11 PUBLIC HEALTH CHEMISTRY 



Calculate out strength of solution by method similar to 

 above. 



Thus, say that 25 c.c. of N/i soda are required to 

 neutralize 5 c.c. of sample, said to be 25 per cent sulphuric 

 acid. 



Then i c.c. N/i soda = or is chemically equivalent 



to i c.c. N/i sulphuric acid. 



But N/i H 2 SO 4 = 98/2 or 49 grm. per litre. 



Hence i c.c. ,, ,, = 0-049 grm. H 2 SO 4 

 Hence i c.c. N/i NaOH = 0-049 S rm - H 2$O 4 

 And 25 c.c. = 25 X 0-049 = 1*225 grm. 



H 2 S0 4 . 



But 25 c.c. N/i NaOH were required to neutralize 5 c.c. 

 of sample ; therefore 5 c.c. of sample contain 1-225 g rm - 

 of sulphuric acid, and 100 c.c. of sample contain 24-5 grm. 

 of sulphuric acid ; that is, if 100 c.c. are taken as 100 grm., 

 24*5 per cent. The sources of error are as before. 



WEIGHING AND MEASURING. 



Measuring of Solutions. Small quantities, like i c.c., 

 2 c.c., etc., up to 25 c.c. or 50 c.c., are most accurately 

 measured by pipette, or on some occasions by burette. For 

 larger amounts, like 100 c.c., 250 c.c., 500 c.c., and 1000 c.c., 

 flasks with a narrow neck and a mark thereon are the best. 

 When extreme accuracy is not essential, the ordinary 

 graduated measure is quite efficient. Be careful in pipet- 

 ting certain liquids not to get any drawn into the mouth. 

 Never pipette strong sulphuric acid or ammonia by mouth 

 suction. 



On Weighing with the Chemical Balance. To weigh a 

 certain quantity of a substance the necessary weights are 

 placed in the right-hand pan of the balance. Some of the 

 substance is then placed in the left-hand pan, or preferably 

 in a watch-glass of known weight, or balanced by a similar 

 one in the other pan. The handle or screw is now turned, 

 and the balance put in action. If the pointer swings 

 more on the side away irom the weights, that is more to 

 the left side, the amount of substance is too little. The 

 handle must now be turned down, and the balance thus placed 



