50 Estimations occurring in Agricultural Analysis [78, 79 



Remove the flame, and add i c.c. of HgCl 2 to oxidise the 

 excess of SnCl 2 . Boil for a few seconds. If the operation 

 has been carefully conducted, only a faint precipitate of 

 Hg 2 Cl 2 will be formed. Now cool down the flask, by running 

 cold water round it, to 6o° C, and titrate, exactly as with the 

 ferrous salt, using the whole of the solution in the flask and 

 making a separate reduction for each titration until the exact 

 quantity of bichromate solution required is found. 



78. Permanganate of potassium ^ as before stated, may be substituted 

 for bichromate. The solution of this substance has a very deep purple 

 colour, and as that colour is destroyed by reduction no indicator is 

 required. 



To prepare a decinormal solution of K 2 Mn 2 8 , dissolve about 5 grams 

 of this substance and dilute to a litre. Standardise it by means of oxalic 

 acid solution. The action of oxalic acid on permanganate is as follows : 



5H 2 C,0 4 .2H 2 + K 2 Mn 2 8 + 3H 2 S0 4 

 = ioC0 2 + K 2 S0 4 + 2MnS0 4 + i8H 2 0. 



Dissolve 63*01 grams of dry crystals of oxalic acid in water and make 

 up to a litre. Measure out 10 c.c. into a beaker by means of a pipette ; 

 add 5 c.c. of dilute H 2 S0 4 and dilute with about 50 c.c. of water. Heat 

 this solution to 6o° C. , and run in the K 2 Mn 2 8 solution from a glass- 

 tapped burette, stirring after each addition. As soon as a faint permanent 

 colour is produced, read off the amount that has been added. From this 

 result the amount of K 2 Mn 2 O s per c.c. may be calculated. 



In using K 2 Mn 2 8 solution the following precautions should be re- 

 membered : 



Always acidify with sulphuric acid. 



No HC1 must be present, as it is liable to be oxidised and evolve chlorine. 



Organic matter must be absent. 



Nitric acid should be present only in very small quantities. 



ESTIMATION OF SUGAR 



79. There are many sugars which occur in the vegetable king- 

 dom, but in this article only two are dealt with — viz., cane sugar 

 and glucose. All the determinations which have been described 

 up to this point are dependent on definite chemical reactions 



