Some assayers check their fire-assays of lead by a wet method. For the convenience of those who desire a rapid method to check their fire-assays, the ammonium molybdate method is here given.
This method is based upon the fact that ammonium molybdate, when added to a hot solution of lead acetate, will give a precipitate of molybdate of lead (PbMoO4), which is insoluble in acetic acid. Any excess of ammonium molybdate will give a yellow color with freshly prepared solution of tannin.
Indicator.—A freshly prepared solution of 1 part tannin in 300 parts water.
Standard Solution.—The standard solution of ammonium molybdate is prepared by dissolving 9 grams of ammonium molybdate in 1000 c.c. of water. One c.c. of this solution will equal about 0.01 gram lead. If the solution is not clear, it can be made so by adding a few drops of ammonium hydrate.
Standardizing.—Weigh out 0.300 gram of pure sulphate of lead, and dissolve it in hot ammonium acetate; then acidify with acetic acid, and dilute with hot water to 250 c.c. Heat to boiling, and add from a burette the molybdate solution, prepared as above mentioned, until all the lead is precipitated as a white precipitate. This is ascertained by placing drops of tannin solution upon a porcelain plate, and then to these drops is added a drop of the solution tested, after each addition of ammonium molybdate. As long as the lead is in excess, no coloration is produced ; but as soon as the molybdate is in excess, a yellow color is produced with the tannin (0.300 gram PbSO4 x 0.68317 = 0.20495 gram Pb). The solution in the beaker should be stirred after each addition of molybdate solution before the drop-test is made. From the number of molybdate solution used, the value of one c.c. is calculated in the usual way.
Assay.—To determine the lead in ore or other material, weigh out 0.5 or 1.0 gram of the substance, according to the percentage of lead. If the substance contains 30% or more lead, 0.5 gram will be sufficient. Treat the sample weighed out in a porcelain casserole with 15 c.c. strong nitric acid and 10 c.c. strong sulphuric acid. Heat until all the nitric acid is expelled, which is indicated by fumes of sulphuric anhydride coming off; then allow it to cool, and dilute with cold water; stir, then boil until all soluble sulphates are brought into solution. How filter, leaving as much of the precipitate in the casserole as possible. Now wash twice with hot dilute sulphuric acid and once with cold water. The sulphate of lead remaining in the casserole is next dissolved with hot ammonium acetate; pour the hot solution on the filter and allow it to run into a clean beaker. This operation is repeated until all the sulphate of lead is dissolved. Wash out the casserole thoroughly with hot water into the filter. Acidify the solution with acetic acid, dilute up to 250 c.c. with hot water. Now heat to boiling and run in from a graduated burette the standardized solution of ammonium molybdate until all the lead is precipitated, stirring the solution after each addition of molybdate, and testing a drop of the solution, after each addition of molybdate, on a porcelain plate with the tannin solution. From the number of c.c. of the molybdate solution used, calculate the per cent of lead.
The lead determination can easily be made in 30 minutes.
Arsenic, antimony and phosphorus do not interfere with this method, as they pass through the filter in solution.