Two coffees have the same pH but one tastes much sharper. What's different?

Answer: Sharper one has more total acid

Sharper one is colder — Temperature shifts mouthfeel but doesn't explain the sharp acidity at the same pH. A colder cup can mute sweetness perception, but the 'sharpness' the question is about traces back to acid quantity, not coldness.

Sharper one has more total acid ✓ — Right — pH is a snapshot of acid strength, not the total acid stored in the cup. Two coffees can share pH yet differ wildly in titratable acidity, the load the tongue actually has to deal with. More titratable acid means more 'sharpness'. Cold brew typically lands at similar pH to hot coffee but with substantially less titratable acid — that's the chemistry behind why it tastes smooth despite the same pH reading.

Sharper one used dark roast — Roast level changes aroma and body, but pH alone doesn't track roast. Two cups can match pH across roast levels. The sharpness you actually taste is closer to the total quantity of acid (titratable acidity), not the degree of roasting.