The acidity of phenols depends on the stability of the phenoxide ion formed after the loss of a proton.

1. Electron Withdrawing Groups (EWG): Substituents like the nitro group ($-NO_2$) are electron-withdrawing due to resonance ($-R$) and inductive ($-I$) effects . These groups disperse the negative charge on the phenoxide ion, thereby stabilizing it and increasing the acidic strength.
2. Number and Position: The stabilizing effect is most pronounced when EWGs are at ortho and para positions. Greater the number of EWGs, higher is the acidity.
3. Picric Acid: 2,4,6-Trinitrophenol contains three nitro groups at ortho and para positions. The combined strong electron-withdrawing influence makes it the most acidic among standard phenolic compounds, even stronger than some carboxylic acids.