pOH Scale — Overview & Calculations

What does the “p” in pOH stand for?

The “p” actually stands for the German word for power: potenz. This is the same “p” that we see in the pH scale.

pH and pOH are both the scales that scientists use to determine the concentration of protons and hydroxide ions in solution. The stronger an acid or base, the more ions they have in solution. Strong acids and strong bases dissociate fully in solutions, weak acids and bases do not.

pOH tells us what the concentration of hydroxide ions in a solution is. Within any acidic or basic solution, a combination of hydroxide ions and protons are found. The more protons there are in a solution, the more acidic the solution is. The more hydroxide ions in a solution, the more basic a solution is. The chemical equation:

$$pH + pOH = 14$$

is a mathematical representation of the relationship between pH, pOH, and a numerical value on both scales. As one increases, the other must decrease; the pH and pOH scales have an inverse relationship.

What is the formula for pOH?

Just as we calculated the pH based on the molar concentration of protons in an aqueous solution, the formula for pOH is dependent on the molar concentration of hydroxide ions.

$$pOH=-log[OH]$$

$\textit{Note}$: When brackets are seen in chemistry equations, this represents molar concentration.

Ok, but what does the pOH scale tell us?

Both the pH and the pOH scale range from zero to 14 as shown in equation 1, above. A pOH of 0 represents the strongest base and a pOH of 14 represents the weakest base. I know that’s a little counterintuitive when you see it. But remember, the pOH scale is not the pH scale. A pH of 14 indicates a solution is very basic, not a pOH. Let me explain why.

$\textit{Note}$: When given a new equation it’s always super useful to plug in a bunch of different numbers and try to really understand the relationship between all of the variables and what they mean.

Based on equation 2, when pOH is 14, this means:

$$14=-log[OH]$$ $$-14=log[OH]$$ $$10^{-14}=[OH]$$

There are .00000000000001 moles of hydroxide ions in every liter of the solution! That’s a super small number, right? Let’s make the pOH zero and see what happens.

$$0=-log[OH]$$ $$0=log[OH]$$ $$10^{0}=[OH]$$ $$1=[OH]$$

When the pOH is zero, we have 1 mole of hydroxide ions in every liter of solution, which’s a lot more than $10^{-14}$ M, showing us why a solution with a lower pOH is a much stronger base.

All of this information backs up some of our previous knowledge too! Because strong bases and strong acids dissociate fully in solution, the more ions there are in solution and the higher their concentrations!

References: 1

(Video) pH and pOH: Crash Course Chemistry #30

by CrashCourse

In this video, Hank describes the pH and pOH scales. You'll learn where these scales come from, what they mean, and how to calculate them. This is a fun video for learning pH and pOH!