The ionic strength of a solution relates to the concentration of the ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions and the interactions between the ions (with same charge and opposite charge) with water forms an “ionic atmosphere”, which behaves like an electric shield. Therefore, the ionic strength is a property of the solution which is a measure of the intensity of the ionic atmosphere. The ionic atmosphere slows down the rate of the ions reaction, which is partly electrical in nature and partly due to the increased viscous drag, as the reacting ion moves through the atmosphere of hydrated ions of opposite charge.
As a result, as the ionic strength changes, the ion activity is affected as the ions in solution will not react in the same way. Only a portion of the total ions will be able to participate in reactions(activity) instead of the total amount of ions (concentration) in solution. Therefore, activity is usually lower than the concentration of the ions in solution. In other words, the higher the ionic strength, the higher the ionic atmosphere, the lower the activity of the ion and the lower the ability of the ion to participate in reactions.
The pH is proportional to the concentration of the protons in solution. When the ionic strength increases, the number of ions increases, the ionic atmosphere increases in intensity and so the activity of the protons decreases, therefore, the pH increases.
If the ionic strength is affecting the pH, then it will affect other important properties related with the pH measurement, for instance the ionization constant.
The ionization constant (or pKa)determines the degree of ionization of a drug in a given media1. As the pKa influences other drug properties, such as permeability, solubility andlipophilicity2,any interaction of the environment with the drug, such as the ionic strength, will directly impact any of the PhysChem properties related with the drug and its salts.
As a consequence, it is imperative that every pKa value reported or published must specify the media conditions such as the temperature and the ionic strength.
Ionic atmosphere image (chemwiki.ucdavis.edu)
1 See blog: What is pKa and how is it used in drug development?
2See blog; Is the pKa of a drug affected by the biorelevant media?
