Synthesis of In‐House Produced Calibrated Silver Phosphate with a Large Range of Oxygen Isotope Compositions
Résumé
The large range of stable oxygen isotope values of phosphate-bearing minerals and dissolved phosphate of inorganic or organic origin requires the availability of in-house produced calibrated silver phosphate of which isotopic ratios must closely bracket those of studied samples. We propose a simple protocol to synthesise Ag3PO4 in a wide range of oxygen isotope compositions based on the equilibrium isotopic fractionation factor and the kinetics and temperature of isotopic exchange in the phosphate-water system. Ag3PO4 crystals were obtained from KH2PO4 that was dissolved in water of known oxygen isotope composition. Isotopic exchange between dissolved phosphate and water took place at a desired and constant temperature into PYREX (TM) tubes that were placed in a high precision oven for defined run-times. Samples were withdrawn at desired times, quenched in cold water and precipitated as Ag3PO4. We provide a calculation sheet that computes the delta O-18 of precipitated Ag3PO4 as a function of time, temperature and delta O-18 of both reactants KH2PO4 and H2O at t = 0. Predicted oxygen isotope compositions of synthesised silver phosphate range from -7 to +31 parts per thousand VSMOW for a temperature range comprised between 110 and 130 degrees C and a range of water delta O-18 from -20 to +15 parts per thousand VSMOW.
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