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Mercury Vapor Pressure Correlation

Summary:


An apparent difference between the historical mercury vapor concentration equations used by the mercury atmospheric measurement community and a new vapor pressure correlation proposed by NIST has generated significant controversy. The validity of both of these relationships has been experimentally assessed using state-of-the-art measurement methods, and it has been concluded that both are correct. The discrepancy may result from the underlying assumption that mercury vapor at room temperature behaves ideally.

Description:


Several mercury vapor concentration data sets/equations (mostly based on the work of Knudsen in 1909) have been in use by the mercury atmospheric measurement community over the past several decades. These predict the equilibrium vapor concentration of mercury as a function of temperature, and have been validated over the years by numerous chemical measurements. Recently a new NIST vapor pressure correlation was proposed by Huber et al. This was based on a survey of published data on high-quality mercury vapor pressure measurements all having an assessed uncertainty of <1 %. The proposed correlation, which follows a Wagner-type form, predicts the mercury vapor pressure from the triple point to the boiling point of mercury, and is thermodynamically consistent. This has caused some disarray in the mercury measurement community because the mercury vapor concentration calculated from the new correlation using the ideal gas law is some 7 % higher than that predicted using the accepted equations/data sets. This has critical implications for the entire mercury measurement community, because the EU WG25 working group, which is working on a European total gaseous mercury (TGM) standard, is considering adopting the new NIST correlation. In doing so, it would automatically create a 7 % calibration bias in all of the historical atmospheric mercury measurements made over the last three decades. This issue has become a high profile one, with several requests to NIST from the user and manufacturers communities to resolve this and assess which of the two predictors is correct.

Additional Technical Details:


The approach taken was to make independent ambient measurements of the equilibrium saturated mercury vapor concentration using isotope dilution mass spectrometry, and equilibrium saturated vapor pressure measurements using a calibrated low-pressure ultrasonic interferometer manometer (UIM) located in the Advanced Measurement Laboratory (AML) at NIST Gaithersburg. The vapor concentration measurements were made using a commercial calibration vapor saturation instrument. The saturated headspace of this system was sampled with a NIST-traceable calibrated digital gas syringe and the vapor concentration measured at ambient conditions using isotope dilution inductively coupled plasma mass spectrometry. The data from these measurements, which has a relative uncertainty of approximately 2 %, indicated that the existing database sets predicting the mercury concentration as a function of temperature are, in fact, valid. However, the UIM measurements on the saturated vapor pressure of mercury also provided experimental evidence supporting the validity of the new NIST correlation. These data have a relative uncertainty of approximately 1 %. Apparatus for measurement of mercury vapor pressure

 

Apparatus for measurement of mercury pressure

 

Major Accomplishments:  

  • High-accuracy mercury vapor concentration measurements have been completed using a static headspace generator and isotope dilution ICP-MS.
  • Mercury vapor pressure measurements have been completed using a state-of-the-art NIST ultrasonic interferometer manometer primary standard.