|Isotope||Atomic mass (Da)||Isotopic abundance (amount fraction)|
|35Cl||34.968 8527(3)||[0.755, 0.761]|
|37Cl||36.965 9026(4)||[0.239, 0.245]|
In 1961, the Commission recommended Ar(Cl) = 35.453(1), based on the atomic weight of silver and on Ag/AgCl equivalent ratios determined chemically by many well-known authorities. Following its 1967 discussion of Ag, Cl, and Br, which were important in the chemical determination of atomic weights of several other elements, the Commission did not assign any new atomic-weight values defined purely on the basis of chemical measurements.
Since 1985, relative isotope-ratio mass spectrometry has yielded abundant evidence for variability in the atomic weight of chlorine in both natural and artificial substances, which resulted in the 1999 decision to increase the uncertainty and remove a significant figure from the atomic weight of chlorine, and to introduce the interval notation in 2009.
The basis of the delta scale for relative Cl isotope-ratio measurements currently is standard mean ocean chloride (SMOC), with δ37Cl = 0 ‰, despite reported evidence for variability in n(37Cl)/n(35Cl) of seawater chloride.
Chlorine isotopes in the chloride ion may be fractionated in nature and the laboratory by diffusion, ion filtration, and halide mineral precipitation. The lighter isotope 35Cl diffuses more rapidly in aqueous solutions, whereas the concentration of the heavier isotope 37Cl is higher in halide minerals than in coexisting solutions. Chlorine isotopes also have been fractionated photochemically in the laboratory. Environmental chloride samples are reported to have δ37Cl values ranging from about −7.7 ‰ to +7.5 ‰, corresponding to x(37Cl) = 0.2408 to 0.2436 and Ar(Cl) = 35.450 to 35.455.
Chlorinated organic solvents from different commercial sources commonly have different Cl isotopic compositions. The chlorine isotopes of those compounds may also be fractionated by biochemical degradation reactions. Chlorinated solvents are reported to have δ37Cl values ranging from at least −6.0 ‰ to +4.4 ‰, corresponding to x(37Cl) = 0.2411 to 0.2430 and Ar(Cl) = 35.450 to 35.454. The range of Cl atomic weights in nature and in laboratory reagents is larger than the range indicated by the standard atomic-weight uncertainty value. Larger ranges of variation may be found as measurements are made on a wider range of environments and of Cl-bearing species.
The radioactive isotope 36Cl decays to 36Ar with a half-life of 301(2) ka. It is produced both naturally and artificially by slow-neutron reactions with 35Cl. Large quantities of 36Cl were injected into the atmosphere as a by-product of nuclear bomb tests in the oceans. Both natural cosmogenic 36Cl and bomb-produced 36Cl from the atmosphere have been useful as environmental tracers in hydrologic studies; however, the concentrations normally encountered are too low by several orders of magnitude to have a measurable effect on the atomic weight of chlorine.
Atomic weights of the elements 2009 by M.E. Wieser and T.B. Coplen. Pure Appl. Chem. 2011 (83) 359-396