Brown Richard J C, Brown Andrew S
Analytical Science Team, National Physical Laboratory, Teddington, Middlesex, UK.
Analyst. 2008 Nov;133(11):1611-8. doi: 10.1039/b806860g. Epub 2008 Aug 21.
Almost all measurements of mercury vapour, for example those to determine mass concentration in air, are currently ultimately traceable to the vapour pressure of mercury, usually via a bell-jar calibration apparatus. This allows a saturated concentration of mercury vapour in air to develop in a confined space in equilibrium with ambient conditions, from which a known mass of mercury can be removed for calibration purposes. Setting aside the uncertainty in the vapour pressure of mercury at a given temperature, the accuracy of vapour phase mercury determinations depends critically on fully understanding the operation and sensitivities of the mercury bell-jar apparatus. This paper discusses the thermodynamic and kinetic considerations that must be taken into account when using the bell-jar apparatus, provides the theoretical basis for understanding the operation of the bell-jar, and presents experimental data demonstrating the systematic biases which may be obtained if the bell-jar is used incorrectly. These biases depend on the temperature difference between the mercury vapour in the bell-jar and the syringe used to remove the mercury vapour from the bell-jar, but they may be well in excess of 10% under some operating conditions. The results from this study have been used to propose best practice solutions for mercury vapour calibrations using the bell-jar.
几乎所有汞蒸气的测量，例如那些用于测定空气中质量浓度的测量，目前最终都可追溯到汞的蒸气压，通常是通过钟罩校准装置来实现。这使得空气中汞蒸气的饱和浓度能在与环境条件达到平衡的密闭空间中形成，从中可以取出已知质量的汞用于校准目的。撇开给定温度下汞蒸气压的不确定性不谈，气相汞测定的准确性关键取决于对汞钟罩装置的操作和灵敏度的充分理解。本文讨论了使用钟罩装置时必须考虑的热力学和动力学因素，为理解钟罩的操作提供了理论基础，并给出了实验数据，证明如果不正确使用钟罩可能会出现的系统偏差。这些偏差取决于钟罩内汞蒸气与用于从钟罩中取出汞蒸气的注射器之间的温差，但在某些操作条件下，它们可能会超过10%。这项研究的结果已被用于提出使用钟罩进行汞蒸气校准的最佳实践解决方案。
