The Enthalpy of Solution of Low Quartz (α-quartz) in Aqueous Hydrofluoric Acid

An adiabatic solution calorimeter was used to measure the enthalpy of solution of the NBS Standard Reference Material No. 1654 (α-quartz or low quartz) for use as a reference standard for HF solution calorimetry. The enthalpy of solution of this material at a concentration of 5 grams in 1000 cm³ of 24.4 weight percent HF(aq) is This value also applies to the enthalpy of solution of pure low quartz within the uncertainty limits given. The enthalpy of solution of quartz (in J · g^-1) as a function of the temperature, T, of reaction in 24.4 weight percent HF in the range 298 to 358 K, is Thus, the average ΔCp for the reaction over the range, 298 to 358 K, is - 1.586 ± 0.025 J · g^-1 · K^-1. As a function of the concentration of HF solution in the range, 18 to 30 weight percent, From measurements of the enthalpies of solution of several samples of quartz and fused silica, the enthalpy difference between low quartz and silica glass at 298.15 K was determined to be 162.2 ± 4.9 J · g^-1 (2.330 ± 0.070 kcal · mol^-1).