Thermal expansion of isobutyric acid+water near the upper critical solution temperature

We report the precise measurement of volume changes as a function of temperature near the critical solution point of isobutyric acid–water. We find not only evidence of a critical anomaly in the thermal expansion but also a small range of temperature in which it is negative, a requirement for thermodynamic consistency. Although the uncertainty in α, the critical exponent determined from these measurements, is on the same order as α itself, we find that α is consistently in the range 0.08–0.14; that is comparable to the Ising value of 1/8.