Treatment of Dissociating Compounds

In the case of dissociating or ionizing organic chemicals such as organic acids and bases, e.g., phenols, carboxylic acids and amines, it is desirable to calculate the concentrations of ionic and non-ionic species, and correct for this effect. A number of authors have discussed and reviewed the effect of pH and ionic strength on the distribution of these chemicals in the environment, including Westall et al. (1985), Schwarzenbach et al. (1988), Jafvert et al. (1990), Johnson and Westall (1990) and the text by Schwarzenbach, Gschwend and Imboden (1993). 

The following treatment has been suggested by Shiu et al. (1994) and is reproduced briefly below. The simplest, first-order approach is to take into account the effect of dissociation by deducing the ratio of ionic to non-ionic species I, the fraction ionic x and the fraction non-ionic xN for the chemical at both the pH and temperature of experimental data determination (/D, xID, xND) and at the pH and temperature of the desired environmental simulation (/E, xIE, xNE). It is assumed that dissociation takes place only in aqueous solution, not in air, organic carbon, octanol or lipid phases. Some ions and ion pairs are known to exist in the latter two phases, but there are insufficient data to justify a general procedure for estimating the quantities. No correction is made for the effect of cations other than H+. This approach must be regarded as merely a first correction for the dissociation effect. An accurate evaluation should preferably be based on experimental 

Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals 

Vapor pressure Ps is not affected, but the apparent Henry s law constant Hv must also be adjusted to HvlxN, being PS/CN or Ps/(Cx-xN). 

CN and Kown can be applied to environmental conditions with a temperature adjustment if necessary. Values of IE xb and xNE can be deduced from the environmental pH and the solubility and Kow of the total ionic and non-ionic forms calculated. 

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