70 52. BASES AND ACIDS WITH KEAGENTS. 



dust can get into it, particularly if the open end of the 

 shorter tube is closed with a plug of cotton, and its 

 strength will not change perceptibly in two or three 

 months. 



c. The following volumetric method of estimating fer- 

 ric oxide has given satisfactory results ( Oudemans^ Fre- 

 senius's Zeitschrift, 6, 129), and is very easily executed. 



Prepare a standard solution of sodic hyposulphite, by 

 dissolving 24.8 grms. of the pure crystallized salt in one 

 litre of water ; this gives a ^ Ij^ normal solution, since 248 ^ 

 is the equivalent of the crystallized salt. Determine the 

 strength of a solution of ferric chloride containing no 

 traces of free chlorine, as carefully as possible, by precipi- 

 tation with ammonia (a). 



To a quantity of this solution, accurately measured, 

 containing about 0.2 grm. of iron, add a littl6 hydrochlo- 

 ric acid, one or two drops of a concentrated solution of 

 cupric sulphate, and the same quantity of potassic sulpho- 

 cyanate ; heat this blood-red liquid to about 40° C, and 

 allow the standard solution of hyposulphite to flow from 

 a burette into it with constant stirring, until the red color 

 disappears, leaving a clear, colorless liquid ; towards the 

 end of the operation, when the color of the solution has 

 become quite pale, wait a few seconds between each ad- 

 dition of a few drops of the hyposulphite. Divide the 

 quantity of ferric oxide corresponding to the amount of 

 ferric chloride taken, by the number of cubic centimetres 

 of the solution of hyposulphite required in this trial, and 

 the quotient will give the amount of ferric oxide which 

 the sodic hyposulphite in one cubic centimetre of the 

 standard solution is able to reduce to protoxide. 



Having in this way determined the value of the solu- 

 tion of hyposulphite with reference to ferric oxide, this 

 oxide may be determined in any solution containing it or 

 the corresponding chloride, in the manner described 



