| Metric | Value |
|---|
Plotly Graph
Calculator Inputs
CO₂* represents dissolved carbon dioxide plus hydrated carbonic acid. Default constants approximate freshwater conditions near 25°C.
Example Data Table
| Scenario | pH | DIC | Unit | pKa₁ | pKa₂ | Expected dominant species |
|---|---|---|---|---|---|---|
| Acidified groundwater | 5.80 | 1.20 | mmol/L | 6.35 | 10.33 | CO₂* |
| Neutral river water | 7.40 | 1.80 | mmol/L | 6.35 | 10.33 | HCO₃⁻ |
| Alkaline process water | 10.60 | 3.50 | mmol/L | 6.35 | 10.33 | CO₃²⁻ rising |
Formula Used
Ka₁ = 10-pKa₁ and Ka₂ = 10-pKa₂
[H⁺] = 10-pH
D = [H⁺]² + Ka₁[H⁺] + Ka₁Ka₂
α₀ = [H⁺]² / D for CO₂*
α₁ = Ka₁[H⁺] / D for HCO₃⁻
α₂ = Ka₁Ka₂ / D for CO₃²⁻
Species concentration = α × CT, where CT is total inorganic carbon.
The alkalinity estimate uses Alk ≈ [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] − [H⁺]. It is useful for screening but does not replace a full ionic-strength or borate-corrected model.
How to Use This Calculator
- Enter the measured pH of your water or solution.
- Input total inorganic carbon as either mmol/L or mg/L as carbon.
- Keep the default pKa values for standard aqueous conditions, or enter custom constants for your matrix.
- Click Calculate Speciation to display the result block above the form.
- Review fractions, species concentrations, and the estimated alkalinity outputs.
- Download the result set as CSV or PDF for reporting.
Carbon Equilibrium Across Water Systems
Carbon speciation controls pH stability, mineral scaling, corrosion tendency, and biological carbon availability. In freshwater near 25°C, bicarbonate often dominates between pH 6.5 and 10.3. Below that interval, dissolved carbon dioxide becomes more important. Above it, carbonate increases quickly and can influence precipitation risks.
Why pH Drives Species Distribution
The distribution fractions α₀, α₁, and α₂ depend on hydrogen ion activity relative to the first and second dissociation constants. At pH 8.3 with pKa values 6.35 and 10.33, bicarbonate usually carries more than 95% of dissolved inorganic carbon. This makes moderate pH shifts operationally significant.
Interpreting DIC With Fractions
Total inorganic carbon alone does not show process behavior. A sample containing 2.5 mmol/L DIC may still act very differently at pH 6.0 versus pH 10.5. Multiplying DIC by the calculated fractions converts one bulk measurement into species-specific concentrations used in treatment, geochemistry, and monitoring workflows.
Operational Relevance of Alkalinity
Estimated alkalinity links carbon chemistry with buffering performance. Higher bicarbonate and carbonate concentrations usually increase acid neutralizing capacity. In treatment plants, this affects coagulant demand, softening efficiency, and post-treatment stabilization. In natural waters, alkalinity supports resilience against rapid pH swings after rainfall or biological activity.
Where This Model Helps Most
This calculator is practical for groundwater screening, aquaculture review, process-water checks, laboratory teaching, and field surveys. It is especially useful when analysts already know pH and DIC but need rapid conversion into CO₂*, HCO₃⁻, and CO₃²⁻ values. Export options also simplify reporting and recordkeeping.
Limits and Good Practice
The model assumes a simplified carbonate system and user-supplied constants. Real samples may require temperature corrections, ionic strength adjustments, and contributions from borate, phosphate, ammonia, or organic alkalinity. For compliance or design work, treat this tool as a screening model and confirm critical decisions with full equilibrium software.
FAQs
What does CO₂* mean in the results?
CO₂* represents dissolved carbon dioxide plus the small hydrated carbonic acid portion commonly grouped together in aqueous carbonate calculations.
Can I enter DIC in mg/L as carbon?
Yes. Select the mg/L as C option and the calculator converts that value into molar concentration before calculating species fractions and concentrations.
Why is bicarbonate often the dominant species?
In many natural waters, pH falls between the first and second carbonate dissociation constants. That range favors bicarbonate over carbon dioxide and carbonate.
Does temperature change the results?
Yes. Temperature changes equilibrium constants and gas solubility. This version lets you record temperature, but pKa adjustments must be entered manually.
Is the alkalinity value exact?
No. It is an estimate from carbonate species and water autoionization. Samples with other acid-base contributors may differ from measured alkalinity.
When should I use a more advanced model?
Use a full equilibrium model for saline waters, high ionic strength samples, compliance studies, precipitation prediction, or systems with multiple buffering species.