The pH and pOH Scales of Acidity

Due to the large range of proton concentration values ([H+]) in aqueous solution (typically from 10-15 to 10 M), a logarithmic scale of acidity makes the most sense to put the values into manageable numbers. The pH scale of acidity defines pH as the negative common logarithm of the concentration of H+: pH = - log [H+].

Before proceeding with our discussion on pH, let's review some properties of Logarithms (affectionately called "logs"). A common logarithm is a function that computes what exponent would be on 10 to obtain the input number. For example, the logarithm of 100 (written "log 100") is 2 because 102 = 100. Likewise, log 1,000,000 = 6. To add logs, multiply their arguments: log 100 + log 1000 = log (100 x 1000) = log (100,000) = 5. To subtract logs, divide their arguments: log 100 - log 1000 = log (100/1000) = log (0.1) = - 1. Given those simple rules, you can now manipulate logs well enough to do any problem involving logs in acid-base chemistry. For this section of chemistry, you will get to know and love the log key on your calculator almost as much as you did in kinetics.

The pH scale in water is centered around 7, which is called neutral pH. This value is not randomly selected, but rather it comes from the fact that the [H+] in pure water is 10-7 (recall that Kw = 10-14). An acidic solution has a pH value less than 7 because [H+] is greater than that of pure water. A basic solution has a pH greater than 7 because there is a lower [H+] than that of pure water (and consequently a larger [OH-]).

Chemists use a pOH scale analogous to the pH acidity scale to gage hydroxide ion concentrations of aqueous solutions. pOH is defined as the negative common logarithm of the concentration of OH-: pOH = - log [OH- ]. In the pOH scale, 7 is neutral, less than 7 is basic, and greater than 7 is acidic. A useful relationship (which you should be able to derive using the definition of Kw) is that pH + pOH = 14. This formula will allow you to readily convert between values of pH and pOH. A comparison of the pH and pOH scales is provided in . Note that because Kw is constant, the product of [H+] and [OH-] is always equal to 10-14.

Figure %: A comparison of the pH and pOH scales of acidity

As you have probably guessed by now, the prefix "p" in front of a symbol means "take the negative log". We have defined pH and pOH as values to describe the acidic or basic strength of solutions. Now we can talk about pKa (- log Ka) and pKb (- log Kb) as measures of acidity and basicity, respectively, for standard acids and bases. An acid with a pKa less than zero is called a strong acid because it almost completely dissociates in water, giving an aqueous solution a relatively low pH. Acids with pKa's greater than zero are called weak acids because they only partially dissociate in water, to make solutions with larger pH's than those strong acids produce at the same concentration. Similarly, bases with pKb's less than zero are strong bases and bases with pKb's greater than zero are called weak bases.

Factors Affecting Acidity

Now that we have the tools to be able to describe the acidity and basicity of compounds with known pKa's, let's analyze trends in acidity and basicity to reach an intuitive understanding of those trends.