Detection and Estimation of Carbon Dioxide

Carbon dioxide is detected by its action on lime-water, which it turns milky owing to the precipitation of calcium carbonate. Baryta water may be used instead of lime-water. Owing to its formation of a feeble acid with water, carbon dioxide turns the colour of blue litmus solution, or moistened litmus paper, to a port-wine red. On boiling the solution, or drying the paper, the blue colour returns, because of the decomposition of the carbonic acid.

Carbon dioxide may be estimated: (i) gasometrically, (ii) gravimetrically, (iii) titrimetrically.

1. The proportion of carbon dioxide in a gaseous mixture is estimated by the diminution of volume which takes place when a measured volume of the gas is shaken with concentrated potash or soda solution, or allowed to stand in contact with the moistened solid alkali. All other gases which combine with alkali, such as sulphur dioxide, nitrogen peroxide, and chlorine, must, of course, be absent from the mixture.
   
   The estimation is carried out by the use of Hempel's or other apparatus for gas analysis.
2. Carbon dioxide is estimated gravimetrically (a) by the loss of weight consequent upon the evolution of the gas from a carbonate, (b) by the direct weighing of such evolved gas after its absorption by alkali, (c) by converting the evolved carbon dioxide into barium carbonate, which is weighed.
   
   
   1. The amount of carbon dioxide in a carbonate or mixture of carbonates may occasionally be estimated by ignition. For instance, the proportion of chalk in a mixture of this substance with quicklime might be so estimated; likewise the proportion of sodium bicarbonate mixed with carbonate, by reckoning the loss on ignition as carbonic acid (H₂CO₃). This method, however, is of limited application unless means are taken to retain the water which is frequently present in basic carbonates, and would also be evolved on ignition. More often the carbon dioxide is evolved by the action of acid upon the carbonate contained in a specially constructed apparatus, such as that of Geissler or Schrotter. When the action of the acid is completed the resulting solution is warmed, and air is then aspirated through the apparatus to displace the remaining carbon dioxide, loss of water-vapour being prevented by causing the escaping air to bubble through concentrated sulphuric acid. The loss in weight of the whole apparatus is the weight of the carbon dioxide expelled.
   2. A more accurate, and indeed the most satisfactory, way of estimating carbon dioxide is to cause the evolved gas to pass through drying tubes containing sulphuric acid or calcium chloride, and then into strong potash solution contained in " potash bulbs," to which is attached a small calcium chloride tube to retain water-vapour carried forward by the passing gas. This method is employed for the estimation of carbonates, which are decomposed by acid; and also for determining the carbon and hydrogen present in an organic compound. For this purpose the compound is burnt in a stream of air or oxygen in a combustion tube with the aid of copper oxide, the resulting water- vapour being absorbed in calcium chloride and the carbon dioxide in potash. Soda-lime contained in a U-tube together with calcium chloride, to absorb water-vapour from the soda-lime, may be employed in place of potash.
   3. The amount of carbon dioxide in a gaseous mixture or in aqueous solution can be estimated by causing it to react with ammoniacal barium chloride solution, and then carefully collecting, washing with CO₂-free water, drying, and weighing the resulting barium carbonate.
3. The carbon dioxide in a gaseous mixture - as, for instance, in air - is determined by titration (Pettenkofer). A measured volume of standard baryta water is added in excess to, and shaken with, a known volume of the gas in a large bottle, and the remaining baryta is then titrated with standard acid in presence of phenolphthalein. From the amount of baryta carbonated the amount of carbon dioxide in the gas is calculated.

Alkali carbonates in solution are also estimated by well-known volumetric methods.

It has been shown by Warburg that a hot solution of barium hydroxide absorbs carbon dioxide far more rapidly than a cold solution, and that by this means small quantities such as 0.001 gram may be estimated.

Davies and McLellan have described a modification of the Lunge-Zeckendorf method of estimating carbon dioxide in the air, depending upon the number of strokes of a pump driving 50 c.c. of air per stroke through standard baryta water containing phenolphthalein, which are necessary to decolorise the solution.