white lead may be assayed for colour and covering power by the usal methods (see Chapter X.).,Dry white lead. — The purity of this pigment is ascertained by dissolving some of the lead in pure dilute nitric acid (1 acid, 2 water) ; strong nitric acid does not dissolve white lead owing to the insolubility of the lead nitrate which is formed in the acid; the ordinary commercial nitric acid contains sulphuric acid, which would lead to the formation of the insoluble sulphate of lead, the production of which might lead to the condemnation of a pure sample. ,On adding dilute sulphuric acid to the solution, after diluting it with water and filtering off the precipitate of lead sulphate thus obtained, no further precipitate should be obtained on successively adding ammonia, ammonium sulphide, and ammonium oxalate to the filtrate. ,A white precipitate with ammonium sulphide would indicate the presence of zinc white, which is a rare thing to find with white lead ; a white precipitate with ammonium oxalate would indicate the presence of whiting. ,The insoluble residue, if any, will consist most probably of barytes, as other adulterants (for reasons already pointed out) are rarely used ; still any lead sulphate, china clay, gypsum, or strontium sulphate which may be used would also be left as an insoluble residue on treating white lead with dilute nitric acid. ,To distinguish these bodies, boil the residue in hydrochloric acid and place the solution on one side to cool ; if crystals of lead chloride separate out and the solution gives a white precipitate with barium chloride, then lead sulphate is present. ,The hydrochloric acid solution should be diluted with water and sulphuretted hydrogen passed through it ; the black precipitate of lead sulphide which may be obtained can be disregarded : this is filtered off and the filtrate boiled for some time to concentrate it and to drive off the sulphuretted hygrogen it contains. Then ammonia is added, when a white precipitate of alumina may be obtained indicating the presence of china clay ; this is filtered off, and to the filtrate is added ammonium carbonate, which will precipitate any calcium that may have been added, in the form of gypsum or whiting. ,A little of the insoluble residue from the hydrochloric acid should be held on a piece of platinum wire in the lower part of a Bunsen flame when, if it contains barytes, the flame will be coloured green ; if strontiam sulphate is present a crimson flame will be obtained. This test is not always easy to carry out, but ,with a little care the coloured flames can be obtained, and they are good proof of the presence of the pigments named. ,A quantitative analysis of white lead may be made as follows : — ,Weigh out 2 grammes and dissolve them in a beaker with the smallest possible quantity of pure dilute nitric acid, remove the insoluble matter by filtering, wash the residue well with warm water, adding the first wash waters to the filtrate, then dry the residue, place the filter paper and its contents in a weighed crucible and burn the paper ; when completely burnt allow the crucible to cool in a desiccator and then weigh it. From the weight so obtained deduct the weight of the crucible and of the filter-paper ash, the diflerence is the weight of the insoluble residue. ,To the filtrate add dilute sulphuric acid and a little alcohol, filter off the precipitate of lead sulphate which is obtained, wash it dry and burn it in a crucible as before. By multiplying the weight of the lead sulphate so obtained by 0.73554 the weight of lead oxide in the white lead can be found. ,The carbonic acid can be ascertained by treating 2 grammes of the white lead with nitric acid in a Schrotter's or other form of apparatus for the estimation of carbonic acid. ,The water may be determined by taking the difference between the amounts of lead oxide and carbonic acid thus found and 100. Hygroscopic water can be ascertained by heating 2 grammes in an oven at 110o to 120o C. until no further loss of weight occurs. ,If the white lead be adulterated with barytes, lead sulphate, china clay or some of the other insoluble white pigments, these will be left behind as an insoluble residue on treatment with nitric acid ; their amount is ascertained by filtering off, washing, drying, and burning the residue in a weighed crucible in the usual way. Soluble adulterants like whiting, strontium carbonate, barium carbonate, and magnesite will be dissolved ; if the presence of these is suspected, to the filtrate should be added more dilute sulphuric acid, which will precipitate the lead and barium ; this precipitate can be filtered off, the two can be separated by boiling with hydrochloric acid, which dissolves the lead sulphate but not the barium sulphate. To the filtrate ammonia and ammonium oxalate are added ; this precipitates the calcium and the strontium, while the magnesite (if present) will remain in solution, and can be precipitated by sodium phosphate. It is not ,neceasary to describe in detail the methods of separating these adulterants any furtherl some notes bearing on this point will be found in the descriptions of each individual pigment, while reference should be made to works on quantitative chemical analysis, such as that of Prof. Sexton, for fuller details. ,By multiplying the weight of the carbonic acid by 5.05 the amount of lead oxide with which it is combined can be calculated; the two amounts added together give the quantity of lead carbonate in the white lead. Deducting the amount of lead oxide combined with the carbonic oxide from the total present, and ,multiplying this difference by 0.077, gives the amount of water combined with it to form lead hydroxide, and the two amounts added together gives the amount of the latter body present. ,,Paste White Lead can be quantitatively examined as follows : — Two grammes are treated with strong nitric acid at a gentle heat; this converts all the oil into an insoluble greasy matter, of which no account need be taken ; the process then becomes identical with that for dry white lead. Should it be desired to ascertain the amount of the oil, then 10 grammes must be weighed into a filter paper and placed in a Soxhlet or other form of fat-extractor and the oil extracted by means of petroleum ether ; the ethereal solution is then run into a weighed glass, the ether evaporated off and the oil weighed. If a fat-extractor is not available it will suffice to agitate the white lead with some petroleum ether in a beaker, allowing the pigment to settle, pouring off the liquid into a weighed glass, again pouring on more ether, and again allowing the white lead to settle ; the ether is poured off into the glass, or the mass may be filtered. Finally, the ether is evaporated off as before. ,It is sometimes recommended to burn off the oil from the white lead in a crucible, but this course is not so satisfactory as treating it with petroleum ether, as not only is the oil burnt off, but the white lead is decomposed ; whereas in the ether method the lead is left in its original form for further examination if necessary.