We did the experiment as Roy suggests on seawater samples.  Using 2.5 mL
sample in 15 mL vessel with 800 mb CO2 isotopic equilibrium was achieved
within 200 minutes.  Samples are gently shaken (not stirred) at 25.0 C.

regards

Russell Frew


> Dear Dachun,
> It is my belief that if one "rocks" the samples, overnight is long enough.
> If CO2 sits statically over water, isotopic equilibration can take longer.
> Rocking breaks up any microlayer of whatever might be on the surface and
> also provides more effective interaction between the bulk of the water and
> the CO2. I recall that some years ago, someone claimed that isotopic
> equilibration of CO2 and H2O could be speeded up considerably by using
> ultrasonic agitation. Perhaps another reader can comment. I think that
this
> would heat  the water and one would need to reproduce the thermal changes
> during the equilibrations  to obtain good precision. It is easy to check
how
> long it takes to get sufficiently close to  equilibrium conditions. Simply
> measure the delta value of CO2 that has been in contact with water for
> progressively longer times and see when any further change in time is
> smaller than your measurement error.
> Regards,
> Roy Krouse
>
> > Hi, Glen:
> >
> > Is it enough for one overnight equilibration between water and CO2?  One
> > article says it takes five days. We use three days.
> >
> > Regards.
> >
> > Dachun Zhang
> > Isotope Lab
> > Zymax
> >
> >
> > "Glen E. Martin" wrote:
> >
> > > Adam Porowski,
> > >
> > > We place  1 mL of sample water in a vacuum flask. Freeze the water,
and
> > > evacuate the air.
> > > The water is melted, and the vacuum reaction flask (internal volume
> > > 10-20mL) is filled with CO2(g) to ambient (atmospheric pressure) minus
5
> or
> > > 10 cms Hg (using a mercury manometer).The sealed sample is
equilibrated
> at
> > > 25.3 degrees C overnight.
> > >
> > > The CO2(g) is extracted/purified the next day for analysis.
> > >
> > > G. Martin
> > >
>