What do you mean by ppm Peracetic Acid?

Peracetic acid is typically used in solution at ppm concentrations and the vapor exposure risk to workers is expressed at ppm vapor; however the two ppms are not the same. This article describes how to convert from ppm PAA in solution to ppm PAA in the vapor and some of the pitfalls that arise with that conversion.

Peracetic acid (also called peroxyacetic acid, PAA) is used in many industries including water healthcare, and the food industry as an antimicrobial and as a bleaching agent in textile, laundry and pulp and paper. In both cases the oxidizing properties of peracetic acid make it useful. Peracetic acid is a strong oxidant and as such contact of the solution with skin and eyes etc. causes irritation to burns depending on the concentration and duration of exposure and inhalation of the vapor can cause irritation to the respiratory system, again the extent depending on concentration and exposure.

While there are no OSHA permissible exposure limits, (as discussed on this blog before, OSHA moves very slowly and has not yet developed PELs for peracetic acid), there are acute exposure guidelines (AEGLs) from the EPA. The lowest AEGL for peracetic acid 0.52 mg/m3 can be converted to 0.17 ppm calculated as a time weighted average from 10 minutes to 8 hours. In addition, the ACGIH has proposed a short term exposure limit of 0.4 ppm.

Most toxic gas concentrations are expressed as parts per million (ppm) or as mg/m3. A concentration is the amount of gas per unit volume and so mg/m3 is a true concentration. A ppm (better described as a ppm per volume or ppmv) is not a true concentration but a relative concentration, similar to a percent.

At constant pressure the two can be equated, but if the pressure changes then the ppm, as a relative concentration will remain constant, but the actual concentration (mg/m3) will change with the pressure. This is the same with other gases, for example a runner who happily cruises along ocean beaches may soon be out of breath if running at high altitude. The relative concentration of oxygen is the same (20.9% by volume), but the actual amount of oxygen present (mg/m3) will have decreased by 10% at 1,000 m (3,300 ft) and 25% at 2,500 m (8,200 ft) above sea level.

To convert from ppm to mg/mis a fairly straightforward calculation using the ideal gas law and the molecular weight of the gas or vapor (for more details see earlier blog discussion on ppm).

If this sounds too easy, and you miss the challenge, don’t worry, there is a small wrinkle to help add confusion. Peracetic acid solution is typically used at concentrations of 2 to 500 ppm. This ppm however is a liquid ppm and not the vapor concentration. It is still a relative unit, but now it is the parts per million per unit mass of the solution. If a solution contains 100 ppm peracetic acid in water and the density is 1 kg/liter, then there are 0.1 g of peracetic acid per liter in the solution. The confusion comes when people try to equate the solution concentration with the vapor concentration since 100 ppm solution does not equal 100 ppm vapor.

If one has a solution of peracetic acid, an estimate can be made of the vapor pressure of peracetic acid over that solution. The vapor pressure of a component of a mixture is proportional to the mole fraction of that component (Raoult’s law), so if there solution of PAA in water, one needs to calculate the concentration of the PAA in moles/liter, the concentration of hydrogen peroxide, acetic acid and water and then calculate the mole fraction (moles of PAA/(moles of PAA + moles of everything else)).

The next step is to multiply the mole fraction of PAA by the vapor pressure for PAA (look it up, in for example the CRC Handbook of Chemistry with Physics, 76th Ed). The result of this calculation is an estimate of the equilibrium vapor pressure of the PAA. The equilibrium vapor pressure is the vapor concentration of PAA over a static solution in a sealed container that has had sufficient time to come to steady state. Most practical applications are not at equilibrium and many are far from it. Therefore, this calculation is at best only rough estimate and may be completely inaccurate. For example, when using PAA in a well ventilated area, the atmospheric PAA concentration will be well below the equilibrium value.

Once the vapor pressure of PAA is obtained, then concentration in ppm can be readily found:

ppm = 1,000,000(vapor pressure) / (atmospheric pressure).

Another wrinkle in the calculation is that peracetic acid can ionize to peracetate ion and H+.

CH3C(O)OOH  =  CH3C(O)OO +  H+

The vapor pressure of the ionized form on the right is negligible, and so only the unionized form (left hand side) is relevant. the pKa for peracetic acid is 8.2 and so the extent of ionization depends on the pH of the solution. If the solution is alkaline, pH 10 or above, then almost all of the PAA will be in the ionized form and there will be little or not PAA vapor. Some PAA mixtures are formulated to be alkaline and therefore have essentially no PAA vapor. If the solution is acidic, pH 6 or below, then essentially all the PAA will be in the non-ionized form. Therefore when calculating the equilibrium vapor pressure, the mole fraction of the non-ionized form should be used.

Using the method above, one can also estimate the concentration of hydrogen peroxide and acetic acid in the vapor phase.

Lastly, it is worth pointing out that if the PAA is being used as a spray, or a fog, then the equilibrium concentrations are not applicable since it is not longer a simple vapor over a liquid. If droplets evaporate, then the relative concentration in the vapor phase will probably reflect the liquid composition if derived from evaporation of liquid droplets, rather than the vapor pressure/concentration ratios estimated from the vapor pressure calculations.

In summary, it is possible to estimate the vapor pressure of a PAA solution and from this calculate an estimate PAA concentration in ppmv. but the bigger problem is that  equilibrium concentrations are rarely seen in PAA applications and so these calculations are at best only rough estimates. It is important that anyone estimating the PAA vapor concentration consider their circumstances to determine to what extent these calculations are applicable or if they can be used at all.