pH+scale

Origins of the pH Scale
The acidity of a solution depends on the concentration of H + ions in the solution.

Water is used as the standard benchmark for neutral solutions. So what is the concentration of H + in water?

Water will undergo an //auto-ionization// reaction:



At 25 o C, this reaction has an equilibrium constant of 1.00 x 10 -14.

This really small K value (in this case, labeled K w ) means that the vast majority of the time, water is in its molecular form, not ionized.

To determine the concentration of H +, look at the equilibrium law:

K W = [H + ][OH - ] Because one water molecule will form one H + ion and one OH - ion, we can substitute a variable for each:

K W = [x][x] = x 2 Inserting the value for K W, and solving for x, you get:

1.00 x 10 -14 = x 2

x = 1.00 x 10 -7 M

So the H + ion concentration, [H + ], is equal to 1.00 x 10 -7 M.

Now, the pH scale is based on powers of ten (logarithmic). So, using the logarithm, we report pH = - log[H + ], which for water at 25 o C, is 7.00, the reference pH of a neutral solution!

And because the OH - ion concentration is mathematically related to H + through the KW expression, pOH values run opposite those of pH.