8 Taylor, Volumetric Deterviinaiion of Cobalt. 



amount of liquid in my experiments, with from 'oy to i 

 of cobalt present, has generally been about 1 50 c.c. The 

 solution is then stirred at intervals for ten minutes, at the 

 end of which time the whole of the cobalt is precipitated. 

 The liquid is now filtered (preferably by the aid of a filter- 

 pump) and the precipitate washed, partly by decantation, 

 until the washings give no blue coloration on the addition 

 of potassium iodide and starch, together with a few drops 

 of acid. The precipitate settles and washes fairly well ; 

 it is much finer and denser than that produced in cobalt 

 solutions by alkalies and bromine water. The filter 

 paper and precipitate together are then transferred to a 

 moderately large beaker, a little water poured on, and then 

 some solution of potassium iodide and dilute hydrochloric 

 acid, which must be added gradually. If a great excess of 

 calcium carbonate has been used there will be a violent 

 effervescence on the addition of the acid, and care must 

 be taken that this does not cause a loss of iodine. The 

 oxide of cobalt, as well as the excess of carbonate, rapidly 

 dissolves and liberates a corresponding amount of iodine. 

 The filter paper may be broken up by a stirring rod so as 

 to admit the acid and iodide more freely to the precipitate. 

 The process may now be finished by diluting with more 

 water and then titrating in the usual way with a deci- 

 normal solution of sodium thiosulphate. Usually, however, 

 I have preferred to pour off the iodine solution from the 

 fragments of paper into another beaker, washing these 

 until they are quite white and free from iodine by succes- 

 sive quantities of water containing a little potassium 

 iodide, and then titrating the moderately clear solution. 

 The whole process, from the time of adding the carbonate 

 and bromine water, can easily be finished within an hour. 

 As already stated, my experiments have not enabled 

 me to decide definitely the composition of the pre- 



