How can you measure Peracetic Acid separately from Hydrogen peroxide, Isn’t there hydrogen peroxide in PAA?

We receive questions similar to the title frequently and the questioner is partly right. Peracetic acid (PAA) is formed in an equilibrium mixture with hydrogen peroxide and acetic acid, so in a mixture (of PAA, hydrogen peroxide, and acetic acid), PAA is being continually formed by the reaction of hydrogen peroxide and acetic acid, and peracetic acid is continually breaking down to reform hydrogen peroxide and acetic acid.

CH3CO2H  +  H2O2  => CH3COOOH  +  H2O

CH3COOOH  +  H2O =>CH3CO2H  +  H2O2

The kinetics of this reaction are quite slow, so absent a catalyst, the reaction takes days or weeks, which for most gas detection applications, means that the mixture behaves as a simple solution of the three components, i.e. the PAA does not turn into hydrogen peroxide in the timescale of gas detection. The solution thus contains both PAA and hydrogen peroxide, and both will contribute to the vapor, but PAA is the more hazardous component. The PAA sensor thus needs to detect PAA selectively and not respond to hydrogen peroxide.

PAA and hydrogen peroxide are both strong oxidizing agents and so the bare sensor will respond to both of them, however ChemDAQ sensors have a chemical filter in front of the sensor which removes at least 99% of the hydrogen peroxide vapor. Thus ChemDAQ’s PAA sensors do indeed respond just to the PAA and not the hydrogen peroxide.