68 § 52. BASES A>,'D ACIDS WITH REAGENTS. 



druggists in one litre of water ; as this solution is changed 

 by exposure to the air, its strength must be determined 

 from time to time, and the oftener, the more imperfectly 

 it is protected from such exposure. 



To determine its strength, weigh out accurately about 

 1.4 grms. of ammonio-ferrous sulphate, dissolve the salt 

 in about 200 c.c. of distilled water, to which about 20 c.c. 

 of dilute sulphuric acid have been added. To protect the 

 salt more completely from oxidation while the solution is 

 taking place, heat it with a part of the water in a small 

 flask closed by a cork through which two short glass 

 tubes pass ; fasten the flask in an inclined position in a 

 retort-holder, and heat its contents while a slow current 

 of carbonic acid is conducted through the upper part of 

 it. When the solution is completed, let the liquid cool in 

 the current of carbonic acid, transfer it quickly to a beak- 

 er or a larger flask, rinse the flask out with the rest of 

 the water, set the vessel over white paper, and immedi- 

 ately begin to add the solution of permanganate from a 

 burette, with constant stirring of the liquid. At first, 

 the red drops disappear the instant that they come in 

 contact with the solution, and the latter gradually takes 

 a yellowish tint ; add the permanganate more and more 

 carefully as the drops begin to disappear less readily, and 

 stop when the last drop gives an unmistakable reddish 

 color to the whole liquid. 



Ammonio-ferrous sulj^hate contains just one-seventh of 

 its weight of iron, and hence the amount of permanganic 

 solution used in this trial will convert a w^eight of 

 iron from ferrous to ferric oxide, equal to one-seventh of 

 the weight of the salt taken. 



The concentration of the solution of permanganate 

 should be such, that from 20 to 30 c.c. is required for 1.4 

 .grms. of ammonio-ferrous sulphate, or 0.2 grm. of metal- 

 lic iron. 



Now, to determine iron by this process the ferric salt 



