A Text-book of Assaying: For the Use of Those Connected with Mines Part 39

~The t.i.tration.~--The solution, containing not much more than 0.1 gram of nickel, and free from the interfering metals, must be cooled. It is next neutralised and then made strongly alkaline with a solution of soda (NaHO); an excess of 20 or 30 c.c. suffices. This will produce a precipitate. The cyanide solution is now run in from a burette until the solution clears, after which an excess of about 20 c.c. is added. It is well to use some round number of c.c. to simplify the calculation. Add a few drops of pota.s.sium iodide solution, and run in the standard solution of silver nitrate from a burette. This should be done a little at a time, though somewhat rapidly, and with constant shaking, till a permanent yellow precipitate appears. If the addition of the cyanide did not result in a perfectly clear solution, this is because something besides nickel is present. The residue may be filtered off, though with a little practice the finishing-point may be detected with certainty in the presence of a small precipitate. If the student has the slightest doubt about a finish he should run in another 5 c.c. of the cyanide and again finish with silver nitrate. The second result will be the same as the first. For example, if 40 c.c. of cyanide and 30 c.c. of silver nitrate were required at the first t.i.tration, then the 45 c.c. of cyanide in the second t.i.tration will require 35 c.c. of silver nitrate.

The difference between the quant.i.ties of the two solutions used in each case will be 10 c.c. It is this difference in the readings of the two burettes which measures the quant.i.ty of nickel present. Each c.c. of the difference is equal to .0025 gram of nickel. But if the cyanide solution is not exactly equal in strength to the silver nitrate, the quant.i.ty of cyanide used should be calculated to its equivalent in silver nitrate before making the subtraction.

The following experimental results ill.u.s.trate the accuracy of the a.s.say and the effect upon it of varying conditions. A solution containing 1 gram of nickel sulphate (NiSO_{4}.6H_{2}O) in 100 c.c. was used. By a separate a.s.say the sulphate was found to contain 22.25 per cent. of nickel. For the sake of simplicity the results of the experiments are stated in weights of nickel in grams.

~Effect of varying excess of Cyanide Solution.~--In each experiment there was 20 c.c. of the nickel solution, equal to .0445 gram of nickel.

There were also 10 c.c. of soda solution, 3 or 4 drops of pota.s.sium iodide and sufficient water to bring the bulk to 100 c.c. before t.i.trating.

Cyanide in excess 6 c.c. 4 c.c. 8 c.c. 12 c.c. 25 c.c.

Nickel found .0434 .0436 .0440 .0442 .0444

Although the difference between the highest and lowest of these results is only 1 milligram, their meaning is quite obvious. The excess of cyanide should not be less than 20 c.c.

~Effect of varying the quant.i.ty of Soda.~--There were two series of experiments, one with 2 c.c. of nickel solution (= .0044 gram of nickel), the other with 20 c.c. The conditions were as before, except that the quant.i.ty of soda was varied.

Soda added 5 c.c. 15 c.c. 30 c.c.

Nickel found, 1st series .0037 .0042 .0045 " " 2nd series .0444 .0444 .0442

These show that the presence of much soda, though it has only a small effect, is beneficial rather than otherwise. Ammonia has a bad effect, if present in anything like the same quant.i.ties.

~Effect of varying the Nickel.~--In experiments with 10, 20, and 40 c.c.

of the nickel solution, the results were:--

Nickel present .0222 .0445 .0890 Nickel found .0220 .0442 .0884

~Effect of Zinc.~--In these experiments 20 c.c. of nickel solution (= .0445 gram of nickel), 10 c.c. of soda, 6 drops of pota.s.sium iodide and water to 100 c.c. were used. The excess of cyanide was purposely kept at from 10 to 15 c.c., which is hardly sufficient.

Zinc added 0 .25 gram. .5 gram.

Nickel found .0442 .0440 .0407

On increasing the excess of cyanide to over 20 c.c. and doubling the quant.i.ty of soda, the experiment with 0.5 gram of zinc gave 0.441 gram of nickel. Hence the t.i.tration is satisfactory in the presence of zinc provided that not fewer than 20 or 30 c.c. of soda are used, and that the excess of cyanide is such that not fewer than 20 or 30 c.c. of silver nitrate are required in the t.i.tration. Moreover, these precautions should be taken whether zinc is present or not.

~Effect of other Metals.~--If metals of the first and second groups are present they should be removed by pa.s.sing sulphuretted hydrogen and filtering. If _iron_ is present it must be removed, since ferrous salts use up much cyanide, forming ferrocyanides, and ferric salts yield ferric hydrate, which obscures the end reaction. Hence the sulphuretted hydrogen must be boiled off and the iron removed as basic ferric acetate by the method described on p. 233. If the precipitate is bulky it should be dissolved in a little dilute acid, neutralised and again precipitated as basic acetate. The nickel will be in the two filtrates. In the absence of manganese and cobalt the t.i.tration may be made without further separation.

_Manganese_ does not directly interfere, but the precipitated hydrate, which rapidly darkens through atmospheric oxidation, obscures the end reaction. It may be removed by pa.s.sing sulphuretted hydrogen through the filtrate from the acetate separation: sulphides of nickel, cobalt and zinc will be precipitated, whilst manganese remains in solution: the addition of more sodium acetate may a.s.sist the precipitation. The precipitate must be filtered off and dissolved in nitric acid: the solution should be evaporated to dryness. The filtrate may retain a little nickel; if so, add ammonia till alkaline, then acidify with acetic acid and again filter; any small precipitate obtained here should be added to that first obtained.

It is only when _cobalt_ is present that any further separation is required. Cobalt hydrate takes up oxygen from the air, and on adding pota.s.sium cyanide some may refuse to dissolve; and the solution itself acquires a brown colour, which becomes deeper on standing. At this stage the cobalt is easily separated. The solution containing the nickel and cobalt with no great excess of acid, is made alkaline by adding 20 c.c.

of soda exactly as in preparing for a t.i.tration. So, too, the solution of cyanide is added so as to have an excess of 20 or 30 c.c.; the solution may have a brown colour, but if it is not quite clear it _must_ be filtered. Then warm (boiling is not needed) and add from 50 to 100 c.c. of bromine water. This throws down all the nickel as black peroxide in a condition easy to filter. Filter it off and wash with water. The precipitate can be dissolved off the filter with the greatest ease by a little warm sulphurous acid. The filtrate and washings, boiled till free from sulphurous acid, yield the nickel as sulphate in a clean condition.

~Determination of Nickel in Nickel Sulphate Crystals.~--Take 0.5 gram of the salt, dissolve in 50 c.c. of water and add 25 c.c. of solution of soda. Run in from a burette, say, 60 c.c. "cyanide." Add a few drops of pota.s.sium iodide and t.i.trate back with "silver nitrate." Suppose 15.5 c.c. of the latter is required. Then 15.5 c.c. subtracted from 60 c.c.

leaves 44.5 c.c., and since 100 c.c. = 0.25 gram of nickel, 44.5 c.c.

will equal 0.11125 gram of nickel. This in 0.5 gram of the salt equals 22.25 per cent.

~Determination of Nickel in German Silver.~--Weigh up 0.5 gram of the alloy, and dissolve in a dish with 5 or 10 c.c. of dilute nitric acid.

Add 5 c.c. of dilute sulphuric acid and evaporate till all the nitric acid is removed. Cool, take up with 50 c.c. of water, and when dissolved pa.s.s sulphuretted hydrogen through the solution. Filter off the precipitate and wash with water containing sulphuretted hydrogen and dilute sulphuric acid. Boil down the filtrate and washings to get rid of the excess of the gas; add some nitric acid and continue the boiling.

Cool, neutralise the excess of acid with soda, add 1 gram of sodium acetate and boil. Filter off the precipitate which contains the iron.

The filtrate, cooled and rendered alkaline with soda, is ready for the t.i.tration.

COBALT

Occurs less abundantly than nickel. Its chief ores are smalt.i.te and cobalt.i.te, which are a.r.s.enides of cobalt, with more or less iron, nickel, and copper. It also occurs as a.r.s.eniate in erythrine, and as oxide in asbolan or earthy cobalt, which is essentially a wad carrying cobalt.

It is mainly used in the manufacture of smalts for imparting a blue colour to gla.s.s and enamels. The oxide of cobalt forms coloured compounds with many other metallic oxides. With oxide of zinc it forms "Rinman's green"; with aluminia, a blue; with magnesia, a pink. This property is taken advantage of in the detection of substances before the blow-pipe.

The compounds of cobalt in most of their properties closely resemble those of nickel, and the remarks as to solution and separation given for the latter metal apply here. Solutions of cobalt are pink, whilst those of nickel are green.

The detection of cobalt, even in very small quant.i.ty, is rendered easy by the strong blue colour which it gives to the borax bead, both in the oxidising and in the reducing flame. It is concentrated from the ore in the same way as nickel, and should be separated from that metal by means of pota.s.sic nitrite in the way described. The dry a.s.say of cobalt has been given under _Nickel_.

GRAVIMETRIC METHOD.

The yellow precipitate from the pota.s.sium nitrite, after being washed with the acetate of potash, is washed with alcohol, dried, transferred to a weighed porcelain crucible, and cautiously ignited with an excess of strong sulphuric acid. The heat must not be sufficient to decompose the sulphate of cobalt, which decomposition is indicated by a blackening of the substance at the edges. The salt bears a low red heat without breaking up. If blackening has occurred, moisten with sulphuric acid, and ignite again. Cool and weigh. The substance is a mixture of the sulphates of cobalt and potash (2CoSO_{4} + 3K_{2}SO_{4}), and contains 14.17 per cent. of cobalt.

Cobalt is also gravimetrically determined, like nickel, by electrolysis, or by precipitation with sodic hydrate. In the latter case, the ignited oxide will be somewhat uncertain in composition, owing to its containing an excess of oxygen. Consequently, it is better to reduce it by igniting at a red heat in a current of hydrogen and to weigh it as metallic cobalt.

PRACTICAL EXERCISES.

1. In the dry a.s.say of an ore containing cobalt, nickel, and copper, the following results were obtained. Calculate the percentages. Ore taken, 5 grams. Speise formed, 0.99 gram. Speise taken. 0.99 gram. a.r.s.enides of cobalt, nickel, and copper got, 0.75 gram. a.r.s.enide of nickel and copper got, 0.54 gram. Gold added, 0.5 gram. Gold and copper got, 0.61 gram.

2. Calculate the percentage composition of the following compounds: Co_{2}As, Ni_{2}As, and Cu_{2}As.

3. A sample of mispickel contains 7 per cent. cobalt. What weight of the mixed sulphates of potash and cobalt will be obtained in a gravimetric determination on 1 gram of the ore?

4. 0.3157 gram of metal was deposited by the electrolysis of a nickel and cobalt solution. On dissolving in nitric acid and determining the cobalt 0.2563 gram of pota.s.sium and cobalt sulphates were got. Find the weights of cobalt and nickel present in the deposit.

5. What should be the percentage composition of pure cobalt.i.te, its formula being Coa.s.s?

ZINC.

Zinc occurs in nature most commonly as sulphide (blende); it also occurs as carbonate (calamine) and silicate (smithsonite). Each of these is sufficiently abundant to be a source of the metal.

The metal is known in commerce as "spelter" when in ingots, and as sheet zinc when rolled. It is chiefly used in the form of alloys with copper, which are known as bra.s.ses. It is also used in the form of a thin film, to protect iron goods from rusting--galvanised iron.

Ores of zinc, more especially blende, are met with in most lead, copper, gold, and silver mines, in larger or small quant.i.ties scattered through the lodes. Those ores which generally come under the notice of the a.s.sayer are fairly rich in zinc; but alloys and metallurgical products contain it in very varying proportions.

Zinc itself is readily soluble in dilute acids; any residue which is left after boiling with dilute hydrochloric or sulphuric acid consists simply of the impurities of the metal; this is generally lead.

All zinc compounds are either soluble in, or are decomposed by, boiling with acids, the zinc going into solution. Zinc forms only one series of salts, and these are colourless. Their chief characteristic is solubility in an alkaline solution, from which sulphuretted hydrogen produces a white precipitate of zinc sulphide. Zinc is detected by dissolving the substance in hydrochloric or nitric acid, boiling, and adding sodic hydrate in excess, filtering, and adding ammonic sulphide to the filtrate. The precipitate contains the zinc, which can be dissolved out by boiling with dilute sulphuric acid, and detected by the formation of a white precipitate on the addition of pota.s.sic ferrocyanide.

The dry a.s.say of zinc can only be made indirectly, and is unsatisfactory. Zinc is volatile, and at the temperature of its reduction is a gas. It is impracticable to condense the vapour so as to weigh the metal, consequently its amount is determined by loss. The following method gives approximate results: Take 10 grams of the dried and powdered ore and roast, first at a low temperature and afterwards at a higher one, with the help of carbonate of ammonia to decompose the sulphates formed; cool and weigh. The metals will be present as oxides.

Mix with 2 grams of powdered charcoal and charge into a black-lead crucible heated to whiteness, cover loosely, and leave in the furnace for about a quarter of an hour. Uncover and calcine the residue, cool and weigh. The loss in weight multiplied by 8.03 gives the percentage of zinc in the ore.

WET METHODS.

Solution and separation may be effected as follows: Treat 1 or 3 grams of the substance with 10 or 30 c.c. of hydrochloric acid or aqua regia; evaporate to dryness; take up with 10 c.c. of hydrochloric acid and dilute to 100 c.c.; heat nearly to boiling; saturate with sulphuretted hydrogen; filter, and wash with water acidulated with hydrochloric acid.

Boil off the sulphuretted hydrogen and peroxidise with a few drops of nitric acid. Cool; add caustic soda till nearly, but not quite, neutralised, and separate the iron as basic acetate by the method described under _Iron_. To the filtrate add ammonia till alkaline, and pa.s.s sulphuretted hydrogen. Allow to settle and decant on to a filter.