Water Scaling & Precipitation Calculator

Screen calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, calcium fluoride, silica and magnesium hydroxide scaling from a water analysis. The calculator also reports ionic strength, activity coefficients, an activity-based Langelier Saturation Index (LSI) and Ryznar Stability Index (RSI).

Enter the water analysis

Use dissolved concentrations in mg/L on the stated reporting basis. Required fields must contain a number; enter zero only when justified by the analysis. Leave optional unknown constituents blank: their mineral rows will be marked “Not assessed”. A non-detect is not automatically zero. All measurements must describe the same water at 25 °C.

Water composition at 25 °C

Parameter	Concentration / value	Required unit
pH at 25 °C *		5–11.5
Total alkalinity *		mg/L as CaCO₃
Calcium, Ca *		mg/L as Ca
Magnesium, Mg *		mg/L as Mg
Sodium, Na *		mg/L as Na
Potassium, K *		mg/L as K
Chloride, Cl *		mg/L as Cl
Sulfate, SO₄ *		mg/L as SO₄
Barium, Ba		mg/L as Ba
Strontium, Sr		mg/L as Sr
Fluoride, F		mg/L as F
Dissolved silica		mg/L as SiO₂
Dissolved orthophosphate		mg/L as P

Include hydroxyapatite as an exploratory calcium-phosphate screen

Alkalinity is as CaCO₃, not mg/L bicarbonate. Sulfate is as SO₄, not as sulfur; silica is as SiO₂, not as silicon; phosphate is dissolved orthophosphate as P, not total phosphorus or mg/L PO₄. Do not enter total hardness in the calcium field. To convert orthophosphate reported as PO₄ to P, multiply by 0.3261; to convert dissolved silica reported as Si to SiO₂, multiply by 2.1393.

Calculate scaling screen Load example water Clear values

Water chemistry and stability indices

Concentrations are used as an approximation to molality for dilute water. Activity coefficients are dimensionless. Charge imbalance uses 100 × (cation charge − anion charge)/(cation charge + anion charge); it is a completeness check, not proof of analytical accuracy.

Mineral saturation at 25 °C

SI = log₁₀(IAP/K); the saturation ratio is IAP/K = 10^SI. A positive SI means supersaturation with respect to the named phase in this model. It does not mean that precipitation is certain or immediate. Results are displayed as ordinary decimal numbers, without e+ notation.

What the results mean

LSI, RSI and calcite saturation

LSI compares the measured pH with the pH at which calcite reaches saturation. Here, pHₛ is found by solving the calcite equilibrium at the same total alkalinity, calcium concentration and activity coefficients, including the modeled phosphate, silica and water contributions to alkalinity. The search range is pH 5–11.5; a solution outside that range is not reported. This activity-based formulation can differ from empirical TDS-based LSI calculators. Calcite SI evaluates the actual entered water, while LSI uses the calculated saturation pH; they need not have identical numerical values.

Positive LSI indicates a thermodynamic tendency toward calcium carbonate precipitation; negative LSI indicates calcite undersaturation. RSI is an empirical stability index calculated as 2pHₛ − pH. It has no direct saturation-ratio meaning and should be interpreted using experience with the particular water system. Neither index is a direct measure of metal corrosion, and a low scaling tendency does not establish that water is non-corrosive.

Ionic strength and activity coefficients

Ionic strength is estimated from I = ½Σcᵢzᵢ². The Davies equation at 25 °C is log₁₀γᵢ = −0.509zᵢ²[√I/(1 + √I) − 0.3I]. Activity is approximated by γ times molar concentration relative to the standard concentration. Neutral dissolved silica is assigned an activity coefficient of one. The calculation iterates ionic strength with carbonate, phosphate and silica acid-base partitioning.

The model is restricted to I ≤ 0.1 mol/L and warns above 0.01 mol/L. This is a screening boundary, not an accuracy guarantee. Ion pairing can matter even below it, especially for sulfate, phosphate and divalent metals. Unknown or unlisted ions can alter ionic strength and charge balance. A full model such as PHREEQC, with an appropriate database and activity model, is needed when those effects matter.

Why mineral phase and reporting basis matter

Calcium sulfate is shown separately as gypsum and anhydrite. Silica is shown as amorphous silica and quartz: quartz supersaturation alone does not establish a risk of rapid amorphous-silica fouling. Silica partitioning accounts for the first two dissociation steps of silicic acid; polymerization, colloids and metal-silicate deposits are not modeled. Hydroxyapatite is one possible calcium-phosphate phase, not a prediction of every phosphate deposit. Other calcium phosphates, metal complexes and reaction kinetics may control real systems.

Saturation is not a precipitation quantity

An SI of +1 corresponds to a saturation ratio of 10 for the reaction as written; SI of −1 corresponds to 0.1. A ratio is not a percentage of scale or a mass concentration. This calculator does not remove precipitated material, recalculate equilibrium after precipitation, predict induction time, calculate antiscalant dose, or estimate deposit mass. Temperature, pressure, gas exchange, surfaces, mixing, inhibitors and residence time can change the outcome. Enter the analysis of the stream being assessed; do not multiply feed concentrations by an RO recovery factor without a suitable concentrate mass balance.

The “near equilibrium” label uses a display band of −0.05 to +0.05 SI to avoid overinterpreting tiny numerical differences. Thermodynamic equilibrium is SI = 0; the display band is not a universal analytical uncertainty or operating limit. A zero input gives a zero ion-activity product and no finite logarithmic SI. Missing optional data gives “Not assessed” instead of a zero result.

Calculation basis and constants

The model uses nominal 25 °C log K values from the USGS-distributed WATEQ4F database. Water activity is assumed to be one. Carbonate partitioning uses pK₂ = 10.329 and water uses pK_w = 14. Orthophosphate uses cumulative protonation log K values of 12.346 and 19.553; fully protonated phosphoric acid is neglected over the supported pH range. Silicic-acid dissociation log K values are −9.83 and −23. Alkalinity is converted using 50 mg CaCO₃ per meq and corrected for modeled hydroxide, hydrogen ions, phosphate and silicate before carbonate is calculated.

For brucite the reaction is Mg(OH)₂ + 2H⁺ ⇌ Mg²⁺ + 2H₂O. For hydroxyapatite it is Ca₅(PO₄)₃OH + 4H⁺ ⇌ 5Ca²⁺ + 3HPO₄²⁻ + H₂O. Their log K values therefore refer to these acid-dissolution reactions, not to a simple hydroxide or phosphate Ksp expression.

References and related calculators

WATEQ4F thermodynamic data used here; USGS activity-coefficient and aqueous-species documentation; USGS mineral-equilibrium documentation.

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Warning: Lenntech BV cannot be held responsible for errors in the calculation, the program itself or the explanation. For questions or remarks please contact Lenntech.