How is the pKa of extremely weak acids determined?

Question

According to Wikipedia, the conjugate acid of LDA has a pKa of about 35 - which is in line with figures I've seen elsewhere. How then can such a weak acid's pKa be determined? If the determination is made in water, how is that even possible? Water would be a stronger acid by far and couldn't it easily mask the acidity of LDA's conjugate acid? Is LDA's pKa value even determined in water? Similarly, how are extremely strong acids' pKa values determined? Wouldn't the leveling effect come into play as one introduces, say, tosic acid into water? How then can we determine the strength of an acid stronger than hydronium ion in water?

Answer 1

You seem to have answered your own question here: you would not rank acids stronger than hydronium, or bases stronger than hydroxide, in water. You might use a solvent such as concentrated sulfuric acid, or liquid ammonia. For example, in liquid ammonia solvent, you could differentiate bases so long as they were weaker than NH₂^-, and you can differentiate acids so long as they are weaker than NH₄⁺. If you rank acid-base strengths in different solvents, the order won't be exactly identical, but it'll be close enough that you can use solvents with an overlapping window to come up with an overall ranking.