A Text-book of Assaying: For the Use of Those Connected with Mines Part 18

Deduct silver in lead 0.0015 "

------ Silver in ore 0.0452 " = 492.2 ozs. per ton.

~Determination of Silver in Silver Precipitate.~--This substance contains, in addition to metallic silver and gold, sulphates of lead and lime; oxides of zinc, copper, and iron; and more or less organic matter.

The sample as received is generally free from "water at 100 C."; and, since it rapidly absorbs water, care should be taken in weighing it.

Since it contains combined water it is not suited for scorifying; therefore the determination of silver and gold (fine metal) is made by pot a.s.say. Weigh up 5 grams of the precipitate, mix with 100 grams of litharge and 1 gram of charcoal. Melt in a crucible at a moderate heat and pour. Detach the slag, replace in the crucible, and, when fused, add a mixture of 20 grams of litharge and 1 gram of charcoal. When the fusion is again tranquil, pour; and cupel the two b.u.t.tons of lead.

In a sample worked in this manner the mean of four determinations gave 0.6819 gram of "fine metal"; deducting 1 milligram for the silver contained in the oxide of lead, and adding 8 milligrams for the cupellation loss, there is got 0.6889 gram or 13.778 per cent. of silver (and gold) in the sample.

~Determination of Silver in Burnt Ores.~ _By Pot a.s.say._--Roasted cupriferous pyrites containing small quant.i.ties of gold and silver comes under this heading. The following mixture will give a fluid slag which is heavy and tough when cold:--

Ore. Borax. Sand. Litharge. Charcoal.

100 50 50 100 7

Mix; place in a large crucible; cover with salt; and melt down under cover. When fused drop in an iron rod for a few minutes, and about a couple of minutes after its withdrawal, pour the charge quickly into a large conical mould. The b.u.t.ton of lead should weigh about 50 grams.

Cupel and weigh the silver. The litharge may be replaced by red lead, in which case another gram of charcoal powder must be added.

In our experience the results obtained by this method are about 20 per cent. less than the actual content of the ore. The results of two a.s.says, after deducting for the silver in the litharge used, were 3.9 and 4.1 milligrams; and a third a.s.say, in which 5.4 milligrams of silver had been added, gave 9.2, which, after deducting the added silver, leaves 3.8 milligrams. The average of the three results is 3.9 milligrams from the 100 grams of ore.

Two lots of 100 grams of the same ore treated in the wet way gave 5.2 and 5.0 milligrams of silver. Burnt ores from Spanish pyrites carry about 0.005 per cent. of silver.

WET METHODS.

Silver is got into solution from its ores by attacking with nitric acid, but it is best, after dissolving, to cautiously add dilute hydrochloric acid, and to carefully avoid excess. If the quant.i.ty of silver is very small the solution is allowed to stand twenty-four hours, but, otherwise, it is warmed and filtered as soon as it clears. Dry the residue and concentrate the silver in a b.u.t.ton of lead by pot method or scorification, according to the amount of stony matter present. Cupel the lead, and the resulting b.u.t.ton will be free from all metals, except perhaps gold. It may be weighed; or dissolved in nitric acid, and the silver determined gravimetrically in the diluted and filtered solution.

It is better to weigh the metal and afterwards to determine the gold in it, estimating the silver by difference. Silver alloys are dissolved in dilute nitric acid (free from chlorides), diluted, and filtered. The solution is then ready for gravimetric determination.

Sulphuretted hydrogen precipitates silver (like copper), completely, even from fairly acid solutions.

GRAVIMETRIC DETERMINATION.

Add dilute hydrochloric acid in small excess to the hot dilute solution, which must contain free nitric acid. Heat and stir until the solution clears. Decant through a small filter, and wash with hot water, acidulated at first with a little nitric acid if bis.m.u.th is suspected to be present. Dry quickly, transfer as much as possible of the precipitate to a watch-gla.s.s; burn and ignite the filter paper, treating the ash first with two drops of nitric acid and then with one of hydrochloric, and again dry. Add the rest of the silver chloride and heat slowly over a Bunsen burner until it begins to fuse. Cool and weigh.

The precipitate is silver chloride (AgCl) and contains 75.27 per cent.

of silver. The moist precipitate is heavy and curdy; it is decomposed by direct sunlight, becoming violet under its influence. When heated it is yellowish; and, since it is volatile at a high temperature, it must not, in drying, be heated above its fusing point. The fused chloride can be removed from the crucible (to which it adheres strongly) by digesting with dilute acid and zinc.

For the determination of silver in nearly pure bullion the following process is used:--Weigh up 1.5054 gram of the alloy. With this amount of alloy each 2 milligrams of silver chloride formed is equivalent to 1 degree of fineness, so that the weight of the silver chloride obtained (stated in milligrams and divided by 2) will give the degree of fineness. Transfer to a bottle (known as "bottles for the Indian mint a.s.say") and dissolve in 10 c.c. of dilute nitric acid, then make up with water to 200 c.c. and add 3 c.c. of dilute hydrochloric acid. Allow to stand a few minutes and then shake. Fill the bottle completely with water, allow to settle, and syphon off the clear liquid; pour on more water, shake gently to break up the lumps, and again fill the bottle with water. Invert over the mouth of the bottle a porous Wedgwood crucible, somewhat similar to those used in gold parting. Take firm hold of the crucible and bottle, and invert promptly so that the silver chloride may be collected in the crucible. Allow to stand a little while for the precipitate to settle, and then carefully remove the crucible under water.[14] Drain off most of the water and break up the silver chloride with the help of a well-rounded gla.s.s rod. This greatly facilitates the subsequent drying. Dry first on the water bath and then on the iron plate. Remove the dried silver chloride, by inverting the crucible, and weigh it.

As an example, 3 determinations of silver in a coin carried out in this way gave:--

(1) 1.8500 gram AgCl = 925.0 fineness.

(2) 1.8498 " = 924.9 "

(3) 1.8502 " = 925.1 "

~Determination of Silver in Burnt Ores.~--Take 100 grams of the ore and place in a large beaker of 2-1/2 litres capacity, and cover with 375 c.c. of hydrochloric acid. Boil for half an hour until the oxides are dissolved and the residue looks like sand and pyrites; then add 20 c.c.

of nitric acid, and boil till free from nitrous fumes. Dilute to 2 litres with water, and pa.s.s a current of sulphuretted hydrogen till the iron is reduced, the copper and silver precipitated, and the liquor smells of the gas. This takes about one hour and a half.

Filter off the precipitate (rejecting the solution) and wash with warm water. Dry and transfer to an evaporating dish, adding the ashes of the filter paper. Heat gently with a Bunsen burner until the sulphur burns, and then calcine until no more sulphurous oxide comes off. When cold add 30 c.c. of nitric acid, boil and dilute to 100 c.c. Add 1 c.c. of very dilute hydrochloric acid (1 to 100),[15] stir well, and allow to stand overnight. Decant on to a Swedish filter paper, dry and calcine.

Mix the ashes with 100 grams of litharge and 1 gram of charcoal, and fuse in a small crucible. Detach the b.u.t.ton of lead and cupel. Weigh and make the usual corrections. As an example, 100 grams of ore treated in this way gave 5.8 milligrams of silver; deducting 0.8 for the silver added in the oxide of lead leaves 5 milligrams obtained from the ore.

Another experiment on 100 grams of the same ore to which 5 milligrams of silver had been added gave 11.0 milligrams. Deduct 5.8 for the silver added; this leaves 5.2 milligrams as the silver obtained from the ore.

These give, as a mean result, 0.0051 per cent., or 1.66 ounce per ton.

~Determination of Silver in Commercial Copper.~--For the method of doing this, with an example and experiment, see under the heading of _Examination of Commercial Copper_.

VOLUMETRIC METHODS.

There are two of these, one adapted for the determination of silver in alloys of approximately known composition, and the other of more general application. The first of these, generally known as "Gay-Lussac's"

method is, as regards its working, perfect in principle; but it requires a practically constant quant.i.ty of silver, that is, one which varies by a few milligrams only in each determination. It is a confirmatory method rather than a determinative one. The other is known as "Volhard's," and resembles in principle and method an ordinary volumetric process.

~Gay-Lussac's method~ is based on the precipitation of silver from a nitric acid solution by a solution of sodium chloride. The point at which the whole of the silver is precipitated being recognised by the standard solution ceasing to give a precipitate. The process depends for its success upon, (1) the ease which silver chloride separates out from the solution leaving it clear after shaking, and, (2), the cloudiness produced by the reaction of very small quant.i.ties of silver nitrate and sodium chloride. In working, a quant.i.ty of the sodium chloride solution equal to 1 gram of silver is added at once to the a.s.say; and, when the solution has been rendered clear by shaking, the residual silver (which should not exceed a few milligrams) is estimated with the help of a weaker solution of sodium chloride. The success in working evidently depends upon the accuracy with which the first addition of the salt solution is made. On this account the standard solution is run in from a special pipette capable of delivering a practically invariable volume of solution. It is not so important that this shall deliver exactly 100 c.c. as that in two consecutive deliveries the volume shall not differ by more than 0.05 c.c. The dilute salt solution is one-tenth of the strength of that first run in, and 1 c.c. of it is equivalent to 1 milligram of silver. Ordinarily it is run in 1 c.c. at a time (and an ordinary burette may be used for this purpose), shaking between each addition until it ceases to give a precipitate. If many such additions have to be made the operation not only becomes tedious, but the solution also ceases to clear after shaking, so that it becomes impossible to determine the finis.h.i.+ng point.

If the a.s.say contains less than one gram of silver the first addition of the dilute salt solution of course produces no precipitate. Five milligrams of silver in solution (5 c.c.) is then added, and the a.s.say proceeded with in the usual way; 5 milligrams of silver being deducted from the amount found.

There is required for the a.s.say a _standard solution of sodium chloride_, which is prepared by dissolving 5.4162 grams of the salt (made by neutralizing carbonate of soda with hydrochloric acid) in water and diluting to one litre. 100 c.c. of this is equivalent to 1 gram of silver.

The weaker solution of salt is made by diluting 100 c.c. of the stronger one to one litre. One c.c. of this will equal 1 milligram of silver, or 0.1 c.c. of the stronger solution.

A _standard solution of silver_ equivalent to the dilute salt solution is made by dissolving 1 gram of fine silver in 10 c.c. of dilute nitric acid, and diluting with water to one litre.

[Ill.u.s.tration: FIG. 44.]

The solution of salt is standardised as follows:--Weigh up 1.003 gram of fine silver and dissolve in 25 c.c. of dilute nitric acid in a bottle provided with a well-fitting flat-headed stopper. Heat on the water bath to a.s.sist solution, resting the bottle in an inclined position. When dissolved blow out the nitrous fumes with the help of a gla.s.s tube bent at right angles. Run in from a stoppered pipette (as shown in fig. 44) 100 c.c. of the standard salt solution, and shake vigorously until the solution clears. Fill an ordinary burette with the weaker standard salt solution, and run 1 c.c. into the a.s.say bottle, letting it run down the side so that it forms a layer resting on the a.s.say solution. If any silver remains in solution a cloudy layer will be formed at the junction where the two liquids meet. This is best observed against a black background If a cloudiness is seen, shake, to clear the liquid, and run in another c.c. of salt, and continue this until a cloudiness is no longer visible. Deduct 1.5 c.c. from the amount of the weaker sodium chloride solution run in. Divide the corrected reading by 10, and add to the 100 c.c. This will give the volume of strong salt solution equivalent to the silver taken.

If the first addition of the weaker salt solution causes no cloudiness add 5 c.c. of the silver solution from an ordinary pipette, shake, and then run in the weaker salt solution, working as before. These 5 milligrams of silver added must be allowed for before calculating. As an example:--1.0100 gram of fine silver was taken for standardising a solution and 4 c.c. of the weaker salt solution were run in. Deducting 1.5 and dividing by 10 gives 0.25 c.c. to be added to the 100 c.c.

100.25 : 1.0100 :: 100 : _x_ _x_ = 1.0075

which is the standard of the salt solution.

The method of working an a.s.say may be gathered from the following example:--In the determination of silver in some b.u.t.tons left after cupellation, it was a.s.sumed that these would contain 99.5 per cent. of silver. For the a.s.say it was necessary to take a quant.i.ty that should contain a little more than 1.0075 grams of silver; then

99.5 : 100 :: 1.0075 : _x_ _x_ = 1.0125

To ensure a slight excess, there was taken 1.0150 gram of the b.u.t.tons, which was treated in exactly the same way as for the standardising. The quant.i.ty of the weaker salt solution required was 7 c.c.; deducting 1.5 c.c., and dividing by 10, gives 100.55 c.c. of strong salt solution, which is equivalent to 1.0130 gram of silver. This being obtained from 1.015 gram of alloy, is equal to 99.8 per cent., or 998.0 fine.

~The Effect of Temperature.~--The standardising and the a.s.say must be done at the same time, since a difference of 5 C. makes a difference of 0.1 c.c. in measuring the 100 c.c. of strong solution of salt. It is always best to prepare a standard with each batch of a.s.says.

~SULPHOCYANATE METHOD.~--Volhard's process is based upon the precipitation of silver in nitric acid solutions with pota.s.sium sulphocyanate, the finis.h.i.+ng point being the development of a reddish-brown colour, produced by the action of the excess of sulphocyanate upon ferric sulphate. The white sulphocyanate settles readily, leaving the liquor clear; and a persistent brown coloration in the liquid indicates the finish. The a.s.say must be carried out in the cold; and water free from chlorides[16] must be used.

_The standard sulphocyanate of pota.s.sium_ solution is made by dissolving 4-1/2 or 5 grams of the salt (KCyS) in water, and diluting to 1 litre.

100 c.c. are about equivalent to 0.5 gram of silver.

_The standard silver nitrate solution_ is made by dissolving 5 grams of fine silver in 50 c.c. of dilute nitric acid, boiling off nitrous fumes, and diluting to 1 litre.

The _indicator_ is a saturated solution of iron alum, or a solution of ferric sulphate of equivalent strength made by t.i.trating acid ferrous sulphate with pota.s.sium permanganate. Use 2 c.c. for each a.s.say.