A Text-book of Assaying: For the Use of Those Connected with Mines Part 40

Dissolve off the precipitate from the filter with hot dilute hydrochloric acid. The solution will contain the zinc, together with any manganese the ore contained, and, perhaps, traces of nickel and cobalt.

If the zinc is to be determined volumetrically, and manganese is present, this latter is separated with carbonate of ammonia, as described further on; but if a gravimetric method is used, and only small quant.i.ties of manganese are present, it is better to proceed as if it were absent, and to subsequently determine its amount, which should be deducted.

GRAVIMETRIC DETERMINATION.

The solution containing the zinc is contained in an evaporating dish, and freed from sulphuretted hydrogen by boiling, and, if necessary, from an excess of acid by evaporation. The evaporating dish must be a large one. Cautiously add sodium carbonate to the hot, moderately dilute solution, until the liquid is distinctly alkaline, and boil. Allow the precipitate to settle, decant on to a filter, and wash with hot water.

Dry, transfer to a porcelain crucible (cleaning the paper as much as possible), add the ash, ignite, and weigh. The substance weighed is oxide of zinc, which contains 80.26 per cent. of the metal. It is a white powder, becoming yellow when heated. It must not show an alkaline reaction when moistened. If it contains manganese this metal will be present as sesquioxide (Mn_{2}O_{3}). Its amount can be determined by dissolving in dilute acid and boiling with an excess of sodic hydrate.

The oxide of manganese will be precipitated, and can be ignited and weighed. Its weight multiplied by 1.035 must be deducted from the weight of oxide of zinc previously obtained. The results yielded by the gravimetric determination are likely to be high, since the basic carbonate of zinc frequently carries down with it more or less soda which is difficult to wash off.

VOLUMETRIC DETERMINATION

This method is based on the facts that zinc salts in an acid solution decompose pota.s.sium ferrocyanide, forming a white insoluble zinc compound; and that an excess of the ferrocyanide can be detected by the brown coloration it strikes with uranium acetate. The method resembles in its working the b.i.+.c.hromate iron a.s.say. The standard solution of pota.s.sium ferrocyanide is run into a hot hydrochloric acid solution of the zinc until a drop of the latter brought in contact with a drop of the indicator (uranium acetate) on a white plate strikes a brown colour.

The quant.i.ty of zinc in the solution must be approximately known; run in a little less of the ferrocyanide than is expected will be necessary; test a drop or two of the a.s.say, and then run in, one or two c.c. at a time, until the brown colour is obtained. Add 5 c.c. of a standard zinc solution, equivalent in strength to the standard "ferrocyanide,"

re-t.i.trate, and finish off cautiously. Of course 5 c.c. must be deducted from the reading on the burette. The precipitate of zinc ferrocyanide formed in the a.s.say solution is white; but if traces of iron are present, it becomes bluish. If the quant.i.ty of ferrocyanide required is known within a few c.c., the finis.h.i.+ng point is exactly determined in the first t.i.tration without any addition of the standard zinc solution.

Unfortunately this t.i.tration serves simply to replace the gravimetric determination, and does not, as many volumetric processes do, lessen the necessity for a complete separation of any other metals which are present. Most metals give precipitates with ferrocyanide of pota.s.sium in acid solutions. If the conditions are held to, the t.i.tration is a fairly good one, and differences in the results of an a.s.say will be due to error in the separation. Ferric hydrate precipitated in a fairly strong solution of zinc will carry with it perceptible quant.i.ties of that metal. Similarly, large quant.i.ties of copper precipitated as sulphide by means of sulphuretted hydrogen will carry zinc with it, except under certain nicely drawn conditions. When much copper is present it is best separated in a nitric acid solution by electrolysis. The t.i.tration of the zinc takes less time, and, with ordinary working, is more trustworthy than the gravimetric method.

_The standard ferrocyanide solution_ is made by dissolving 43.2 grams of pota.s.sium ferrocyanide (K_{4}FeCy_{6}.3H_{2}O) in water, and diluting to a litre. One hundred c.c. are equal to 1 gram of zinc.

_The standard zinc solution_ is made by dissolving 10 grams of pure zinc in 50 c.c. of hydrochloric acid and 100 or 200 c.c. of water, and diluting to 1 litre, or by dissolving 44.15 grams of zinc sulphate (ZnSO_{4}.7H_{2}O) in water with 30 c.c. of hydrochloric acid, and diluting to 1 litre. One hundred c.c. will contain 1 gram of zinc.

_The uranium acetate solution_ is made by dissolving 0.2 gram of the salt in 100 c.c. of water.

To standardise the "ferrocyanide" measure off 50 c.c. of the standard zinc solution into a 10 oz. beaker, dilute to 100 c.c., and heat to about 50 C. (not to boiling). Run in 47 or 48 c.c. of the "ferrocyanide" solution from an ordinary burette, and finish off cautiously. Fifty divided by the quant.i.ty of "ferrocyanide" solution required gives the standard.

In a.s.saying ores, &c., take such quant.i.ty as shall contain from 0.1 to 1 gram of zinc, separate the zinc as sulphide, as already directed.

Dissolve the sulphide off the filter with hot dilute hydrochloric acid, which is best done by a stream from a wash bottle. Evaporate the filtrate to a paste, add 5 c.c. of dilute hydrochloric acid, dilute to 100 c.c. or 150 c.c., heat to about 50 C., and t.i.trate. Manganese, if present, counts as so much zinc, and must be specially separated, since it is not removed by the method already given. The following method will effect its removal. To the hydrochloric acid solution of the zinc and manganese add sodium acetate in large excess and pa.s.s sulphuretted hydrogen freely. Allow to settle, filter off the zinc sulphide and wash with sulphuretted hydrogen water. The precipitate, freed from manganese, is then dissolved in hydrochloric acid and t.i.trated.

The following experiments show the effect of variation in the conditions of the a.s.say:--

~Effect of Varying Temperature.~--Using 20 c.c. of the standard zinc solution, 5 c.c. of dilute hydrochloric acid, and diluting to 100 c.c.

Temperature 15 C. 30 C. 70 C. 100 C.

"Ferrocyanide" required 20.6 c.c. 20.3 c.c. 20.3 c.c. 20.3 c.c.

The solution can be heated to boiling before t.i.trating without interfering with the result; but it is more convenient to work with the solution at about 50 C. Cold solutions must not be used.

~Effect of Varying Bulk.~--These were all t.i.trated at about 50 C., and were like the last, but with varying bulk.

Bulk 25.0 c.c. 50.0 c.c. 100.0 c.c. 200.0 c.c.

"Ferrocyanide" required 20.2 " 20.4 " 20.3 " 20.4 "

Any ordinary variation in bulk has no effect.

~Effect of Varying Hydrochloric Acid.~-- With 100 c.c. bulk and varying dilute hydrochloric acid the results were:--

Acid added 0.0 c.c. 1.0 c.c. 5.0 c.c. 10.0 c.c. 20.0 c.c.

"Ferrocyanide"

required 24.4 " 20.2 " 20.3 " 20.3 " 20.7 "

~Effect of Foreign Salts.~--The experiments were carried out under the same conditions as the others. Five grams each of the following salts were added:--

Salt added { Ammonic Ammonic Sodium Sodium { chloride. sulphate. chloride. sulphate.

"Ferrocyanide"

required 20.3 c.c. 20.5 c.c. 20.6 c.c. 20.4 c.c.

Salt added { Pota.s.sium Magnesium Nil.

{ Nitrate. sulphate.

"Ferrocyanide"

required 20.2 c.c. 20.4 c.c. 20.4 c.c.

In a series of experiments in which foreign metals were present to the extent of 0.050 gram in each, with 20 c.c. of zinc solution and 5 c.c.

of dilute hydrochloric acid, those in which copper sulphate, ferrous sulphate, and ferric chloride were used, gave (as might be expected) so strongly coloured precipitates that the end reaction could not be recognised. The other results were:--

"Ferrocyanide"

required.

With nothing added. 20.3 c.c.

" 0.050 gram lead (as chloride) 20.9 "

" 0.050 " manganese (as sulphate) 25.5 "

" 0.050 " cadmium (as sulphate) 23.5 "

" 0.050 " nickel (as sulphate) 26.2 "

~Effect of Varying Zinc.~--These were t.i.trated under the usual conditions, and gave the following results:--

Zinc added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Ferrocyanide"

required 1.1 " 10.2 " 20.3 " 50.6 " 101.0 "

~Determination of Zinc in a Sample of Bra.s.s.~--Take the solution from which the copper has been separated by electrolysis and pa.s.s sulphuretted hydrogen until the remaining traces of copper and the lead are precipitated, filter, boil the solution free from sulphuretted hydrogen, put in a piece of litmus paper, and add sodic hydrate solution in slight excess; add 10 c.c. of dilute hydrochloric acid (which should render the solution acid and clear); warm, and t.i.trate.

A sample of 0.5 gram of bra.s.s treated in this manner required 16.4 c.c.

of "ferrocyanide" (standard 100 c.c. = 0.9909 zinc), which equals 0.1625 gram of zinc or 32.5 per cent.

~Determination of Zinc in Blende.~--Dissolve 1 gram of the dried and powdered sample in 25 c.c. of nitric acid with the help of two or three grams of pota.s.sium chlorate dissolved in the acid. Evaporate to complete dryness, taking care to avoid spirting. Add 7 grams of powdered ammonium chloride, 15 c.c. of strong ammonia and 25 c.c. of boiling water; boil for one minute and see that the residue is all softened. Filter through a small filter, and wash thoroughly with small quant.i.ties of a hot one per cent. solution of ammonium chloride. Add 25 c.c. of hydrochloric acid to the filtrate. Place in the solution some clean lead foil, say 10 or 20 square inches. Boil gently until the solution has been colourless for three or four minutes. Filter, wash with a little hot water; and t.i.trate with standard ferrocyanide.

~Determination of Zinc in Silver Precipitate.~--This precipitate contains lead sulphate, silver, copper, iron, zinc, lime, &c. Weigh up 5 grams of the sample, and extract with 30 c.c. of dilute sulphuric acid with the aid of heat. Separate the copper with sulphuretted hydrogen, peroxidise the iron with a drop or two of nitric acid, and separate as acetate. Render the filtrate ammoniacal, pa.s.s sulphuretted hydrogen; warm, and filter. Dissolve the precipitated zinc sulphide in dilute hydrochloric acid, evaporate, dilute, and t.i.trate. Silver precipitates carry about 2.5 per cent. of zinc.

GASOMETRIC METHOD.

Metallic zinc is readily soluble in dilute hydrochloric or sulphuric acid, hydrogen being at the same time evolved.[74] The volume of the hydrogen evolved is obviously a measure of the amount of zinc present in the metallic state. The speed with which the reaction goes on (even in the cold) and the insolubility of hydrogen renders this method of a.s.say a convenient one. It is especially applicable to the determination of the proportion of zinc in zinc dust. The apparatus described in the chapter on gasometric method is used. The method of working is as follows: Fill the two burettes with cold water to a little above the zero mark, place in the bottle about 0.25 gram of the substance to be determined, and in the inner phial or test tube 5 c.c. of dilute sulphuric acid; cork the apparatus tightly and allow to stand for a few minutes; then bring the water to the same level in the two burettes by running out through the clip at the bottom. Read off the level of the liquid in the graduated burette. Turn the bottle over sufficiently to spill the acid over the zinc, and then run water out of the apparatus so as to keep the liquid in the two burettes at the same level, taking care not to run it out more quickly than the hydrogen is being generated.

When the volume of gas ceases to increase, read off the level of the liquid, deduct the reading which was started with; the difference gives the volume of hydrogen evolved. At the same time read off the volume of air in the "volume corrector," which must be fixed alongside the gas burettes. Make the correction. For example: A piece of zinc weighing 0.2835 gram was found to give 99.9 c.c. of gas at a time when the corrector read 104 c.c.[75] Then the corrected volume is

104 : 100 :: 99.9 : _x_.

_x_ = 96.0 c.c.

100 c.c. of hydrogen at 0 C. and 760 mm. is equivalent to 0.2912 gram of zinc; therefore the quant.i.ty of zinc found is

100 : 96 :: 0.2912 : _x_.