Thermal-Conductivity Measurements of Aqueous Orthophosphoric Acid Solutions in the Temperature Range from (293 to 400) K and at Pressures up to 15 MPa

A new improved guarded parallel-plate thermal-conductivity cell for absolute measurements of corrosive (chemically aggressive) fluids under pressure has been developed. Using the new modified guarded parallel-plate apparatus the thermal conductivity of aqueous orthophosphoric acid solutions was measured over the temperature range from (293 to 400) K and pressures up to 15 MPa. Measurements were made for three compositions of H3PO4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{3}\text {PO}_{4}$$\end{document} (8 mass%, 15 mass%, and 50 mass%) along three isobars of (0.101, 5, and 15) MPa. The combined expanded uncertainty of the thermal-conductivity (λ)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\lambda )$$\end{document} measurements at the 95 % confidence level with a coverage factor of k=2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k=2$$\end{document} is estimated to be 2 %. The uncertainties of the temperature, pressure, and concentration measurements were 15 mK, 0.05 %, and 0.01 %, respectively. The temperature, concentration, and pressure dependences of the thermal conductivity of the solution were studied. The measured values of thermal conductivity were compared with the available reported data and the values calculated from various correlation and prediction models. A new wide-range correlation model (extended Jones–Dole type equation with pressure-dependent coefficients) for the H3PO4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{3}\text {PO}_{4}$$\end{document}(aq) solution was developed using the present experimental data.