There are several factors that affect the pH of a solution. The first factor is the type of acid or base that is present in the solution. In general, a strong acid like nitric acid will produce a lower pH solution than a weak acid like acetic acid at the same analytical concentration because the strong acid will react to a greater extent to from hydrogen ions.
Similarly, a strong base like NaOH will produce a higher pH than a weak base like ammonia at the same analytical concentration because the strong base will create more hydroxide ions. A second factor that affects the pH is the concentration of the acid or base.
In the case of acid rain, as we increase the concentration of nitric acid or sulfuric acid the amount of hydrogen ions that is produced will increase and the pH will increase and the pH will decrease. In the same manner, an increase in the concentration of either a strong or weak base will result in more hydroxide ions and increase the pH of the solution.
A third item that is important in determining the pH of solution is the solvent. We learlier in Section 8.20C that it is the solvent that determines the relative strength of an acid or base and the degree to which such substances can donate or accept hydrogen ions. Thais affects not only the strength of the acid or base but also the range of pH values that can be obtained.
This is because the pH scale is determined by the autoprotolysis constant for the solvent (Kauto for a general solvent, or Kw for water). In water, Kw is equal to 1.0 x 10-14 at 25 .C, so the pH scale in water at this temperature extends up to -log (Kw ) =14.00.If we were to instead use deuterium oxide (D2 O) as the solvent, Kauto at 25.C would be 1.1 x 10-15 and the pH scale would extend up to -log (Kauto ) = 14.96.
If ethanol or ammonia were the solvent, Kauto would be 7.9 x 10-20 or 2.0 x 10-28 and the pH scale would be represented by a range that went up to 19.1 or 27.7.
Because temperature can affect a chemical equilibrium, it will affect acid-base reactions and the pH and hydrogen ion activity that result from these reactions. One way temperature can alter these processes is by changing the values for Ka or Kb.
As an example, the pKa for phosphoric acid changes from 2.15 to 2.28 when going from 25.C to 50.C. That means this acid will transfer more hydrogen ions to water as we increase the temperature over this range. Another item affected by temperature is the autoprotolysis constant of the solvent. For water, Kw has a change of about ten-fold between 24.C and 60.C.
This change will affect the pH scale slightly (going from a maximum of 13.997 to 13.017, according to Table 8.4) and will alter the degree to which water can act as an acid or base.