Potassic iodide added1.5 gram3 grams5 grams"Hypo" required20.0 c.c.20.0 c.c.20.0 c.c.
In these the iodide was added direct to the solution containing the copper, which was afterwards diluted to 100 c.c. and titrated.In another series the iodide was added after the dilution to 100 c.c., and the results were:—
Potassic iodide added1.5 gram3 grams5 grams10 grams"Hypo" required20.0 c.c.20.1 c.c.20.0 c.c.20.0 c.c.
Effect of Varying Bulk.—In these experiments, 20 c.c. of copper sulphate were taken, 3 grams of potassic iodide added, and also water to the required bulk.
Bulk20.0c.c.100.0c.c.200.0c.c.500.0c.c."Hypo" required20.0"20.0"20.0"19.9"
In the last of these experiments the colour was discharged at 18 c.c., but gradually returned until 19.9 c.c. had been run in. It will be seen that considerable variation in bulk does not interfere.
Effect of Acetic Acid.—These experiments were like the last series mentioned, but the bulk was 100 c.c., and varying amounts of acetic acid were added.
Acetic acid added0c.c.1.0c.c.5.0c.c.10.0c.c.20.0c.c."Hypo" required20.0"20.1"20.1"20.0"20.2"
Acetic acid, then, does not interfere to any serious extent.
Effect of Varying Sodium Acetate.—These experiments were like those last mentioned, but without acetic acid, and with varying amounts of sodium acetate.
Sodium acetate added0 gram1 gram2 grams5 grams10 grams"Hypo" required20.0 c.c.20.0 c.c.20.2 c.c.19.3 c.c.18.2 c.c.
In the 5 grams experiment, when the finishing point had been apparently reached the colour slowly returned; but as the results generally on titrating were not satisfactory a repetition of the experiment was made with the addition of 5 c.c. of acetic acid, which gave an equally bad result.
Effect of Foreign Salts.—The conditions of these experiments were the same as before. The salts were added and dissolved before the addition of the potassium iodide. Using 5 grams (or in the case of the acids, 5 c.c.), the results were as follows:—
Salt added—dilute H2SO4Acetic acidNaAcNaCl"Hypo" required20.0 c.c.20.0 c.c.20.1 c.c.19.3 c.c.20.1 c.c.Salt addedKNO3Na2SO4AmClAm2SO4"Hypo" required20.2 c.c.18.7 c.c.20.0 c.c.19.9 c.c.
The low result with the sulphate of soda was evidently due to the formation of a sparingly soluble double salt, which removed copper from the solution; on adding a little acetic acid the full amount of "hypo" was required. The effect of the presence of certain metals is important. The method of determining it was to add the substance to the solution containing the copper, andpartly precipitate with soda solution; then treating with 1 c.c. of acetic acid, adding the iodide, and proceeding as before.
Substance Added."Hypo" Required,-20. c.c0.050 gram arsenic as As2O520.0 "0.050 " antimony as SbCl519.8 "0.050 " lead as Pb(NO3)220.1 "
A similar experiment with 0.050 gram of bismuth nitrate could not be determined, the end-reaction being masked. Bismuth iodide is soluble in potassic iodide, forming a brown solution, the colour of which is very similar to that produced by iodine; and although it does not strike a blue colour with starch, "hypo" has an action on it.
A similar experiment with 0.050 gram of iron as ferric chloride required 22.3 c.c. of "hypo," and the colour returned on standing. This shows that ferric acetate liberates iodine under the conditions of the assay. Trying to counteract this, by adding to a similar solution 0.5 gram of phosphate of soda dissolved in a little water, 19.7 c.c. of "hypo" were required instead of 20.0, but the assay showed signs of returning colour.
In standardising, the same result was obtained, whether the copper was present as nitrate or sulphate before neutralising.
Effect of Varying Copper.—With the same conditions as before, but with varying amounts of copper and a proportionally increasing quantity of iodide, the results were:—
Copper present1.0c.c.10.0c.c.20.0c.c.50.0c.c.100.0c.c."Hypo" required1.0"10.0"20.0"50.0"100.0"
showing the results to be exactly proportional.[58]
Determination of Copper in Copper Pyrites.—Take 2 grams of the dried and powdered ore and treat in a porcelain dish with 20 c.c. of nitric acid, and evaporate to dryness. Take up with 30 c.c. of hydrochloric acid, dilute, and transfer to a pint flask; make up with water to 200 c.c., warm, and pass sulphuretted hydrogen to excess. Filter, and wash the precipitate with water acidified with sulphuric acid. Wash the precipitate back into the flask, and dissolve with 15 c.c. of nitric acid. Evaporate almost to dryness; add 20 c.c. of water, and boil till free from nitrous fumes; filter off the sulphur and gangue; neutralise with soda, avoiding excess; add 1 or 2 c.c. of acetic acid, and shake till clear. Add 5 grams of potassium iodide, dilute to 100 c.c., and titrate. The following is an example:—
0.5 gram of copper required50.5c.c."hypo."The assay required55.6""
which is equal to 27.5 per cent. of copper.
This is based on the blue coloration of ammoniacal copper solutions. The quantity of copper in 100 c.c. of the assay solution should not be more than 15 milligrams, or less than half a milligram. It is not so delicate as most other colorimetric methods, but nevertheless is a very useful one.
The manner of working is the same as that described under iron.
Standard Copper Solution.—Weigh up 0.5 gram of electrotype copper, dissolve in 10 c.c. of nitric acid, boil off nitrous fumes, and dilute to 1 litre. 1 c.c. = 0.5 milligram.
In nearly all cases it will be necessary to separate the copper with sulphuretted hydrogen from a solution of about 5 grams of the material to be assayed. The filter paper containing the sulphide (and, probably, much sulphur) is dried and burnt. The ashes are dissolved in 5 c.c. of dilute nitric acid, 10 c.c. of dilute ammonia added, and the solution filtered through a coarse filter into a Nessler tube, washing the paper with a little dilute ammonia.
The estimation of the colour and calculation of the result are made in the way described on page 44.
The effect of varying conditions on the assay may be seen from the following experiments.
Effect of Varying Temperature.—The effect of increased temperature is to slightly decrease the colour, but this can only be observed when a fair quantity of copper is present.
1.0c.c. at 15°showed thecolour of1.0c.c. at 70°2.5"""2.7"5.0"""5.0"10.0"""9.0"
Effect of Varying Ammonia.—The solution must, of course, contain free ammonia; about 5 c.c. of dilute ammonia in 50 c.c. bulk is the quantity to be used in the experiments. A larger quantity affects the results, giving lower readings and altering the tint. With small quantities of ammonia the colour approaches a violet; with larger, a sky-blue.
2.5c.c. with25c.c. ofstrong ammoniaread2.2c.c.5.0"""""4.0"10.0"""""8.0"
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 Chloride.With 10 grams Ammonic Sulphate.2.52.52.52.05.05.05.34.310.010.010.08.5
These show that sulphates should be avoided, and either nitrate or chloride solutions be used in the standard as well as in the assay.
Determination of Copper in a Manganese Ore.—Treat 3 grams of the ore with 20 c.c. of hydrochloric acid, and evaporate to dryness. Take up with 10 c.c. of hydrochloric acid; dilute to about 200 c.c., and pass sulphuretted hydrogen until the solution smells of the gas; filter, burn, take up with 5 c.c. of dilute nitric acid, add 10 c.c. of dilute ammonia, and filter into the Nessler tube, and make up with the washings to 50 c.c. Into the "standard" tube put 5 c.c. of dilute nitric acid and 10 c.c. of dilute ammonia. Make up to nearly 50 c.c. with water, and run in the standard copper until the colours are equal. In a determination 4 c.c. (= 2.0 milligrams of copper) were required; this in 3 grams of ore = 0.07 per cent.
Determination of Copper in "Black Tin."—Weigh up 3 grams of the dried ore, boil with 10 c.c. of hydrochloric acid, and afterwards add 1 c.c. of nitric; boil off nitrous fumes, evaporate to about 5 c.c., dilute to 50 c.c., add 20 c.c. of dilute ammonia; stir, and filter. If much iron is present, dissolve the precipitate of ferric hydrate in acid, and reprecipitate with ammonia. Mix the two filtrates, and dilute to 100 c.c. Take 50 c.c. for the test. A sample of 3 grams of an ore treated in this way required 5.2 c.c. of standard copper to produce equality of tint. This gives 0.35 per cent.
Determination of Copper in Tin.—Weigh up 1 gram of the sample, transfer to an evaporating dish, and cover with 30 c.c. of aqua regia. Warm until the metal has dissolved, then evaporate almost to dryness. Take up with a few c.c. of hydrochloric acid and again evaporate.
Dissolve the residue in 10 c.c. of dilute hydrochloric acid and transfer to a 100 c.c. flask. Add 10 c.c. of dilute ammonia and make up with water to the containing mark.
Filter off 50 c.c. of the solution into a Nessler glass and determine the copper in it colorimetrically.
Very pure copper can be obtained in commerce, owing to the demand for metal of "high conductivity" for electrical purposes, which practically means for metal free from impurities.
Much of the metal sold contains as much as one per cent. of foreign substances, of which arsenic is the most important. The other elements to be looked for are bismuth, lead, antimony, silver, gold, iron, nickel, cobalt, sulphur, and oxygen. In "blister copper" (which is the unrefined metal), aluminium, silicon, and phosphorus may be met with.
Oxygen.—All commercial copper carries oxygen; most of it is present as cuprous oxide, which is dissolved by molten copper. The estimation of oxygen is often made "by difference." The copper and the other impurities being determined, the rest is assumed to be oxygen. Probably this is nearly correct, but the whole of the oxygen should not be ascribed to cuprous oxide; for any arsenic the metal contained would be present as cuprous arsenite, since arsenide of copper and cuprous oxide could not exist together at the temperature of fusion without interacting. In the report of the analysis, it is best to state the proportion of oxygen thus:—
Oxygen ——— per cent. by difference.
There is a method of determination by fusing 5 or 10 grams in a brasqued crucible, and counting the loss as oxygen; and another method for the determination of cuprous oxide based on the reaction of this substance with nitrate of silver.[59]About 2 grams of silver nitrate, dissolved in 100 c.c. of water, is allowed to act upon 1 gram of the copper in the cold. The precipitate is filtered off, washed thoroughly with water, and the basic salt dissolved and determined colorimetrically.
One part of copper found represents 1.68 part of cuprous oxide, or 0.19 part of oxygen. Copper generally carries from 0.1 to 0.2 per cent. of oxygen.
Silveris found in most samples, but occurs in variable proportions; when it amounts to 30 ounces per ton it has a commercial value. To determine its amount, dissolve 10 grams of the copper in 35 c.c. of nitric acid and 50 c.c. of water, boil off nitrous fumes, and dilute to about 100 c.c. One or two c.c. of dilute hydrochloricacid (one to 100 of water) are added, stirred in, and the precipitate allowed to settle for twenty-four hours. Filter through a double Swedish paper, dry, burn, and cupel the ashes with one gram of sheet lead.
Ten grams of a sample of copper gave in this way 4.7 milligrams of silver. Ten grams of the same copper, to which 24 milligrams of silver had been added gave 28.2 milligrams.
Gold.—To determine it, dissolve 10, 20, or 50 grams of the sample in 35, 70, or 175 c.c. of nitric acid and an equal volume of water, boil till free from nitrous fumes, and dilute to double its volume. Allow to stand for some time, decant on to a filter, dry, burn, and cupel the ashes with 1 gram of sheet lead. If silver is present, owing to traces of chlorides in the re-agents used, "parting" will be necessary. (SeeGold.)
Working in this way on 20 grams of copper, to which 1.8 milligram of gold had been added, a button weighing 2.0 milligrams was obtained.
Antimonyis not a frequent impurity of copper: it can be detected in quantities over 0.1 per cent. by a white residue of Sb2O4, insoluble in nitric acid. With material containing only small quantities of antimony the white oxide does not show itself for some time, but on long-continued boiling it separates as a fine powder. It is best (when looking for it) to evaporate the nitric acid solution to the crystallising point, to add a little fresh nitric acid and water, and then to filter off the precipitate. After weighing it should be examined for arsenic and bismuth.
Lead.—Refined coppers are often free from lead, anything more than traces being seldom found; in coarse coppers it is sometimes present in considerable quantities.
Its presence may be detected in the estimation of the copper electrolytically, the platinum spiral becoming coated with a brown or black deposit of lead dioxide. The depth of colour varies with the lead present, and obviously could be made the basis of an approximate estimation. The colour shows itself within an hour or so, but is best observed when all the copper has been deposited.
Electrolysing a solution of one gram of pure copper, to which 0.5 milligram of lead had been added, the deposit was dark brown; in a similar solution with 1 milligram of lead it was much darker, and with 2 milligrams it was black. Under the conditions of the assay the dioxide cannot be weighed, as it partly dissolves on breaking the current. When lead has been found, its quantity may be estimated by evaporating to dryness the nitric acid solution to which an excess of sulphuric acid has been added, taking up with water, and filtering off and weighing the lead sulphate.
The separation of traces of lead as chromate is a fairly good one. Dissolve 5 grams of the copper in 17 c.c. of nitric acid and an equal volume of water; boil off nitrous fumes, neutralise with soda, and afterwards acidulate with acetic acid; and dilute to a litre. Add 20 grams of sodium acetate, warm, and precipitate the lead with a dilute solution of potassium chromate. Copper chromate (yellow) may be at the same time thrown down, but it is readily soluble on diluting. Filter off the precipitate; wash it into a beaker and pass sulphuretted hydrogen; oxidise the sulphide and weigh as lead sulphate. Treated in this way 5 grams of copper yielded sulphate of lead equal to 2.0 milligrams of lead. Five grams of the same sample to which 10 milligrams of lead were added gave 11.4 milligrams.
Nickel and Cobalt.—Nickel is always present in larger or smaller quantities in commercial copper, and, perhaps, has an influence on the properties of the metal. It is determined as follows:—Dissolve 10 grams of the copper in 35 c.c. of nitric acid and an equal bulk of water, boil off nitrous fumes and neutralise with soda, add 2 grams of carbonate of soda dissolved in water, boil, and filter. Acidify the filtrate with 2 or 3 c.c. of dilute nitric acid and dilute to 1 or 1-1/2 litres. Pass sulphuretted hydrogen through the cold solution till the copper is all down and the liquid smells of the gas. Filter and evaporate the filtrate to a small bulk, and determine the nickel by electrolysing the solution rendered ammoniacal, or by precipitating as sulphide and weighing as sulphate. (See underNickel.) The precipitate, after weighing, should be tested for cobalt. If present it is separated with potassium nitrite as described underCobalt. Ten grams of copper gave 6.0 milligrams of nickel; and another lot of 10 grams of the same copper, to which 10.0 milligrams of nickel had been added, gave 17.2 milligrams.
Sulphur.—The amount of sulphur in refined copper is very small, seldom exceeding 0.005 per cent. In coarse copper, as might be expected, it is found in larger quantities.
In determining it, it is first converted into sulphuric acid, and then precipitated and weighed as barium sulphate. The precipitation cannot be effected from a nitric acid solution. Ten grams of copper are dissolved in nitric acid, as for the other determinations, and then boiled with excess of hydrochloric acid till the nitric acid is completely removed. There is then added a few drops of a dilute solution of baric chloride, and the solution is allowed to stand for some hours. The baric sulphate is filtered off and weighed.
The necessity for precipitating from a hydrochloric acid solution is seen from the following determinations. In each experiment10 grams of copper was used, and a known weight of sulphur, in the form of copper sulphate, added.
Sulphur added.Sulphur found in Hydrochloric Acid Solution.Sulphur found in Nitric Acid Solution.5 milligrams8 milligrams0.03 milligrams10 "11 "0.03 "15 "17 "12.00 "
Bismuth.—Nearly all samples of copper contain bismuth, but only in small quantities. It is best determined colorimetrically as described underBismuth. The method of concentrating and preparing the solution for colorimetric assay is as follows. Dissolve 10 grams of copper in nitric acid, as in the other determinations; neutralise with soda; add 1 or 1.5 grams of bicarbonate of soda and boil for ten minutes; filter, dissolve the precipitate in hot dilute sulphuric acid; add sulphurous acid and potassium iodide in excess, and boil till free from iodine. Filter and dilute to 500 c.c. Take 50 c.c. of the yellow solution for the determination. A few c.c. of a dilute solution of sulphurous acid (1 in 100) will prevent the liberation of iodine. The following experiments test the method of separation. Ten grams of copper were treated as above and precipitated with 1.5 gram of "soda;" the precipitate contained 0.6 milligram of bismuth (= 0.006 per cent.). The filtrate treated with another 1.5 gram of "soda" gave a precipitate which was free from bismuth. To the filtrate from this was added 1.0 milligram of bismuth, and another fraction was precipitated with 1.5 gram of "soda." In this precipitate was found 1.0 milligram of bismuth. To the filtrate another milligram of bismuth was added and the separation with "soda" repeated. The bismuth was separated from this precipitate with ammonic carbonate before determination, and 0.9 milligram was found.
Arsenic.—The proportion of arsenic in copper varies from 0.01 to 0.75 per cent. whilst in coarse copper it may amount to 2 or even 3 per cent. To determine it, dissolve 5, 10, or 20 grams of the copper (according to the amount of arsenic present) in 18 c.c., 35 c.c., or 70 c.c. of nitric acid, and an equal volume of water. Boil off the nitrous fumes, dilute to 100 c.c. and neutralise with soda; add 1.5 or 2 grams of carbonate of soda dissolved in a little water, and boil. Filter (washing is unnecessary) and dissolve back into the flask with a little dilute hydrochloric acid; add 30 c.c. of dilute ammonia and 25 c.c. of "magnesia mixture," and allow to stand overnight. The whole of the arsenic is precipitated as ammonic-magnesic arsenate in one hour, but it isadvisable to leave it longer. The precipitate may be dried and weighed, or, better, titrated with uranium acetate. (SeeArsenic.) To test this method of separation 10 grams of pure copper were taken and 0.200 gram of arsenic dissolved with it. The arsenic was determined by titration with uranium acetate, and 0.200 gram was found. Two other similar experiments with 0.080 and 0.010 gram of arsenic added, gave 0.079 and 0.012 gram respectively.
Antimony or bismuth may be present without interfering with the titration. With 0.100 gram of antimony and 0.100 gram of arsenic, 0.100 gram of arsenic was found; and in another case, with 0.100 gram of bismuth and 0.060 gram of arsenic, 0.060 gram was found. In these experiments the antimony and bismuth were present in the assay solution when titrated. For a gravimetric determination they would require to be removed before precipitating with "magnesia mixture."
Phosphorus, if present, counts as arsenic in the proportion of 1 to 2.4; but, except in the case of coarse coppers, it is always absent.
Iron, if present, interferes by forming a white flocculent precipitate of ferric arsenate after the addition of the sodium acetate and preliminary to the titration. Each milligram of iron abstracts, in this way, 1.3 milligrams of arsenic.
Iron.—Refined coppers carry traces of iron, varying from 0.001 to 0.01 per cent. It is best determined during the arsenic estimation. The precipitate of the ammonic-magnesic arsenate will contain the whole of the iron as ferric hydrate. On dissolving in hydrochloric acid, neutralising with ammonia, adding 5 c.c. of sodic acetate, diluting, and boiling, it reappears as a white precipitate of ferric arsenate. It is filtered off (the arsenic being estimated in the filtrate), dissolved in warm hydrochloric acid, and determined colorimetrically as described underIron. A series of experiments testing the separation is there given.
Phosphorus.—Refined coppers do not carry phosphorus, although it may be present in "coarse copper" up to 1 per cent. or more. In such samples the following method is adopted for the estimation of both phosphorus and arsenic. Dissolve 10 grams of copper and 0.1, 0.2, or 0.3 gram of iron wire (according to the amount of arsenic and phosphorus present) in 35 c.c. of nitric acid and an equal volume of water. Add soda till the free acid is nearly neutralised. Next add a strong solution of sodium acetate, until the solution ceases to darken on further addition, then dilute with water to half a litre. The solution is best contained in a large beaker; it is next heated to the boiling point, and at once removed and allowed to settle. If the precipitate islight coloured it is evidence that sufficient iron has not been added, or, if it is green, from basic copper salts, it shows that the solution was not sufficiently acid. In either case start afresh. Filter off the precipitate and wash with hot water containing a little sodium acetate, dissolve it off the filter with hot dilute hydrochloric acid, add ammonia in excess, and pass sulphuretted hydrogen for five minutes. Warm at about 70° C. for a quarter of an hour. Filter. The clear yellow filtrate contains the arsenic and phosphorus. Add dilute sulphuric acid in excess; filter off the yellow precipitate of sulphide of arsenic, dissolve it in nitric acid, and titrate with uranium acetate, as described underArsenic.
The filtrate from the sulphide of arsenic is rendered alkaline with ammonia and "magnesia mixture" added. The solution is stirred, and allowed to stand overnight. The precipitate of ammonic-magnesic phosphate is filtered off, dissolved, and titrated with uranium acetate, using the same standard solution as is used in the arsenic assay: 0.5 gram of arsenic equals 0.207 gram of phosphorus.
Copper.—The method of determining this has been described underElectrolytic Assay.
In the method of concentration by fractional precipitation with sodic carbonate (which is adopted in most of these determinations) the precipitate will contain all the bismuth, iron, and alumina; the arsenic and phosphorus as cupric arsenate and phosphate; and the greater part of the lead, antimony, and silver. The nickel and cobalt, and the sulphur as sulphuric acid, will remain in solution with the greater part of the copper.
1. According to a wet assay 2 grams of a certain ore contained 0.3650 gram of copper. What would you expect the dry assay produce to be?
2. A standard solution is made by dissolving 25 grams of potassic cyanide and diluting to a litre. Assuming the salt to be 98 per cent. real cyanide, what would 100 c.c. of the solution be equivalent to in grams of copper?
3. How would you make a solution of "hypo" of such strength that 100 c.c. shall equal 0.633 gram of copper?
4. What weight of ore, containing 17.0 per cent. of copper, would you take in order to get about 0.5 gram of copper in solution for electrolysis?
5. The solution of copper in nitric acid is effected by the following reaction:—
3Cu + 8HNO3= 3Cu(NO3)2+ 4H2O + 2NO.
What volume of nitric acid will be required to dissolve 1 gram of copper?
The chief ore of lead is galena, a sulphide of lead, common in most mining districts, and frequently associated with blende and copper-pyrites. It always carries more or less silver; so that in the assay of the ore a silver determination is always necessary. Carbonate (cerussite), sulphate (anglesite), and phosphate (pyromorphite) of lead also occur as ores, but in much smaller quantities.
Lead ores are easily concentrated (owing to their high specific gravity, &c.) by mechanical operations, so that the mineral matter sent to the smelter is comparatively pure.
Lead is readily soluble in dilute nitric acid. The addition of sulphuric acid to this solution throws down heavy, white, and insoluble lead sulphate.
Galena is soluble in hot hydrochloric acid, sulphuretted hydrogen being evolved; but the action is retarded by the separation of the sparingly soluble lead chloride. If a rod of zinc is placed in this solution, metallic lead is precipitated on it as a spongy mass, the lead chloride being decomposed as fast as it is formed. The opening up of the ore is thus easily effected, the sulphur going off as sulphuretted hydrogen, and the lead remaining in a form easily soluble in dilute nitric acid. Galena itself is readily attacked by nitric acid, part of the lead going into solution, and the rest remaining as insoluble lead sulphate. The sulphate is due to the oxidation of the sulphur by nitric acid; its amount will vary with the quantity and concentration of the acid used. Sulphate of lead is soluble in solutions of ammonium or sodium acetate; or it may be converted into carbonate by boiling with carbonate of soda. The carbonate, after washing off the sulphate of soda, dissolves easily in nitric acid. The precipitation of lead from acid solutions with sulphuric acid, and the solubility of the precipitate in ammonium acetate, distinguishes it from all other metals. The addition of potassium chromate to the acetate solution reprecipitates the lead as a yellow chromate.
The dry assay of lead is largely used, but it is only applicable to rich or concentrated ores, and even with these only gives approximate results. Both lead and lead sulphide are sensibly volatile at a moderately-high temperature; hence it is necessary to obtain a slag which is easily fusible. As a reducing agent iron is almost always used, and this is added either in the form of an iron rod, or the crucible itself is made of this metal. The flux used is carbonate of soda.
When a clay crucible is used, the method of working is as follows:—Weigh up 25 grams of the dry and powdered ore, mix with an equal weight of "soda" and 2 grams of tartar; place in a crucible (E. Battersea round), and then insert a piece of iron rod about half an inch in diameter, and of such a length that it will just allow the crucible to be covered. The rod should be pushed down so as to touch the bottom of the crucible, and the mixture should be covered with a sprinkling of borax. Place in a furnace heated to, but not above, redness, and cover the crucible. In about twenty minutes the charge will be fused: the fusion is complete when bubbles of gas are no longer being evolved; and then, but not till then, the iron is withdrawn, any adhering buttons of lead being washed off by dipping the rod a few times in the slag. Cover the crucible, leave it for a minute or two, and then pour. Detach the slag, when cold, by hammering. The weight of the button multiplied by 4 gives the percentage. The commoner errors of students in working the process are too high a temperature and too quick a withdrawal.
A sample of ore treated in this manner gave on duplicate assay 17.5 and 17.6 grams of lead, equalling 70.0 and 70.4 per cent. respectively. By wet assay the sample gave 73.3 per cent. Using an iron crucible, the results will be 1 per cent. or so higher. The crucible must be made of wrought iron; and, if it has been previously used, should be cleaned by heating to dull redness and scraping the scale off with a stirrer. Take 30 grams of the ore, mix with 30 grams of "soda" and 3 grams of tartar; put the mixture in the crucible, and cover with a sprinkling of borax; heat for about twenty minutes at not too high a temperature, and then scrape down the slag adhering to the side with a stirrer. Leave in the furnace till action has ceased. Before pouring, tap the pot gently, and then tilt it so as to make the slag wash over the part of the crucible along which the charge is to be poured. Pour; and, when cold, clean and weigh the button of metal. A crucible may be used from ten to twenty times.
These assays are for ores containing the lead chiefly as sulphide. For oxidised ores, charcoal or tartar is employed as the reducing agent. The student may practise on red lead as follows:—Take 30 grams of red lead; mix with 10 grams each of borax and "soda" and about 1.5 gram of powdered charcoal; place in a small clay crucible with a cover (C. Battersea round), fuse at a gentle heat, and pour when action ceases. This assay will only take a few minutes.
Where lead is present as phosphate (as in the case of pyromorphite), or mixed with phosphates (as sometimes happens), carbonate of soda is a suitable flux; but the phosphate of sodawhich is formed makes a thick tenacious slag, which is very apt to be carried out of the pot by the escaping gas. A wide-mouthed clay pot is taken and a little fluor spar added. For the assay of pyromorphite the following charge may be used:—Ore, 20 grams; "soda," 25 grams; tartar, 7 grams; and fluor spar, 5 grams; and 2 grams of borax as a cover. This will melt down in about ten minutes, and should be poured as soon as tranquil.
In the case of galena the best method of getting the lead into solution is to treat with hydrochloric acid and zinc. Put 1 gram of the ore in an evaporating dish 4 inches across, and cover with 10 c.c. of dilute hydrochloric acid. Heat till the evolution of sulphuretted hydrogen becomes sluggish, and then drop in a piece of zinc rod. If the solution effervesces too strongly, dilute it. Continue the heating until the sulphide is seen to be all dissolved; when the lead is all precipitated, pour off the liquid and wash twice with cold water. Peel off the precipitated lead with the help of a glass rod, and then clean the zinc. Cover the lead with 20 c.c. of water and 5 c.c. of dilute nitric acid, and heat gently till dissolved; all the lead will be in solution, and, when filtered off from the gangue, will be ready for a gravimetric determination. For volumetric work this filtering is unnecessary.
The chief objection to this method is that commercial zinc carries considerable quantities of lead. Although this can be determined and allowed for, the correction required is in most cases too large to be satisfactory. The following method is applicable in all cases, but is more troublesome:—Treat 1 gram of the ore with 10 c.c. of dilute nitric acid in an evaporating dish covered with a clock-glass, and evaporate till nearly dry. Take up with 50 c.c. of water, and add 10 c.c. of dilute sulphuric acid. Filter. The residue contains the lead as sulphate, together with the insoluble matter of the ore and globules of sulphur. Warm with a solution of ammonium acetate, and filter. The lead will be in the filtrate, and is recovered in a state fit for direct gravimetric estimation by the addition of dilute sulphuric acid. If the volumetric method is to be used, the lead sulphate should be dissolved out with a solution of sodium acetate instead of with the ammonium salt solution.
The lead is separated and precipitated as sulphate, as already described. The solution must be allowed to stand, and the clearliquid be decanted through a filter. Transfer the precipitate, and wash with very dilute sulphuric acid (1 or 2 c.c. in 100 c.c. of water). The acid must be completely removed with one or two washes with cold water, and then with alcohol. The volume of liquid required for washing is small, as the precipitate is dense and easily cleaned; but the washing must be carefully done, since if any acid remains it will, on drying, char the paper, and render the subsequent work troublesome. Dry, transfer to a watch-glass, and burn the filter paper, collecting its ash in a weighed porcelain crucible. The filter paper must be freed as much as possible from the lead sulphate before burning, and the ash treated with a drop or two of nitric and sulphuric acids. Transfer the lead sulphate to the crucible; ignite gently, keeping the temperature below redness; cool, and weigh. The precipitate will contain 73.6 per cent. of lead oxide or 68.3 per cent. of lead.
Determination of Lead in Commercial Zinc.—Take 10 grams of zinc, and treat (without heating) with 60 c.c. of dilute hydrochloric acid. When the zinc is nearly all dissolved, decant off the clear liquid, and dissolve the residue in 2 c.c. of dilute nitric acid. Evaporate till most of the acid is removed; dilute to 20 or 30 c.c. with water, and add 10 c.c. of dilute sulphuric acid. Filter off, and weigh the lead sulphate. Ten grams treated in this way gave—0.1610 gram of lead sulphate, equivalent to 1.10 per cent. of lead.
This is based upon the reaction between chromate of potash and soluble lead salts in neutral solutions, whereby an insoluble yellow chromate of lead is produced.[60]An excess of the chromate is required to complete the reaction, so that the point at which an indicator shows the presence of undecomposed chromate cannot be satisfactorily taken as the finish. Therefore an excess of the standard chromate must be run in, and such excess determined.
Chromate of lead is not precipitated from strong nitric acid solutions, and only incompletely from dilute ones. Acids generally are detrimental to the precipitation, and must be neutralised before titrating. If the lead is present as sulphate in sodic acetate solution, it is well to render it distinctly alkaline with ammonia.
Lead chromate precipitated in the cold is a lemon-yellow, light precipitate, very difficult to filter: on heating to 40° C. the colour becomes orange; at 60° C. it assumes a deeper hue, and becomesflocculent; and at a boiling temperature it still further darkens and settles readily. These changes in colour are not due to any chemical change, as will be seen by testing the filtrate for chromium or lead: this is an advantage to the assay, since it is only at the higher temperature that the precipitate can be easily filtered. The lead is not completely precipitated, but the amount remaining in solution is only 2 or 3 milligrams, which is just sufficient to give a dark coloration with sulphuretted hydrogen.
The standard chromate of potash solutionis made by dissolving 7.13 grams of bichromate of potash and 2.0 grams of caustic soda in water, and diluting to 1 litre; or 9.40 grams of the neutral chromate (K2CrO4) may be dissolved and diluted to 1 litre: 100 c.c. will be equivalent to 1.000 gram of lead.
Standard Lead Solution.—16 grams of nitrate of lead (Pb(NO3)2) are dissolved in water and diluted to 1 litre; 100 c.c. will contain 1.000 gram of lead.
Acetate of Soda Solution.—250 grams of the crystallised salt (NaAc.3H2O) are dissolved, and diluted to 1 litre. Use 40 c.c. for each assay.
In the titration the assay solution should measure 150 to 200 c.c., and should be boiling or nearly so. It is best contained in a pint flask, and the standard chromate solution used with an ordinary burette. Run in the chromate solution in a steady stream until the whole of the lead has been precipitated. The amount required for this may be calculated: for example, 1 gram of an 80 per cent. ore would require 80 c.c. A little of the assay may be filtered off, and if it does not show a yellow colour in the filtrate run in 2 c.c. more of the standard solution and continue this addition till a colour is shown. After this run in another c.c. to ensure an excess, dilute to 250 c.c., and heat to boiling; allow to settle for three or four minutes, filter off 50 c.c. into a Nessler glass, and determine the excess of chromate colorimetrically. The excess found in the 50 c.c. must, of course, be multiplied by five, and then be deducted from the quantity of chromate originally run into the assay solution. The quantity to be deducted should not exceed 3 c.c. Where a number of determinations are made the colorimetric estimation is facilitated by using a series of standard phials similar to those described under theElectrolytic Copper Assay. The determination is rendered sharper and less liable to error by the addition of a few drops of acetic acid to convert the chromate into bichromate. The same chromate solution must be used in this determination as was used in the precipitation.
In standardising the chromate solution, the standard lead nitrate solution is used. A quantity containing about as muchlead as the assay is supposed to contain is measured off, rendered alkaline with dilute ammonia, and then neutralised with acetic acid, using a small piece of litmus paper dropped into the solution as indicator. Then dilute, boil, and titrate. When the lead in the assay has been separated as sulphate and dissolved in sodic acetate, less chromate is apparently required, and in this case it will be necessary to precipitate the lead in the standard with an equivalent of sodic sulphate and redissolve in sodic acetate just as in the assay. In these solutions (although there is considerable chromate in excess) a further addition of 5 or 6 c.c. of the chromate solution will cause a further precipitate. The following experiments show the effect of variation in the conditions of the assay:—
Effect of Varying Temperature.—Twenty c.c. of lead nitrate solution and 10 grams of sodium acetate were used; diluted to 200 c.c., heated to the desired temperature, and titrated. The results were:—