Total Hardness Determination (EDTA Titrimetric Method)
1. Purpose
This Standard Operating Procedure (SOP) describes a validated method for the determination of Total Hardness in water and wastewater samples using the EDTA titrimetric method. The procedure is designed to produce accurate, precise, and reproducible results suitable for routine laboratory analysis and regulatory monitoring.
2. Scope
This method applies to the analysis of drinking water, surface water, groundwater, and wastewater samples in which calcium (Ca²⁺) and magnesium (Mg²⁺) are the primary contributors to hardness.
3. Principle of the Method
Total hardness in water is caused mainly by dissolved calcium and magnesium salts. The determination is based on complexometric titration using Ethylenediaminetetraacetic Acid (EDTA) as the titrant.
At a controlled pH of 10.0 ± 0.1, calcium and magnesium ions react with Eriochrome Black T (EBT) indicator to form a wine‑red colored complex. During titration, EDTA preferentially complexes with calcium and magnesium ions. Once all metal ions are bound by EDTA, the indicator is released, producing a distinct color change from wine‑red to sky blue, which signifies the endpoint.
4. Responsibilities
- Laboratory Analyst: Perform the analysis in accordance with this SOP and accurately record all observations and results.
- Laboratory Supervisor: Ensure availability, calibration, and validation of equipment, reagents, and standards.
5. Apparatus and Equipment
The following equipment is required:
- Calibrated pH meter
- Burette (50 mL capacity)
- Conical flasks (100 mL)
- Volumetric flasks
- Measuring cylinders
- Analytical balance
- Standard laboratory glassware
6. Reagents and Chemicals
All reagents shall be of Analytical Reagent (AR) grade.
6.1 Distilled Water
Used for reagent preparation and dilution.
6.2 Buffer Solution (pH 10)
Dissolve 16.9 g ammonium chloride (NH₄Cl) in 143 mL ammonium hydroxide, and dilute to 250 mL with distilled water.
6.3 Eriochrome Black T Indicator
Thoroughly mix 0.5 g Eriochrome Black T with 100 g sodium chloride (NaCl).
6.4 Inhibitor Solution (If Required)
For samples containing interfering ions, dissolve 4.5 g hydroxylamine hydrochloride in 100 mL of 95% ethyl alcohol or isopropyl alcohol.
6.5 Standard EDTA Solution (0.01 M)
Dissolve 3.723 g EDTA in distilled water and dilute to 1000 mL. Standardize the solution against 0.01 M zinc sulfate solution.
6.6 Standard Zinc Sulfate Solution (0.01 M)
Dissolve 2.8754 g ZnSO₄ in distilled water and make up to 1000 mL.
6.7 Standard Calcium Solution (1 mL = 1 mg as CaCO₃)
Dry analytical‑grade calcium carbonate at 180°C for 1 hour. Accurately weigh 1.000 g, dissolve using minimal concentrated hydrochloric acid, boil briefly, cool, add methyl red indicator, neutralize to an orange endpoint using 3N ammonium hydroxide, and dilute to 1000 mL with distilled water.
7. Standardization of EDTA Solution
- Pipette 50 mL of standard calcium solution into a conical flask.
- Add 1 mL buffer solution.
- Add 1–2 drops of Eriochrome Black T indicator.
- Titrate with EDTA solution until the color changes from purple to sky blue.
- Record the burette reading and calculate the exact normality of the EDTA solution.
8. Sample Analysis Procedure
- Transfer 50 mL of the water sample (or a suitable aliquot) into a conical flask.
- If interference is anticipated, add 1 mL hydroxylamine hydrochloride solution.
- Add 1–2 mL buffer solution to adjust the pH to 10.0–10.1.
- Add 2–3 drops of Eriochrome Black T indicator; the solution will turn wine‑red.
- Titrate with standardized EDTA solution, stirring rapidly initially and slowly near the endpoint.
- Note the endpoint indicated by a color change from wine‑red to sky blue.
9. Calculation
Total Hardness (as CaCO₃), mg/L
Total Hardness = {A x N x 1000 x 100}/{V}
Where:
- A = Volume of EDTA used (mL)
- N = Normality of EDTA solution
- V = Volume of sample taken (mL)
10. Process Flow (Summary)
- Measure 50 mL of sample into a conical flask
- Add buffer solution
- Add Eriochrome Black T indicator (wine‑red color)
- Titrate with 0.01 N EDTA solution
- Observe endpoint color change to sky blue
- Record titration volume
- Calculate total hardness in mg/L
11. Precautions
- Maintain pH strictly at 10 to ensure accurate endpoint detection.
- Use freshly standardized EDTA solution.
- Ensure all glassware is free from metal contamination.
12. Conclusion
The EDTA titrimetric method is a well‑established and reliable technique for determining total hardness in water and wastewater samples. When performed under controlled conditions, it provides precise and reproducible results essential for water quality assessment and effective treatment process control.
