180 Mr. Brown on the Analysis of Molybdate of Lead. 



XXIX. — On the Analysis of Molybdate of Lead. By Mr. John Brown. 



Molybdate of Lead was first analysed by Klaproth, who proceeded in 

 the following manner: — * 



100 grains of the mineral, finely pounded, were treated with dilute 

 hydrochloric acid, and the whole of the silica was thus separated. Upon 

 cooling, the greater part of the chloride of lead was deposited in fine 

 crj-stals. The clear supernatant liquor was then drawn off, and when 

 sufficiently concentrated, the remaining chloride of lead was deposited. 

 The whole of the chloride was then carefully collected together, dried, 

 and weighed. Its weight was 74*5 grains. From this, the quantity of 

 oxide of lead was ascertained, which was 64*42 grains. Every 100 grains 

 of molybdate of load contain, therefore, 64*42 grains of oxide of lead. 

 When the solution had thus been freed from lead, it was concentrated by 

 evaporation. Nitric acid was then added to the solution, which imme- 

 diately became of a fine blue colour; when sufficiently concentrated, a 

 quantity of molybdic acid separated. The solution was then evaporated 

 to dryness, and the molybdic acid remained in the form of a fine citron- 

 yellow powder, which when completely dried weighed 34*25 grains. 



The constituents, therefore, of 100 parts of the purest crystals of 

 Carinthian molybdate of lead, are, according to Klaproth : — 



Oxide of lead, 64*42 59*59) corrected from 



Molybdic acid, 34*25 34*25) the chloride. 



As Klaproth did not know the true composition of chloride of lead, the 

 quantity of oxide of lead given above is wrong. Calculating the quantity 

 of oxide from the quantity of chloride which he obtained, we get 59*59 

 per cent, of oxide of lead, which is near the theoretical quantity, or 

 60*87. But the great error is in the molybdic acid. What Klaproth 

 considered as silica, was very probably molybdic acid, as that acid is not 

 entirely soluble in hydrochloric acid, and as he apparently deducted this 

 as impurity, he gets too little molybdic acid. He also does not mention 

 how he washed out the molybdic acid from the chloride of lead. It could 

 not well have been done with water, for chloride of lead is soluble to a 

 great extent. This is a point of imperfection in the analysis. 



II. This mineral was next subjected to a close examination by Charles 

 Hatchett, Esq. whose analysis is recorded in the Philosophical Transac- 

 tions (vol. xviii. abridgment), from which the following is an extract : — 



250 grains of the ore, freed from as much impurity as possible, were 

 put into a glass flask and digested for some time under a strong heat with 

 dilute sulphuric acid. When the solution cooled, the clear liquor was 

 drawn off, and the residual sulphate of lead washed by subsidence. This 

 process was repeated several times. The acid solutions were then filtered, 



* Beitrage zur chemischen Kentniss der Mineral Ktfrper. I. 265. 



