XI, A, 4 Witt: Analysis of Babbitt Metal 173 



Chemically pure metallic lead or lead sulphate should be used 

 for standardizing the ammonium molybdate solution. Weigh 

 the lead with the sample and cari-y through in parallel. Place 

 1 gram of the alloy in a 250 cubic centimeter beaker and add 30 

 cubic centimeters of 1.20 specific gravity nitric acid. After 

 the acid has boiled down to about 10 or 15 cubic centimeters, 

 dilute with 30 to 40 cubic centimeters of water and boil the mix- 

 ture for ten minutes. After allowing the precipitate to settle, 

 decant the liquid through a filter paper to which has been added 

 a little paper pulp and wash the filter two or three times with 1 

 per cent hot nitric acid. Treat the precipitate which remains in 

 the beaker a second time with acid and water and filter as 

 before.^ After transferring the precipitate to the filter, wash 

 it eight or ten times with dilute nitric acid, taking care to keep 

 the volume of the filtrate and washings down to about 150 to 

 200 cubic centimeters. Evaporate very carefully to 20 or 30 

 cubic centimeters and cool. Add excess sulphuric acid and heat 

 the mixture until the water has been expelled and white fumes 

 appear. Dilute with 50 cubic centimeters of water and allow 

 the lead sulphate precipitate to settle. Filter out the precipi- 

 tate and wash thoroughly with very dilute sulphuric acid. Place 

 the filter paper containing the precipitate in the original beaker 

 and dissolve in 75 cubic centimeters of hot ammonium acetate 

 solution and an equal volume of hot water. When the solution 

 is complete, titrate with ammonium molybdate solution, using 

 tannic acid as an outside indicator. It is very important 

 that the lead solution should be almost boiling at the beginning 

 of the titration to insure complete precipitation of the lead; 

 otherwise it is easy to get a false end point and report wrong 

 results. The standard is titrated in exactly the same way, and 

 a blank is run on the ammonium acetate solution. To deter- 

 mine the value of the ammonium molybdate solution in terms 

 of lead, the weight of the lead in grams, in the standard, is 

 divided by the number of cubic centimeters of ammonium 

 acetate solution used in the titration of the standard, minus the 

 blank titration. This method seems long, but not much time 



' This second treatment with dilute nitric acid is very important and 

 should never be omitted. Experiments have shown that after only one 

 treatment with acid, considerable lead nitrate remains with the precipitate 

 and cannot be completely washed out with dilute nitric acid. 



It is also important not to evaporate the solution almost to dryness (as 

 is done in the case of determining tin as stannic oxide) , for not all of the 

 lead will remain in solution. In case traces of antimony and tin go into 

 solution, they do not interfere with the titration. 



