48 Estimations occurring in Agricultural Analysis [73 



has been dried in a desiccator, dissolve in water, and make up 

 to a litre exactly as described in the case of AgN0 3 ; mix well. 

 73. Estimation of Iron in Solutions of Ferrous Salts. 

 It is necessary that all the iron in the solution shall be in the fer- 

 rous state. If any ferric salts be present, they must be reduced 

 before titration as described in the next article. To acquire 

 proficiency in the actual estimation, Fe(NH 4 ) 2 (S0 4 ) 2 .6H 2 

 may be used. Weigh out about a gram of this salt, dissolve 

 it in water, add 2 drops of strong H 2 S0 4 , and make up to 

 250 c.c. in a flask of that capacity. After shaking up well, remove 

 25 c.c. with a pipette into a beaker. In another beaker make 

 up a very dilute solution of potassic ferricyanide, K 3 Fe(CN) G , 

 by dissolving a piece of this salt, about the size of a hemp seed, 

 in 50 c.c. of distilled water. Place about a dozen separate drops 

 of this solution on a white tile. This is most easily done by 

 dipping a glass rod into the solution and touching the plate 

 with it. A drop of any acid liquid containing iron in the 

 ferrous state will turn one of these drops blue, whilst ferric salts 

 will produce no visible change. 



Warm the beaker containing the ferrous solution to about 

 6o° C. Run in the decinormal bichromate solution from a 

 burette, half a c.c. at a time, stirring well after each addition. 

 After stirring, take out a drop on the end of a stirring rod and 

 touch one of the K 3 Fe(CN) 6 drops on the tile. As soon as 

 no colour is formed read off the amount added. This gives 

 a rough estimate of the quantity required ; but, seeing that we 

 have added half a c.c. at a time, this reading may be '4 c.c. 

 out, as the liquid is apt to absorb oxygen from the air. Throw 

 away the liquid, wash out the beaker, and measure out another 

 25 c.c. Warm up to about 6o° C. as before. This time the 

 bichromate may be run in until *5 c.c. less than before has 

 been added. Then add the solution, drop by drop, testing 

 after each addition, until the exact amount is discovered. 



