A Ph Primer

For the benefit of those who are not chemists, it is worth pointing out that both of these ranges are only slightly off neutral (pH 7.0). The degree of acidity/alkalinity should not be misunderstood as being comparable to that of strong acids like hydrochloric acid and and strong alkalis like sodium hydroxide like lye.

To further elaborate, the pH is the negative of the logarithm (common) of the concentration of H3O+, written as [H3O+]. In "neutral" (pure) water (H2O), [H3O+] = 10^(-7) M [where ^() means that the stuff in parentheses is an exponent of the preceding number, namely, 10. Thus,

10^(+7) = 10x10x10x10x10x10x10 and

10^(-7) = 1/(10x10x10x10x10x10x10)

Since

[H3O+] = 10^(-pH) (definition of pH)

10^(-7) = 10^(-pH) (for pure water or a neutral solution)

so pH = 7 (ditto)

A value of the pH below 7 represents and acidic solution.

A value of the pH above 7 represents a basic solution.

If pH = 8, then [H3O+] = 10^(-8), which is 1/10th of 10^(-7) (neutral). At all times (in a dilute aqueous solution at 77 degrees F), [H3O+][-OH] = 10^(-14) Thus, in a neutral solution, [H3O+] = [-OH] = 10^(-7). When pH = 8 and [H3O+] = 10^(-8), [-OH] = 10^(-6). Then [-OH] is 100 times [H3O+], instead of being equal to it, as in a neutral solution. Hydroxide ion, -OH, is what makes a solution basic. When its amount is not balanced by an equal amount of H3O+ (which makes a solution acidic), then the net result is a basic solution.

The acidity or basicity of a solution is a different concept from the stregth of an acid or base. When a strong acid or base is dissolved in water, every molecule reacts with water to produce H3O+ (acid) or -OH (base). If the amount of acid or base (per liter of solution) is large, then the solution will be strongly acidic, but if the amount is small, such as 10^(-6) M, then the solution will be weakly acidic, pH = 6 (approximately). A similar result might be obtained by making a more concentrated solution of a weak acid, for which only a small fraction of the dissolved molecules react with water to form H3O+.

A difference in pH of 0.3 implies that [H3O+] varies by a factor of 2.

A difference in pH of 0.5 implies that [H3O+] varies by a factor of 3.

Thus, pH = 6.7 implies that [H3O+] is 2x10^(-7) M.

If the pH of EPS is 6.5, then [H3O+] = 3x10^(-7) M.

All of the considerations above are for equilibrium, referring to a condition under which no change in the composition of the solution will occur, no matter how long you wait. The chemical reactions involved reach equilibrium in about 10^(-12) seconds (or even a little less), so you really don't have to worry about waiting for it to happen.

My EPS, at my first massage, had pH = 7.0. That appears to be a little too basic, but I'm not sure what the normal variation and experimental error are.