pH and pOH

Of course, when we discuss acids and bases, we are usually dealing with aqueous solutions. For reference, aqueous solutions are solutions where the solvent(the substance that other species dissolve in) is water.


When we deal with aqueous solutions, molar concentrations become important, especially with regards to acids and bases where the molar concentrations(moles per unit volume) of certain species can determine many properties of the solution.


Scientists have a slightly different metric to use when analyzing molar concentrations, however. Since concentrations range across different magnitudes of 10, a logarithmic scale is necessary to easily quantify them. Technically, this is a more "quality of life" thing as you don't need this scale but it still helps a lot.


The scale is known as the pH-pOH scale, and can help us analyze the relative amounts of hydrogen ions(protons) and hydroxide ions in a solution at a given temperature.


p(anything) is defined as:


p(anything) = -log(anything)

Note that the logarithm is a common logarithm(base 10) which isn't explicitly formulated but implied. Now, let's apply this to molar concentrations of hydrogen ions and hydroxide ions.

pH = -log(H+) = -log(H3O+)

pOH = -log(OH-)

Note that the concentration of hydronium(water with an extra hydrogen) equals that of hydrogen ions. This is because in most acid-base reactions, the solution is implied to be aqueous so we don't technically need to represent water in the equation.


The two quantities listed out above are related by the following:

pH + pOH = 14.00

which is equivalent to:

Note that these relations only hold at 25 degrees Celsius(298 K). In pure water, the concentration of hydronium and hydroxide are the same, so both have concentrations of 1.00 * 10-7 M. This equality between the concentrations of both holds in pure water of any temperature. The difference is that while they're equal, they'll both have a different value because Kw is temperature-dependent. Kw is basically an equilibrium constant for both of these concentrations and so by Le Chatelier's Principle, since the ionization of water is endothermic, the concentrations of hydronium and hydroxide increase as temperature increases, meaning pH and pOH decrease(both scales are reverse logarithmic).


The last important concept to note with regards to pH and pOH is that if:


pH > pOH(pH>7): the solution is considered acidic(more hydrogen ions than hydroxide ions)


pH <pOH(pH<7): the solution is considered basic(more hydroxide ions than hydrogen ions)


pH = pOH(pH = 7): the solution is consideredd neutral(equal proportions of hydroxide and hydrogen ions)


The last condition is that of pure water at any temperature. The parentheses on all the conditions are for if the solution is at 25 C.

Citations/Attributions

Chemistry 2e. Provided by: Openstax. Located at: https://openstax.org/books/chemistry-2e/pages/1-introduction. License: CC BY 4.0