⚗️ Water alkalinity
In short
Titrimetric determination of total and partial water alkalinity by titration with hydrochloric acid solution to pH 8. The test is performed according to PN-EN ISO 9963-1:2001; Measurement range: 0.4–20 mmol/L. The procedure comprises 7 steps (about 11–21 min in total); it is used for: Drinking water quality control—buffering capacity assessment, Water treatment technology (coagulant, lime dosing), Water corrosivity assessment in installations (Langelier index).
At a glance
- Standard: PN-EN ISO 9963-1:2001
- Category: Physicochemistry
- Procedure steps: 7
- Total time of stages: 11 min–21 min
- Measurement range: 0.4–20 mmol/L
- p-alkalinity endpoint: pH 8.3
- m-alkalinity endpoint: pH 4.5
Overview
Water alkalinity is a measure of its ability to neutralize acids—it reflects the content of bicarbonates (HCO₃⁻), carbonates (CO₃²⁻), and hydroxides (OH⁻). It is a key water buffering parameter—it determines water resistance to pH changes under the influence of acidic substances.
The PN-EN ISO 9963-1 standard describes a titrimetric method for determining alkalinity. The sample is titrated with standard hydrochloric acid (HCl) solution to established pH values: 8.3 (phenolphthalein alkalinity—p-alkalinity) and 4.5 (total alkalinity—m-alkalinity). These pH values correspond to equivalence points of three main components: bicarbonate, carbonate, and hydroxide.
Application range: 0.4–20 mmol/L. Method intended for analysis of natural, treated, and wastewaters. Alkalinity is an important parameter in water treatment technology (coagulation, disinfection), water corrosivity assessment, and environmental monitoring.
Method principle
The water sample is titrated with standard hydrochloric acid solution (HCl 0.1 mol/L). The endpoint is established potentiometrically (pH meter) or visually with color indicators. Titration to pH 8.3 (phenolphthalein—colorless below pH 8.3) indicates phenolphthalein alkalinity (p)—includes hydroxides and half of carbonates. Titration to pH 4.5 (methyl orange or mixed indicator—color change from yellow to orange/pink) indicates total alkalinity (m)—includes all three components: HCO₃⁻, CO₃²⁻, OH⁻.
Applications
- Drinking water quality control—buffering capacity assessment
- Water treatment technology (coagulant, lime dosing)
- Water corrosivity assessment in installations (Langelier index)
- Surface water monitoring—acid rain neutralization capacity
- Wastewater control—neutralization processes
- Groundwater monitoring—calcium-carbonate equilibrium
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.4–20 mmol/L |
| p-alkalinity endpoint | pH 8.3 |
| m-alkalinity endpoint | pH 4.5 |
| Accuracy | ±2% (at concentration >2 mmol/L) |
| Indicators | Phenolphthalein (pH 8.3), methyl orange (pH 4.5) |
| Analysis time | 5–10 minutes per sample |
Standard
- Standard number
- PN-EN ISO 9963-1:2001
- Title (PL)
- Jakość wody — Oznaczanie zasadowości — Część 1: Oznaczanie zasadowości ogólnej i zasadowości wobec fenoloftaleiny
- Title (EN)
- Water quality — Determination of alkalinity — Part 1: Determination of total and composite alkalinity
Step-by-step procedure
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Sample preparation
Analyze sample as soon as possible after collection (max 24h). Do not filter, do not acidify. Bring to room temperature.
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Sample measurement
Pipette 100 mL sample into 250 mL Erlenmeyer flask. For waters with high alkalinity—reduce volume.
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p-alkalinity determination (optional)
Add 2–3 drops phenolphthalein. If solution turns pink—titrate with HCl 0.1 mol/L until decolorization (pH 8.3). Record V₁.
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m-alkalinity determination
To same sample (or new portion) add 2–3 drops methyl orange. Titrate with HCl 0.1 mol/L until color change from yellow to orange (pH 4.5). Record V₂ (or Vtotal).
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Alternative—potentiometric method
Immerse pH electrode in sample. Titrate with HCl recording pH. Read HCl volumes corresponding to pH 8.3 and pH 4.5 from titration curve.
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Calculations
Alkalinity [mmol/L] = (V_HCl × c_HCl × 1000) / V_sample. p-alkalinity from V₁, m-alkalinity from V₂ (or Vtotal). Convert to mg CaCO₃/L by multiplying by 50.04.
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Quality control
Blank with boiled deionized water. Duplicates—difference max ±0.1 mL. CRM control sample each series.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Burette 25 mL (class A) | Brand, Hirschmann, with PTFE stopcock | 200–600 PLN |
| pH meter (for potentiometric endpoint determination) | Mettler Toledo SevenCompact S220, WTW inoLab pH 7310 | 3,000–15,000 PLN |
| Erlenmeyer flasks 250 mL | DURAN / Simax borosilicate | 10–30 PLN/piece |
| Volumetric pipettes 100 mL (class A) | Brand, Hirschmann | 40–100 PLN/piece |
| Magnetic stirrer | IKA C-MAG MS 4 | 1,500–4,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Hydrochloric acid (HCl) 0.1 mol/L | 7647-01-0 | Standardized titrant solution. Prepare from fixanal or standardize against Na₂CO₃ (borax). Store tightly closed. |
| Phenolphthalein—indicator solution | 77-09-8 | 1% solution in 60% ethanol. Color change: colorless (pH<8.3) → pink (pH>8.3). Add 2–3 drops. |
| Methyl orange—indicator solution | 547-58-0 | 0.1% solution in water. Color change: yellow (pH>4.4) → orange/pink (pH<3.1). Add 2–3 drops. |
| Deionized water | — | Freshly boiled and cooled (CO₂ removal)—free from CO₂. Conductivity <0.1 mS/m. |
| Sodium carbonate (Na₂CO₃) p.a. | 497-19-8 | Primary standard substance for HCl standardization. Dried at 270°C for 1 h. |
Health and safety (OHS)
- Hydrochloric acid (HCl) 0.1 mol/L—diluted, but avoid eye contact. Use glasses.
- Phenolphthalein—solution in ethanol—flammable. Keep away from fire.
- Methyl orange—azo dye, suspected mutagen. Use gloves.
- When preparing HCl from concentrated acid—work under fume hood (HCl vapors irritating).
- Safety glasses and lab coat required.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 9963-1:2001 — “Water quality — Determination of alkalinity — Part 1: Determination of total and composite alkalinity”.
How does this method work?
The water sample is titrated with standard hydrochloric acid solution (HCl 0.1 mol/L).
What is the measuring range and accuracy?
Measurement range: 0.4–20 mmol/L; p-alkalinity endpoint: pH 8.3; m-alkalinity endpoint: pH 4.5; Accuracy: ±2% (at concentration >2 mmol/L).
How long does the test take?
The times given for the individual stages add up to approximately 11 min–21 min. The procedure comprises 7 steps.
What equipment is required?
Burette 25 mL (class A), pH meter (for potentiometric endpoint determination), Erlenmeyer flasks 250 mL, Volumetric pipettes 100 mL (class A), Magnetic stirrer. Indicative cost of the core instrument (Burette 25 mL (class A)): 200–600 PLN.
Where is this test used?
Drinking water quality control—buffering capacity assessment; Water treatment technology (coagulant, lime dosing); Water corrosivity assessment in installations (Langelier index); Surface water monitoring—acid rain neutralization capacity; Wastewater control—neutralization processes; Groundwater monitoring—calcium-carbonate equilibrium.
What safety precautions apply?
Hydrochloric acid (HCl) 0.1 mol/L—diluted, but avoid eye contact. Use glasses.; Phenolphthalein—solution in ethanol—flammable. Keep away from fire.; Methyl orange—azo dye, suspected mutagen. Use gloves.; When preparing HCl from concentrated acid—work under fume hood (HCl vapors irritating).; Safety glasses and lab coat required.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 9963-1.