text: If we consider a solvent which can be dissociated into a positive species X and a negative species Y: XY ↔ X+ + Y- or 2XY ↔ X2Y+ + Y- or 2XY ↔ X+ + XY2- a compound causing an incréase in X+ (or X2Y+) and a decréase in Y- (or XY2-) is an acid and one causing the reverse is a base