Fig. 49.
The outer pot of each cell is made of sheet copper, and must be clean and free from solder on the inside. It is provided near the top with a perforated copper shelf in the shape of a ring, into which the inner or porous cell loosely fits. It is charged with a saturated solution of copper sulphate, and crystals of this salt must be added, and always kept in excess. When the battery is at work copper is being deposited on the inner surface of this pot.
The inner or porous pot contains the zinc rod, and is charged with a dilute acid, made by diluting one volume of sulphuric acid up to ten with water. The object of the porous pot is to prevent the mixing of the acid and copper sulphate solutions, without interrupting the flow of electricity. The copper sulphate solution will last for months, but the acid must be emptied out and recharged daily.
The zinc rods must be well amalgamated by rubbing with mercury under dilute acid until they show a uniformly bright surface. They should not produce a brisk effervescence when placed in the acid in the porous pot before coupling up.
The battery when working is apt to become dirty from the "creeping" of the copper and zinc sulphate solution. It must be kept away from the working bench, and is best kept in a box on the floor.
Fig. 50.
The connection of the battery with, and the fixing of, the electrodes may be made by any suitable arrangement, but the following is a very convenient plan. The wire from the zinc is connected by means of a binding screw with a piece of stout copper wire, which, at a distance sufficiently great to allow of easy coupling with the battery, is led along the back of a pieceof hard wood. This is fixed horizontally about one foot above the working bench. The general arrangement is shown in fig. 50, in which, however, for the sake of economy of space, the battery is placed on the working bench instead of on the floor. The piece of wood is one inch square and three or four feet long. It is perforated from front to back at distances of six inches by anumber of small holes, in which are inserted screws like that shown in fig. 51. These are known as "terminals," and may be obtained of any electrician. The head of each screw is soldered to the wire mentioned above as running along the back and as being connected with the zinc end of the battery. These terminals serve to fix the electrodes on which the copper is to be deposited. The wire from the copper end of the battery is similarly connected by a connecting screw (fig. 52) with another wire (H in fig. 53), which runs along the top of the rod and has soldered to it, at distances of six inches, cylindrical spirals of copper wire. These should project from the rod at points about half-way between the terminals already described. They may be made by wrapping copper wire around a black-lead pencil for a length of about three inches.
Fig. 51.
Fig. 52.
Fig. 53.
The rod is perforated from top to bottom with a series of small holes, one in advance of each terminal but as near it as possible. Into these short pieces of glass tube are inserted to ensure insulation. These receive the other electrodes, which are connected with the wire leading to the copper end of the battery, through the spirals, with the help of a binding screw. The figure will make this clear. (Fig. 53.)
Fig. 54
The electrodesconsist of a platinum spiral and cylinder. The spiral should have the shape shown in A, fig. 54. When in work it is passed through one of the holes fitted with glass tubes and connected with the copper end of the battery. The thickness of the wire of which it is made is unimportant, provided it is stout enough to keep its form and does not easily bend. The spiral will weigh about 8 grams. The cylinder (C, fig. 54) will weigh about 12 grams. It should have the shape shown in the figure. In working it is clamped to one of the terminals, and on it the copper is deposited. A cylinder will serve for the deposition of from 1 to 1.5 gram of copper. It is made by rivetting a square piece of foil on to a stiff piece of wire, and then bending into shape over a glass tube or piece of rounded wood. Each cylinder carries a distinctive number, and is marked by impressingRoman numerals on the foil with the blade of a knife. The weight of each is carefully taken and recorded. They lose slightly in weight when in use, but the loss is uniform, and averages half a milligram per month when in daily use. The cylinders are cleaned from deposited copper by dissolving off with nitric acid and washing with water; and from grease by igniting.
Thebeakers, to contain the solution of copper to be electrolysed, are ordinary tall beakers of about 200 c.c. capacity, and are marked off at 100 c.c. and 150 c.c. They are supported on movable stands, consisting of wooden blocks about six inches high and three inches across. The bar of wood which carries the connecting wires and electrodes is permanently fixed over the working bench, at such a height that, with the beakers resting on these blocks, the electrodes shall be in position for working.
To fix the electrodes to the rod, remove the stand and beaker and pass the long limb of the spiral up through one of the glass tubes. Connect it with the free end of the copper spiral by means of a connecting screw (fig. 52), and then draw out and bend the copper spiral so that the platinum one may hang freely. Screw the wire of the cylinder to the terminal, and, if necessary, bend it so that the cylinder itself may be brought to encircle the rod of the spiral in the manner shown in fig. 53.
Thegeneral method of workingis as follows:—The quantity of ore to be taken for an assay varies with the richness of the ore, as is shown in the following table:—
Percentage of Copper in the Ore.Quantity of Ore to be taken.1 to 55 grams5 to 103 "10 to 302 "30 to 501.5 "50 to 1001 "
The weighed quantity of ore is dissolved by evaporating with nitric acid and taking up with hydrochloric, as already described. Any coloured residue which may be left is generally organic matter: it is filtered off, calcined, and any copper it contains is estimated colorimetrically. Nearly always, however, the residue is white and sandy. The copper is separated from the solution as sulphide by means of a rapid current of sulphuretted hydrogen. The liquid is decanted off through a filter, the precipitate washed once with hot water and then rinsed back into the flask (the filter paper being opened out) with a jet of water from a wash bottle. Fifteen c.c. of nitric acid are added to the contents of the flask, which are then briskly boiled until the bulk is reduced toless than 10 c.c. The boiling down is carried out in a cupboard free from cold draughts, so as to prevent the condensation of acid and steam in the neck of the flask. Twenty c.c. of water are next added, and the solution is warmed, and filtered into one of the beakers for electrolysis. The filtrate and washings are diluted with water to the 100 c.c. mark, and the solution is then ready for the battery. It must not contain more than 10 per cent. by volume of nitric acid.
The number and weight of the platinum cylinder having been recorded, both electrodes are fixed in position and the wooden block removed from under them. The beaker containing the copper solution is then brought up into its place with one hand, and the block replaced with the other so as to support it. All the assays having been got into position, the connecting wires are joined to the battery. If everything is right bubbles of oxygen at once stream off from the spiral, and the cylinder becomes tarnished by a deposit of copper. If the oxygen comes off but no copper is deposited, it is because the assay solution contains too much nitric acid. If no action whatever takes place, it is because the current is not passing. In this case examine the connections to see that they are clean and secure, and the connecting wires to see that they are not touching each other.
The action is allowed to go on for sixteen or seventeen hours, so that it is best to let the current act overnight. In the morning the solutions will appear colourless, and a slow stream of oxygen will still be coming off from the spiral.
A wash-bottle with cold distilled water and two beakers, one with distilled water and the other with alcohol, are got ready. The block is then removed, the spiral loosened and lowered with the beaker. The cylinder is next detached and washed with a stream of water from the wash-bottle, the washings being added to the original solution. The current from the battery is not stopped until all the cylinders are washed. After being dipped in the beaker of water and once or twice in that with the alcohol, it is dried in the water-oven for about three minutes, and then weighed. The increase in weight is due to deposited copper. This should be salmon-red in colour, satin-like or crystalline in appearance, and in an even coherent deposit, not removed by rubbing. It is permanent in air when dry, but sulphuretted hydrogen quickly tarnishes it, producing coloured films. With ores containing even very small proportions of bismuth, the deposited copper has a dark grey colour, and when much of this metal is present the copper is coated with a grey shaggy deposit.
It still remains to determine any copper left undeposited in the solution. This does not generally exceed four or five milligrams,and is estimated colorimetrically. Thirty c.c. of dilute ammonia (one of strong ammonia mixed with one of water) are added to the electrolysed solution, which is then diluted up to the 150 c.c. mark with water. It is mixed, using the spiral as stirrer, and, after standing a few minutes to allow the precipitate to settle, 100 c.c. of it are filtered off through a dry filter for the colorimetric determination. Since only two-thirds of the solution are taken for this, the quantity of copper found must be increased by one-half to get the quantity actually present.
Fig. 55.
Thecolorimetric determinationmay be made in the manner described under that head, but where a number of assays are being carried out it is more convenient to have a series of standard phials containing known amounts of copper in ammoniacal solution. By comparing the measured volume of the assay solution with these, the amount of copper present is determined at a glance. These standard bottles, however, can only be economically used where a large number of assays are being made daily.
A convenient plan is to get a quantity of white glass four-ounce phials, like that in fig. 55, and to label them so that they shall contain 100 c.c. when filled up to the bottom of the labels. The labels should be rendered permanent by coating with wax, and be marked with numbers indicating the milligrams of copper present. The bottles are stopped with new clean corks, and contain, in addition to the specified quantity of copper, 6 c.c. of nitric acid and 10 c.c. of strong ammonia, with sufficient water to make up the bulk to 100 c.c. The copper is best added by running in the requisite amount of a standard solution of copper, each c.c. of which contains 0.001 gram of the metal.
The standard bottles should be refilled once every three or four months, since their colorimetric value becomes slowly less on keeping. The following determinations of a set which had been in use for three months will illustrate this. The figures indicate milligrams of copper in 100 c.c.: the first row gives the nominal and the second row the actual colorimetric value of the standards. The difference between the two shows the deterioration.
1234681012141233.75.57.591113
The amount of copper in the assay is got by increasing that found colorimetrically by one-half and adding to that found on the platinum cylinder. The percentage is calculated in the usualway. The following examples will illustrate this, as well as the method of recording the work in the laboratory book:—
_____________________________________________Cylinder I. + Cu 9.5410Cylinder I. 9.5170——————0.0240By colour 100 c.c. = 0.0015}0.0007} 0.0022—————— ——————0.0022 0.0262IX. Sample. Took 5 grams.Copper = 0.52%_____________________________________________Cylinder VI. + Cu 10.5705Cylinder VI. 10.0437——————0.5268By colour, 100 c.c. = 0.0070}0.0035} 0.0105—————— ——————0.0105 0.5373Matte, No. 1070. Took 1.5 gram.Copper = 35.82%_____________________________________________Cylinder XIII. + Cu 12.0352Cylinder XIII. 11.0405——————0.9947By colour 100 c.c. = 0.0005}0.0002} 0.0007—————— ——————0.0007 0.9954X. Sample, Cake copper. Took 1.0053 gram.Copper = 99.00%____________________________________________
In the electrolytic assay of metals, alloys, precipitates, and other bodies rich in copper, the preliminary separation of the copper by sulphuretted hydrogen is unnecessary. It is sufficient to dissolve the weighed sample in 10 c.c. of nitric acid, boil off nitrous fumes, dilute to 100 c.c. with water, and then electrolyse.
General Considerations.—In the preliminary work with the copper sulphide there is a small loss owing to its imperfect removal in washing the filter paper, and another small loss in dissolving in nitric acid owing to the retention of particles in the fused globules of sulphur. To determine its amount the filter-papers and sulphur were collected from forty assays, and the copper in them determined. The average amount of copper in each assay was 0.175 gram; that left on the filter paper was 0.00067 gram; and that retained by the sulphur 0.00003 gram; thus showing anaverage loss from both sources of 0.00070 gram. The determinations from another lot of forty-two similar assays gave on an average
Copper left on filter paper0.00070gramCopper retained by sulphur.0.00004"
The loss from these sources is trifling, and need only be considered when great accuracy is required.
The deposition of the copper under the conditions given is satisfactory, but, as already stated, if the solution contain more than 10 per cent. of nitric acid it is not thrown down at all; or if a stronger current is used, say that from three Bunsen cells, it will be precipitated in an arborescent brittle form, ill adapted for weighing. It may be noted here that increasing the size of the cells does not necessarily increase the intensity of the current.
In two determinations on pure electrotype copper the following results were obtained:—
Copper Taken.Copper Found.0.8988 gram0.8985 gram0.8305 "0.8303 "
The presence of salts of ammonia, &c., somewhat retards the deposition, but has no other ill effect.
The organic matter generally present in copper ores interferes more especially in the colorimetric determination of the residual copper. It can be detected on dissolving the ore as a light black residue insoluble in nitric acid. It is filtered off at once, or, if only present in small amount, it is carried on in the ordinary process of the assay and separated in the last filtration before electrolysis.
The following experiments were made to test the effect of the presence of salts of foreign metals in the solution during the precipitation of copper by electrolysis:—
Copper Taken.Other Metal Added.Copper Found.0.1000 gram0.1000 gram of silver0.18000.1050 "0.1000 " "0.20000.1030 "0.1000 " mercury0.20100.1037 "0.1000 " "0.20150.1020 "0.1000 " lead0.10200.1030 "0.1000 " "0.10280.1010 "0.1000 " arsenic0.10100.1007 "0.1000 " "0.10220.1030 "0.1000 " antimony0.10500.1034 "0.1000 " "0.10570.0990 "0.1200 " tin0.09900.1014 "0.1000 " "0.10150.1000 "0.1000 " bismuth0.16620.1040 "0.1000 " of cadmium0.10520.1009 "0.1300 " zinc0.10170.1014 "0.1000 " nickel0.10070.1079 "0.1200 " iron0.10890.1054 "0.1000 " chromium (Cr2O3)0.10350.1034 "0.1000 " " (K2CrO4)0.10100.1075 "0.1000 " aluminium0.10780.1010 "0.1000 " manganese0.0980
It will be seen from these that mercury, silver, and bismuth are the only metals which are precipitable[52]along with the copper under the conditions of the assay. Mercury, which if present would interfere, is separated because of the insolubility of its sulphide in nitric acid.
Bismuth is precipitated only after the main portion of the copper is thrown down. It renders the copper obviously unsuitable for weighing. It darkens, or forms a greyish coating on, the copper; and this darkening is a delicate test for bismuth. In assaying ores containing about three and a half per cent. of copper, and known to contain bismuth in quantities scarcely detectable in ordinary analysis, the metal deposited was distinctly greyish in colour, and would not be mistaken for pure copper. Ten grams of this impure copper were collected and analysed, with the following results:—
Copper99.46per cent.Bismuth00.30"Iron00.14"Arsenic00.10"———100.00
The quantity of copper got in each assay was 0.175 gram, and consequently the bismuth averaged 0.00053 gram.
To separate the bismuth in such a case the deposit is dissolved off by warming it in the original solution. The bismuth is precipitated by the addition of ammonic carbonate, and the solution, after filtering and acidifying with nitric acid, is re-electrolysed.
Determination of Copper in Commercial Copper.—Take from 1 to 1.5 gram, weigh carefully, and transfer to a beaker; add 20 c.c. of water and 10 c.c. of nitric acid; cover with a clockglass, and allow to dissolve with moderate action; boil off nitrous fumes, dilute to 100 c.c., and electrolyse. The cylinder must be carefully weighed, and the electrolysis allowed to proceed for 24 hours. The weight found will be that of the copper and silver. The silver in it must be determined[53]and deducted.
Determination of Copper in Brass, German Silver, or Bronze.—Treat in the same manner as commercial copper. If nickel is present, the few milligrams of copper remaining in the electrolysed solution should be separated with sulphuretted hydrogen, the precipitated sulphide dissolved in nitric acid, and determined colorimetrically.
There are two of these in use, one based on the decolorising effect of potassic cyanide upon an ammoniacal copper solution, and the other upon the measurement of the quantity of iodine liberated from potassic iodide by the copper salt. The cyanide process is the more generally used, and when carefully worked, "on certain understood and orthodox conditions," yields good results; but probably there is no method of assaying where a slight deviation from these conditions so surely leads to error. An operator has no difficulty in getting concordant results with duplicate assays; yet different assayers, working, without bias, on the same material, get results uniformly higher or lower; a difference evidently due to variations in the mode of working. Where a large number of results are wanted quickly it is a very convenient method. The iodide process is very satisfactory when worked under the proper conditions.
The process is based upon the facts—(1) that when ammonia is added in excess to a solution containing cupric salts, ammoniacal copper compounds are formed which give to the solution a deep blue colour; and (2) that when potassic cyanide is added in sufficient quantity to such a solution the colour is removed, double cyanides of copper and potassium or ammonium being formed.[54]In the explanation generally given the formation of cuprous cyanide is supposed[55]; but in practice it is found that one part of copper requires rather more than four parts of cyanide, which agrees with the former, rather than the latter, explanation.
Reliance on the accuracy of the process cannot rest upon the supposition that the cyanide required for decoloration is proportional to the copper present, for varying quantities of ammonia salts, ammonia and water, and differences of temperature have an important effect. The results are concordant and exact only when the cyanide is standardised under the same conditions as it is used. It is best to have the assay solution and that used for standardising as nearly as possible alike, and to titrate the two solutions side by side. This demands an approximate knowledge of the quantity of copper contained in the ore and a separation of the bulk of the impurities.
For the titration there is required a standard solution of potassium cyanide made by dissolving 42 grams of the salt, known to dealers as Potassium Cyanide (Gold), in water and diluting to one litre: 100 c.c. of this will be about equivalent to one gram of copper. For poor ores the solution may conveniently be made half this strength.
The solution of the ore and the separation of the copper as sulphide are effected in the same ways as have been already described for electrolysis. Similarly, too, the sulphide is attacked with 15 c.c. of nitric acid and the assay boiled down to 10 c.c. Add 20 c.c. of water and warm, filter into a pint flask, wash well with water, and dilute to about 150 c.c.; add 30 c.c. of dilute ammonia, and cool.
Prepare a standard by dissolving a quantity of electrotype copper (judged to be about the same as that contained in the assay) in 20 c.c. of water and 10 c.c. of nitric acid, boil off the nitrous fumes, and dilute to 150 c.c.: add 30 c.c. of dilute ammonia and cool.
Fill a burette with the standard cyanide solution. The burette with syphon arrangement, figured on page 52, is used. A number of titrations can be carried on at the same time provided the quantity of copper present in each is about the same. This is regulated in weighing up the ore. The flasks must of course be marked, and should be arranged in series on a bench in front of a good light and at such a height that the liquid can be looked through without stooping. Supposing about 50 c.c. of cyanide will be required, 30 c.c. should be run into each, and each addition be recorded as soon as made; then run 15 c.c. into each. The solutions will now probably show marked differences of tint: add 1 c.c. of cyanide to the lighter ones and more to the darker, so as to bring the colours to about the same depth of tint. They should all be of nearly equal tint just before finishing. At the end add half a c.c. at a time until the colours are completely discharged. A piece of damp filter paper held between the lightand the flask assists in judging the colour when nearly finished. Overdone assays show a straw yellow colour which deepens on standing.
The following will illustrate the notes recorded of five such assays and one standard:—
(1)30c.c.15c.c.5c.c.2c.c.1c.c.1/2c.c.—c.c.=53-1/2c.c.(2)30"15"1"1"1"1/2"—"=48-1/2"(3)30"15"3"1"1"1/2"—"=50-1/2"(4)30"15"5"2"1"1/2"1/2"=54"(5)30"15"2"1"1"1/2"—"=49-1/2"(6)30"15"2"1"1"1/2"1/2"=50standard
Three grams of ore were taken, and the standard contained 0.480 gram of copper.
In this series the difference of half a c.c. means about 0.15 per cent. on the ore; with a little practice it is easy to estimate whether the whole or half of the last addition should be counted.
To get satisfactory results, the manner of finishing once adopted must be adhered to.
The following experiments show the effect of variation in the conditions of the assay:—Usea solution of copper nitrate, made by dissolving 10 grams of copper in 50 c.c. of water and 35 c.c. of nitric acid, and diluting to a litre. 100 c.c. = 1 gram of copper.
Effect of Varying Temperature.—In these experiments 20 c.c. of copper nitrate were used, with 10 c.c. of nitric acid, 30 c.c. of dilute ammonia, and water to 200 c.c. The results were—
Temperature15°30°70°100°Cyanide required21.5 c.c.20.8 c.c.19.7 c.c.18.8 c.c.
The temperature is that of the solutionbeforetitrating. These show the importance of always cooling before titrating, and of titrating the assay and standard at the same temperature.
Effect of Varying Bulk.—The quantities of copper, acid, and ammonia were the same as in the last-mentioned experiments. The results were:—
Bulk100.0c.c.200.0c.c.300.0c.c.400.0c.c.Cyanide required23.3"21.7"21.4"21.4"
These show that large variations in bulk must be avoided.
Effect of Varying Ammonia.—The quantities of copper and acid were the same as in the series of experiments last noticed. The bulk was 200 c.c. The results were:—
Dilute ammonia20.0c.c.30.0c.c.50.0c.c.100.0c.c.Cyanide required20.9"21.7"22.3"24.6"
Effect of Varying Acid.—The quantities of copper and water were the same as in the last-noticed set of experiments: 30 c.c. of dilute ammonia were used.
Nitric acid5.0c.c.10.0c.c.15.0c.c.Cyanide required21.6"21.7"21.5"
On adding nitric acid to the solution it combines with a portion of the ammonia to form ammonic nitrate; it will be seen from the last series of experiments that the lessening of the amount of free ammonia will decrease the quantity of cyanide required; but, on the other hand, the ammonic nitrate which is at the same time formed will increase the amount required; under the conditions of the assay these two effects neutralise each other, and such differences in the quantity of acid as are likely to occur are unimportant.
Effect of Varying Ammonic Salts.—The quantities of copper, water, and ammonia were the same as in the last mentioned set of experiments, but no nitric acid was used.
Ammonic nitrate added1 gram5 grams10 grams20 gramsCyanide required21.2 c.c.22.1 c.c.23.1 c.c.24.1 c.c.
These show that combined ammonia seriously affects the titration, and that the principle sometimes recommended of neutralising the acid with ammonia, and then adding a constant quantity of ammonia, is not a good one, because there is then an interference both by the ammonia and by the variable quantity of ammonic salts.
The same quantity of combined ammonia has the same effect, whether it is present as sulphate, nitrate, chloride, or acetate, as the following experiments show. Four lots of 20 c.c. of "copper nitrate" were taken, and 20 c.c. of dilute ammonia added to each. These were carefully neutralised with the respective acids, rendered alkaline with 30 c.c. more of ammonia, cooled, diluted to bulk, and titrated. The results were:—
Withsulphuric acid22.5c.c. ofcyanide"nitric acid22.6"""hydrochloric acid22.6"""acetic acid22.5""
Effect of Foreign Salts.—Sulphates, nitrates and chlorides of sodium or potassium have no action, whilst the hydrates, carbonates, bicarbonates, sulphites, and nitrites have an important effect. The interference of ammonic salts has already been shown.
Salts of silver, zinc, and nickel react with cyanide just as copper does, and consequently interfere. Ferrous salts are sure to be absent, and ferric salts yield ferric hydrate with the ammonia, which is not acted on by the cyanide, but, owing to its bulkiness, it settles slowly; this lengthens the time required for titration, and so modifies the manner of working.An assay should not be worked with ferric hydrate present, unless the standard contains about the same amount of it.On mines it is often inconvenient to separate the copper by means of sulphuretted hydrogen; hence it is customary to titrate without previousseparation. In this case, instead of standardising the cyanide with electrotype copper, a standard ore should be used. This should be an ore (of the same kind as those being assayed) in which the copper has been carefully determined.
Effect of Varying Copper.—In these experiments 10 c.c. of nitric acid, 30 c.c. of ammonia, and water to 200 c.c. were used.
Copper nitrate present1.0c.c.10.0c.c.20.0c.c.50.0c.c.100.0c.c.Cyanide required0.7"11.2"21.7"54.5"108.1"
These results show that under the conditions laid down the various causes of disturbance nearly neutralise one another, and the results within a fair range are practically proportional.
Determination of Copper in Copper Pyrites.—Weigh up 2 grams of the dried and powdered ore, and place in an evaporating dish about four inches in diameter. Cover with 20 c.c. of nitric acid and put on a hot plate. Evaporate to dryness without further handling. Allow to cool and take up with 30 c.c. of hydrochloric acid, boil, dilute, and transfer to a pint flask, filtering if necessary. Make up the bulk with the washings to about 150 c.c. Precipitate with sulphuretted hydrogen, filter, and wash back the precipitate into the flask. Add 15 c.c. of nitric acid, and boil down rapidly to 10 c.c. Dilute, add 30 c.c. of dilute ammonia, make up to 150 c.c., and cool. For the standard, weigh up 0.5 gram of copper, more or less, according to the quantity judged to be present in the assay. Dissolve in 20 c.c. of dilute nitric acid, boil off nitrous fumes, add 30 c.c. of dilute ammonia, make up to the same bulk as that of the assay, and cool. Titrate the two solutions side by side and as nearly as possible in the same manner.
Since the assay solution is often turbid from the presence of small quantities of lead and of iron from incomplete washing, and since this slight precipitate is very slow in settling, the standard can hardly be compared strictly with the assay. This can be counteracted by precipitating in both solutions a mixture of ferric and aluminic hydrates, which settles readily and leaves the supernatant liquor clear. To effect this, boil the nitric acid solutions with 30 c.c. of a solution containing 15 grams each of alum and ferrous sulphate to the litre. In an actual determination 2 grams of the ore were taken and compared with 0.5 gram of copper. The assay required 57.7 c.c. of cyanide and the standard 52.5 c.c.
52.5 : 0.5 :: 57.7 : 0.5495
This on 2 grams of ore = 27.47 per cent.; the same sample by electrolysis gave 27.60 per cent. of copper.
Determination without Previous Separation.—Dissolve up 2 grams as before, but, instead of passing sulphuretted hydrogen, add 30 c.c. of dilute ammonia, shake well, and cool. Prepare a standard by dissolving 0.5 gram of copper in 1 c.c. of nitric acid, add 0.6 gram of iron in the form of ferric chloride and 20 c.c. of hydrochloric acid, dilute to about 150 c.c., add 30 c.c. of dilute ammonia, and cool. Titrate the two solutions side by side. In a determination on the sample last used, 58 c.c. were required for the assay and 53 c.c. for the standard, which indicates 27.3 per cent. of copper.
This method of working is somewhat rough.
This is based upon the fact that when potassic iodide in excess is added to a strong solution of a cupric salt in a faintly acid solution, cuprous iodide is formed and an equivalent of iodine liberated.[56]The iodine is measured by titrating with a solution of sodium hyposulphite,[57]using starch paste as indicator. The iodine is soluble in the excess of potassium iodide, forming a deep brown solution; the hyposulphite is added until this brown colour is almost removed. Starch paste is then added, and strikes with the remaining iodine a dirty blue colour. The addition of the "hypo" is continued until the blue colour is discharged. The end reaction is sharp; a drop is sufficient to complete it.
As regards the titration, the process leaves little to be desired; the quantity of "hypo" required is strictly proportional to the copper present, and ordinary variations in the conditions of working are without effect. The presence of salts of bismuth masks the end reaction because of the strong colour imparted to the solution by the iodide of bismuth. Under certain conditions there is a return of the blue colour in the assay solution after the finishing point has apparently been reached, which is a heavy tax on the patience and confidence of the operator. This is specially apt to occur when sodium acetate is present, although it may also be due to excessive dilution.
The standard "hypo" solutionis made by dissolving 39.18 grams of the crystallised salt (Na2S2O3.5H2O) in water and diluting to one litre. One hundred c.c. will equal one gram of copper.
The starch solution is made by mixing 1 gram of starch into a thin paste with cold water, pouring it into 200 c.c. of boilingwater, and continuing the boiling for a minute or so. The solution must be cold before use, and about 2 c.c. is used for each assay. It should not be added until the bulk of the iodine has been reduced.
To standardise the "hypo," weigh up 0.3 or 0.4 gram of pure copper, dissolve in 5 c.c. of dilute nitric acid, boil off nitrous fumes, and dilute with an equal bulk of cold water. Add "soda" solution until a permanent precipitate is obtained, and then 1 c.c. of acetic acid. This should yield a clear solution. Fill an ordinary burette with the "hypo." Add 3 grams of potassium iodide crystals to the copper solution, and, when these are dissolved, dilute to 100 c.c. with water. Run in the "hypo" solution rather quickly until the brown colour is nearly discharged—i.e., to within 3 or 4 c.c. of the finish. Add 2 c.c. of the starch solution, and continue the addition of the "hypo" a few drops at a time until the tint suddenly changes to a cream colour. The blue colour must not return on standing three or four minutes. Calculate the standard in the usual way.
In assaying ores, the copper is dissolved and separated with sulphuretted hydrogen as in the other processes, but the sulphide should be washed more completely to ensure the absence of iron salts.
The following experiments show the effect of variation in the conditions of the assay. Use a solution of copper sulphate containing 39.38 grams of copper sulphate crystals (CuSO4.5H2O) in the litre. 100 c.c. equal 1.00 gram of copper.
Effect of Varying Temperature.—The assay after the addition of the potassic iodide must be kept cold, else iodine may be volatilised.
Effect of Varying Potassium Iodide.—In various descriptions of the process the amount of iodide required is variously stated at from "a few crystals" to as much as 10 grams. The proportion required by theory for 1 gram of copper is a little over 5 grams: an excess, however, is required to keep the liberated iodine in solution. On economic grounds this excess should not be extravagant; if the student uses 10 parts of the iodide for each part of copper in the assay he will have sufficient. In the experiments there were used 20 c.c. of the copper sulphate, with varying amounts of potassic iodide, and the following results were got:—