Alkalinity not converging - alkalinity input as C(4)

[pvignoni]

Dear all,

I'm working with saline water samples (between 36 and 95 psu) with high B content (14 to 40 mmol/l). I need to calculate SI (particularly for carbonate minerals) and the distribution of species. The water pH varies between 7 and 8.6. I'm using the pitzer database. With some samples I get the error message "Alkalinity has not converged" and the question "Is non carbonate alkalinity greater than total alkalinity?". Considering the high B content and the fact that if I do not include B everything works, B is contributing a lot to total alkalinity. I read another comment where someone had similar problems with As, and a possible solution was to specify a total concentration of C(4).

I only have alkalinity values that were measured by end point titration until pH 4.5. The measured values are clearly not the total alkalinity since I get the message is non carbonate alkalinity greater than total alkalinity. Is it right to assume that the measured alkalinity is the carbonate alkalinity? How do I get C(4) from my alkalinity values or how should I input alkalinity in a way that it calculates total alkalinity considering B?

SOLUTION 1 LT_2021
    temp      12.8
    pH        8.58
    pe        4
    redox     pe
    units     mmol/l
    density   1
    Alkalinity 7.12 meq/l
    Ca        166.16
    Cl        1972.7
    K         27.84
    Mg        113.61
    Na        1182.42
    S(6)      22.4
    B         37.68
    Li        43.68
    Sr        2.75
    Si        2.14

Thanks!

[dlparkhurst]

An alkalinity titration should give total alkalinity, not just carbonate alkalinity. In your analysis both B and Si would make contributions to the total alkalinity, and it would be more than your alkalinity measurement.

The stated analysis is an NaCl brine with specific conductance of about 100000 uS/cm. Just checking. I'm wondering if the units of your analysis are mg/L rather than mmol/L with a specific conductance of about 5000?

If the analytical units are correct, I don't think you can do much to estimate the carbon concentration. You could define the following instead of Alkalinity, which would give the carbon concentration necessary to produce calcite equilibrium. That might give you some idea of what a reasonable TDIC concentration would be.

Code: [Select]

C(4) 1 calcite 0


[pvignoni]

Dear David,

Thanks for your answer. The analytical units are correct, it is indeed a NaCl brine with a specific conductance of 144000 uS/cm. I will do as you suggest and define a C concentration to produce calcite equilibrium and calcite precipitation as it is happening in this brine.

Thanks for your time!

Paula