157 



The Estimation or Lead by the Titration of Lead Chro- 



mate. 



By W. C. Brooks. 



The first experiments were made to test the accuracy of the method. 

 Weighed amounts of pure lead nitrate were dissolved in water and the lead 

 was precipitated by potassium dichromate in slight excess. The lead chro- 

 mate was filtered, was washed and was dissolved in hydrochloric acid. 

 The resulting chromic acid was determined by adding potassium iodide 

 and then by titrating the free iodine with sodium thiosulphate. The re- 

 sults of several titrations are given below : 



Pb(N0 3 ) 2 



0.2 Na 2 S 2 3 



Lead Found. 





sed. Grams. 



Used. cc. 



Per cent. 



Error. 



0.1 



18.15 



62.56 



+0.05 



0.1 



18.16 



62.56 



+0.05 



0.2 



36.35 



62.60 



+0.09 



0.2 



36.3 



62.55 



+0.04 



A method for the estimation of lead in ores was worked out from 

 these results. Weigh out 0.2 gms. of the ore, dissolve in 10 to 15 cc. of 

 nitric acid and evaporate almost to dryness. Add 50 cc. of water and 5 

 gms. of ammonium acetate and heat to boiling. Precipitate the lead from 

 the boiling solution with a slight excess of potassium dichromate. Filter, 

 wash and dissolve the precipitate with 20 cc. of 1 :1 hydrochloric acid, the 

 solution being received into the original beaker. Dilute the solution and 

 washings to 150 cc, add 0.3 to 0.5 gms. of potassium iodide and titrate the 

 iodine that is liberated with sodium thiosulphate. 



The only element that has been found to interfere with this method is 

 barium, which gives high results. Experiments are being carried on to 

 obtain a simple means of removing this metal. Although the work upon 

 this method is not complete, it promises to become an accurate and a rapid 

 means for the estimation of lead in the presence of all of the common 

 elements. 



