A Text-book of Assaying: For the Use of Those Connected with Mines Part 29

~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 pota.s.sium iodide. Using 5 grams (or in the case of the acids, 5 c.c.), the results were as follows:--

dilute Acetic Salt added -- H_{2}SO_{4} acid NaAc NaCl "Hypo" required 20.0 c.c. 20.0 c.c. 20.1 c.c. 19.3 c.c. 20.1 c.c.

Salt added KNO_{3} Na_{2}SO_{4} AmCl Am_{2}SO_{4} "Hypo" required 20.2 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, and partly 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.c 0.050 gram a.r.s.enic as As_{2}O_{5} 20.0 "

0.050 " antimony as SbCl_{5} 19.8 "

0.050 " lead as Pb(NO_{3})_{2} 20.1 "

A similar experiment with 0.050 gram of bis.m.u.th nitrate could not be determined, the end-reaction being masked. Bis.m.u.th iodide is soluble in pota.s.sic 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 a.s.say.

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 a.s.say 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 quant.i.ty of iodide, the results were:--

Copper present 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Hypo" required 1.0 " 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 pa.s.s 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 pota.s.sium iodide, dilute to 100 c.c., and t.i.trate. The following is an example:--

0.5 gram of copper required 50.5 c.c. "hypo."

The a.s.say required 55.6 " "

which is equal to 27.5 per cent. of copper.

COLORIMETRIC PROCESS.

This is based on the blue coloration of ammoniacal copper solutions. The quant.i.ty of copper in 100 c.c. of the a.s.say 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 a.s.sayed. 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 coa.r.s.e filter into a Nessler tube, was.h.i.+ng 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 a.s.say 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 quant.i.ty of copper is present.

1.0 c.c. at 15 showed the colour of 1.0 c.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 quant.i.ty to be used in the experiments. A larger quant.i.ty affects the results, giving lower readings and altering the tint. With small quant.i.ties of ammonia the colour approaches a violet; with larger, a sky-blue.

2.5 c.c. with 25 c.c. of strong ammonia read 2.2 c.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 | With 10 grams | With 10 grams | Ammonic Nitrate.| Ammonic Chloride.| Ammonic Sulphate.

--------------+-----------------+------------------+------------------ 2.5 | 2.5 | 2.5 | 2.0 5.0 | 5.0 | 5.3 | 4.3 10.0 | 10.0 | 10.0 | 8.5 --------------+-----------------+------------------+------------------

These show that sulphates should be avoided, and either nitrate or chloride solutions be used in the standard as well as in the a.s.say.

~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 pa.s.s 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 was.h.i.+ngs 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 gla.s.s and determine the copper in it colorimetrically.

EXAMINATION OF COMMERCIAL COPPER.

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 a.r.s.enic is the most important. The other elements to be looked for are bis.m.u.th, 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 a.s.sumed to be oxygen. Probably this is nearly correct, but the whole of the oxygen should not be ascribed to cuprous oxide; for any a.r.s.enic the metal contained would be present as cuprous a.r.s.enite, since a.r.s.enide of copper and cuprous oxide could not exist together at the temperature of fusion without interacting. In the report of the a.n.a.lysis, 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.

~Silver~ is 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 hydrochloric acid (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.