💧 Total water hardness
In short
Titrimetric determination of the sum of calcium and magnesium (total hardness) in ground water, surface water and drinking water with EDTA at pH 10 against Eriochrome Black T. The test is performed according to PN-ISO 6059:1999; Lowest determinable concentration: 0.05 mmol/l; test portions above 3.6 mmol/l are diluted. The procedure comprises 6 steps; it is used for: Ground waters, surface waters and drinking waters (clause 1), Water intended for human consumption — hardness 60–500 mg/l CaCO₃ as a recommended value (Journal of Laws 2026, item 748, Annex 1 Part D), Monitoring of surface and ground waters — titrimetric method for total hardness (Journal of Laws 2021, item 1576).
At a glance
- Standard: PN-ISO 6059:1999
- Category: Physicochemistry
- Procedure steps: 6
- Lowest determinable concentration: 0.05 mmol/l; test portions above 3.6 mmol/l are diluted
- Titration pH: 10.0 ± 0.1 (4 ml of buffer per 50 ml test portion)
- Standard volumetric solution: EDTA 10 mmol/l
Overview
STATUS (as of 29.10.2026). PN-ISO 6059:1999 — Polish version, published 28.07.1999, implements ISO 6059:1984 [IDT]; it replaced PN-C-04554-01:1971, PN-C-04554-02:1971, PN-C-04554-10:1971, PN-C-04562-00:1975 and PN-C-04562-01:1975. The standard is current — no withdrawal note. ISO 6059:1984 is current in the ISO catalogue, with no newer edition or draft.
SCOPE (clause 1). The sum of the calcium and magnesium concentrations in ground waters, surface waters and drinking waters. The method is not intended for effluents or for waters with a high salt content, such as sea waters. The lowest concentration that can be determined is 0.05 mmol/l.
WATER HARDNESS. Total hardness is the total concentration of calcium and magnesium; carbonate hardness is the part of it equivalent to the carbonate and hydrogen carbonate content (informative Annex A). The result is given as an amount-of-substance concentration; if calcium has been determined separately, the mass concentration of magnesium can be calculated (clause 3). Scales from Annex A: 1 mmol/l = 100 mg/l CaCO₃ = 10 French degrees = 7.02 English (Clark) degrees = 5.61 German degrees — the standard describes German degrees as no longer in use. When hard water is heated, calcium carbonate and magnesium hydroxide precipitate and form scale. Waters are classified by hardness as very soft (up to 0.89 mmol/l), soft (0.89–1.79 mmol/l), of medium hardness (1.79–2.68 mmol/l), of considerable hardness (2.68–3.57 mmol/l), hard (3.57–5.35 mmol/l) and very hard (above 5.35 mmol/l).
INTERFERENCES (clause 10). Aluminium, barium, lead, iron, cobalt, copper, manganese, tin and zinc interfere — they are titrated together with calcium and magnesium or obscure the colour change at the end point. Orthophosphate and carbonate may precipitate calcium at the pH of the titration; some organic matter may also interfere. Iron up to 10 mg/l is masked by adding 250 mg of sodium cyanide to the test portion (the solution must be alkaline before cyanide is added); cyanide also minimizes the interference of zinc, copper and cobalt. If the interferences cannot be eliminated, an atomic absorption method is used.
PRECISION (clause 9). Repeatability ± 0.04 mmol/l, corresponding to about 2 drops of the EDTA solution.
LEGAL REQUIREMENTS. Water intended for human consumption: hardness 60–500 mg/l expressed as calcium carbonate — a value recommended for health reasons that does not oblige the supplier to supplement (Regulation of the Minister of Health of 22 May 2026, Journal of Laws 2026, item 748, Annex 1 Part D Table 2 item 3); measurement uncertainty up to 20 % (Annex 4 Part B). In the monitoring of surface and ground waters, PN-ISO 6059 is the titrimetric method for total hardness (Journal of Laws 2021, item 1576, Annex 8, indicator 3.3.8).
Method principle
Calcium and magnesium are titrated at pH 10 with an aqueous solution of the disodium salt of EDTA. The indicator — Eriochrome Black T (in the standard: mordant black 11) — forms a claret or violet colour with calcium and magnesium ions. During the titration EDTA reacts first with the free calcium and magnesium ions and then, at the equivalence point, with the ions bound to the indicator, liberating it: the colour changes from claret or violet to blue (clause 3).
Applications
- Ground waters, surface waters and drinking waters (clause 1)
- Water intended for human consumption — hardness 60–500 mg/l CaCO₃ as a recommended value (Journal of Laws 2026, item 748, Annex 1 Part D)
- Monitoring of surface and ground waters — titrimetric method for total hardness (Journal of Laws 2021, item 1576)
- Not for effluents or waters with a high salt content such as sea water (clause 1)
Key parameters
| Parameter | Value |
|---|---|
| Lowest determinable concentration | 0.05 mmol/l; test portions above 3.6 mmol/l are diluted |
| Titration pH | 10.0 ± 0.1 (4 ml of buffer per 50 ml test portion) |
| Standard volumetric solution | EDTA 10 mmol/l |
| Colour change | claret or violet → blue (the last reddish tinge disappears) |
| Repeatability | ± 0.04 mmol/l (about 2 drops of EDTA) |
| Result | in mmol/l, to the nearest 0.02 mmol/l |
Standard
- Standard number
- PN-ISO 6059:1999
- Title (PL)
- Jakość wody — Oznaczanie sumarycznej zawartości wapnia i magnezu — Metoda miareczkowa z EDTA
- Title (EN)
- Water quality — Determination of the sum of calcium and magnesium — EDTA titrimetric method
Step-by-step procedure
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Sampling
In accordance with the relevant parts of ISO 5667 (clause 6).
-
Preparation of the test portion
Samples normally need no pretreatment; samples with gross amounts of particulate matter are filtered through a 0.45 µm filter as soon as possible after collection (risk of losing some calcium and magnesium). Test portions with a total calcium and magnesium content above 3.6 mmol/l are diluted and the dilution factor F is recorded. Samples acidified for preservation are neutralized with the calculated amount of 2 mol/l NaOH; dilution by acid or alkali is taken into account in the calculation (7.1).
-
Titration
Pipette 50.0 ml into a 250 ml conical flask, add 4 ml of buffer solution and 3 drops of indicator — claret or violet colour, pH 10.0 ± 0.1. Titrate immediately with EDTA solution under continuous stirring: rather rapidly at the beginning, slowly towards the end; add EDTA dropwise when the colour starts to change to blue. End point — the last reddish tinge has disappeared and another drop of EDTA no longer changes the colour (7.2.1).
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Second titration
To a second 50.0 ml test portion add EDTA in an amount 0.5 ml less than that consumed in the first titration, then 4 ml of buffer and 3 drops of indicator, and add EDTA dropwise to the end point (7.2.2).
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Choice of test portion
If the EDTA consumption is less than 4.5 ml, titrate a larger test portion and increase the volume of buffer in proportion; if it exceeds 20 ml, titrate a smaller test portion made up with water to 50 ml (7.2.3).
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Calculation
c(Ca + Mg) = c₁ · V₃ / V₀ mmol/l, where c₁ is the EDTA concentration (mmol/l), V₃ the volume of EDTA used in the titration (ml, 7.2.2) and V₀ the volume of the test portion (normally 50 ml); for a diluted test portion the factor F is applied. The result is reported in mmol/l to the nearest 0.02 mmol/l; conversion to mg/l CaCO₃: × 100 (clauses 8, 11, Annex A).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Burette, 25 ml | graduated in divisions of 0.05 ml, ISO 385-1 class A or equivalent (clause 5) | — |
| Conical flasks, 250 ml | 7.2.1 | — |
| Pipette, 50 ml | for the 50.0 ml test portion (7.2.1) | — |
| Stirring | titration under continuous stirring (7.2.1) | — |
| pH meter | for checking the buffer — 10 ml diluted to 100 ml must give pH 10 ± 0.1 (4.1) | — |
| Polyethylene bottles | for the buffer solution and the EDTA solution (4.1, 4.2) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Buffer solution pH 10 | — | dissolve 67.5 g of ammonium chloride in 570 ml of 25 % (m/m) ammonia solution (ρ = 0.910 g/ml), add 5.0 g of the disodium magnesium salt of EDTA and dilute to 1000 ml with water; store in a polyethylene bottle (limited shelf-life); if 10 ml diluted to 100 ml does not give pH 10 ± 0.1, discard the solution (4.1) |
| EDTA, standard volumetric solution c(Na₂EDTA) = 10 mmol/l | 6381-92-6 | dry disodium EDTA dihydrate at 80 °C for 2 h, dissolve 3.725 g in water and dilute to 1000 ml in a volumetric flask; store in a polyethylene bottle and check the concentration regularly; standardize against the calcium standard solution (20.0 ml diluted to 50 ml, procedure as in clause 7): c₁ = c₂ · V₁ / V₂ (4.2) |
| Calcium standard solution, c(CaCO₃) = 10 mmol/l | 471-34-1 | dry calcium carbonate at 150 °C for 2 h and cool in a desiccator; moisten 1.001 g with water in a 500 ml conical flask, add 4 mol/l hydrochloric acid dropwise until dissolved, avoiding excess; add 200 ml of water, boil for a few minutes (expelling CO₂), cool, add a few drops of methyl red and 3 mol/l ammonia solution until orange; transfer quantitatively to a 1000 ml flask and make up to the mark; 1 ml contains 0.4008 mg (0.01 mmol) of calcium; commercial solutions may be used (4.3) |
| Eriochrome Black T (mordant black 11) — indicator | 1787-61-7 | dissolve 0.5 g in 100 ml of triethanolamine; up to 25 ml of ethanol may replace triethanolamine to reduce the viscosity. To facilitate end-point detection, 0.17 g of methanil salt (sodium salt of 4-anilinoazobenzenesulfonic acid) may be added — the colour then changes from red to pale grey or green (4.4) |
| Sodium hydroxide, 2 mol/l | 1310-73-2 | for neutralizing samples acidified for preservation (7.1) |
| Water | — | distilled water or water of equivalent purity; reagents of recognized analytical grade (clause 4) |
Health and safety (OHS)
- Sodium cyanide (for masking iron) is a poison — take the necessary precautions when handling and disposing of it; solutions containing sodium cyanide must not be acidified (clause 10)
- Handle 25 % ammonia solution, 4 mol/l hydrochloric acid and triethanolamine in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 6059:1999 — “Water quality — Determination of the sum of calcium and magnesium — EDTA titrimetric method”.
How does this method work?
Calcium and magnesium are titrated at pH 10 with an aqueous solution of the disodium salt of EDTA. The indicator — Eriochrome Black T (in the standard: mordant black 11) — forms a claret or violet colour with calcium and magnesium ions.
What is the measuring range and accuracy?
Lowest determinable concentration: 0.05 mmol/l; test portions above 3.6 mmol/l are diluted; Titration pH: 10.0 ± 0.1 (4 ml of buffer per 50 ml test portion); Standard volumetric solution: EDTA 10 mmol/l; Colour change: claret or violet → blue (the last reddish tinge disappears).
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Burette, 25 ml, Conical flasks, 250 ml, Pipette, 50 ml, Stirring, pH meter, Polyethylene bottles.
Where is this test used?
Ground waters, surface waters and drinking waters (clause 1); Water intended for human consumption — hardness 60–500 mg/l CaCO₃ as a recommended value (Journal of Laws 2026, item 748, Annex 1 Part D); Monitoring of surface and ground waters — titrimetric method for total hardness (Journal of Laws 2021, item 1576); Not for effluents or waters with a high salt content such as sea water (clause 1).
What safety precautions apply?
Sodium cyanide (for masking iron) is a poison — take the necessary precautions when handling and disposing of it; solutions containing sodium cyanide must not be acidified (clause 10); Handle 25 % ammonia solution, 4 mol/l hydrochloric acid and triethanolamine in accordance with their safety data sheets.
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-ISO 6059.