Dear Colleagues,

I don t know whether I should continue on this matter or not. It looks that there are
some who are interest in knowing it. In my opinion, too, the Isogeochem Server is a
good opportunity to discuss our viewpoints and improve our understanding with the
comments of others. Let me write first that I am a regular reader of GCA and it
publishes excellent material. Of course, it is well established. My comments were only
that the reviewers should put more attention when a mistake had been pointed out.
People at least I use the information published there as granted.

I worked out the problem using the activity approach. The data are given in the
attached file. I use  Error and Trail method  to solve a non-linear equation. I do it
in excel without writing any programming code. I don t think that anyone is
interested in that. The basis mistake in the reply by Halas et al. (GCA, 64, 575-576,
2000) was that their equation 5 should be K3=a(H)a(OH)/a(H2O). The values are given
for their system (10 g Na2CO3+CO2 from 1g of Na2CO3). Again, there could be small
differences if you use different constants. I tried to use the same constants as were
given by Halas et al. I did the calculation without re-check. So,
there could be some mistakes. I think it could be better if Halas et al. change
equation 5 and re-calculate the parameters.

Recently, we have started a project to study rainwater acidity around geothermal
systems. We observed that it was really difficult to measure pH of pure water like
rainwater with a pH-meter (rainwater characterization is based on pH around the world).
Similarly there are problems in measuring pH of highly
acidic or alkaline (more in case of alkaline) waters. (we are working to understand this).
Additionally, the IAEA has
conducted three interlab calibrations (Giggenbach, IAEA-TECDOC-641, Vienna,
p.439-456, 1989 and two are very recent). If you look these reports, you will find
that there is some improvement in the analysis after 10 years; but the quality of analysis
of
especially SiO2, HCO3 and CO3 is far bellow the expectations. That is different topic
how to improve it. But the results indicate that in my viewpoint one should not
design a fundamental experiment (like in discussion) where there is need of
correcting isotopic measurements with chemical analysis. As I tried to explain it in
my comments. The calculated  ratio of HCO3:CO3 is 18:82. So there is need of correcting the
HCO3 contribution. One can calculate the propogation of error in their isotopic data due to
the error in the chemical analysis.

Secondly, the partial pressure of CO2 in vapor phase is very low (much less than
10^-4 bars). If you are extracting 10 micromoles, you can calculate how much volume
should be of the experimental vessel and how the vapor phase CO2 can be extracted
only without changing its isotopic composition. The amount of water vapor will be
much higher than that of CO2. Check the steam tables. When you condense this vapor
some part of CO2 will get dissolved in the water. So there will be some fractionation
in this process, too. So, there are many points one has to consider for this work. I
do agree that some thing is better than nothing. This way the calculated values (with
spectroscopic data) could be more accurate than the experimentally measured.  The
measurement of isotopic
fractionation for CO2(g) and carbonate is a challenging subject. Definitely, it is
hard job to work on this type of problems. So, we should do encourage the people who are
working such types of problems. My comments are just some suggestions,
which may be considered during their work.

Best regards

Mahendra