David Watling

), in the form of a vapour, has become the compound of choice for gaseous biodecontaminations in the pharmaceutical industry and some sectors of health care. The vapour is generated by evaporating an aqueous solution of H

 gas concentration, but also the water content of the carrier gas. As some processes operate at vapour concentrations below the dew point, to avoid condensation, and some at concentrations that cause the formation of dew, it is important to understand the relationship between the vapours and to have a method of establishing the dew point. 	

, and the point at which saturated vapours occur and hence when condensation may be expected has been investigated. An analysis of the equations governing equilibrium vapour pressures of these gas mixtures has been done and a method of calculating the dew point has been derived. Somewhat surprisingly, results have shown that the dew point will frequently be reached during a hydrogen peroxide biodecontamination when the relative humidity is only 40%. This is important because it has implications for the understanding of the fundamental mechanisms of H

 biodecontamination. There has, for some time, been a debate regarding whether gaseous H

 is a wet or dry process, and the purpose of preparing this paper is to give those who are interested in the process a tool to allow them to know at what vapour concentrations and temperature saturated vapour will be reached.	

Condensation will occur when vapour mixtures become saturated. This may happen at conditions when there is a high water content and a low H

 content, or vice versa, or indeed at a range of intermediate values. Raising the temperature of the gas mixture will also raise the saturated vapour pressures.	

Regardless of how the vapours are generated, once the saturated vapour pressures have been reached, condensation will occur (depending only on the temperature and the vapour concentrations). If water and H

 vapour are injected in sufficient quantities into a closed space, the dew point will be reached and condensation will form. The condensation formed will be at a concentration that ensures that the liquid condensate and vapour are in equilibrium.	

Saturated water vapour pressure may be calculated using the following equation by Keyes

Scatchard et al.2 developed a similar equation for H2O2 (Equation 2): 	

Equation 2, for the saturated vapour pressure of H

 Using Raoult's Law it is possible to calculate the equilibrium vapour pressure of a perfect multicomponent liquid. Unfortunately, because of the bond formed between H

 and water molecules, Raoult's Law has to be modified by the addition of an activity coefficient (which is always less than unity), and is then expressed as follows (Equation 3): 	

The activity coefficients for water and H

  and then later reviewed and published by Scumb (Equations 4 and 5).

 From Raoult's Law, it is clear that the saturated vapour pressure of water above a solution of H

 will always be less than that of pure water at the same temperature. In any liquid system, the sum of the mol fractions of the components must be unity; thus, for an aqueous solution of H

, it is not possible to directly calculate the vapour pressures or gas concentration at which dew will form in a mixture of H

 and water vapour. Not only will this be temperature dependent, but some vapour mixtures will condense at high water content and others at high H

 content. It is the interaction of the two vapours, at any specified temperature, that will determine if the mixture of vapours is saturated and will therefore condense.	

Condensation will occur when the vapour and liquid are in equilibrium. Consider a closed vessel into which a mixture of water and H

 vapour is put. If the concentration of the vapours is increased then, at some point, condensation will form and the condensate and vapour will be in equilibrium. To find the point of condensation we must find the equilibrium point (that is, a concentration of condensate that has the same vapour pressures as the vapours that have been built up in the closed vessel). 	

Articles

Hydrogen peroxide has become the compound of choice for gaseous biodecontamination in the pharmaceutical industry. As some processes operate at vapour concentrations below the dew point, to avoid condensation, and others actually form dew, it is important to understand the relationship between the vapours and to have a method of establishing the dew point.