Critical resistivity in impurity doped binary liquid mixtures

The electrical resistance of the critical binary liquid system C₆H₁₂₊(CH₃CO)₂O is measured both in the pure form and when the system is doped with small amounts (≈ 100 ppm) of H₂O impurities. Near T_c, the resistance varies as dR/dT = A₁+A₂ (T-T_c)^-b with b ≈ 0.35. Neither the critical exponent b nor the amplitude ratio A₁/A₂ are affected by the impurities. A sign reversal of dR/dT is noticed at high temperatures T ? T_c.