Dear Dave and others,

Adding to the large experience of Arndt, I like to add the review I made
on this 'volume effect' for my book on SI analytical techniques. This
also includes a number of publications discussing this volume effect,
which I hope are useful for all who are interested in this matter.
Indeed, a constant volume (isn't that advisible for many reactions for
SI analysis?!?) of Zn shot is highly recommended as commented by others.

The review text (part of a section on Zn quality and suppliers):

 A fractionation effect related to the volume of Zn used in a reaction
is found (Schiegl & Vogel, 1970; Florkowski, 1985; Tanweer et al., 1988;
Tanweer, 1993). This effect is thought to depend on dissolving of H2 in
the Zn, with larger fractionations for larger mass of Zn; smaller
effects are found for Zn with a grey surface (BDH Chemicals Zn shot)
compared with grains with a shiny surface from other producers
(Florkowski, 1985). Different mass effects were found for the different
qualities of the BDH Zn shot (see above)(Tanweer, 1993). The size of the
effect is related to the amount of Zn and temperature; a steady decrease
at a rate of 1.5‰ per g of water reduced is reported by Schiegl & Vogel
(1970). Karhu (1997) tested the ‘excess Zn’ effect with Zn-Na mixtures,
and found no systematic effect; Eventual effects are related to H2
absorption and impurities in the Zn. Flow-trough reactors suffer memory
effects, with dependency on size of water samples (mass-effect)(Demény,
1995).
[NOTE: now the comments by Arndt, with reference to this isogeochem list
discussion, will be added to this review]

References:

Demény, A., 1995, H isotope fractionation due to hydrogen-zinc reactions
and its implications on D/H analysis of water samples. Chem. Geol., v.
121, p. 19-25.

Florkowsky, T., 1985, Sample preparation for hydrogen isotope analysis
by mass spectrometry. Int. J. Appl. Radiat. Isot., v. 36, p. 991-992.

Karhu, J. A., 1997, Catalytic reduction of water to hydrogen for
isotopic analysis using zinc containing traces of sodium. Anal. Chem.,
v. 69, p. 4728-4730.

Schiegl, W. E., and J. C. Vogel, 1970, Deuterium content of organic
matter. Earth Planet. Sci. Lett., v. 7, p. 307-313.

Tanweer, A., G. Hut, and J. O. Burgman, 1988, Optimal conditions for the
reduction of water to hydrogen by zinc for mass spectrometric analysis
of deuterium content. Chem. Geol. (Isot. Geosc. Sect.), v. 73, p.
199-203.

Tanweer, A., 1993, Investigation of BDH zinc for hydrogen isotope
analysis by mass spectrometry. Z. Naturforsch., v. 48a, p. 739-740.

I hope this is of some value to the users of the Zn reduction method.
Best wishes,
Pier.
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