What does it mean if a solution has a pH of 6.0?

Oct 23, 2017

That it contains $10$ times more hydronium cations than a neutral solution at room temperature.

Explanation:

As you know, the $\text{pH}$ of a solution is a measure of the concentration of hydronium cations, ${\text{H"_3"O}}^{+}$, present in this solution.

More specifically, to find the $\text{pH}$ of a solution, you need to take the negative log base $10$ of the concentration of hydronium cations.

"pH" = - log(["H"_3"O"^(+)])

You can rewrite this equation as

$\left[\text{H"_3"O"^(+)] = 10^(-"pH}\right)$

Now, pure water at room temperature has

$\text{pH} = 7$

This implies that the concentration of hydronium cations in pure water at room temperature is equal to

["H"_3"O"^(+)] = 10^(-7)color(white)(.)"M"

["H"_3"O"^(+)] = 1 * 10^(-7)color(white)(.)"M"

In order for the $\text{pH}$ of the solution to decrease by $1$ unit, the concentration of hydronium cations must increase by an order of magnitude, i.e. $10$-fold.

So for $\text{pH} = 6.0$, you have

["H"_3"O"^(+)] = 10^(-6.0)color(white)(.)"M"

["H"_3"O"^(+)] = 1 * 10^(-6)color(white)(.)"M"

This corresponds to an increase by an order of magnitude in the concentration of hydronium cations, since

(["H"_ 3"O"^(+)]_ "pH = 6.0")/(["H"_ 3"O"^(+)]_ "pH = 7") = (1 * 10^(-6)color(red)(cancel(color(black)("M"))))/(1 * 10^(-7)color(red)(cancel(color(black)("M")))) = 10

You can thus say a solution that has a $\text{pH}$ equal to $6.0$ contains $10$ times more hydronium cations than a solution that has $\text{pH} = 7$, i.e. than a neutral solution at room temperature.

This, of course, implies that you are dealing with a solution that is slightly acidic, since that is what $\text{pH} < 7$ at room temperature implies.