ELEMENTARY PHYSIOLOGICAL CHEMISTRY 245 



pieces of granulated zinc are now added to prevent bumbing, and it is 

 then mixed with a concentrated solution of pure caustic soda till faintly 

 alkaline. The amount of the caustic alkali solution necessary to 

 exactly neutralise the quantity of acid used in the incineration should 

 have been previously determined by titration, and a little more than 

 the necessary amount then carefully poured down the side of the 

 distilling flask, so as to form a layer under the acid ammonium sul- 

 phate solution. By this means any ammonia gas which might be 

 produced where the two fluids are in contact, is at once absorbed by the 

 overlying layer of acid. The flask is now connected with the distilling 

 apparatus (see Fig. 150), the other end of this being attached to a tube 



which touches the surface of a measured quantity of decinormal ( j 



sulphuric acid contained in another Erlenmeyer flask (&). 



When all is ready the contents of the distilling flask are shaken up 

 so as to mix them, the burner is lighted underneath it, and distillation 

 allowed to proceed till no more ammonia gas is given off. The chemical 

 reaction which now ensues is that the excess of alkali in the distilling 

 flask reacts with the ammon. sulph. liberating free ammonia, which 



distills over and is at once absorbed by the acid placed to receive it. 



3rd Stage Titration. Before describing this it will be necessary to 

 explain what is meant by a normal or decinormal solution. 



A normal solution is one which contains the molecular weight of the 

 substance in grammes dissolved in 1000 c.c. of water. Sodium 

 hydrate has the molecular weight : Na^O^Hj = 40, therefore a normal 

 solution (n) would contain 40 grammes per 1000 c.c., and a deci- 



(n \ 

 - j 4-0 grammes. Hydrochloric acid has the molecular 



weight: H 1 C1 35 . 5 = 36'5, therefore a normal solution would contain 36*5 

 grammes per 1000 c.c., and a decinormal 3'65 grammes. 



In other words, a measured quantity (say 50 c.c.) of yr: acid 



(Yl 



would exactly neutralise the same amount of TQ alkali. In the case 



of an acid which forms two kinds of salts i.e. a dibasic acid such as 

 sulphuric, which forms an acid KHS0 4 and a neutral salt K 2 S0 4 

 the molecular weight must be halved in order to form a normal 

 solution. The reason of this is quite evident when we consider that 

 instead of only forming one salt, as does a monobasic acid, it forms two. 

 The molecular weight of H 2 S0 4 is 98, therefore x a normal solution 



equals 49 grammes per 1000 c.c. and yxr = 4'9 grammes per 1000 c.c. 



