Calculator Inputs
Example Data Table
| Sample | pH | Alkalinity | Unit | Temperature | Expected dominant species |
|---|---|---|---|---|---|
| Soft lake water | 7.20 | 65 | mg/L as CaCO₃ | 18 °C | HCO₃⁻ |
| Tap water | 8.30 | 120 | mg/L as CaCO₃ | 25 °C | HCO₃⁻ |
| High pH process stream | 10.40 | 4.10 | meq/L | 30 °C | CO₃²⁻ rising strongly |
| Marine-like sample | 8.05 | 2.30 | meq/L | 22 °C | HCO₃⁻ with some CO₃²⁻ |
These rows show realistic input styles for practice and validation.
Formula Used
Hydrogen ion: [H⁺] = 10-pH
Dissociation constants: K₁ = 10-pKa1, K₂ = 10-pKa2
Distribution denominator: D = [H⁺]² + K₁[H⁺] + K₁K₂
Species fractions:
α₀ = [H⁺]² / D
α₁ = K₁[H⁺] / D
α₂ = K₁K₂ / D
Total alkalinity approximation: TA ≈ [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] − [H⁺]
DIC solution: DIC = (TA − [OH⁻] + [H⁺]) / (α₁ + 2α₂)
Species concentrations:
[CO₂*] = α₀ × DIC
[HCO₃⁻] = α₁ × DIC
[CO₃²⁻] = α₂ × DIC
This model estimates carbonate partitioning from pH and alkalinity. It excludes borate, phosphate, silicate, and activity corrections used in advanced marine packages.
How to Use This Calculator
- Enter the measured sample pH.
- Provide total alkalinity and choose the correct unit.
- Set temperature and optional salinity.
- Keep default constants, or override them for specialized work.
- Click the calculation button.
- Review DIC, carbonate species, fractions, and the chart.
- Export the results as CSV or PDF.
- Compare outputs with laboratory or field observations.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates dissolved inorganic carbon and its partitioning into dissolved carbon dioxide, bicarbonate, and carbonate using pH, alkalinity, and selected equilibrium constants.
2. Why is bicarbonate often dominant?
Near neutral and mildly alkaline pH, bicarbonate usually dominates because the first dissociation is favored while the second remains limited until pH rises further.
3. What does CO₂* mean here?
CO₂* represents hydrated dissolved carbon dioxide plus carbonic acid grouped together. Many practical carbonate calculations treat them as one combined species.
4. Why can results become nonphysical?
Conflicting pH and alkalinity inputs can force negative DIC. That often signals unit mistakes, poor measurements, or chemistry outside this simplified alkalinity model.
5. Should I override pKa values?
Use defaults for general educational work. Override constants when your method, ionic strength, salinity, or reference data requires specific equilibrium values.
6. Does this include all buffering ions?
No. This simplified tool focuses on carbonate alkalinity and water dissociation. It does not include borate, phosphate, silicate, ammonia, or organic alkalinity.
7. Which alkalinity unit is safest to use?
Use the exact unit reported by your lab. mg/L as CaCO₃ is common, while meq/L is often clearer for equilibrium calculations.
8. When is this calculator most useful?
It is useful for water treatment screening, aquaculture checks, teaching acid-base equilibria, and quick validation of carbonate distribution trends.