PhreeqcUsers Discussion Forum
Beginners => PHREEQC Basics => Topic started by: deso on February 15, 2016, 08:31:05 AM

Hi PHREEQC users, I know this is covered somewhat in other posts, but I've read these, plus other stuff on the net and it's still not clear to me how to input alkalinity.
I know that the notation for Alkalinity = Alkalinity as CaCO3. So is that Total alkalinity as CaCo3? What if I only have HCO3 mg/L?
As a general rule I have been entering HCO3 mg/L as Alkalinity by converting HCO3 mg/L to HCO3 as CaCO3 (HCO3 as CaCo3 = HCO3 mg/L / 1.22).
Is this correct? I'm not following the other conversions in other threads/posts.
Alkalinity as CaCO3 (to me) can mean several things, Total Alkalinity as CaCO3, CO3 mg/L as CaCO3 and HCO3 mg/L as CaCO3, so I'm getting confused when looking at multiple discussions....I'm not sure they're always calling things the same term.
I'm running basic inverse modelling scenarios. I notice sometimes if I use C(4) instead the solutions still balance (although I know this is wrong as C(4) is total C, but then why would it balance?). Also, while entering Alkalinity as HCO3 as CaCO3 (as described above) generally works, sometimes a solution wont balance, and so the models wont run.

In PHREEQC, Alkalinity is the total alkalinity from all species that are titrated in an alkalinity titration (including some negative contributions, like H+). It includes contributions from HCO3 and CO32, but also possibly OH, H3SiO4, H2BO3, etc. There is an alkalinity option in PRINT that writes all of the aqueous species that contribute to the calculated alkalinity. The total alkalinity is printed either in the listing of total concentrations, or in the "Description of solution".
PHREEQC actually needs alkalinity as eq/kgw, but you can define alkalinity in a number of ways that will be converted into these units. Your confusion arises from the conversion of mg/L to meq/L (ignoring the small conversion from L to kgw). You need a factor that is mg/meq, which depends on the way that alkalinity is reported in the analysis. If alkalinity is reported as HCO3, then the factor is 61. If the alkalinity is reported as CaCO3, then the factor is 50.
Rarely, both HCO3 and CO32 are reported. If there is no additional value for total alkalinity reported for the analysis, the alkalinity must be summed using the factors 61 mg/meq for HCO3 and 30 mg/meq for CO32.
As long as total alkalinity is reported, you do not need to convert the alkalinity to mg/L CaCO3. The facility exists in SOLUTION and SOLUTION_SPREAD to define any of the following:
Alkalinity x meq/L
Alkalinity y mg/L as HCO3 (alternatively, Alkalinity y mg/L gfw 61)
Alkalinity z mg/L as CaCO3 (alternatively, Alkalinity z mg/L gfw 50)
Once you determine the units of your analyses, I think one of these three units will be appropriate.
At pH ~8 there is not too much difference between C(4) (TDIC, total dissolved inorganic carbon) and HCO3 because the contribution of CO32 and CO2(aq) to TDIC are small and usually there are few other alkalinity species than HCO3. In this case, entering either meq/L Alkalinity or mmol/L C(4) would give similar results.

Hi dlparkhurst, thanks for your reply. I know you must explain this issue a lot.
For simplicity, lets forget about total alkalinity. I never have total alkalinity and rarely see it reported. Almost always I have HCO3 mg/L, usually without any recorded data for CO3 mg/L.
I don't understand entirely what you mean by a factor. Do you mean convert the HCO3 mg/L to meq/L. So HCO3/61 = meq/L? Then list that as "alkalinity". This works sometimes, but not always. In fact, I find I have more success by using mg/L and converting the HCO3 to HCO3 as CaCO3 (HCO3/1.22) then using the "alkalinity" notation. In some cases I have data in meq/L and using a C(4) or alkalinity notations for HCO3 dont work, but if I convert the HCO3 to mg/L then divide by 1.22 (i.e. convert to HCO3 as CaCO3, then solutions balance). I don't understand why this would happen, especially when C(4) and alkalinity notation work fine for other HCO3 meq/L data
I don't understand your list or notations, Is "alkalinity y" a notation or just have a y after it because its in the list. I am comparing scenarios from different papers. Sometimes the HCO3 is a tritration, sometimes it is some undisclosed measurement and it's ambiguous as to whether or not it is alkalinity or HCO3 or HCO3 as CaCO3. Sometimes the HCO3 is measured using a in field meter (?)
Is there any way to ask PHREEQC to adjust the alkalinity so the solution balances before running the inverse modelling? This way, if I assume all data is HCO3 as mg/L and convert to HCO3 as CaCO3 and enter this as "Alkalinity" then I will be at least using a consistent approach.

I think you always have total alkalinity, it can just be reported in different ways.
Alkalinity is usually measured by titration with an acid. The measurement is really the number of equivalents added to a sample to attain an endpoint, usually in the neighborhood of pH 4.5. This titration includes any aqueous species that will accept an H+ at a pH above the endpoint. The total alkalinity is then reported in various units, such as mg/L as HCO3, or mg/L as CaCO3. This does not mean that there is literally that many mg/L of HCO3 ion or CaCO3 in solution; it is the number of equivalents of total alkalinity expressed in these units. Often the titrant is normalized to express alkalinity in these units, so the number of meq is never actually specified, but conceptually, you are using an acid of a certain normality and the titration is meq of acid added to the solution.
You seem to think that you have a measurement of the ion HCO3, but I do not think this is the case. I would be very surprised if you have HCO3 concentration measured by electrode in any of your analyses. If you are certain that you do have measurements of HCO3 rather than alkalinity, then we can talk about it, but I doubt it.
x, y, and z were intended to be alkalinity concentrations taken from the analysis. I will be more explicit. If you have a solution with 1 meq/L, then all of the following are equivalent:
SOLUTION
units mg/L
Alkalinity 1 meq/L
END
SOLUTION
units mg/L
Alkalinity 61 mg/L as HCO3
END
SOLUTION
units mg/L
Alkalinity 61 mg/L gfw 61
END
SOLUTION
units mg/L
Alkalinity 50 mg/L as CaCO3
END
SOLUTION
units mg/L
Alkalinity 50 mg/L gfw 50
END
(I include units mg/L because the default units are mmol/kgw, and it is not allowed to mix units of /L with units of /kgw).
Note that in PHREEQC there is no unit meq/L for any element other than Alkalinity. If you specify meq/L for Ca (or as the default, units meq/L), the concentration of Ca+2 will be interpreted as mmol/L. If you use PhreeqcI to define SOLUTION input, meq/L is not in the list of default units. I think perhaps your problem is not with alkalinity but with assuming meq/L can be applied to divalent ion concentrations.
Please look at the output file to see the description of your solutions. The output will include the molalities of the elements and the Alkalinity, plus if you use PRINT; alkalinity true, you will see all of the species that contribute to alkalinity. Look at how the input data have been converted to molalities and, for Alkalinity, eq/kgw. Do some experimentation. Skip the INVERSE_MODELING. you need to get your solution definitions correct before inverse modeling. The charge balance is given in the printout; look at it.
Saying "HCO3 as CaCO3" does not really make sense. It is either Alkalinity as HCO3 or Alkalinity as CaCO3, and you are making needless conversions because PHREEQC will do a conversion to equivalents if you use the correct version of the SOLUTION definitions above.
I do not think it is a good idea, but you can charge balance on C(4).

Also, if you are using SOLUTION_SPREAD, this is the way to define Alkalinity in mg/L HCO3, where \t indicates a tab.
SOLUTION_SPREAD
units mg/l
pH\t Alkalinity \t Na
\t mg/l as HCO3\t
8.0\t 61\t 23

Thanks dlparkhurst for taking the time to explain that. I think it's clear to me now. I appreciate you're time.
BTW  PHREEQC forum often doesn't open for me.....not sure if its bug...but I have the same prob at work and home. Hence my delays in being able to respond sometimes.