SULPHURIC ACID 



71 



about 25 c.c. to a J -liter flask containing some distilled water, 

 and weigh the small flask again. Now add distilled water to 

 the J -liter flask till the liquid measures J -liter at 15° C. 

 Calculate the strength of the dilute acid, i.e. the weight of pure 

 sulphuric acid in i c.c. of the liquid. 



Into a small wide-mouthed flask quickly place several small 

 dry lumps of caustic soda, cork the flask, and weigh. 

 Quickly transfer i or 2 grams of the caustic soda to a 

 350 c.c. flask containing about 50 c.c. of distilled 

 water, recork the weighing flask, and weigh again 

 to find the exact weight of caustic soda taken. 

 Weigh out a second quantity of caustic soda 

 into a second flask containing water, and marked 

 with a file to distinguish it from the first. Allow 

 the caustic soda to dissolve in the water in each 

 flask, and add a few drops of solution of litmus 

 till the liquid is just rendered blue. Fill a burette 

 with the diluted sulphuric acid till the top mark 

 exactly coincides with the bottom of the meniscus 

 of the liquid when viewed exactly on the level, 

 the burette being perfectly upright. Allow the 

 acid to run into the caustic soda, constantly 

 shaking the flask meanwhile, till the blue colour of the 

 litmus is changed to purple. Boil the liquid and add a 

 few more drops of acid till again perfectly neutral; repeat the 

 process till a permanent purple colour is obtained. Having 

 read off the amount of acid used, add one more drop of acid. 

 If previously neutral, this will turn the liquid red. Treat the 

 second solution of caustic soda in the same way. If the 

 results be not concordant, a third and fourth determination 

 must be carried out. Calculate the reacting proportion of 

 sulphuric acid and caustic soda. Does it agree with that 

 expressed by the formulae — 



Na2O.H2O-fSO3.H2O 

 or, 2NaOH-f-H2S04? 



Fig. 29. 



