Dear Tomasz,
We try to understand science or we are just trying to justify ourselves. It is good
that I am not infront of you, otherwise....
OK, give me chance, by Next Monday I will do the calculation using the activity approach
and show you that both the methods (approximation to molar concentration and activity)
provide the same results as I presented. In the meantime please look on the points I
presented. (1) Think why there is no bicarbonate and OH terms in the ionic force
expression. (2) How Halas et al got the activity of water in the solution to 0.98? (3)
How precisely can we analyze chemical species in a solution to correct isotopic
analysis? Oh, Newton's method is also an approximation method. You should use some
analytical (exact) solution for this. Newton's method is just an approximation method to
solve an algebraic equation. It is used in PHREEQ. There are many more numerical methods
to work out this problem. Read some book on numerical analysis.
But promise me if both methods give same results for this problem, you will change your
wordings. You want me to work on this problem. Please reply me first the above mentioned
points like a scientist with fundamental arguments.
looking forward to your reply
Mahendra
Tomasz Durakiewicz escribis:
> Dear Dr. Verma,
>
> Calm down, please. The referees and authors of GCA are not a bunch of idiots, I
> presume. Your simplified calculations are wrong, as is the graph method you propose.
> The question is not whether one should use the graph method or concentration method,
> but rather if the activity-based calculations shoud be performed in the framework of
> Debye-Huckel (as Halas did) or Pitzer model (due to hign concentration). I suggest
> you to repeat your calculations in the framework of one of above models. If you are
> not familiar with solving equations by Newton's method, you may use the web-based
> solver WEB-PHREEQ by NDSU.
> There are usually differences in pH values calculated by use of different models.
> Therefore if possible, one should measure pH and use the measured value to calculate
> the solution. So did Halas in the course of his 3 years work on this problem.
> Your remarks on "electronic problems" are chaotic and irrelevant. As physicist with
> 40 years hands-on experience with experiment, Halas is probably the best scientist
> of choice to perform this difficult work, in which the measurement of pH was really
> not the most complicated task. Also their activities seem to be correct, if you
> calculate it well.
> The detailed answer to your problems is given in the reply (GCA, 64, 575-576, 2000)
> and I say with confidence that there is no need to discuss this any more. So, I
> recommend to consider the data from Halas et al. (GCA, 61, 2691-2695, 1997) as
> reliable.
>
> With regards,
>
> Dr. Tomasz Durakiewicz
> Department of Earth and Planetary Sciences
> University of New Mexico
> Northrop Hall, 200 Yale Blvd. NE
> Albuquerque, NM 87131
> USA
>
> Mahendra Pal Verma wrote:
>
> > Dear Colleagues,
> >
> > I found that the article of Halas et al. (GCA, 61, 2691-2695, 1997) was
> > recommended as a reference for isotopic fractionation between CO2(s) and
> > carbonate, etc. I have gone through the article of Halas et al. (GCA, 61,
> > 2691-2695, 1997). They have problems in the chemistry of the system; they used
> > to measure the isotopic fractionation coefficient. I wrote a comment for it
> > (GCA, 64, 573-574, 2000). I got surprised to read their reply (GCA, 64,
> > 575-576). During writing the comment I thought that the error was a mistake by
> > the authors and was escaped by the reviewers. It can be done by anyone. After
> > reading the reply my opinion has changed. It is really incredible that no one
> > checks the basic chemistry and mathematics even after pointing the error. I
> > still do not believe that it could be possible in such a reputed Journal (GCA).
> > They have problem in chemistry of the system, so their data may not be reliable.
> >
> > Let me write first, how I solved the chemistry of the system considered by Halas
> > et al. The procedure is well established in the basic books on chemistry like
> > Aquatic Chemistry by Stumm and Morgan, 1981.
> >
> > There were seven equations in total to solve a system with seven variables (H,
> > OH, H2CO3, HCO3, CO3, alk(Na), pCO2 (g)). The five equations, which were written
> > by Halas et al. (2000) and one for total dissolved carbonic concentration
> > (CT=[H2CO3]+[HCO3]+ [CO3]+ in vapor phase (negligible for alkaline solution))
> > and other alk=Na(given). Halas et al. indirectly used the equations.
> >
> > These equations can be solved with sufficient accuracy for this system with
> > considering molar concentrations instead of activity. As I did. The curves for
> > the variation of carbonic species with pH are well standardized and can be found
> > in all the books on the carbonic chemistry. If the ratio (or values) of [HCO3]
> > and [CO3] is known, one can read the pH through the figure (graphic method
> > presented by Stumm and Morgan). A question comes in mind why there is
> > inconsistency in pH values with Halas et al. calculations. They are using
> > activity instead of molar concentration. Their results should be more accurate,
> > if they worked out correctly. The concentration of HCO3 and CO3 are sufficiently
> > close together even using slightly different values of the constants. So both
> > the approaches (molar concentration and activity) should give similar results.
> > Why is there difference in the pH values? Let me point out their basic problems.
> >
> > 1. In the equation 8 of their reply, they should include terms corresponding to
> > HCO3 and OH, as they have calculated the proportion of HCO3 and CO3 as 0.18 and
> > 0.82, respectively. At high pH the concentration of H is insignificant, but it
> > is not the case for OH.
> >
> > 2. Secondly I did not check their whole mathematics. They got the activity of
> > H2O as 0.98, which is impossible for this system. It is the activity of H2O in
> > seawaters.
> >
> > 3. I wrote the values in two decimal places in my calculation. It was just for
> > the sake of comparison. The accuracy depends on the values of the constants and
> > analytical errors. The values of these constants are pK1=6.35, pK2=10.33,
> > pKW=13.998 in the Handbook of Chemistry and Physics (David R. Lide, CRC press).
> > The values of Henry constant is not given in this form, but the accuracy may not
> > be better than the three significant digits. If you fit a regression equation,
> > it does not mean that the accuracy improve in the values of constants as
> > reported by Halas et al. Anyway the calculation will not affect much.
> >
> > 4. It is good that the pH meters do not have 10 digits display. Otherwise it
> > could be possible do isotope geochemistry with chemical analysis. I wish that
> > the authors do some exercise to calculate the error in the fractionation
> > coefficient values even with an error of 0.01 in pH, when the concentration of
> > HCO3 and CO3 are in the proportion 0.1827:0.8172. There are many problem
> > associated the pH measurement with an electrode. For example, at pH=10.2, the
> > concentration of H will be 10^-10.2, so you are measuring very low concentration
> > with an electrode. The pH-meter works better for buffer solutions. They have
> > electronics problems. Giggenbach (IAEA-TECDOC-641, Vienna, p.439-456, 1989) did
> > an interlab comparative study of geothermal water analysis. The values of HCO3
> > and SiO2 were having maximum discrepancy. The values of pH were reported only up
> > to first decimal place. For example, the values were in the range 7.6 to 8.4 for
> > a samples and had error (Standard Deviation) much higher than 0.1.
> >
> > With the above arguments I say with confidence that their values of the
> > fractionation coefficient are not reliable. So, I recommend giving a second
> > thought to use the data from Halas et al. (GCA, 61, 2691-2695, 1997).
> >
> > Cheers,
> >
> > Mahendra P. Verma
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