« Back to Glossary Index

pKa is a measure of the strength of an acid. pKa is the pH @ which 1/2 of the acid molecules have given up a H⁺. It’s a measure of “how extreme” (i.e. basic/alkaline) conditions must be in order for an acid to give up its H⁺ 

  • at any pH above an acid’s pKa, any particular molecule of that acid is more likely to be deprotonated than protonated (cookie jar’s running low so cookie monsters donate or at least don’t steal)
  • at any pH below an acid’s pKa, any particular molecule of that acid is more likely to be protonated than deprotonated (cookies galore!)

The chances of cookie donation increase the further above pKa you are (e.g. 1% deprotonated @ 2 pH units below pKa & 99% deprotonated @ 2pH units above pKa 

Different acids have different “willingness thresholds” 

  • STRONGER acids are more generous & have LOWER pKas – even a slight deficit of H⁺ in their surroundings & they’ll give one up
  • WEAKER acids are “greedier” –  they have HIGHER pKas meaning they won’t give up an H⁺ until there’s a big deficit 

For example, say you have 3 acids: X w/pKa of 3, Y (pKa of 7), & Z (pKa of 9)

  • @ neutral pH (7), most of X will be deprotonated, 1/2 of Y will be deprotonated & almost none of Z will be deprotonated
  • But if we raise the pH (so there’s less free H⁺ available) even the stingy Z will give up its H⁺ 

⚠️ Don’t confuse pKa w/pH! pH is a measure of the *total* concentration of protons ([H⁺]) from *any* source. This includes:

  • water itself: H₂O ⇌ H⁺ + OH⁻
  • AND/OR acid(s)(HA) dissolved in it: HA ⇌ H⁺ + A⁻

⚠️ pKa & pH are different: pKa is a (constant) property of a *molecule* while pH is a (changeable) property of a *solution*