Wednesday, February 6, 2013

Peracetic Acid: Parts Per Million in Water and in Air

One of the most common sources of confusion when talking to people about workplace exposure limits is the difference between liquid ppm and gas phase ppm. In both cases ppm stands for Parts Per Million.

Using Peracetic Acid (PAA) as an example, PAA is commonly used in dilute solution e.g. ~0.2% by weight solution. This means that for every 1000g or solution, 2g of it is PAA, the rest is water, hydrogen peroxide, acetic acid, surfactants etc. The ppm is just a fractional weight, similar to a percentage (parts per hundred).

For a gas or vapor, ppm is still parts per million, but now it is parts per million by volume because we do not normally deal with the weights of gases (except perhaps atmospheric pressure, ~14 psi, is the weight of the atmosphere on us and a liter of air weights about 1.2g). Instead we normally deal with gas pressures and volumes. The pressure of a gas mixture is the sum of the pressure of its components, at least to a reasonable approximation (Ideal Gas Law) and so it is more convenient to work in parts per million volume than weight.

The key message therefore is that a ppm gas is not the same as a ppm in liquid and so again using PAA as an example, while the EPA Acute Exposure Guide Line for PAA vapor is 0.17 ppm (AEGL 1, 10 min to 8 hr time weighted average); it does NOT mean that 2,000 ppm solution (0.2%) solution is immediately deadly. It is possible to estimate the gas or vapor ppm from a liquid ppm, but it is a little involved (To avoid details, skip to last two paragraphs).

A gas and vapor are very similar, in that both relate to chemicals in the gas state, but a vapor is a chemical whose liquid is below its boiling point. Thus if water evaporates from a glass of water, that is water vapor. If the water and air is heated above the boiling point of water (100 oC), then the water would be present as a gas.

Some liquids evaporate easily (alcohol for example), others barely evaporate at all (olive oil). There is an equilibrium between the liquid state and the vapor state known as the vapor pressure and as the temperature of a liquid rises towards the boiling point the vapor pressure increases until at the boiling point the vapor pressure equals atmospheric pressure. Vapor pressures for many compounds have been measured and tabulated in chemical handbooks and so values are readily available.

If the liquid is a mixture, such as PAA solution, then the vapor pressure of a component is proportional to the mole fraction of that component (fractional number of molecules of the component compared to all molecules, see Raoult’s Law).

If we know the vapor pressure of our component (Vapor Pressure of PAA = 1.93 kPa at 25oC, CRC Handbook of Chemistry and Physics, 76th Ed, p 6-80), we can calculate the PAA vapor pressure of various PAA solutions. Similar calculations can be be done for the other components such as hydrogen peroxide as well.

PAA Concentrations

        Liquid (%, ppm)                     Vapor(ppm)

  • 0.05%   500 ppm                   2.5 ppm
  • 0.1%       1,000 ppm               4.9 ppm
  • 0.2%      2,000 ppm               9.9 ppm
  • 0.5%       5,000 ppm              25 ppm
  • 1%          10,000 ppm              50 ppm
  • 5%>        50,000 ppm            260 ppm
  • 10%         100,000 ppm         540 ppm
These PAA vapor concentrations are in ppm. The estimates also depend on the concentrations of other components in the mixture such as acetic acid and hydrogen peroxide (always found in PAA solutions) since they affect the mole fraction calculation discussed above. For the estimates in the table above the acetic acid and hydrogen peroxide concentrations were 5 and 10% w/w respectively.

These estimates though are only approximations. In particular, they represent equilibrium values which are not found in most applications where PAA is used. For example if the PAA vapor is being swept away by a ventilation system then the vapor concentrations will not reach the levels in the table above. These vapor pressure calculations are useful estimating of the potential concentration of PAA vapor and this allow a rational basis for determining the risk of exposure to PAA and what means should be employed to keep workers safe.

Note: if the PAA is being sprayed, then the concentrations included aerosols or the complete evaporation of droplets in which case the vapor pressure calculations do not apply.

If the PAA vapor concentration has the potential to exceed safe levels, then the PAA vapor should be monitored. Continuous monitors for PAA and many other compounds with potentially hazardous vapors are readily available. Even if the PAA is controlled with ventilation or is used within dedicated equipment, the potential exists for it escape into the environment. Any equipment can fail from wear and tear, mechanical failure or user error. Even though there is no OSHA PEL for PAA, the EPA has issued Acute Exposure Guidelines for PAA as discussed above, the ACGIH is considering a 15 minute short term exposure limit of 0.4 ppm and even manufacturers of PAA such as Solvay recommend a TWA exposure limit of 0.2 ppm.

In summary, there is a lot of confusion between ppm vapor concentrations and liquid concentrations for compounds like PAA. The two are different but are related by a somewhat involved vapor pressure calculation. The calculated vapor pressures though are only estimates but are useful in determining if there is a risk of over exposure. If there is a significant risk of over exposure, then continuous monitors for PAA should be employed.