Measurement of the coexistence curve of the binary mixture isobutyric acid-water (I-W) with increasing amounts of ions impurity showed essentially a linear dependence of the critical temperature and critical composition with concentration of (K+, Cl-) ions. Each coexistence curve of the electrolyte mixture is characterized by a new effective critical exponent beta = (0.251 - 0.771) is neither compatible with the Ising value beta = 0.325. The experimental technique used here is the visual method. (C) 2002 Elsevier Science B.V. All rights reserved.