PROCEEDINGS OF SECTION B. 101 



comparatively slow, and in some cases cumbrous. The process I now 

 propose to use, though not perhaps as absolutely accurate as more 

 lengthy methods, is quite near enough for all practical purposes of the 

 hygienist. It is extre^tiely simple, and may be carried out well within 

 the hour from obtaining samples, and in as small a quantity as 100 c.cs. 

 It is as follows : — 



A series of 100 c.o. flasks (say, ten in number), well stoppered and 

 filled vrith water free from carbon dioxide, &c. (preferably distilled 

 water), are emptied at the place where the sample is required to be taken. 

 The stoppers are inserted and the samples taken to the laboratory 

 or tested on the spot. Into each 100 c.c. flask a standard solution of 

 sodium carbonate coloured with phenol phthalein is rapidly run, in 

 arithmetically progressive quantities until a limit is reached beyond 

 which it is unnecessary to go. For instance, in the first flask 4 c.cs. 

 are placed ; in the second, 5 c.cs. ; in the third, 6 c.cs. ; and so on. 

 Two or three flasks should be reserved for final adjustment if necessary. 

 The standard solution of sodium carbonate is of such a strength that 

 each c.c. is equivalent to 001 c.c of carbon dioxide. (As the air contains 

 normally about 0*04 per cent, of CO2 it will be seen at once why I 

 begin with 4 c.c. of the standard solution.) The only factor in the 

 process which requires special care is the making up of the standard 

 solution. The calculation and equation used in doing this are here 

 given : — 



44 

 1 c.c. of CO2 99590 — '001971 gramme 



and according to the equation 



CO2 + H2O + NaoCOg = 2 NaHCOg 



1 c.c. of CO2 converts '^^^^''^^'^ ^^^ = '004748 gramme of NagCOg 

 into NaHCOg. 



Thus a solution is made by dissolving 0*4748 gramme of sodium 

 carbonate in 100 c.c. of water, a ^ gramme of phenol phthalein being 

 added as the indicator. This solution is carefully adjusted to its proper 



strength by titration \\ath — acid in the usual manner. If properly 



made 1 c.c. will equal 1 c.c. of carbon dioxide (N.T.P.). It can be 

 kept for months unimpaired. The working solution is made by diluting 

 this 100 times with distilled water free from COg, say, 10 c.cs. to a litre. 

 A standard solution is thus obtained, 1 c.c. of which is equivalent to 

 •01 c.c. of COg. The 100 c.cs. flasks containing the air to be tested 

 and the standard solution are Avell and continuously shaken for twenty 

 minutes. The amount of CO 2 expressed in percentage by volume 

 is readily indicated by the number of c.cs. of standard solution 



