Contradictions in acidity and solubility of phenol derivatives
In the case of o-nitrophenol, the acidic hydrogen is hydrogen-bonded to the nitro group's oxygen atom, making it less acidic. In the case of catechol, one acidic hydrogen is hydrogen-bonded to the adjacent OH group's oxygen atom, but the other, more acidic hydrogen is not hydrogen bonded. This hydrogen is more acidic than that of hydroquinone. The resulting anion is even more strongly hydrogen-bonded than neutral catechol, stabilizing it even further. It is the second acidic hydrogen of catechol that is analogous to the acidic hydrogen of o-nitrophenol.