Estimation of Hardness of Water by EDTA (Complexometric Titration)
Comprehensive IITs/NITs and other college‑level notes: principle, reagents, apparatus, procedure, derivations, solved example, diagrams, exam tips and revision box. By Mohan Dangi
1. Objective
Determine total hardness (Ca²⁺ + Mg²⁺) of a water sample and express it in mg/L (ppm) as CaCO3 using EDTA titration (complexometric titration).
2. Definitions & Units
- Total hardness: concentration of multivalent cations (mainly Ca²⁺ and Mg²⁺) expressed as mg/L as CaCO3.
- 1 ppm = 1 mg/L.
3. Principle (with Equations)
EDTA (ethylenediaminetetraacetic acid) in its chelating form EDTA4− forms 1:1 complexes with most divalent metal ions. The general reaction:
$\mathrm{M^{2+} + EDTA^{4-} \rightarrow M(EDTA)^{2-}}$
For calcium and magnesium:
$\mathrm{Ca^{2+} + EDTA^{4-} \rightarrow Ca(EDTA)^{2-}}$
$\mathrm{Mg^{2+} + EDTA^{4-} \rightarrow Mg(EDTA)^{2-}}$
Moles of EDTA consumed at endpoint = total moles of hardness (as Ca²⁺ equivalents) because of the 1:1 stoichiometry.
4. Indicator & Buffer
Indicator: Eriochrome Black T (EBT). At pH ≈ 10 (ammonia buffer), free EBT is blue; EBT forms a wine‑red complex with Ca²⁺/Mg²⁺. During titration EDTA removes metal from indicator → indicator turns blue at the endpoint.
Buffer: Ammonia–ammonium chloride buffer (pH ≈ 10). pH must be controlled to maintain EDTA speciation and indicator behaviour.
5. Reagents & Standard Solutions (typical)
| Reagent | Purpose / Notes |
|---|---|
| 0.01000 M EDTA (disodium EDTA) | Titrant; standardize before use |
| Eriochrome Black T indicator (0.5% in alcohol) | 2–4 drops per titration |
| Ammonia–ammonium chloride buffer (pH 10) | Maintain pH |
| Primary standard: CaCO3 (or standard Ca2+ solution) | For EDTA standardization |
6. Apparatus
Burette (25 or 50 mL), conical flask (250 mL), pipettes (10–50 mL), volumetric flasks, magnetic stirrer (optional), glassware cleaned and rinsed.
7. Procedure — Total Hardness (Stepwise)
- Pipette a known volume of water sample (commonly 50.00 mL) into a 250 mL conical flask.
- Add ~5–10 mL ammonia buffer (pH 10).
- Add 2–3 drops Eriochrome Black T—the solution turns wine‑red if Ca/Mg are present.
- Titrate with standard EDTA (e.g., 0.01000 M) with swirling until colour changes permanently from wine‑red → blue. Record burette reading.
- Repeat for two concordant readings.
8. Standardization of EDTA (Brief)
Standardize EDTA using a primary standard such as accurately weighed CaCO3 dissolved/diluted to prepare a known Ca2+ solution. Pipette an aliquot, add buffer and indicator, titrate to obtain Vstd. Compute EDTA molarity from known moles of CaCO3 in the aliquot:
$\displaystyle M_{EDTA} = \frac{\text{moles CaCO}_3}{V_{EDTA(\mathrm{L})}}$
9. Calculations — Formulae & Worked Example
Derivation (clear)
Moles EDTA used = $M_{EDTA}\times V_{EDTA(\mathrm{L})}$. Because stoichiometry is 1:1, moles CaCO3 equivalent = moles EDTA. Mass of CaCO3 = moles × 100.087 g·mol−1. Convert to mg and divide by sample volume in L for mg·L−1.
Compact formula (useful)
Let $M$ = EDTA molarity (mol·L−1), $V_t$ = titrant volume (mL), $V_s$ = sample volume (mL), $M_{CaCO_3}=100.087$ g·mol−1:
$\displaystyle \text{Hardness (mg·L}^{-1}\text{ as CaCO}_3) = \frac{M\times V_t(\text{mL})\times 100.087}{V_s(\text{mL})} \times 1000$
Worked numerical example (step-by-step)
Given: $M=0.01000$ M EDTA, $V_s=50.00$ mL, $V_t=12.35$ mL.
Step 1 — Convert V_t to L: V_t = 12.35 mL = 0.01235 L
Moles EDTA = M * V_t(L) = 0.01000 * 0.01235 = 0.0001235 mol
Step 2 — Mass of CaCO3 equivalent (g) = moles * 100.087 = 0.0001235 * 100.087 = 0.01236074 g
Convert to mg = 12.36074 mg
Step 3 — Hardness (mg/L as CaCO3) = mass (mg) / sample volume (L)
Sample volume = 50.00 mL = 0.05000 L
Hardness = 12.36074 / 0.05000 = 247.2148 mg/L ≈ 247.21 mg/L as CaCO3
10. Interferences & Precautions
- Interfering ions (Fe, Al, Mn, Cu, Zn, Pb, etc.) may complex EDTA. Use masking agents or pretreatment when required.
- Maintain pH ≈ 10 (critical).
- Filter turbid samples; organic matter may obscure endpoint.
- Standardize EDTA and perform blanks if necessary.
11. Exam / Interview Tips (IIT professor perspective)
- Always state units clearly (mg/L as CaCO3).
- Mention the standardization step in calculations.
- Remember 1:1 stoichiometry of EDTA with Ca2+/Mg2+.
- Know behaviour of Eriochrome Black T at the endpoint and the role of pH.
- Memorize the rule-of-thumb: 1 mL of 0.01000 M EDTA ≈ 1.0009 mg CaCO3.
12. Concise Revision Box
- Principle: EDTA forms 1:1 chelates with Ca2+/Mg2+ — moles EDTA = moles hardness.
- Indicator: Eriochrome Black T (wine‑red → blue at pH ≈ 10).
- Standardize EDTA with primary standard (CaCO3).
- Calculation: moles EDTA × 100.087 g·mol−1 → mg CaCO3 → mg/L.
Prepared for IITs/NITs and other college Students
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