Apparatus, Reagents.—The usual apparatus. For the standard solution, ammonium molybdate is required [(NH4)2MoO4], or by a simple calculation the heptamolybdate (NH4)6Mo7O24,4H2O which is non-deliquescent, or the trioxide MoO3 dissolved in NH4HO may be used. For an indicator a solution of 1 gm. tannin in 500 c.cs. water is used. For analysis the student may take a siliceous galena ore, containing roughly about 50% galena and 50% quartz.
Method, Reactions.—When (NH4)2MoO4 is added to a hot solution of Pb(C2H3O2)2 the following reaction takes place,
Pb(C2H3O2)2 + (NH4)2MoO4 = PbMoO4 + 2NH4C2H3O2
To determine the end point of this precipitation, an external indicator, ‘ tannin,’ is used; a drop on a white porcelain plate giving a yellow colour with any excess of the molybdate of ammonia. Knowing the strength and volume of the standard solution used, the quantity of lead is easily calculated. The ore is broken up with HNO3 and H2SO4, and the lead obtained as the sulphate, which is dissolved in hot NH4C2H3O2.
Preparation of the Standard Solution.—An N/20 solution will be found most convenient. Dissolve 9.8 gms. (NH4)2MoO4 (or equivalents of (NH4)6.Mo7O24,4H2O or MoO3 in NH4HO; in distilled water and make up to one litre at 16° C. If the solution is not clear a few drops of strong ammonia will generally clarify it.
1 c.c. N/20 (NH4)2MoO4 = about .01 gm. lead
Checking the Standard.—Weigh out two portions, each of .2 gm. pure lead sulphate. Transfer to 300 c.c. beakers. Dissolve in a little hot 5E. NH4C2H3O2, adding a little at a time, and heating till solution is complete. Acidify with acetic acid. Dilute each lot to 200 c.cs. with hot water. Titrate with the molybdate solution, testing a drop every now and then with a drop of the tannin solution on a porcelain plate. Immediately the molybdate is in excess a yellow colour is obtained. Note the number of c.cs. used.
Assume that 13.62 c.cs. are required for .2016 gm. PbSO4. But .2016 gms. PbSO4 contain .2016 x 207/303 = .1377 gm. Pb.
Therefore 1 c.c. N/20 (NH4)2MoO4 = .1377/13.62 = .01011 gm. Pb
Duplicates should agree within .00005 gm. Pb.
The Analysis.—Weigh out .5 gm. of the sampled and finely powdered galena. Transfer to a casserole or a porcelain dish with an inverted funnel placed inside. Cautiously add 15 c.cs. 16E. HNO3 and 10 c.cs. 36E. H2SO4. Heat just to boiling, and continue at this temperature till white fumes of SO3 appear. The excess of HNO3 is then expelled. Remove from the heat and wash down the lid or cover when cool. Add 50 c.cs. cold distilled water. Stir well with a glass rod, breaking up any residue. Gently boil to dissolve all soluble sulphates.
Let settle, and pour off most of the liquid through a filter paper. Wash by decantation twice with hot E. H2SO4 and once with hot water. To the white residue in the beaker add a little hot NH4C2H3O2 and heat a few minutes. Decant through the filter, having placed a clean 300 c.c. beaker under the funnel. Repeat the treatment till the PbSO4 is all dissolved. Test by removing a drop of the solution from the beaker and bringing it into contact with a drop of NH4HS on a porcelain plate. Wash out the beaker with hot water and wash the filter till the lead is all through.
Acidify the filtrate with acetic acid and dilute to 200 c.cs. Titrate as before with the (NH4)2MoO4. Note the number of c.cs. used and calculate the percentage of lead.
Repeat the estimation on another sample of the ore. The duplicate should agree within .1%.
Note.—This method (see the Engineering and Mining Journal, vol. lv., No. 13, 1893) is well suited for the estimation of lead in ores and mattes, being both accurate and quick. With some practice a complete estimation can be made in about half an hour.