182 Mr. Brown on the Analysis of Molybdate of Lea</. 



dryness; when perfectly dry, a small quantity of nitric acid was added, 

 and the solution was again dried. The residue was then heated to redness 

 in a close vossel, and weighed ; its weight was found to be 40*5 grains. 

 100 grains contain therefore — 



Oxide of lead, 59*0 58*0") corrected from 



Molybdic acid, 40*5 40'5j the chloride. 



99-5 98-5 



This method is essentially the same as that used by Klaproth. The 

 result, however, is much nearer the truth; but Gobel gets too much 

 molybdic acid, and too little oxide of lead; this was probably owing 

 to some of the chloride of lead not being obtained, as it is soluble 

 to a great extent in water, (1 in 152 of water,) * and the analyst does not 

 state how he washed the chloride of lead free from molybdic acid. 



TV. The methods hitherto employed being liable to very great objec- 

 tions, the molybdate of lead was analysed by another method, which had 

 proved successful in the hands of Mr. William Parry last year, in the 

 College Laboratory. 



26*84 grains of the mineral, finely pounded, were boiled for a con- 

 siderable time with nitric acid, and filtered. The undecomposed mineral, 

 along with a quantity of molybdic acid, remained on the filter. This was 

 then completely washed; ammonia was then poured into the filter. The 

 molybdic acid was thus dissolved, and the insoluble matter remained on 

 the filter. This was then washed, dried, ignited, and weighed. The 

 weight of the insoluble matter in 26*84 grains was 1*15 grains. 



The solution containing the molybdate of ammonia was then evaporated 

 to dryness, and heated to redness in a close vessel. The greater part of 

 the molybdic acid was thus obtained. Its weight was 6*76 grains. 



The first washings from the molybdic acid and insoluble matter were 

 then concentrated. Caustic ammonia was added, in order to neutralise 

 the excess of acid, and afterwards sulphohydret of ammonia was added 

 in excess. In this manner the lead was precipitated in the form of 

 sulphuret, while the tersulphuret of molybdenum was re-dissolved by the 

 excess, giving the solution a deep red colour. The sulphuret of lead was 

 then thrown on a filter and washed with water containing sulphohydret of 

 ammonia. When completely washed, the sulphuret of lead was dissolved 

 in muriatic acid, and after boiling for some time was filtered to get rid of 

 the sulphur. The filtered liquor was then concentrated, and the lead 

 precipitated by means of oxalate of ammonia ; the precipitated oxalate of 

 lead was then thrown on a filter, washed, and dried. By ignition the 

 oxalate of lead was converted into the oxide; the quantity of which in 

 26*84 grains was thus found to be 16*20 grains, which is equivalent to 

 60*35 per cent, of oxide of lead. 



* In two experiments 3963 grains of water at 60°, dissolved 26*2 grains PbCl,= 1 in 151 

 grains; and 4260 grains of water dissolved 27*6 grains PbCl,= 1 in 154 grains of water. 



