310 Mr. R. W. Pearson on the Determination of Bismuth 



required in excess to indicate the complete precipitation of the 

 bismuth. The bichrome test necessary to effect this object will 

 vary in amount, being greater as the volume of the liquid ope- 

 rated upon increases. The amount of bichromate in excess is 

 calculated as being in combination with the bismuth, therefore 

 the results obtained by this method are a little too high. The 

 intensity of colour so peculiar to bichromate of potash I'educes 

 the required excess to an infinitely low figure, however. By rever- 

 sing the course of procedure, i. e. adding a solution of bismuth 

 to a solution of bichromate of potash, a contrary result naturally 

 occurs. In the latter case we add a few drops of bismuth solu- 

 tion in excess to ensure the complete precipitation ; consequently 

 since the excess so added, as in the preceding case, is calculated 

 as being combined with chromic acid, it follows that the quan- 

 tity of bismuth deduced will be a little too low. We may get as 

 accurate results as it is probably possible to obtain by combining 

 the two methods in the following manner : — 



Method I. — 1000 grains of the solution in which bismuth is to 

 be estimated are poured into a flask, heated to ebullition, and small 

 portions at a time of a graduated solution of bichromate of pot- 

 ash added from a burette until a precipitate ceases to form and 

 the supernatant liquid is coloured by an excess of the bichrome 

 test. Suppose 250 gi-ains of bichrome test B have been required 

 to effect this object. 



Method II. — To the ascertained volume of bichromate of pot 

 ash, which we have supposed to be 250 grains, a graduated solu- 

 tion of nitrate of bismuth is added vice versa until the colour of 

 the hquid entirely disappears. The number of measures used is 

 read off. Suppose 1025 grains of the solution of bismuth have 

 been added, then the true quantity is as follows : — 



There have been used for — 

 I. 1000 grs. of solution of bismuth, 250 grs. of bichrome test B. 

 II. 1025 ... ... 250 



2025 500 



Since 100 grains of bichrome test B represents 0*1 grain of 

 bismuth, the 500 grains used indicate "1 x 5 = 0*5 grain of me- 

 tallic bismuth as the quantity contained in 2025 grains of the 

 solution of bismuth. 



The following is an equally simple and sometimes more con- 

 venient modification : — 



Method III. — As in the preceding cases, the bichrome test is 

 added to a solution of bismuth of unknown strength until an 

 excess of the precipitant remains in solution. A standard solu- 

 tion of bismuth is now added to the same solution from which 

 the bismuth has been precipitated, until the colom* of the super- 



