Abstract
Calcium alkali phosphates Ca(Na,K)PO4 are main constituents of bioceramics and thermochemically produced phosphorus fertilizers because of their bioavailability. Sparse thermodynamic data are available for the endmembers CaNaPO4 and CaKPO4. In this work, the missing data were determined for the low-temperature phase modifications of the endmembers CaNaPO4 and CaKPO4 and three intermediate Ca(Na,K)PO4 compositions. Standard enthalpy of formation ranges from − 2018.3 ± 2.2 kJ mol−1 to − 2030.5 ± 2.1 kJ mol−1 and standard entropy from 137.2 ± 1.0 J mol−1 K−1 to 148.6 ± 1.0 J mol−1 K−1 from sodium endmember β-CaNaPO4 to potassium endmember β′-CaKPO4. Thermodynamic functions are calculated up to 1400 K for endmembers and the sodium-rich intermediate phase β-Ca(Na0.93K0.07)PO4. Functions above 640 K are extrapolated because of the phase transition from low- to high-temperature phase. Impurities in the synthesized intermediate phases γ-Ca(Na0.4K0.6)PO4 and γ-Ca(Na0.35K0.65)PO4 and one additional phase transition around 500 K impeded the determination of high-temperature thermodynamic functions. In general, data for phase transition temperatures agree with the previously reported phase diagrams.
Highlights
Calcium alkali phosphates Ca(Na,K)PO4 are of interest for fertilizer production and bioceramics used for bone tissue reconstructions
Standard enthalpy of formation ranges from - 2018.3 ± 2.2 kJ mol-1 to - 2030.5 ± 2.1 kJ mol-1 and standard entropy from 137.2 ± 1.0 J mol-1 K-1 to 148.6 ± 1.0 J mol-1 K-1 from sodium endmember b-CaNaPO4 to potassium endmember b0-CaKPO4
The chemical compositions of the synthesis products were determined by inductively coupled plasma mass spectrometry (ICP-MS) and compared to the nominal elemental mass fractions calculated from the ratios of the starting chemicals (Table 1)
Summary
Calcium alkali phosphates Ca(Na,K)PO4 are of interest for fertilizer production and bioceramics used for bone tissue reconstructions. The earliest description of CaNaPO4 and CaKPO4 dates back to the time of the development of the Rhenania process for thermochemical treatment of rock phosphate in the 1920s [6, 7] and the research continues until today [4, 8, 9]. Based on these procedures, a comparable thermochemical process was developed to treat phosphorus-rich sewage sludge ash for fertilizer production [5, 10, 11]
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