Critical behavior of the binary-fluid isobutyric acid-water with added ions (K$^+$,Cl$^-$)

Measurements of the coexistence curve of the binary-fluid mixture isobutyric $\tx{acid} + \tx{water}$ with added KCl salt at various concentrations show an important dependence on the critical temperature and the critical exponent $\beta$. The phase-transition region shifts as a function of the salt concentration. The effective -critical exponent $\beta ^* = (0.266\tx{--}0.293)$ is neither compatible with the Ising value $\beta =0.325$ nor with the Fisher renormalized value $\sim 0.365.