A Text-book of Assaying: For the Use of Those Connected with Mines Part 31

WET a.s.sAY.

In the case of galena the best method of getting the lead into solution is to treat with hydrochloric acid and zinc. Put 1 gram of the ore in an evaporating dish 4 inches across, and cover with 10 c.c. of dilute hydrochloric acid. Heat till the evolution of sulphuretted hydrogen becomes sluggish, and then drop in a piece of zinc rod. If the solution effervesces too strongly, dilute it. Continue the heating until the sulphide is seen to be all dissolved; when the lead is all precipitated, pour off the liquid and wash twice with cold water. Peel off the precipitated lead with the help of a gla.s.s rod, and then clean the zinc.

Cover the lead with 20 c.c. of water and 5 c.c. of dilute nitric acid, and heat gently till dissolved; all the lead will be in solution, and, when filtered off from the gangue, will be ready for a gravimetric determination. For volumetric work this filtering is unnecessary.

The chief objection to this method is that commercial zinc carries considerable quant.i.ties of lead. Although this can be determined and allowed for, the correction required is in most cases too large to be satisfactory. The following method is applicable in all cases, but is more troublesome:--Treat 1 gram of the ore with 10 c.c. of dilute nitric acid in an evaporating dish covered with a clock-gla.s.s, and evaporate till nearly dry. Take up with 50 c.c. of water, and add 10 c.c. of dilute sulphuric acid. Filter. The residue contains the lead as sulphate, together with the insoluble matter of the ore and globules of sulphur. Warm with a solution of ammonium acetate, and filter. The lead will be in the filtrate, and is recovered in a state fit for direct gravimetric estimation by the addition of dilute sulphuric acid. If the volumetric method is to be used, the lead sulphate should be dissolved out with a solution of sodium acetate instead of with the ammonium salt solution.

GRAVIMETRIC DETERMINATION.

The lead is separated and precipitated as sulphate, as already described. The solution must be allowed to stand, and the clear liquid be decanted through a filter. Transfer the precipitate, and wash with very dilute sulphuric acid (1 or 2 c.c. in 100 c.c. of water). The acid must be completely removed with one or two washes with cold water, and then with alcohol. The volume of liquid required for was.h.i.+ng is small, as the precipitate is dense and easily cleaned; but the was.h.i.+ng must be carefully done, since if any acid remains it will, on drying, char the paper, and render the subsequent work troublesome. Dry, transfer to a watch-gla.s.s, and burn the filter paper, collecting its ash in a weighed porcelain crucible. The filter paper must be freed as much as possible from the lead sulphate before burning, and the ash treated with a drop or two of nitric and sulphuric acids. Transfer the lead sulphate to the crucible; ignite gently, keeping the temperature below redness; cool, and weigh. The precipitate will contain 73.6 per cent. of lead oxide or 68.3 per cent. of lead.

~Determination of Lead in Commercial Zinc.~--Take 10 grams of zinc, and treat (without heating) with 60 c.c. of dilute hydrochloric acid. When the zinc is nearly all dissolved, decant off the clear liquid, and dissolve the residue in 2 c.c. of dilute nitric acid. Evaporate till most of the acid is removed; dilute to 20 or 30 c.c. with water, and add 10 c.c. of dilute sulphuric acid. Filter off, and weigh the lead sulphate. Ten grams treated in this way gave--0.1610 gram of lead sulphate, equivalent to 1.10 per cent. of lead.

VOLUMETRIC METHOD.

This is based upon the reaction between chromate of potash and soluble lead salts in neutral solutions, whereby an insoluble yellow chromate of lead is produced.[60] An excess of the chromate is required to complete the reaction, so that the point at which an indicator shows the presence of undecomposed chromate cannot be satisfactorily taken as the finish.

Therefore an excess of the standard chromate must be run in, and such excess determined.

Chromate of lead is not precipitated from strong nitric acid solutions, and only incompletely from dilute ones. Acids generally are detrimental to the precipitation, and must be neutralised before t.i.trating. If the lead is present as sulphate in sodic acetate solution, it is well to render it distinctly alkaline with ammonia.

Lead chromate precipitated in the cold is a lemon-yellow, light precipitate, very difficult to filter: on heating to 40 C. the colour becomes orange; at 60 C. it a.s.sumes a deeper hue, and becomes flocculent; and at a boiling temperature it still further darkens and settles readily. These changes in colour are not due to any chemical change, as will be seen by testing the filtrate for chromium or lead: this is an advantage to the a.s.say, since it is only at the higher temperature that the precipitate can be easily filtered. The lead is not completely precipitated, but the amount remaining in solution is only 2 or 3 milligrams, which is just sufficient to give a dark coloration with sulphuretted hydrogen.

_The standard chromate of potash solution_ is made by dissolving 7.13 grams of b.i.+.c.hromate of potash and 2.0 grams of caustic soda in water, and diluting to 1 litre; or 9.40 grams of the neutral chromate (K_{2}CrO_{4}) may be dissolved and diluted to 1 litre: 100 c.c. will be equivalent to 1.000 gram of lead.

_Standard Lead Solution._--16 grams of nitrate of lead (Pb(NO_{3})_{2}) are dissolved in water and diluted to 1 litre; 100 c.c. will contain 1.000 gram of lead.

_Acetate of Soda Solution._--250 grams of the crystallised salt (NaAc.3H_{2}O) are dissolved, and diluted to 1 litre. Use 40 c.c. for each a.s.say.

In the t.i.tration the a.s.say solution should measure 150 to 200 c.c., and should be boiling or nearly so. It is best contained in a pint flask, and the standard chromate solution used with an ordinary burette. Run in the chromate solution in a steady stream until the whole of the lead has been precipitated. The amount required for this may be calculated: for example, 1 gram of an 80 per cent. ore would require 80 c.c. A little of the a.s.say may be filtered off, and if it does not show a yellow colour in the filtrate run in 2 c.c. more of the standard solution and continue this addition till a colour is shown. After this run in another c.c. to ensure an excess, dilute to 250 c.c., and heat to boiling; allow to settle for three or four minutes, filter off 50 c.c. into a Nessler gla.s.s, and determine the excess of chromate colorimetrically. The excess found in the 50 c.c. must, of course, be multiplied by five, and then be deducted from the quant.i.ty of chromate originally run into the a.s.say solution. The quant.i.ty to be deducted should not exceed 3 c.c. Where a number of determinations are made the colorimetric estimation is facilitated by using a series of standard phials similar to those described under the _Electrolytic Copper a.s.say_. The determination is rendered sharper and less liable to error by the addition of a few drops of acetic acid to convert the chromate into b.i.+.c.hromate. The same chromate solution must be used in this determination as was used in the precipitation.

In standardising the chromate solution, the standard lead nitrate solution is used. A quant.i.ty containing about as much lead as the a.s.say is supposed to contain is measured off, rendered alkaline with dilute ammonia, and then neutralised with acetic acid, using a small piece of litmus paper dropped into the solution as indicator. Then dilute, boil, and t.i.trate. When the lead in the a.s.say has been separated as sulphate and dissolved in sodic acetate, less chromate is apparently required, and in this case it will be necessary to precipitate the lead in the standard with an equivalent of sodic sulphate and redissolve in sodic acetate just as in the a.s.say. In these solutions (although there is considerable chromate in excess) a further addition of 5 or 6 c.c. of the chromate solution will cause a further precipitate. The following experiments show the effect of variation in the conditions of the a.s.say:--

~Effect of Varying Temperature.~--Twenty c.c. of lead nitrate solution and 10 grams of sodium acetate were used; diluted to 200 c.c., heated to the desired temperature, and t.i.trated. The results were:--

Temperature 15 30 50 100 "Chromate" required 19.8 c.c. 19.5 c.c. 19.3 c.c. 19.2 c.c.

The first two of these filtered badly, the precipitate coming through the filter; the last was very satisfactory in the working.

~Effect of Varying Bulk.~--Using 20 c.c. of lead nitrate, and 10 grams of sodium acetate as before, diluting to the required bulk, heating to boiling, and t.i.trating, the results were:--

Bulk 100.0 c.c. 200.0 c.c. 500.0 c.c. 1000.0 c.c.

"Chromate" required 19.6 " 19.3 " 19.4 " 19.4 "

~Effect of Varying Acetic Acid.~--Since the experiments are carried out in the presence of sodic acetate, acetic acid is the only acid whose effect need be considered. Working as before, but with 200 c.c. bulk and varying amounts of the acid, the results were:--

Acid present -- 10.0 c.c. 20.0 c.c. 40.0 c.c.

"Chromate" required 19.7 c.c. 19.1 " 18.5 " 17.3 "

These experiments show that only slight quant.i.ties of acid are admissible.

~Effect of Varying Sodium Acetate.~--With the same conditions as before, but with varying weights of sodium acetate, the results were:--

Sodium acetate present -- 5 grams 10 grams 25 grams 50 grams "Chromate" required 19.7 c.c. 19.6 c.c. 19.6 c.c. 18.8 c.c. 17.8 c.c.

These experiments show that excessive quant.i.ties of sodium acetate must be avoided. Ammonium acetate interferes to a greater extent, and if both acetic acid and this salt are present, each exerts its disturbing influence. With 10 grams of ammonium acetate, 19.4 c.c. of the chromate solution were required instead of 19.7 c.c. in the absence of this salt; with 10 grams of the acetate and 10 c.c. of acetic acid, only 18.6 c.c.

were required.

~Effect of Foreign Salts.~--As already stated, sulphates interfere.

Twenty c.c. of the lead nitrate solution were taken, precipitated with sulphate of soda, and the precipitate dissolved in 10 grams of sodium acetate and t.i.trated as before. Duplicate experiments required 18.6 c.c.

and 18.7 c.c. of the chromate solution. A similar experiment with 40 c.c. of lead nitrate required 37.4 c.c. of chromate. If the sulphate had not been present, the results would have been 19.7 c.c. and 39.4 c.c.

respectively.

~Effect of Varying Lead.~--In these experiments the conditions were as before, but with varying amounts of lead.

Lead nitrate solution present 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

Chromate solution required. 9.4 " 19.7 " 48.8 " 98.2 "

~Determination of Lead in Galena.~--Weigh up 1 gram of the powdered and dried ore, and boil in an evaporating dish with 10 c.c. of dilute hydrochloric acid. When the action becomes sluggish, dilute with an equal bulk of water, and add a weighed piece of zinc rod about 1 inch long and quarter-inch across. Keep up a moderate action by warming till the ore is seen to be completely attacked and the lead precipitated.

Decant off the solution, wash once, strip off the lead, wash and weigh the remaining zinc. Dissolve the lead in 5 c.c. of dilute nitric acid, and 5 c.c. of water with the aid of heat. Dilute and transfer to a pint flask; add a slight excess of dilute ammonia, and render faintly acid with acetic acid. Dilute to 150 c.c., heat to boiling, and run in the standard chromate in slight excess, noting the amount required, and make up to 250 c.c. with water. Boil the solution, allow to settle for a minute or so, filter off 50 c.c., and determine the excess of chromate colorimetrically. As an example, 1 gram of an impure galena was precipitated with 75 c.c. of standard chromate (100 c.c. = 1.020 gram lead). The excess found in 50 c.c. was 0.3 c.c., which, multiplied by 5, gives 1.5 c.c. as the excess in the whole solution. The remaining 73.5 c.c. of "chromate" required by the a.s.say, are equivalent to 0.7497 gram of lead. The zinc used up weighed 1.5 grams, and contained 0.0165 gram of lead. Thus we get--

Lead in the a.s.say 0.7497 gram Lead from the zinc 0.0165 "

------ .'. Lead in the galena 0.7332 "

Equivalent to 73.3 per cent.

Another sample, in which the galena was accompanied with a large quant.i.ty of pyrites, gave the following results:--Three grams were treated with 30 c.c. of dilute hydrochloric acid and a rod of zinc. The zinc and lead were carefully transferred to another vessel, the zinc cleaned, and the lead (dissolved in 5 c.c. of dilute nitric acid and 20 c.c. of water) treated as before.

14.5 c.c. of the chromate were required = 0.1479 gram lead Lead in 2 grams of zinc = 0.0220 "

------ .'. Lead in 3 grams of the ore = 0.1259 "

Equivalent to 4.20 per cent.

The same ore gave by separation of the lead with sulphuretted hydrogen, and conversion into sulphate, 4.16 per cent.

With fairly pure ores, free from sulphate, the a.s.say may be made more quickly as follows: Dissolve 1 gram of the finely powdered ore by boiling gently with 40 c.c. of dilute hydrochloric acid for 15 minutes; cool; add a few drops of permanganate; neutralise with ammonia, add acetic acid and a little sodium acetate. t.i.trate with standard chromate.

COLORIMETRIC PROCESS.

This is based upon the brown coloration produced in very dilute solutions of lead by the action of a solution of sulphuretted hydrogen.

The quant.i.ty of lead in the 50 c.c. of the a.s.say solution must not much exceed 0.5 milligram, nor be less than 0.01. The sulphuretted hydrogen is used in the form of a solution, and is not bubbled through the a.s.say.

The principle of working is the same as previously described.

_Standard Lead Solution._--Each c.c. of this should contain 0.1 milligram of lead. It is made by diluting 10 c.c. of the solution of lead nitrate, described under the volumetric process, to 1 litre.