💧 Total water hardness
Complexometric determination of total water hardness (sum of Ca²⁺ and Mg²⁺) by titration with EDTA against Eriochrome Black T at pH 10.
Overview
Total water hardness is the sum of calcium (Ca²⁺) and magnesium (Mg²⁺) ion concentrations, usually expressed in mmol/L or mg CaCO₃/L (°dH—German degrees). It is one of the most frequently determined water quality parameters—it affects taste, tendency to form boiler scale and deposits in installations, and the effectiveness of washing and laundry.
The PN-EN ISO 6059 standard describes a complexometric titration method with versenate (EDTA) in pH 10 buffer medium against Eriochrome Black T indicator. The method is used for groundwater, surface water, and drinking water. The lowest determinable concentration is 0.05 mmol/L.
Water classification: very soft <1.8 mmol/L (<100 mg CaCO₃/L), soft 1.8–3.6 mmol/L, moderately hard 3.6–5.4 mmol/L, hard 5.4–7.0 mmol/L, very hard >7.0 mmol/L (>400 mg CaCO₃/L). Recommended value for drinking water: 60–500 mg CaCO₃/L.
Method principle
EDTA (ethylenediaminetetraacetic acid, disodium salt—disodium versenate) forms stable, colorless chelate complexes with Ca²⁺ and Mg²⁺ ions in a 1:1 molar ratio. At pH ~10 (ammonia buffer NH₃/NH₄Cl), the indicator Eriochrome Black T (EBT) forms a reddish-violet complex with Mg²⁺ and Ca²⁺ ions. During titration, EDTA first binds free Ca²⁺ and Mg²⁺ ions, then "takes" ions from the indicator—causing color change from reddish-violet to blue (endpoint).
Applications
- Drinking water quality control (recommended 60–500 mg CaCO₃/L)
- Groundwater and surface water monitoring
- Water softening process control
- Water testing in boilers and heating installations
- Food industry—process water
- Water control in pools and spas
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.05 mmol/L and higher |
| Accuracy | ±1–2% (at concentration >1 mmol/L) |
| Titration pH | 10.0 ±0.1 (ammonia buffer) |
| Indicator | Eriochrome Black T (EBT) |
| Color change | Reddish-violet → blue |
| Analysis time | 5–10 minutes per sample |
Standard
- Standard number
- PN-ISO 6059:1999
- Title (PL)
- Jakość wody — Oznaczanie sumarycznej zawartości wapnia i magnezu — Metoda miareczkowa z EDTA
- Title (EN)
- Water quality — Determination of the sum of calcium and magnesium — EDTA titrimetric method
Step-by-step procedure
1. Workstation preparation
Rinse burette 3× with EDTA solution. Fill burette and remove air bubbles. Set meniscus to 0.00 mL.
2. Sample measurement
Pipette 50 mL water sample into 250 mL Erlenmeyer flask. For hard water reduce volume (e.g., 25 mL + 25 mL H₂O).
3. Ammonia buffer addition
Add 1 mL of pH 10 ammonia buffer. Mix by swirling.
4. Indicator addition
Add 3–5 drops of EBT solution (or spatula of dry mixture with NaCl). Solution will turn reddish-violet.
5. Titration
Titrate with EDTA solution, stirring continuously, until persistent color change from reddish-violet to pure blue (without pink tint). Endpoint should be sharp.
6. Reading and calculations
Read EDTA volume on burette to 0.05 mL accuracy. Calculate hardness: T [mmol/L] = (V_EDTA × c_EDTA × 1000) / V_sample. Convert to mg CaCO₃/L by multiplying by 100.09.
7. Duplicate measurement
Repeat titration for second sample portion. Result difference should not exceed 0.1 mL EDTA.
8. Quality control
Each series titrate control sample (CRM or hardness standard). Perform blank with deionized water—EDTA consumption should be <0.05 mL.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Burette 25 mL (class A) | Brand, Hirschmann, with PTFE stopcock | 200–600 PLN |
| Erlenmeyer flasks 250 mL | DURAN / Simax borosilicate | 10–30 PLN/piece |
| Volumetric pipettes 50 mL (class A) | Brand, Hirschmann | 30–80 PLN/piece |
| Graduated cylinder 100 mL | Class A, borosilicate | 30–60 PLN |
| Magnetic stirrer (optional) | IKA C-MAG MS 4 | 1,500–4,000 PLN |
| pH meter (for buffer pH control) | Any laboratory pH meter | 1,000–5,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| EDTA-Na₂ solution 0.01 or 0.02 mol/L | 6381-92-6 | Standardized solution of disodium EDTA salt. Prepare from p.a. substance or ready fixanal. Standardize against CaCO₃. |
| Ammonia buffer pH 10 | 12125-02-9 | Dissolve 67.5 g NH₄Cl in 570 mL NH₃ 25% and fill to 1 L with water. pH = 10.0 ±0.1. Store tightly closed (volatile NH₃). |
| Eriochrome Black T (EBT) | 1787-61-7 | Metallochromic indicator—0.5% solution in triethanolamine or dry indicator mixture with NaCl (1:100). Color change: reddish-violet → blue. |
| CaCO₃ standard solution | 471-34-1 | For EDTA standardization. Dissolve 1.000 g CaCO₃ (dried 150°C, 2h) in minimum HCl, boil, cool, fill to 1 L. |
| Deionized water | — | Free from Ca and Mg, conductivity <0.1 mS/m. |
Health and safety (OHS)
- Ammonia buffer (NH₃ 25%)—corrosive, irritating to respiratory system (H314, H335). Prepare under fume hood.
- EDTA-Na₂—irritating to eyes (H319). Use safety glasses.
- Eriochrome Black T—may stain skin. Use gloves.
- HCl (for CaCO₃ standard preparation)—corrosive (H314). Work under fume hood.
- Safety glasses and lab coat required.