Accurate density measurements of reference liquids by a magnetic suspension balance

Abstract

A density measurement system with a magnetic suspension balance has been developed for accurate density measurements of reference liquids. In this system, the buoyancy force of a sample liquid acting on a sinker of silicon crystal is measured through a magnetic suspension coupling. For the accurate density measurements, the volume of the silicon sinker has been calibrated with a relative combined standard uncertainty of 1.4 × 10−6 by the pressure-of-floatation method in a manner that is traceable to the International System of Units. The temperature of the sample liquid is controlled within a fluctuation of less than 1 mK. The effect of the magnetic property of the sample liquid on the density measurement has been examined. Theoretical calculation of the magnetic interaction in the densimeter has clarified that the diamagnetic property of n-tridecane, if not corrected, will result in a relative systematic error of 60 × 10−6 in the density measurement. The densities of some candidates for the reference liquids, iso-octane, n-nonane, n-tridecane, water, 2,4-dichlorotoluene, 3,4-dichlorotoluene and bromobenzene have been measured. The relative combined standard uncertainty in the density measurement of bromobenzene has been estimated to be 6.7 × 10−6 at 20 °C under atmospheric pressure.