Singularity in the degree of dissociation of isobutyric acid, water solutions at the critical solution point

A recent prediction that the temperature derivative of the ion concentration in isobutyric acid, water solutions should diverge at the critical solution point with an exponent (δ-1)δ is in error because the author fails to properly account for the variables held constant in taking the derivative, and in the path of approach to the critical point. The correct behavior is a standard weak divergence governed by the exponent α.