Calculator Inputs
Example Data Table
The table below shows how the carbonate system shifts as pH changes while total inorganic carbon remains 2.50 mmol/L.
| Scenario | pH | CO2* (%) | HCO3− (%) | CO3²− (%) | Alkalinity (mg/L as CaCO3) | Dominant species |
|---|---|---|---|---|---|---|
| Acidic sample | 6.30 | 52.87 | 47.12 | 0.00 | 58.89 | CO2* / H2CO3 |
| Near-neutral sample | 8.30 | 1.10 | 97.99 | 0.91 | 124.87 | HCO3− |
| Alkaline sample | 10.50 | 0.03 | 40.34 | 59.66 | 215.38 | CO3²− |
Formula Used
Hydrogen and equilibrium constants
[H+] = 10−pH
Ka1 = 10−pKa1
Ka2 = 10−pKa2, Kw = 10−pKw
Distribution denominator
D = [H+]2 + Ka1[H+] + Ka1Ka2
Species fractions
α0 = [H+]2 / D for CO2* / H2CO3
α1 = Ka1[H+] / D for HCO3−
α2 = Ka1Ka2 / D for CO3²−
Concentrations and carbonate alkalinity
Species concentration = α × CT
[OH−] = Kw / [H+]
Alkalinity ≈ [HCO3−] + 2[CO3²−] + [OH−] − [H+]
How to Use This Calculator
- Enter the measured pH for the sample.
- Input total inorganic carbon and choose the correct concentration basis.
- Set sample volume if you want total moles for a collected volume.
- Review pKa1, pKa2, and pKw. Keep defaults for common 25 °C work or adjust them for your chosen chemistry reference.
- Choose the number of decimals to display.
- Click the calculate button to see the result block above the form, then export the data to CSV or PDF.
FAQs
1. What does this calculator estimate?
It estimates how total inorganic carbon distributes among dissolved carbon dioxide, bicarbonate, and carbonate at a selected pH and equilibrium constant set.
2. Why is bicarbonate often dominant near neutral pH?
Between the first and second dissociation constants, bicarbonate is thermodynamically favored. That is why many natural waters around pH 7 to 9 show bicarbonate as the main carbonate species.
3. Does temperature automatically change the constants here?
No. Temperature is displayed for reporting. To reflect temperature effects, update pKa1, pKa2, and pKw with values from your laboratory method or reference source.
4. What is CO2* in the results table?
CO2* is the combined dissolved carbon dioxide and hydrated carbonic acid term commonly used in carbonate-system calculations and water-chemistry balances.
5. Why can alkalinity differ from a lab report?
This page estimates carbonate alkalinity from equilibrium species. Real samples may include borate, phosphate, silicate, ammonia, organic acids, or ionic-strength effects that change measured total alkalinity.
6. Which input unit should I choose?
Choose the basis that matches your dataset. For example, use mmol/L for direct molar data, mg/L as C for carbon reporting, or mg/L as CaCO3 for hardness-style conventions.
7. Can I use this for seawater or brines?
It is best for simplified aqueous calculations. High-salinity systems may need ionic-strength corrections, activity coefficients, and seawater-specific carbonate models for better accuracy.
8. What do the ratio outputs help me see?
The ratios show how strongly the system leans toward bicarbonate over dissolved carbon dioxide, or carbonate over bicarbonate, which helps interpret buffering and pH sensitivity.