38 Estimations occurring in Agricultural Analysis [58 



in a solution of this substance. The method would be to 

 measure out a quantity of the KHO solution, mix with it a 

 few drops of litmus solution, and add the sulphuric acid until 

 the liquid became neutral. 



If we could measure the volume of H 2 S0 4 solution added, 

 we should know the weight of pure H 2 S0 4 required to neutra- 

 lise the KHO in the KHO solution. From the weight of 

 H 2 SO! it would be easy to calculate the weight of KHO. 



A solution of known strength, such as the sulphuric acid in 

 the above example, is called a standard solution. 



A substance which tells when the reaction is completed, as 

 the litmus in the example, is called an indicator. 



The instrument by which the amount of sulphuric acid is 

 added is a burette. 



58. Standard Solutions. — For convenience in calcu- 

 lating, standard solutions are generally made up to certain 



strengths known as normal (N), seminormal ( — J, and deci- 

 normal ( — ) . 



A normal solution of a monobasic acid, or of a monacidic 

 alkali, is one of which 1 litre contains the molecular weight 

 in grams of the acid or alkali. 



A normal solution of any other acid is one of which 1 litre 

 will exactly neutralise 1 litre of normal monacidic alkali solu- 

 tion. In the same way, a normal solution of any other alkali 

 would be one of which 1 litre will exactly neutralise 1 litre of 

 normal monobasic acid solution. Thus, a litre of a normal 

 solution of caustic potash, KHO, would contain 56 grams of 

 potash, and, according to the equation 



HC1 + KHO = KC1 + H 2 

 36-5+ 56 = 74-5 + 18, 



a litre of normal potash would exactly neutralise 36*5 grams 



