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Drinking water · Drinking water, borewell water

Total Hardness by EDTA Titration (Eriochrome Black T)

Calcium and magnesium ions are titrated together with EDTA at pH 10. The indicator Eriochrome Black T turns from wine-red to clear blue when all the metal ions have been complexed. The result is expressed as mg/L of calcium carbonate.

  • Environmental & water
  • Complexometric titration with 0.01 M disodium EDTA at pH 10, Eriochrome Black T indicator
  • Burette & glassware
  • pH meter
  • Analytical balance

Reference: Typical method based on APHA Standard Methods 2340 C (EDTA titrimetric) and the IS 3025 part for total hardness. Follow your lab SOP.

PRINCIPLE

Why this method works

EDTA forms stable 1 : 1 complexes with Ca²⁺ and Mg²⁺. At pH 10 (ammonia buffer), Eriochrome Black T (EBT) binds to magnesium to give a wine-red complex. As EDTA is added it takes up free Ca²⁺ and Mg²⁺ first, and finally pulls Mg²⁺ away from the indicator, releasing the free blue indicator. Because each mole of EDTA reacts with one mole of Ca²⁺ or Mg²⁺, hardness is reported as the equivalent mass of CaCO₃ (molar mass 100.09 g/mol).

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Standard EDTA titrant0.01 M disodium EDTA dihydrate, standardised against a calcium carbonate standard
Ammonia buffer pH 10Ammonium chloride + ammonia solution, usually containing a little Mg-EDTA so a sharp endpoint is seen even in Mg-free samples
IndicatorEriochrome Black T, as a dry powder mixture with NaCl or a fresh solution
Inhibitor (if needed)Sodium cyanide or sodium sulphide inhibitors for interfering metals — only under strict safety controls, or as per SOP
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Measure sample

    Pipette 50 mL into a conical flask.

  2. 02

    Buffer

    Add 1–2 mL buffer to reach pH 10.0 ± 0.1.

  3. 03

    Indicator

    Add EBT; solution turns wine-red.

  4. 04

    Titrate

    Titrate with 0.01 M EDTA to clear blue.

  5. 05

    Blank

    Titrate distilled water the same way.

Procedure

  1. 1Pipette 50.0 mL of sample (or a smaller volume diluted to 50 mL if hardness is high, so the titre stays below about 15 mL) into a 250 mL conical flask.
  2. 2Add 1–2 mL of pH 10 buffer and check that the pH is 10.0 ± 0.1.
  3. 3Add a small amount (about 0.1–0.2 g) of EBT indicator powder; the solution becomes wine-red.
  4. 4Titrate with standard 0.01 M EDTA, swirling continuously. Slow down near the end and add the last drops at 3–5 second intervals until the last reddish tinge disappears and the solution is pure blue.
  5. 5Finish the titration within about 5 minutes of adding the buffer, so that calcium carbonate does not precipitate.
  6. 6Run a blank on 50 mL of distilled water and subtract the blank titre.

Standard preparation

  1. 1Standard calcium solution: dissolve 1.000 g of dried CaCO₃ in a little dilute HCl, boil off CO₂, neutralise and make up to 1000 mL (1.00 mL = 1.00 mg CaCO₃).
  2. 2Titrate an aliquot of the calcium standard with the EDTA solution under the same conditions to establish its exact molarity.

Notes for analysts

  • Calcium hardness alone can be found by titrating at pH 12–13 with murexide or a calcium indicator; magnesium hardness = total − calcium hardness.
  • A dull, slow or purple endpoint usually means interfering metals, old indicator or wrong pH.
  • Rough classification often used: below 75 mg/L soft, 75–150 moderately hard, 150–300 hard, above 300 very hard (as CaCO₃).
TEST CONDITIONS

Method parameters (typical)

Titrant0.01 M EDTA (1 mL ≈ 1.001 mg CaCO₃)
pH10.0 ± 0.1 (ammonia buffer)
IndicatorEriochrome Black T
EndpointWine-red → pure blue
Sample volume50 mL (smaller aliquot diluted to 50 mL for hard water)
Titration timeComplete within about 5 min of buffer addition
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
EDTA standardisationReplicate titres agree within 0.05 mL (or %RSD as per SOP)
Check standardKnown CaCO₃ standard recovered within the SOP limit (e.g. 95–105%)
Duplicate samplesAgree within the SOP limit

Typical sequence

  1. 1Standardise EDTA against calcium standard
  2. 2Blank
  3. 3Sample in duplicate
  4. 4Check standard
CALCULATION

Calculation with a worked example

Formula
Total hardness (mg/L as CaCO₃) = (V_s − V_b) × M × 100.09 × 1000 ÷ V

Where

V_s
EDTA titre for the sample (mL)
V_b
EDTA titre for the blank (mL)
M
Molarity of EDTA (mol/L)
100.09
Molar mass of CaCO₃ (g/mol)
V
Volume of sample taken (mL)

Given

V
50.0 mL
V_s
8.40 mL
V_b
0.10 mL
M
0.0100 M

Working

  1. 1

    Net titre = 8.40 − 0.10 = 8.30 mL

  2. 2
    Moles of EDTA = 8.30 × 0.0100 ÷ 1000 = 8.30 × 10⁻⁵ mol (= moles of Ca + Mg)
  3. 3

    Mass as CaCO₃ = 8.30 × 10⁻⁵ × 100.09 g = 8.307 mg in 50 mL

  4. 4
    Hardness = 8.307 × 1000 ÷ 50 = 166.1 mg/L as CaCO₃

Total hardness = 166 mg/L as CaCO₃ (within the IS 10500 acceptable limit of 200 mg/L)

ACCEPTANCE CRITERIA

What result passes

  • As per IS 10500:2012: acceptable limit 200 mg/L, permissible limit 600 mg/L as CaCO₃ in the absence of an alternative source; check the current edition.
  • WHO has no health-based guideline for hardness; it is an acceptability (taste, scaling) parameter.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
AccuracyRecovery of calcium standard spikes
PrecisionDuplicate titrations on real samples
InterferencesIron, manganese, copper, aluminium and others can block the indicator; assess and use inhibitors or ICP results where needed
In LimsPulse · Sample Management

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