Recommendation 2 of the 94th CIPM (2005)

Clarification of the definition of the kelvin, unit of thermodynamic temperature

The International Committee for Weights and Measures (CIPM),

considering

• that the kelvin, unit of thermodynamic temperature, is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water,
• that the temperature of the triple point depends on the relative amount of isotopes of hydrogen and oxygen present in the sample of water used,
• that this effect is now one of the major sources of the observed variability between different realizations of the water triple point,

decides

• that the definition of the kelvin refer to water of a specified isotopic composition,
• that this composition be:
  0.000 155 76 mole of ²H per mole of ¹H,
  0.000 379 9 mole of ¹⁷O per mole of ¹⁶O, and
  0.002 005 2 mole of ¹⁸O per mole of ¹⁶O,
  which is the composition of the International Atomic Energy Agency reference material Vienna Standard Mean Ocean Water (VSMOW), as recommended by IUPAC in “Atomic Weights of the Elements: Review 2000”.
• that this composition be stated in a note attached to the definition of the kelvin in the SI brochure as follows:
  “This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of ²H per mole of ¹H, 0.000 379 9 mole of ¹⁷O per mole of ¹⁶O and 0.002 005 2 mole of ¹⁸O per mole of ¹⁶O”.

DOI : 10.59161/CIPM2005REC2E

NOTE

The kelvin was redefined by the 26th CGPM in 2018 (Resolution 1).