The amount of acetic acid in vinegar is determined by the volumetric titration of acetic acid with standardized NaOH solution. When atmospheric carbon dioxide is absorbed by deionized water used in the titration, it would react with water and form carbonic acid.
[tex] CO_{2}(aq) + H_{2}O(l) <===> H_{2}CO_{3}(aq) [/tex]
When the deionized water is contaminated with carbonic acid, it would result in the consumption of a higher amount of NaOH for neutralization. Therefore, the amount of acetic acid determined would be too high than it should be.