EngiMetric Arena / Interpret & Validate
Does a 6.5 % charge-balance gap matter?
Competency Evidence interpretation primary Quantitative reasoning supporting
What you should take away: Interpret a charge-balance deviation as an analytical quality flag and decide when an analysis must be rechecked before design use.
The situation
What is happening
A laboratory reports calcium 90 mg/L, magnesium 20 mg/L, sodium 110 mg/L, potassium 5 mg/L, bicarbonate 220 mg/L, sulfate 90 mg/L and chloride 150 mg/L. The charge balance is 6.5 % cation-excess, outside the typical +/-5 % guideline. A designer wants to use the analysis directly for an RO projection. Choose the defensible interpretation.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Calcium | 90 | mg/L | Reported as Ca. |
| Magnesium | 20 | mg/L | Reported as Mg. |
| Sodium | 110 | mg/L | Reported as Na. |
| Bicarbonate | 220 | mg/L | Reported as HCO3. |
| Sulfate | 90 | mg/L | Reported as SO4. |
| Chloride | 150 | mg/L | Reported as Cl. |
| Charge balance | 6.5 | % | Cation excess; guideline is about +/-5 %. |
Assumptions this challenge makes
- The reported ion concentrations are total dissolved concentrations.
- The charge balance uses the equivalent weights in the water-analysis engine.
- A 5 % deviation is a typical screening guideline, not a universal acceptance limit.
Your call
Your answer
Hints
- The engine reports both the cation and anion sums in meq/L. Compare their difference with the guideline.
- Ask what a charge-balance gap indicates: a real water property, or a data-quality problem?
- Closing the gap by editing one ion would hide the evidence. What should happen instead?
Your answer Engine result
Where that comes from
Each limit, checked
Why
- The engine sums the cations to 11.05 meq/L and the anions to 9.71 meq/L.
- The balance error is (11.05 - 9.71) / (11.05 + 9.71) x 100 = 6.45 %, outside the typical +/-5 % guideline.
- That does not prove the water is impossible; it flags the analysis for verification. Common causes are a missing ion, a reporting unit error or an analytical bias.
- The defensible interpretation is to recheck the laboratory data before using the analysis for design. Adjusting sodium to close the balance would manufacture agreement.
- The charge balance is a screening check. It does not replace a full laboratory audit or a mass-balance reconciliation.
What this challenge does not establish
- The +/-5 % guideline is typical, not universal; some laboratories and standards use different limits.
- The calculation depends on the reported ion forms and equivalent weights.
- A balanced analysis can still be wrong if all ions share a systematic error.
- The check does not identify which ion or measurement is responsible for the gap.