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.

pH Scale

pH Scale and Examples. Downloaded from Wikimedia Commons Fall 2014; Author: OpenStax College;
License: Creative Commons Attribution 3.0 Unported license.

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+].

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