36 PUBLIC HEALTH CHEMISTRY 



chloride dissolved in I litre of aq. dest., then i c.c. = 

 i mgr. CaCO 3 . The soap solution is then tested with 

 50 c.c. of aq. dest. (recently boiled to get rid of CO 2 ), to 

 determine how much of it is required to produce a perma- 

 nent lather ; that is, a lather which remains as a uniform 

 film ^ in. thick on the surface of the water five minutes 

 after the bottle has been laid on its side. The soap solution 

 is added from a burette, -J^ c.c. at a time. The 50 c.c. are 

 contained in a stoppered bottle, 150 c.c. capacity, which 

 is well shaken after each addition, then laid on its side, 

 and the character of the lather noted. If it quickly 

 disappears, then more soap solution is added, until the 

 lather has the permanence described. The amount usually 

 required by 50 c.c. of aq. dest. varies from 0-2 c.c. to 

 0-6 c.c., and should be determined not once for all, but 

 at intervals, as it will vary with the strength of the soap 

 solution, and this latter tends to deteriorate on keeping. 



The soap solution is now standardized against the 

 standard CaCO 3 solution, 5 c.c. of which are added to 

 45 c.c. of recently boiled aq. dest., contained in a glass- 

 stoppered bottle of about 150 c.c. capacity. The soap 

 solution is now added from a burette, i c.c. at a time. 

 Shake briskly after each addition. When the proper 

 lather is formed, the shaking of the bottle produces a soft 

 sound which is different from the hard sound at first 

 emitted, and heard when the bottle is held near the ear. 

 Say that 4 c.c. of standard soap solution were required to 

 produce the permanent lather, and that 0-5 c.c. were 

 necessary for 50 c.c. of aq. dest., then 4 0-5 = 3 '5 c.c. 

 have been used in precipitating the 5 mgr. of CaCO 3 

 contained in the 5 c.c. of standard calcium solution. But 

 we wish the standard soap solution to be 5 c.c. = 5 mgr. 

 or i c.c. = i mgr. calcium carbonate. Hence it is too 

 strong, and we must dilute it (with a mixture of equal 

 parts of methylated spirit and aq. dest.) so as to make 

 every 3-5 c.c. up to 5 c.c., or every 35 c.c. up to 50 c.c. 

 The solution is now of standard strength, but requires to 

 be re-standardized at intervals, as it is somewhat unstable. 



The standardizing can also be done against a standard 

 solution of Ba(NO 3 ) 2 which has a molecular weight of 

 261 compared to 100 for CaCO 3 . Therefore if 2'6i grm. 



