Alternative Gas diffusion method

One method of determining free, dissolved carbon dioxide which avoids the errors outlined in 1. and 2. and which, moreover, is specific to CO2, is described by J. Kegel in Fresenius Z. Anal. Chem. 276, 45 - 50 (1975). 

This method is based on accommodating the water sample under test in one section and C02 free distilled water in a separate section in a gas-tight vessel with a C02-free atmosphere (air, nitrogen or the like). In this way it is impossible for the two waters to become mixed, but gas exchange may take place without hindrance. Within a certain time, which may be considerably shortened by steady, careful stirring of the vessel, the CO2 from the water sample is transferred to the gaseous atmosphere and from there to the distilled water until a solubility equilibrium of the CO2 is estab- 

Taking as a basis the distribution factor of the CO2 between the gas phase and water at the measured temperature, the CO2 content of the pure CO2 solution can now be determined, without external influence. The result can then be converted to yield the content of free, dissolved carbon dioxide in the water sample taken. Hydrogen carbonate ions and alkaline earth ions in the natural water are certain to be excluded from the distilled water. 

Consequently, the titration curve is steeper and the setting of the stoichiometric point of neutrality is more accurate and free from the influence of the activity coefficients. The negative influences mentioned in 2. are also 

This method should be considered for application if a high degree of accuracy is required or if considerable interference is to be expected as a result of the ideal conditions laid down in 2. not being met. 

---