pH Tutorial

First, some ground rules:

Water dissociates in the following reaction:

H2O    H+ + OH-         equation 1

Actually, free H+ never exists in aqueous solution.  Rather, H+ combines with H2O to form H3O+.  So a more correct way to write this equation would be:

H2O   H3O+ + OH-         equation 2

But sometimes it is just easier to use the shorthand of substituting H+ for H3O+

Acid:  a substance that increases the H+ concentration (also symbolized [H+]) of a solution.  An acidic solution has more H+ in it than OH-

Base: a substance that reduces the [H+] in a solution.  This may be done directly or indirectly, as shown in the following examples:

DIRECT:        NH3 + H+ NH4+ equation 3

INDIRECT:    NaOH Na+ + OH- equation 4

The reason that the indirect method reduces the [H+] of a solution is because the OH- released by the base combines              with H+ formed when water dissociates (see equation 1 above) to form H2O, thereby lowering the [H+] of a              neutral solution.  A basic solution has more OH- than H+.

A neutral solution has and equal concentration of H+ and OH-

In other words, [OH-] = [H+]

Notice that some reactions reach an equilibrium, while other reactions go to completion.  We can see that strong acids and strong bases dissociate completely, meaning that there is none of the starting compound left.  Rather, all we get is the ionized acid or base.  However, some compounds are weak acids or weak bases.  These do not dissociate completely, meaning that an equilibrium exists between the dissociated and the non-dissociated forms.  So, for example, a strong acid will readily give up ALL of its H+, while a weak acid will only give up some of its H+, and will remain partly undissociated (it will hold on to some of its H+).  Do you see why we call one “strong” and the other “weak”?

The pH scale is a measure of the acidity (or basicity) of a solution.

A key point to remember:  in any solution, [H+] [OH-] = 10-14M2         equation 5

In a neutral solution, [H+] = [OH-].  So, in the above equation, we can say that

[H+] = [OH-] = X                    equation 6

OR:  (X) * (X) = 10-14M2         equation 7

X2 = 10-14M2

SO:  X = 10-7M = [H+] = [OH-]         equation 8

When we take the pH of something, what we’re doing is measuring how acidic (or basic) it is.   In other words, we are measuring the relative [H+].  How do we do this?  Physically, we can make measurements with a pH meter or with an indicator that changes color at a particular pH.  We can also calculate the pH mathematically if we know the [H+] or the [OH-].  Let’s see how:

pH = -log [H+]          equation 9

In other words, pH is simply a measure of the [H+] in a solution.  We know that acids have a low pH (pH less than 7), that neutral solutions have a pH=7, and that bases have a high pH (pH greater than 7).  We also know:

Acids:[H+] > [OH-]

Bases:[H+] < [OH-]

Neutral solutions :[H+] = [OH-]

So let’s apply all of these facts.  In a neutral solution, as determined above (see equation 8), [H+] = [OH-] = 10-7M.

If we try to figure out what the pH of this solution is, we would do the following:

pH = -log [H+]

substituting, we get:pH = -log [10-7]

if we solve the logarithm, we find that pH = 7

We can do this with a calculator, but it’s EASY to do it in your head:

Remember that these are log10…so the rule is:

log10 (10x) = x

In other words, pH = - (-7) = 7

So, for a solution of pH = 5, the [H+] > [OH-], the solution is acidic!

pH = -log [H+] = 5

therefore, [H+] = 10-5

and since [H+] [OH-] = 10-14M2 (see equation 5 above), we can substitute:

(10-5)([OH-]) = 10-14M2

Remember that the exponents add/subtract,

So, [OH-] = 10-9