pOH
/ pee-oh-AYTCH /
If pH is the dial that reads how acidic a solution is by counting hydrogen ions, pOH is the same kind of dial read from the other end of the room — it counts the hydroxide ions instead. The two are simply two ways of describing the same balance.
Precisely, pOH is the negative base-10 logarithm of the hydroxide-ion activity, or in everyday work the hydroxide concentration in moles per litre. A low pOH means lots of hydroxide and a basic solution; a high pOH means little hydroxide and an acidic one — exactly the reverse of how pH behaves. Because water's autoprotolysis ties the two ions together, at 25 degrees Celsius pH and pOH always add up to about 14.
pOH matters mostly as a convenient shortcut when a problem hands you the hydroxide concentration directly, as with a dissolved strong base: compute pOH first, then subtract from 14 to get pH. The honest caveat is that the tidy sum of 14 holds only near room temperature, because the ion product of water, and hence that total, shifts with temperature.
A 0.001 mole-per-litre solution of sodium hydroxide has a hydroxide concentration of 0.001, so its pOH is 3; subtracting from 14 gives a pH of 11, marking a clearly basic solution.
Find pOH from hydroxide, then subtract from 14 to reach pH.
The pH plus pOH equals 14 rule is a room-temperature convenience, not a law. The true constraint is that the two ion concentrations multiply to Kw, which itself climbs with temperature.