Wachter E A, Hayes J M
Department of Chemistry, Indiana University, Bloomington 47405, USA.
Isot Geosci. 1985;52:365-74. doi: 10.1016/0168-9622(85)90046-6.
The rate of exchange of isotopes of oxygen between solutions of concentrated phosphoric acid and CO2 was measured as a function of temperature, acid strength (pressure of water in equilibrium with the solution), pressure of CO2, and surface area of the reaction vessel. At 75 degrees C, significant exchange was found to occur even for the "anhydrous" phosphoric acids, those in which the nominal percentage of H3PO4 in solution is equal to or exceeds 100%. Exchange is much slower at 25 degrees C, but isotopic shifts as large as 0.1% can be observed in 95% H3PO4 at equilibration times approaching 1000 hr. Rates of exchange were found to be dependent upon the vapor pressure of water in equilibrium with the acid solutions. Exchange was found to occur primarily on the surface of the reaction vessel above the solution, with no dependence on total CO2 pressure. These observations indicate that phosphoric acids with nominal concentrations of H3PO4 approaching 105% are preferable for the minimization of exchange between CO2 samples and acid solutions during phosphorolyses of carbonate materials. Moreover, with such acids, significant time--temperature trade-offs are possible, allowing rapid preparation of CO2 at elevated temperatures.
测量了浓磷酸溶液与二氧化碳之间氧同位素的交换速率，该速率是温度、酸强度（与溶液平衡的水的压力）、二氧化碳压力以及反应容器表面积的函数。在75摄氏度时，即使对于“无水”磷酸（即溶液中H₃PO₄的标称百分比等于或超过100%的磷酸），也发现有显著的交换发生。在25摄氏度时交换要慢得多，但在平衡时间接近1000小时的95% H₃PO₄中，可以观察到同位素位移高达0.1%。发现交换速率取决于与酸溶液平衡的水的蒸气压。发现交换主要发生在溶液上方反应容器的表面，且与总二氧化碳压力无关。这些观察结果表明，对于在碳酸盐材料的磷解过程中使二氧化碳样品与酸溶液之间的交换最小化而言，标称浓度接近105%的磷酸是更可取的。此外，使用这类酸时，显著的时间 - 温度权衡是可能的，这使得在高温下能够快速制备二氧化碳。
