Calcium Carbonate Precipitation Potential (CCPP) Calculator
The Langelier index tells you the direction; CCPP tells you the quantity — how many mg/L of CaCO3 will precipitate (scaling) or can dissolve (corrosive). This is a screening estimate: verify against lab testing before design decisions.
Calcium Carbonate Precipitation Potential
| Item | Value |
|---|
How the calculation works
Method — simplified Caldwell–Lawrence, solved iteratively (screening estimate)
1. Convert: Ca (mol/L) = Ca(mg/L as CaCO3) / 100000; Alk (eq/L) = Alk(mg/L as CaCO3) / 50000.
2. Temperature constants (Plummer & Busenberg, T in Kelvin): pK1, pK2, and pKso for calcite.
3. Ionic strength I ≈ 2.5×10−5 × TDS; activity coefficients from the Davies equation.
4. Conditional constants K1′, K2′, K′so from the activity coefficients.
5. Initial dissolved inorganic carbon CT from measured pH and alkalinity.
6. Bisect for X = mol/L of CaCO3 that must precipitate (X > 0) or dissolve (X < 0) to reach saturation, where precipitating X removes X mol/L calcium, 2X eq/L alkalinity and X mol/L carbon; pH is re-solved from the carbonate equilibrium at each step.
7. CCPP (mg/L as CaCO3) = 100000 × X.
1. Convert: Ca (mol/L) = Ca(mg/L as CaCO3) / 100000; Alk (eq/L) = Alk(mg/L as CaCO3) / 50000.
2. Temperature constants (Plummer & Busenberg, T in Kelvin): pK1, pK2, and pKso for calcite.
3. Ionic strength I ≈ 2.5×10−5 × TDS; activity coefficients from the Davies equation.
4. Conditional constants K1′, K2′, K′so from the activity coefficients.
5. Initial dissolved inorganic carbon CT from measured pH and alkalinity.
6. Bisect for X = mol/L of CaCO3 that must precipitate (X > 0) or dissolve (X < 0) to reach saturation, where precipitating X removes X mol/L calcium, 2X eq/L alkalinity and X mol/L carbon; pH is re-solved from the carbonate equilibrium at each step.
7. CCPP (mg/L as CaCO3) = 100000 × X.
Notes & limitations
Simplified implementation of the Caldwell & Lawrence (1953) / Merrill approach: calcite only, Davies activity model, closed system. Treat the result as a screening estimate — for design, confirm with laboratory testing or established water-chemistry software.
- Positive CCPP = precipitation potential (scaling); negative CCPP = dissolution potential (the water can dissolve that much CaCO3 — corrosive toward cement, concrete and unlined pipe).
- The calculation ignores magnesium, silica, phosphate and other ions that affect real-world scale and corrosion — it answers the calcium-carbonate question only.
- Verify the constants against the current edition of your reference (e.g. AWWA M22) if you use this for regulatory or design work.
- Small input errors in pH move the result noticeably — use a calibrated meter and a fresh, un-aerated sample.
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