Coexistence curve of the n-heptane plus nitrobenzene mixture near its consolute point measured by an optical method

We have measured the coexistence curve of the binary liquid mixture n-heptane+nitrobenzene (C7H16+C6H5NO2) near its consolute point using an optical method. In particular, the critical exponent beta describing the coexistence curve was measured for this system. Previous experimental values of beta for n-heptane+nitrobenzene were higher than the typical theoretically calculated value, an unusual, although not unique, occurrence. In an effort to study this discrepancy, we have used an improved experimental apparatus for our measurements. We have taken special care to minimize temperature gradients and maximize the temperature stability of our thermal control system. We have also exploited features of a known optical method to analyze, thoroughly, sources of systematic errors. We measured an apparent value of beta as 0.367 +/- 0.006 and by a careful study of the known sources of error we find that they are not able to remove the discrepancy between the measured and the theoretical values of beta. We also measured the critical temperature of the system at T-C=(291.80 +/- 0.02) K (18.65 degrees C).