Why do we care about the [H⁺] anyway? What is special about this particular ion? Well, it turns out that either too much or too little H⁺ can cause serious problems to chemical reactions. We use the terms acidic and basic to describe these conditions. If the [H⁺] of the solution is greater than 1.0 x 10^-7, we say the solution is acidic, and if the [H⁺] is less than 1.0 X 10^-7, we say the solution is basic.

Because the [H⁺] is so important and because it is rather cumbersome to say things like, “the [H⁺] of the fluid is 1.0 X 10^-7 Molar,” chemists have developed a shorthand to express the [H⁺]. This shorthand expresses the [H⁺] as the pH of the solution. The pH of a solution is the negative logarithm of the [H⁺] (concentration expressed as moles per liter, M). So, if the [H⁺] is 1.0 x 10^-7 M, the pH of that solution would be 7 (-log 10^-7 is -(-7) or 7). Since this is the pH in which the [H⁺] and [OH^-] are equal, we say that this is a neutral solution. When using pH, one thing that is a little confusing is that as the [H⁺] of a solution goes up, the pH goes down. Suppose that a solution has a [H⁺] of 1.0 X 10^-6 M. The pH of the solution would be 6, but since the math behind pH is log base 10, the change in pH from 7 to a pH of 6 represents a 10-fold increase in hydrogen ions. Moving from a pH of 7 to pH of 5 represents a 100-fold increase. Thus, an acidic solution is any solution with a pH<7. Likewise, any solution that has a pH>7 is a basic solution. Below is an image that shows the pH of some common solutions.

So, there are two important lessons from this; the lower the pH, the higher the [H⁺], and a change in pH of one unit (7 to 6 for example) is a 10-fold change in [H⁺].