CHEMICAL ANALYSIS 11 



(i) Methyl-orange group : are most susceptible to 

 alkalies ; methyl-orange, lacmoid, cochineal, congo-red ; 

 (2) Phenolphthalein group : are most susceptible to acids ; 

 phenolphthalein, turmeric ; (3) Litmus group : are inter- 

 mediate in susceptibility litmus, rosolic acid, phenace- 

 tolin. 



Sensitiveness. Phenolphthalein, lacmoid, rosolic acid, 

 and phenacetolin showed change of colour with one-fifth 

 quantity of acid or alkali required by methyl-orange and 

 litmus ; that is to say, if the two latter required in 100 

 c.c. of acid or alkali 0-5 c.c. to show change of colour, the 

 former required only o-i c.c. 



Neutral point of one indicator does not coincide exactly 

 with that of other indicators. Thus, saliva is generally 

 neutral to litmus, alkaline to lacmoid or congo-red, and 

 acid to turmeric. Fresh milk shows similar variations. 



1. Litmus solution is violet coloured. Acids change it 

 to red ; alkalies to blue. 



In cold solution it may be used for the titration of 



Hydrates of soda, potash, ammonia, lime, baryta, etc. 

 Nitric, sulphuric, hydrochloric, and oxalic acids. 

 Arsenites and silicates of soda and potash. 



In boiling solution 



Carbonates and bicarbonates of K, Na, Ca, Mg, Ba. 

 Sulphides of sodium and potassium. 



2. Methyl-orange is orange-brown in colour. Acids 

 change it to pink ; alkalies to faint yellow. 



Only used in cold solution, and then for titration of 

 Hydrates, carbonates, bicarbonates of K, Na, NH 3 , 



Ca, Mg, Ba, etc. 

 Sulphides, arsenites, silicates, borates of K, Na, NH 3 , 



Ca, Mg, Ba, etc. 

 All the mineral acids. 

 Sulphites. 

 Half the base in the alkaline and earthy alkaline 



phosphates and arseniates. 

 Not for organic acids, nor in presence of nitrous acid 



or nitrites, which decompose it. 



