Improved Estimates of the Isotopic Correction Constants for the Triple Point of Water

Abstract

In 2006, the CIPM clarified the definition of the kelvin by specifying the isotopic composition of the water to be used in the realization of the triple point. At the same time, the Consultative Committee for Thermometry gave recommended values for the isotopic correction constants to be used for water departing from the specified composition. However, the uncertainties in the values for the correction constants were undesirably large due to unresolved differences between the data sets from which the values were determined. This paper derives improved values of the constants by considering additional data from isotopic fractionation measurements and the heats of fusion and freezing points of the relevant water isotopologues. Values of the corrections determined from the expanded data are A D = 671(10) µK, A 18O = 603(3) µK, and A 17O = 60(1) µK. A typical correction made with these values lies just within the expanded uncertainty (k = 2) of the corrections made with the older values, but has about half the uncertainty.

Ice–liquid isotope fractionation factors for 18O and 2H deduced from the isotopic correction constants for the triple point of water

“…White and Tew [24] have used this relation, along with isotope fractionation measurements [10], to deduce the isotopic effect on the temperature shift. However, this was not for the freezing point, but for the TPW: the state at which three phases (gas, liquid, and solid) of water coexist in a thermodynamic equilibrium state.…”

“…A 2H , A 18O and A 17O are the isotopic correction coefficients; δ 2 H, δ 18 O, and δ 17 O are the isotopic deviations from VSMOW expressed as delta values, defined in the usual way (not multiplied by 1000) as δ = R sample /R VSMOW − 1, where R is the isotopic abundance ratio. Until recently, the best values for the A coefficients were based on a compendium of several, rather unsystematic temperature and isotope measurements with real triple point cells [24], and in addition on a clever use of the ice-liquid water fractionation factor reported by Lehmann and Siegenthaler [10]. White and Tew derived the isotopic dependences of the triple point temperature from the isotope fractionation factors between ice and liquid by using the Van't Hoff's relation [24].…”

“…Until recently, the best values for the A coefficients were based on a compendium of several, rather unsystematic temperature and isotope measurements with real triple point cells [24], and in addition on a clever use of the ice-liquid water fractionation factor reported by Lehmann and Siegenthaler [10]. White and Tew derived the isotopic dependences of the triple point temperature from the isotope fractionation factors between ice and liquid by using the Van't Hoff's relation [24]. The values they obtained in that way corresponded well with the directly derived results (and are more accurate), and the mean values presented in their work are recommended for use with the International Temperature Scale of 1990 (ITS-90) [25].…”

“…Now that these coefficients have been established through direct measurements, it is tempting to follow the relation between these coefficients and the isotope fractionation between ice and liquid in the same way White and Tew [24] did, but backwards, and deduce independent, and possibly more accurate values for the ice-liquid isotope fractionation factors. That is the subject of this paper.…”

Thermodynamic analysis and experimental study of the effect of atmospheric pressure on the ice point

“…An isotopic analysis of the water used by the manufacturer of our TPW cell results in a triple point temperature of 273.159 907 K [22] based on White and Tew [23]. The average isotopic distribution of Boulder tap water [24] (which fed the laboratory water system) lowers the freezing point by 0.085 mK.…”