2.2.2. Principle in pH dye-based indicators
The carbon dioxide gas dissolves in the aqueous buffer or
hydrophilic filter layer in strip or membrane type sensor and forms
carbonic acid.
CO2ðgÞ4CO2ðaqÞ
CO2ðaqÞ þ H2O4H2CO3
The two hydrogen atoms from the formed carbonic acid
(diprotic) may dissociate to form hydrogen ions (Hþ) and bicarbonate
ions (HCO3) with pKa of 6.36 at 25 C (Mook & de Vries,
2000).
H2CO34HCO
3
þ Hþ
Then, a hydrogen ion, as a proton, combines with a water
molecule to form a hydronium ion, H3Oþ
Hþ þ H2O/H3Oþ
Hydronium ions react with the basic (dissociated) form (In) of
the indicator dye, resulting in an acid (protonated) form (HIn)
which in turn produces a color change of the indicator containing
solution or label.
Hþ þ In/HIn
There are certain drawbacks are associated with use of
pH-based sensors that CO2 levels cannot be measured directly, but
in its ionic form and, therefore, the pH-value measurement could
be impaired by other volatile acid or basic gases produced during
spoilage. And also, maintenance costs are very high.