﻿2 Prof. 11. Bunsen on the Washing of Precipitates. 



cantations, then 



therefore 



or 



nV = W; (2) 



(l+— >■-** 



W=nv(^'a-1 ( 3 ) 



If we differentiate W with respect to n and make the differential 

 quotient equal to 0, then the minimum value of W becomes, 

 when n= ao, 



W = ?;nat. log. a (4) 



Precipitates obtained in the course of chemical analysis may 

 in all cases be assumed to be sufficiently washed when the im- 

 purity retained by them amounts to no more than the 1Qo 1 ooo 

 part. Making therefore a = 100000 and v = l, it results from 

 equation (4) that the least quantity of fluid required in order to 

 remove the impurity contained in a precipitate to the 1 oOQ 

 part, amounts to eleven and a half times the volume occupied by 

 the precipitate itself in the liquid in which it exists. It is evi- 

 dent, therefore, that the amount of water actually necessary to 

 wash a precipitate the more nearly approaches this minimum the 

 oftener we decant, and the smaller the quantity of washing-water 

 we employ at each decantation. 



Since some of the principal sources of error in analytical work 

 consist in the incomplete or in the too protracted washing of 

 precipitates, it becomes important to know how to ascertain the 

 progress of the washing throughout the several stages of the 

 process. By employing the same volume of water at each suc- 

 cessive addition, and estimating its relation to that of the preci- 

 pitate remaining at the bottom of the vessel or upon the filter, 

 we can find from the following Table, calculated by means of the 

 formula above given, the number of times it is necessary to de- 

 cant in order to diminish the amount of impurity in the precipi- 

 tate to the , nn \ — , — 3_ — — l or — 1 — part. Column I. 



100000* 50000' 20000' 10000 ^ 



shows the relation between the volume of the precipitate and that 

 of the washing-water employed for each successive decantation, 

 column II. the number of decantations required to diminish the 

 amount of impurity to the necessary extent, and column III. the 

 total volume of water obtained from the several decantations. 



