| Abstract
| - Densities of sodium hydroxide solutions at concentrations of 0.1 ≲ m/mol·kg-1 ≲ 8 have been measured byvibrating-tube densimetry over the temperature range 323 ≤ T/K ≤ 573 at a pressure of 10 MPa. The purpose-built densimeter, which is capable of measuring densities of corrosive fluids at temperatures up to 573 K andpressures up to 30 MPa, has a platinum−rhodium vibrating tube with inlet/outlet tubes of platinum and an opticaldetection system. Calibrations were carried out with water and aqueous solutions of sodium chloride at temperaturesup to 523 K. At higher temperatures, nitrogen was substituted as the auxiliary density reference because ofuncertainties in the NaCl(aq) densities. The apparent molar volumes of NaOH(aq) calculated from the measureddensities were fitted with an extended Redlich−Meyer equation. The present results are in good agreement withliterature data but are more precise. The current global accuracy in the densities of NaOH(aq) appears to be about0.1 %, being significantly better at lower concentrations and temperatures but slightly worse at higher concentrationsand temperatures. More accurate knowledge of the densities of the reference materials (other than water) used forcalibrating vibrating-tube densimeters under extreme conditions is probably required to improve the accuracy ofsuch data.
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