Chemistry - Formation of peracetic acid from acetic acid and hydrogen peroxide and its stability in their presence

Solution 1:

The formation of peracetic acid from acetic acid and hydrogen peroxide is an equilibrium reaction and so in order for the peracetic acid to remain at a constant concentration as desired, acetic acid and hydrogen peroxide must also be present so that the reaction can be at equilibrium. $$\ce{CH3COOH + H2O2 <=> CH3COOOH + H2O}$$

what exactly keeps the excess hydrogen peroxide from reacting with the excess acetic acid to simply form more peracetic acid?

Nothing. The hydrogen peroxide reacts with the acetic acid to form peracetic acid and water but the peracetic acid and water also react to reform hydrogen peroxide and acetic acid. At equilibrium the rates of these reactions are equal and so the concentrations of the species do not change. At room temperature the equilibrium constant is somewhere around 2.5 (see references) which indicates that significant amounts of reactants and products are present, supporting the reasoning above.

The paper Chin J Proc Eng 2008 February, 8 (1), 35–41 has some data on the values of the equilibrium constant and the variation in concentration of peracetic acid formed.

Another value for the equilibrium constant on page vii of Unis, Melod (2010) Peroxide reactions of environmental relevance in aqueous solution. Doctoral thesis, Northumbria University.

Solution 2:

But, my understanding was that mixing acetic acid and hydrogen peroxide makes peracetic acid

Exactly! But within the mixture, peracetic acid reacts back to hydrogen peroxide and acetic acid. With other words, the components are in an equilibrium.

\[ \ce{CH3COOH + H2O2 <=> CH3CO3H + H2O} \]

For disinfection purposes, these mixtures are sold as made. This is safe (and cheap) for both the manufacturer and the consumer. The fairly stable and safe to use, while concentrated solutions of peracetic acid tend to decompose - violently.

Phosphonic acid with a (first) pKa around two is the strongest acid within the mix. One might speculate that it is added to adjust the pH, keep more acetic acid protonated and therefore shift the reaction a bit more towards the peracetic acid.