EXAMINATION OF AIR 63 



powerful in this respect as a rise of 2-32 C. or 4-18 F. (see 

 " Gordon Report on Ventilation of Houses of Parliament," 



1905). 



Temperature. By the thermometer. 



Pressure. By the barometer fitted with a vernier 

 (e.g. Fortin's standard barometer). 



Humidity. By the hygrometer (direct or indirect). 

 Daniell's, Regnault's, Dines', Mason's (wet and dry bulb). 

 Absolute humidity, relative humidity, dewpoint, Glaisher's 

 factors, Apjohn's formula. 



Carbonic Acid Gas. The determination of CO 2 affords 

 an important index as to the extent to which other impuri- 

 ties exist. It is usually estimated by Pettenkofer's 

 method. Solutions required : (a) Baryta water 0-5 per 

 cent ; (b) Standard oxalic acid solution i c.c. = 0-5 c.c. 

 CO 2 at normal temperature and pressure (2-822 grm. per 

 litre). 



PROCESS. This consists in exposing a measured quantity 

 of baryta water to a known volume of air enclosed in a jar. 

 The baryta water absorbs the carbonic acid gas, becoming 

 barium carbonate, which is precipitated. The amount of 

 baryta water unused or unchanged is estimated by titra- 

 tion of a measured portion of the baryta water removed 

 from the jar, with standard solution of oxalic acid in the 

 presence of phenolphthalein. Oxalic acid is used in prefer- 

 ence to sulphuric acid, because the latter would attack 

 particles of barium carbonate floating in the liquid, and 

 thus give rise to some degree of error. 



BaH 2 O 2 + CO 2 = BaCO 3 + H 2 O 



BaH 2 O 2 + H 2 C 2 O 4 -2H 2 O = BaC 2 O 4 + 4-H 2 O. 



From these equations we see that one molecule of baryta 

 water is neutralized by one molecule of CO 2 and one 

 molecule of oxalic acid. Hence the two latter are chemi- 

 cally equivalent, and i molecule of oxalic acid measures 

 indirectly i molecule CO 2 ; that is, 126 grm. (cryst.) 

 oxalic measure indirectly 44 grm. CO 2 , or 126 grm. 

 (cryst.) oxalic measure 22-32 litres CO 2 , or 2-822 grm. 

 oxalic measure 0-5 litre CO 2 . Hence, if we dissolve 2-822 

 grm. of crystallized oxalic acid in i litre of distilled 

 water, i c.c. = 0-5 c.c. CO 2 . 



