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Drinking water · Drinking water, raw water

Phenolphthalein and Total Alkalinity (as CaCO₃)

Measures the capacity of water to neutralise acid. The sample is titrated with standard acid first to the phenolphthalein endpoint (pH 8.3) and then to the total alkalinity endpoint (about pH 4.5). From the two titres the hydroxide, carbonate and bicarbonate contributions can be estimated.

  • Environmental & water
  • Acid–base titration with 0.02 N sulphuric acid to pH 8.3 and to about pH 4.5 (indicator or potentiometric)
  • Burette & glassware
  • pH meter
  • Potentiometric titrator

Reference: Typical method based on APHA Standard Methods 2320 B (titration method) and the IS 3025 part for alkalinity. Follow your lab SOP.

PRINCIPLE

Why this method works

In most natural waters alkalinity comes from bicarbonate, carbonate and (rarely) hydroxide. Titrating to pH 8.3 converts hydroxide to water and carbonate to bicarbonate — this is phenolphthalein alkalinity (P). Continuing to about pH 4.5 converts all bicarbonate to carbonic acid — the total is total alkalinity (T). Results are expressed as mg/L CaCO₃ using its equivalent weight of 50.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Standard sulphuric acid0.02 N (0.01 M) H₂SO₄, standardised against sodium carbonate
Phenolphthalein indicatorPink in alkaline solution, colourless below about pH 8.3
Mixed indicatorBromocresol green–methyl red (or methyl orange), endpoint about pH 4.5
Sodium thiosulphateA drop of 0.1 N solution to remove residual chlorine that would bleach the indicator
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Sample

    Fill bottle full, cap, analyse within 24 h.

  2. 02

    Aliquot

    Pipette 100 mL; dechlorinate if needed.

  3. 03

    P endpoint

    Phenolphthalein; titrate pink → colourless.

  4. 04

    T endpoint

    Mixed indicator; titrate to pink/grey endpoint.

  5. 05

    Calculate

    P and T, then OH⁻, CO₃²⁻, HCO₃⁻.

Procedure

  1. 1Collect the sample in a completely filled, tightly capped bottle and analyse within 24 hours (avoid shaking, warming or opening, which lose CO₂).
  2. 2Pipette 100 mL into a conical flask (or beaker if titrating potentiometrically). Add a drop of thiosulphate if the sample contains chlorine.
  3. 3Add 2–3 drops of phenolphthalein. If the solution turns pink, titrate with 0.02 N H₂SO₄ until it just becomes colourless (pH 8.3). Record the titre P. If no pink colour appears, P = 0.
  4. 4Add the mixed indicator to the same flask and continue titrating to the endpoint (about pH 4.5, or as defined in the SOP for the alkalinity level). Record the total titre T from the start.
  5. 5For low alkalinity or coloured samples, use a pH meter and titrate to the defined pH values instead of indicators.

Standard preparation

  1. 1Standardise the 0.02 N H₂SO₄ against dried sodium carbonate (primary standard) or against a 0.02 N carbonate standard, titrating to about pH 4.5.

Notes for analysts

  • Relationships commonly used: P = 0 → all bicarbonate; P < T/2 → carbonate 2P, bicarbonate T − 2P; P = T/2 → all carbonate; P > T/2 → hydroxide 2P − T, carbonate 2(T − P); P = T → all hydroxide.
  • When total alkalinity is less than total hardness, the difference is 'non-carbonate' (permanent) hardness.
TEST CONDITIONS

Method parameters (typical)

Titrant0.02 N H₂SO₄ (1 mL = 1.00 mg CaCO₃)
Phenolphthalein endpointpH 8.3
Total alkalinity endpointAbout pH 4.5 (SOP may specify 4.3–4.9 depending on alkalinity)
Sample volume100 mL
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Acid standardisationReplicate titres agree within the SOP limit
pH meter (if used)Calibrated at pH 4 and 7 (and 10 if needed)
DuplicatesAgree within the SOP limit

Typical sequence

  1. 1Standardise acid
  2. 2Sample in duplicate
  3. 3Check standard (sodium carbonate solution)
CALCULATION

Calculation with a worked example

Formula
Alkalinity (mg/L as CaCO₃) = A × N × 50 000 ÷ V

Where

A
Titre of acid to the relevant endpoint (mL): P to pH 8.3, T to pH 4.5
N
Normality of the acid
50 000
Equivalent weight of CaCO₃ (50 g/eq) × 1000 mg/g
V
Sample volume (mL)

Given

V
100 mL
N
0.02 N H₂SO₄
Titre to pH 8.3 (P)
1.2 mL
Total titre to pH 4.5 (T)
11.6 mL

Working

  1. 1
    Phenolphthalein alkalinity P = 1.2 × 0.02 × 50 000 ÷ 100 = 12 mg/L as CaCO₃
  2. 2
    Total alkalinity T = 11.6 × 0.02 × 50 000 ÷ 100 = 116 mg/L as CaCO₃
  3. 3

    P (12) is less than T/2 (58), so hydroxide = 0

  4. 4

    Carbonate = 2P = 24 mg/L; bicarbonate = T − 2P = 116 − 24 = 92 mg/L (all as CaCO₃)

P alkalinity = 12 mg/L; total alkalinity = 116 mg/L as CaCO₃ (carbonate 24, bicarbonate 92)

ACCEPTANCE CRITERIA

What result passes

  • As per IS 10500:2012: total alkalinity as CaCO₃ acceptable limit 200 mg/L, permissible limit 600 mg/L; check the current edition.
  • The OH/CO₃/HCO₃ split from P and T is an approximation that assumes no other buffering species; borate, silicate or phosphate will distort it.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
AccuracyRecovery of a sodium carbonate/bicarbonate standard
PrecisionDuplicate titrations
EndpointIndicator endpoint confirmed against potentiometric endpoint for the water type
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