Testing for peracetic acid and hydrogen peroxide.

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Testing for peracetic acid and hydrogen peroxide.

Both hydrogen peroxide and peracetic acid (PAA) are strong oxidants and disinfectants. PAA is made from a reaction between hydrogen peroxide and acetic acid. PAA is usually sold in sanitation products that are a mixture of hydrogen peroxide, PAA and acetic acid. The ratio is approximately 5 hydrogen peroxide : 1 PAA : 1.4 acetic acid.

Tests for hydrogen peroxide actually test for presence of the peroxide functional group which is part of each molecule. This test is based on reaction with peroxidase in the presence of a redox indicator. Macherey Nagel Quantofix Peroxide test strips use this method. But PAA also has this same peroxide functional group. Therefore the peroxidase test will also detect PAA.

Measuring hydrogen peroxide in a hydrogen peroxide / PAA mixture will therefore overestimate hydrogen peroxide. However the overestimate is a relatively small percentage of the total. This is because PAA has a molecular weight just over twice that of hydrogen peroxide. So for the same weight (as hydrogen peroxide), PAA has half the number of peroxide groups as hydrogen peroxide. Given that the usual ratio between the two is around 5 : 1 in sanitizing solutions then the ratio of peroxide groups in the mix will be more like 10 (from hydrogen peroxide) to 1(from PAA). So the peroxidase test will overestimate hydrogen peroxide by around 10%. The peroxidase test can't distinguish between hydrogen peroxide and PAA.

This type of test can't be used as a way to reliably estimate PAA in a hydrogen peroxide / PAA mix because the proportion of PAA in these mixes tends to fall over time. Therefore a method designed specifically for PAA should be chosen if PAA levels are critical in an application.

Peracetic acid can be determined with a specific peracetic acid test. One such test relies on the ability of PAA to oxidize iodide to iodine The iodine then reacts with starch to form a gray-purple colour. At least some PAA test strips use this method. Some PAA kits use the iodine method but add a titration step. In a hydrogen peroxide / PAA mix it is still possible to use this test because the PAA tends to react more readily with the iodide. Usually the reaction colour (test strips) or titration amount (test kit) is read within a specified time before the hydrogen peroxide can cause interference with the result. Kits using the iodide iodine method tend to be more accurate at lower temperatures and where there are lower concentrations of hydrogen peroxide.

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