.c. of acetic acid, and dilute to half a litre. Boil. The solution is ready for titrating. Substances which lose sulphur on heating (such as pyrites) are thus treated:—Weigh up 1 gram, and evaporate nearly to dryness with 10 c.c. each of nitric and hydrochloric acids. Take up with 10 c.c. of hydrochloric acid, and again boil down to a small bulk; dilute and transfer to a 9-inch evaporating-dish; add 10 grams of sodium acetate and 5 c.c. of acetic acid, dilute to half a litre, and boil. The solution is ready for titrating. Sulphates may be dissolved up in the dish itself with the help of a c.c. or so of hydrochloric acid; sodium acetate and acetic acid are then added; and, after dilution and boiling, the solutions are at once titrated.

The solution before titration must contain no free mineral acid, but 5 or 10 c.c. of acetic acid should be present. It must contain 10 grams of sodium acetate, or sufficient to convert any free mineral acid into its corresponding sodic salt; or, if chlorides, nitrates or sulphates of the metals are present, sufficient to decompose them. If a precipitation occurs, as is the case with ferric salts, &c., the solution is titrated with the precipitate in it.

The Titration.—The standard solution of barium chlorideis made by dissolving 76.25 grams of the crystallized salt (BaCl2.2H2O) in distilled water, and diluting to 1 litre. 100 c.c. will equal 1 gram of sulphur. As indicator, use dilute sulphuric acid. The strength of the solution may be checked by the titration of 5 grams of ferrous sulphate (oxidized with permanganate of potassium or a few drops of nitric acid), which should require 57.5 c.c. of the barium chloride solution; or any pure sulphate of known composition can be used; anhydrous salts should be preferred.

Fig. 65.

Fill an ordinary 100 c.c. burette with the solution of barium chloride. The evaporating dish containing the assay solution is placed on a round burner (as shown in fig. 65), and the solution is kept steadily boiling. An ordinary Bunsen-burner flame will cause bumping, and should not be used. Run in the standardsolution in quantity known to be insufficient; then withdraw a portion of about 2 c.c., with a pipette, and filter through a fine filter-paper into a test tube. Run in another 0.5 c.c. of the standard solution, and withdraw and filter into a test tube another portion of 2 c.c.; and continue this operation until half-a-dozen or more portions have been drawn off. The test tubes should be arranged in order in a stand resting on a piece of paper, so that each test tube representing 0.5 c.c. of the standard baric chloride may have its value recorded beneath it (fig. 66). Add to each test tube 3 drops of dilute sulphuric acid; that which shows the first appearance of a precipitate marks the point at which the titration is complete. Suppose, for example, that the test tube marked 48.5 c.c. shows no precipitate, while that at 49.0 c.c. shows one, it is evident that the finishing point lies between these readings. With a little practice, one can judge from the appearance of the precipitate in the 49 c.c. tube, whether 1/4 c.c. should be deducted or not.

Fig. 66.

It is better to add dilute sulphuric acid, and to watch for the appearance of a precipitate in the test tube, than to add baric chloride and to look for its non-appearance; besides, baric chloride is much less likely to be present in a test tube as impurity than sulphates are. In this way the chance of error from what are termed "accidental causes" is diminished.

The following experiments show the effect of variation in the conditions of titration:—

Makea standard solution of sulphuric acidby diluting 43.65 grams of sulphuric acid (sp. g. 1.6165) to 1 litre: 100 c.c. will contain 1 gram of sulphur. An equivalent solution may be made by dissolving 100.62 grams of sodium sulphate crystals (Na2SO4.10H2O), or 86.88 grams of ferrous sulphate (FeSO4.7H2O), in water (oxidising the latter), and diluting to 1 litre.

The order in which these experiments are given is that in which they were made in an investigation into the conditions under which the titration could most accurately be effected.

Effect of Hydrochloric and Nitric Acids.—The titrations were performed in the manner already described, but sodic acetateand acetic acid were absent. Twenty c.c. of the standard solution of sulphuric acid were used.

Hydrochloric acid present0.0c.c.1.0c.c.2.0c.c.5.0c.c."Baric chloride" required20.0"20.0"19.7"12.5"Nitric acid present0.0c.c.1.0c.c.2.0c.c.5.0c.c."Baric chloride" required20.0"19.5"18.0"10.0"

These show clearly the interference of free mineral acids, although very dilute hydrochloric acid (1 c.c. in 500 of water) has no effect.

Effect of Acetic and Citric Acids.—A similar series of experiments with these acids gave the following results:—

Acetic acid present0.0c.c.5.0c.c. 50.0c.c. 100.0c.c."Baric chloride" required20.0"20.0"20.0"20.0"

Citric acid present0 gram1 gram5 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.

These acids do not interfere.

Effect of Sodic Acetate and Acetic Acid.—In each of these experiments 5 c.c. of acetic acid was present.

Sodium acetate added0 gram1 gram10 grams50 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.20.0 c.c.

As sodic acetate and acetic acid did not interfere, it became desirable to make some experiments on thefinishing point. The first object sought for was the smallest amount of the standard baric chloride in 500 c.c. of water, required to give an indication when tested in the manner already described.

Conditions of Assay Solution.Baric Chloride required.Water only0.05 c.c.With 10 grams of sodium acetate and 5 c.c. of acetic acid0.05 "The same with 5 grams of nitre0.10 "Like the last, but with 5 grams of salt instead of nitre0.10 "

These show that as small an amount of baric chloride solution as is equal to only 0.000002 gram of sulphur in the 2 c.c. of solution tested yields a decided precipitate on the addition of 3 drops of sulphuric acid.

To determine whether the same finishing point is obtained on testing the filtered portions in the test tubes with baric chloride as is obtained on testing with sulphuric acid, a titration was made with 20 c.c. of standard solution of sulphuric acid, together with the usual quantities of sodic acetate and aceticacid; and two lots of 2 c.c. each were filtered into two sets of test tubes after each addition of the standard baric chloride. To one series 3 drops of baric chloride solution were added, and to the other 3 drops of sulphuric acid. The results were—

"Baric Chloride" added.With Dilute Sulphuric Acid.With Baric Chloride Solution.19.5 c.c.ClearCloudy19.75 "ClearCloudy20.0 "FinishedFinished20.25 "CloudyClear20.5 "CloudyClear

The two methods of testing give the same result. But this balance is disturbed in the presence of much nitre, the indications with baric chloride being disturbed by an opalescence for some c.c. beyond the finishing point. In solutions containing free hydrochloric or nitric acid, a precipitate is obtained with either baric chloride or sulphuric acid.

Effect of Varying Sulphur.—In these and the subsequent experiments the titrations were performed in the presence of 10 grams of sodic acetate and 10 c.c. of acetic acid in the manner already described.

Standard sulphuric acid used5.0 c.c.10.0 c.c.20.0 c.c.50.0 c.c.100.0 c.c."Baric chloride" required5.0 "10.0 "20.0 "50.0 "100.0 "

Effect of Varying Temperature.—With 5 c.c. of standard sulphuric acid titrated at 15° C., 5 c.c. of baric chloride were required; but with larger quantities the results were altogether unsatisfactory when titrated cold.

Effect of Varying Bulk.—

Bulk100.0 c.c.200.0 c.c.500.0 c.c.1000.0 c.c."Baric chloride" required20.0 "20.0 "20.0 "20.5 "

Considerable variation in bulk has no effect, but 500 c.c. is the most convenient volume to work with. It is well to occasionally replace the water boiled off during titration.

Effect of Foreign Salts.—In all these experiments 20 c.c. of "sulphuric acid" were used, and the titration was performed in the ordinary way.

Sodic chlorideadded0 gram5 grams10 grams"Baric chloride" required20.0 c.c.20.0 c.c.19.7 c.c.Ammonic chlorideadded0 gram5 grams10 grams"Baric chloride" required20.0 c.c.20.0 c.c.19.5 c.c.Calcic chlorideadded0 gram1 gram2 grams5 grams"Baric chloride" required20.0 c.c.20.0 c.c.19.2 c.c.19.0 c.c.Zinc chlorideadded0 gram1 gram3 grams5 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.20.0 c.c.Ferrous chlorideadded0 gram1 gram3 grams5 grams"Baric chloride" required20.0 c.c.19.7 c.c.19.5 c.c.19.0 c.c.Ferric chlorideadded0 gram1 gram3 grams5 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.20.0 c.c.Copper chlorideadded0 gram1 gram3 grams5 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.20.0 c.c.Potassic Nitrateadded0 gram1 gram5 grams10 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.19.0 c.c.Potassic Nitriteadded0 gram1 gram5 grams"Baric chloride" required20.0 c.c.20.0 c.c.20.0 c.c.Sodic phosphateadded0 gram1 gram"Baric chloride" required20.0 c.c.22.5 c.c.Sodic arsenateadded0 gram1 gram"Baric chloride" required20.0 c.c.20.5 c.c.

In the absence of ferric salts, phosphates and arsenates count as sulphur.

In two series of experiments for determining the effect of varying amounts of sulphur in the form of ferrous sulphate, we obtained the following results:—In the first series the assay solution was prepared in the manner we have described forPyrites; and in the second series, by fusion with nitre.

Sulphur added0.050 gram0.100 gram0.200 gram"Baric chloride"required(1)5.0 c.c.10.0 c.c.20.0 c.c.""(2)4.7 "10.0 "20.0 "Sulphur added0.500 gram1.000 gram"Baric chloride"required(1)50.0 c.c.100.0 c.c.""(2)50.0 "100.0 "

More than 5 grams of nitre must not be used in an assay; and, since the requisite amount of nitre considerably exceeds that sufficient to oxidise the sulphur, not more than 0.5 gram of unoxidised sulphur should be present in the portion of the sample weighed up for determination. When the amount of sulphur present is not known within reasonable limits, the test portions may be tried with a drop of baric chloride solution instead of sulphuric acid, so that the diminishing quantity of precipitate may give warning of an approach to the finishing point.

Determination of Sulphur in Blende.—Weigh up 1 gramof dried and powdered blende, and mix and fuse with 5 grams of nitre in the manner described. Place the dish and its contents in the titrating-dish, extract with water, add 10 grams of sodium acetate and 10 c.c. of acetic acid, remove and wash the platinum-dish, and dilute to 500 c.c.; boil and titrate. In the example, duplicate determinations required (a) 32.0 c.c., (b) 32.25 c.c., giving an average of 32.1 per cent. of sulphur.

Determination of Sulphur in Chalcopyrite(Yellow Copper Ore).—Take 1 gram of the finely-powdered sample, and 5 grams of nitre. Sprinkle a little of the nitre in a small Wedgwood mortar, place the ore on it, and cover with 2 or 3 grams more of the nitre. Rub up together, and transfer to a small porcelain dish; clean out the mortar with the rest of the nitre, and add to the contents of the dish. Cover with a piece of platinum foil, and heat gently with a Bunsen burner till the nitre melts and the stuff shows signs of deflagrating; remove the heat, and allow the action to go on by itself for a minute or so, then heat over the Bunsen burner for 10 minutes. Cool; transfer the whole to the titrating-dish; boil with 500 c.c. of water; remove the small dish and foil; add sodic acetate and acetic acid, and titrate.

For example, 1 gram required 34.5 c.c. of "barium chloride" (standard = 1.005 gram S), which is equivalent to 34.7 per cent. sulphur. The theoretical percentage is 34.8.

Determination of Sulphur in Chalcocite(Grey Copper Ore).—Proceed as in the last experiment but, since the action with nitre is more moderate, no special precautions need be taken on heating. A platinum dish may be used.

An example which was heated for 30 minutes required 20.5 c.c. of the barium chloride solution. This is equivalent to 20.6 per cent. of sulphur. The theoretical yield is 20.2 per cent.

Determination of Sulphur in Pyrites.—Take 1 gram of the finely-powdered sample, cover with 10 c.c. of nitric acid, and, when action has ceased, evaporate to a small bulk. Add 3 or 4 c.c. of hydrochloric acid, and again evaporate to a paste. Take up with 1 or 2 c.c. of dilute hydrochloric acid, dilute with water, transfer to a titrating-dish, add 10 grams of sodic acetate and 5 c.c. of acetic acid, and dilute with water to 500 c.c. Boil and titrate.

An example with 1 gram of a pure crystallized pyrites required 52.7 c.c. of the barium chloride solution, which is equivalent to 53.0 per cent. of sulphur. Theory requires 53.3 per cent. of sulphur.

Determination of Sulphur in Mispickel.—Take 1 gram of the powdered ore and evaporate with 10 c.c. of nitric acid, and take up with 3 or 4 c.c. of hydrochloric acid. If any globules ofsulphur remain, again evaporate with nitric acid. Dilute, and transfer to the titrating-dish. Add 10 grams of sodic acetate, dilute with water, boil, and titrate. The mispickel carries (according to theory) exactly sufficient iron to precipitate the arsenic as ferric arsenate in an acetic acid solution, so no more iron need be added. The ferric arsenate will separate out as a yellowish-white flocculent precipitate.

An example required, in duplicate experiment, 18.5 c.c. and 18.7 c.c. of barium chloride, equivalent to 18.7 per cent. of sulphur. The formula, FeS2.FeAs2, requires 19.6 percent., but the sulphur generally varies considerably from this amount.

Determination of Sulphur in Burnt Ores.—Take 5 grams of the dried and powdered ore, and rub up with 4 grams of nitre; transfer to the platinum-dish; clean out the mortar with another gram of nitre, and add this as a cover. Heat, and extract with water as before; add the sodium acetate and acetic acid; and titrate. Burnt ores carry from 2.5 to 5 per cent. of sulphur. A series of four determinations gave:—

"Baric Chloride" Required.Percentage of Sulphur.Gravimetric Results.12.6 c.c.2.52 %2.45 %29.9 "5.98 "5.84 "18.1 "3.62 "3.53 "22.0 "4.40 "4.43 "

For ores carrying less than 1 per cent. of sulphur, take 10 grams for the assay.

Determination of Sulphuric Oxide (SO3) in Sulphates.—When the sulphur exists in the sample received by the assayer in an oxidised state as sulphate, it is usual to report it in terms of sulphuric oxide (SO3). In this case, the metal must also be reported as oxide. For example, an analysis of copper sulphate would be thus reported:—

Oxide of copper (CuO)31.8 %Sulphuric oxide (SO3)32.1 "Water36.1 "———100.0

The percentage of sulphur multiplied by 2.5 gives the percentage of sulphuric oxide. Thus a sample of copper sulphate containing 12.85 per cent. of sulphur will contain 12.85 × 2.5 or 32.12 per cent. of sulphuric oxide.

In minerals and metallurgical products, it is common to find the sulphur in both conditions—i.e., as sulphate and sulphide. Generally in these the percentage of sulphur only is wanted; but this will depend entirely on commercial requirements, and not onthe fancy of the assayer. Soluble sulphates are determined separately by extracting with small quantities of cold water, so as to avoid the separation of basic sulphates, or, if the sulphides present are not at the same time attacked, by dilute hydrochloric acid. Lead sulphate may be extracted by boiling with ammonic acetate; whilst barium, strontium, and, perhaps, calcium sulphate, will be mainly found in the residue insoluble in acids.

Weigh up from 2 to 5 grams of the material according to the amount of sulphur judged to be present, and dissolve them in the titrating-dish with 1 c.c. of hydrochloric acid and 50 c.c. of water. Add 10 grams of sodic acetate, and 10 c.c. of acetic acid; dilute, boil, and titrate. In the case of ferric salts, half the quantity of acetic acid will be better, as then the ferric iron will be precipitated, and a colourless solution will be left, in which the end reaction is more readily distinguished.

Determined in this way, 5 gram samples of the following salts gave the results indicated below:—

Salt."Barium Chloride" Required.Sulphuric Oxide.Copper sulphate64.25 c.c.32.12 %Magnesium sulphate65.25 "32.62 "Zinc sulphate56.25 "28.12 "Ferrous sulphate58.25 "29.12 "Sodium sulphate51.25 "25.60 "

Determination of Sulphuric Oxide in Barytes(Heavy spar).—Fuse 2 grams of the powdered mineral with 5 grams of "fusion mixture" for five minutes; and, when cold, extract with water. Filter, acidulate the filtrate with an excess of 10 c.c. of acetic acid, dilute, boil, and titrate. For example, a transparent crystallised sample required 27.0 c.c. of barium chloride, which is equivalent to 13.6 per cent. of sulphur, or 34.0 per cent. of sulphuric oxide. Theory requires 34.3 per cent. of the latter. Since both carbonate of soda and potash are liable to contain sulphates, a blank determination should be made on 5 grams of the "fusion mixture," and the amount found be deducted from that got in the assay.

1. The price of sulphur in an ore being 4-1/2d.per unit in the northern markets, what would be the price of a ton of ore containing 49 per cent. of sulphur? What would be the effect on the price of an error of 0.25 per cent. in the assay?

2. Pyrites carries 50 per cent. of sulphur, and on calcining yields 70 per cent. of its weight of burnt ore. Supposing the burnt ore carries 3.5 per cent. of sulphur, what proportion of the sulphur will have been removed in the calcining?

3. How would blende compare with pyrites as a source of sulphur for sulphuric acid making?

4. How would you determine the percentage of sulphuric oxide in a sample of gypsum? What is sulphuric oxide, and what relation does it bear to sulphur?

5. A mineral contains 20.7 per cent. of water, 32.4 per cent. of lime, and 18.6 per cent. of sulphur. What is its probable composition? What experiment would you try to determine the accuracy of your conclusion?

occurs in nature combined with copper, mercury, and lead, in certain rare minerals. In small quantities it is found in many ores. It is detected in solution by the red precipitate produced on boiling the acid solution with sodium sulphite. This reaction is used for its determination.

Solution.—The solution is effected by boiling with nitric acid or aqua regia, or by fusing with nitre. To separate the selenium, the solution is evaporated with an excess of hydrochloric acid and a little sodium or potassium chloride. This destroys any nitric acid that may be present, and reduces selenic acid (H2SeO4) to selenious (H2SeO3). The solution is diluted with water, and treated with a solution of sulphite of soda. It is warmed, and at last boiled. The selenium separates as a red precipitate, which (on boiling) becomes denser and black. It is collected on a weighed filter, washed with hot water, dried at 100° C., and weighed as pure selenium.

Selenium can be precipitated with sulphuretted hydrogen as a sulphide, which is readily soluble in ammonium sulphide. This sulphide may be oxidised with hydrochloric acid and chlorate of potash; and the selenium separated in the manner described.

Tellurium occurs in nature, native, and in combination with gold, silver, bismuth and lead. It is sometimes met with in assaying gold ores. It may be detected by the purple colour it imparts to strong sulphuric acid when dissolved in the cold, and by the black precipitate of metallic tellurium which its solutions yield on treatment with a reducing agent. Telluric acid is reduced to tellurous (with evolution of chlorine) on boiling with hydrochloric acid.

Solutionis effected by boiling with aqua regia, or by fusing with nitre and sodium carbonate.

Separation.—Tellurium closely resembles selenium in its reactions. It is separated and determined in the same way. Likeit, it forms a sulphide soluble in ammonium sulphide. It is distinguished from selenium by the insolubility, in a solution of cyanide of potassium, of the metal precipitated by sodium sulphite; whereas selenium dissolves, forming a soluble potassic seleno-cyanide.[102]

For the determination, solution is effected by fusing with nitre and sodium carbonate, dissolving out the tellurate of potash with water, and boiling with hydrochloric acid. Tellurous compounds are formed, with evolution of chlorine; and the solution, on treating with a reducing agent (such as sulphurous acid or stannous chloride), yields metallic tellurium; which is washed, dried at 100° C., and weighed.

FOOTNOTES:[101]BaCl2+ Na2SO4= BaSO4+ 2NaCl.[102]Se + KCy = KCySe.

[101]BaCl2+ Na2SO4= BaSO4+ 2NaCl.

[102]Se + KCy = KCySe.

The chief source of the arsenic of commerce is arsenical pyrites, or mispickel, which contains about 45 per cent. of arsenic (As). Arsenic also occurs as a constituent of several comparatively rare minerals; and, as an impurity, it is very widely distributed. White arsenic is an oxide of arsenic, and is obtained by roasting arsenical ores, and refining the material (crude arsenic), which condenses in the flues. Arsenic itself is volatile, and many of its compounds have the same property. It forms two well-defined series of salts, corresponding to the oxides: arsenious oxide (As2O3), and arsenic oxide (As2O5). These combine with bases to form arsenites and arsenates respectively. Boiling with nitric acid converts the lower into the higher oxide; and powerful reducing-agents, such as cuprous chloride, have the opposite effect.

Arsenic may be detected by dissolving the substance in hydrochloric acid, or in aqua regia (avoiding an excess of nitric acid), and adding a little of this solution to the contents of a small flask in which hydrogen is being made by the action of zinc and hydrochloric acid. The ignited jet of hydrogen assumes a blue colour if arsenic is present, and a cold porcelain dish held in the flame (fig. 67) becomes coated with a dark deposit of metallic arsenic. Antimony produces a similar effect, but is distinguished by the insolubility of its deposit in a cold solution of bleaching-powder.

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