Three-terminal capacitance cell for stopped-flow measurements of very dilute solutions

A capacitance cell has been designed, constructed, and tested for stopped-flow measurements of very dilute low-relative permittivity liquid solutions. The capacitance cell utilizes a three-terminal design and is connected to ultrahigh sensitivity capacitance bridge. The cell was designed for operating conditions T = 243.15-373.15 K and P = 0-1 MPa and tested with pure anhydrous cyclohexane and with dilute acetone/cyclohexane solutions at 298.15 K and 0.1 MPa under an argon blanket in a thermostated bath with a temperature variation of <0.001 K. Details of the design and materials of construction are reported. The measured relative permittivity of cyclohexane was 2.015 565, in agreement with the literature, and the long term variation of the measurement was +/- 5 x 10(-6). The relative permittivities of the acetone/cyclohexane solutions were very linear (R2=0.9997) in the measured interval of 0.001-0.008 mole fraction. These measurements confirm that the design specifications for stability and resolution/sensitivity of better than 1 x 10(-5) have been realized. Finally, the present online capacitance cell was connected online to an ultrasensitive densitometer and ultrasensitive refractometer and binary acetone/cyclohexane solutions were measured. The measurements of density, refractive index, and relative permittivity were combined to provide a dipole moment of acetone of 2.750+0.005 D, which is in good agreement with literature.