pOH and pH are ways of describing how acidic or basic an aqueous solution is. Here is the pOH definition, a look at how pOH and pH are related, and examples of calculating pOH.
pOH Definition
The pOH of an aqueous solution is the negative logarithm of the hydroxide ion (OH–) concentration.
pOH = -log[OH – ]
As with pH, pOH values range from 1 to 14, where 7 is neutral. The scales for pH and pOH are opposites of one another. A low pOH value indicates high basicity of alkalinity, while a high pOH value indicates high acidity. In contrast, a low pH indicates high acidity, while a high pH indicates high basicity.
Why Use pOH?
You may wonder why you might want to use pOH, since pH is already such a handy way of describing acids and bases. The main use of pOH is finding hydroxide ion concentration when the pH of a solution is known. Also, it’s easier to calculate pOH of a base and then calculate its pH.
How pH and pOH Are Related
pH and pOH are related. As pH increases, pOH decreases. As pH decreases, pOH increases.
- pH + pOH = 14 (at 25° C)
- pH = 14 – pOH
- pOH = 14 – pH
How to Find pOH
The two ways to find pOH are from the hydroxide ion concentration or from a known pH value.
Find pOH From Hydroxide Ion Concentration
If you know the molarity of a base solution, plug it in as the hydroxide ion concentration in the pOH formula.
For example, find the pOH of a 0.25 M NaOH solution.
The key here is recognizing that sodium hydroxide is a strong base, so its concentration is its hydroxide ion concentration. This is because sodium hydroxide completely dissociates into its ions in water:
NaOH(aq) → Na+(aq)+OH−(aq)
In other words, for every mole of NaOH, there is one mole of OH– in solution. NaOH and the hydroxide ion have the same concentration values:
[NaOH] = [OH–]
So, simply use the sodium hydroxide concentration in the pOH formula:
pOH = -log[OH – ]
pOH = -log(0.25)
pOH = 0.60
Think about the answer and whether it makes sense. This is a strong base solution, so you expect it has a high pH value or a low pOH value. A pOH of 0.60 is very low, so the answer makes sense!
If you are given the concentration of an acid, first calculate pH and then use the value to find pOH.
Find pOH From pH
For example, find the pOH of a solution with a pH of 3.5.
pH + pOH = 14
pOH = 14 – pH
pOH = 14 – 3.5
pOH = 10.5
References
- Covington, A. K.; Bates, R. G.; Durst, R. A. (1985). “Definitions of pH scales, standard reference values, measurement of pH, and related terminology”. Pure Appl. Chem. 57 (3): 531–542. doi:10.1351/pac198557030531
- Feldman, Isaac (1956). “Use and Abuse of pH measurements”. Analytical Chemistry. 28 (12): 1859–1866. doi:10.1021/ac60120a014
- Mendham, J.; Denney, R. C.; Barnes, J. D.; Thomas, M. J. K. (2000). Vogel’s Quantitative Chemical Analysis (6th ed.). New York: Prentice Hall. ISBN 0-582-22628-7.