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 - Δ H soln ( 353.15 K ) = 2362.2 + 1.1 J ⋅ g - 1 . 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 - Δ H soln ( T ) = 2275.0 + 1.586 ( T - 298.15 ) . 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, - Δ H soln ( 353.15 K ) = 2362.10 + 1.429 ( wt % HF - 24.40 ) + 0.069 ( wt % HF - 24.40 ) 2 . 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).