Calculator
Editable inputs
| Species | MW g/mol | Charge z | Concentration mg/L, editable | meq/L calculated | mmol/L calculated |
|---|---|---|---|---|---|
| Ca | 40.078 | +2 | |||
| Mg | 24.305 | +2 | |||
| Na | 22.989769 | +1 | |||
| K | 39.0983 | +1 | |||
| Ba | 137.327 | +2 | |||
| Sr | 87.62 | +2 | |||
| Fe(II) | 55.845 | +2 | |||
| NH4 | 18.03846 | +1 | |||
| Cl | 35.45 | -1 | |||
| SO4 | 96.06 | -2 | |||
| HCO3 | 61.0168 | -1 | |||
| CO3 | 60.0089 | -2 | |||
| NO3 | 62.0049 | -1 | |||
| F | 18.9984 | -1 | |||
| PO4 | 94.9714 | -3 | |||
| SiO2 | 60.0843 | 0 | |||
| B (as B) | 10.81 | 0 |
Calculated outputs
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About this tool
Before sizing any treatment, the lab data has to hang together. Enter ion concentrations in mg/L to check the charge balance, compare the ion sum with reported TDS and get an approximate ionic strength and hardness. It catches missing ions, unit slips and alkalinity errors early.
Review bands are editable, so you can match your own lab or project criteria.
Method and equations
meq/L = (mg/L) / (MW / |z|)CBE (%) = 100 × (Σ cations − Σ anions) / (Σ cations + Σ anions)Δ = (Σ mg/L − TDS) / TDSI = ½ × Σ (c_i × z_i²), c_i in mol/Lmg/L = 50 × (meq Ca + meq Mg)Assumptions
- Concentrations are entered in mg/L as the measured ion (Ca, SO4, HCO3, etc.), with boron entered as B.
- Silica and boron are uncharged in this ledger, so they add to the ion sum but not to the charge balance.
- Review bands default to 5% and 10%. Both are editable.
- Density is recorded with the analysis for your export; it is not used in these screening calculations.
Limitations
- Charge-balance error is a diagnostic for incomplete or inconsistent analyses, not a pass/fail acceptance criterion.
- The 5% and 10% bands are editable review thresholds, not universal laboratory acceptance criteria.
- The tool does not calculate mineral saturation, scaling indices or chemical dosing.
- For concentrated brines, activities, complexation and speciation matter. Use a validated non-ideal model (for example PHREEQC with Pitzer or SIT) when saturation or equilibrium chemistry is required.
- Ion sum and gravimetric TDS are related but different measurements, so a small difference is normal.
Worked example
The default analysis is a high-salinity brine with 22,000 mg/L sodium, 39,000 mg/L chloride, 5,500 mg/L sulfate, 1,500 mg/L magnesium and a reported TDS of 70,000 mg/L.
- Sodium contributes 22,000 / 22.99 = 956.95 meq/L; chloride contributes 39,000 / 35.45 = 1,100.14 meq/L.
- Total cations come to 1,113.58 meq/L and total anions to 1,217.11 meq/L.
- CBE = 100 × (1,113.58 − 1,217.11) / (1,113.58 + 1,217.11) = −4.44%, inside the 5% review band.
- The ion sum is 69,370 mg/L, 0.9% below the reported TDS. Approximate ionic strength is 1.295 mol/L and Ca + Mg hardness is 7,170 mg/L as CaCO₃.
The analysis is internally consistent enough to move on. An anion excess of this size is still worth a question to the lab: is a cation such as Sr, Ba or NH4 missing, or is alkalinity reported on a different basis?
The calculator above opens with these default values. Use Reset to defaults to return to them.
Frequently asked questions
What is an acceptable charge-balance error?
There is no single universal limit. Many labs review results outside about 5%, and high-salinity samples are often harder to close. This tool lets you set your own review bands and treats the result as a prompt to investigate, not as an acceptance decision.
Why does my ion sum not match TDS?
Gravimetric TDS is measured by drying a sample, so it can include or lose material that ion analysis does not see, such as bicarbonate converting to carbonate, hydrated salts or unmeasured species. A difference of a few percent is normal; a large gap usually means a missing ion or a unit error.
Can I use this for saturation or scaling risk?
No. This lightweight version deliberately stops at ionic accounting. Saturation, precipitation and scaling need an appropriate non-ideal speciation model and project-specific evidence.
Should I add sodium or chloride to close the balance?
No. Forcing closure by inventing ions hides the real problem. Go back to the lab report and check missing analytes, units, dilution factors and the alkalinity basis first.
Related ZLD guide chapters
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Brine Consulting screening tool. Results are not a design guarantee, vendor guarantee, regulatory determination or detailed process design, and do not replace project-specific modelling, pilot testing or professional engineering review.