⚗️ Calcium and magnesium in water
In short
Determination of dissolved calcium and magnesium in raw and drinking waters by flame atomic absorption spectrometry, with lanthanum chloride (air–acetylene flame) or caesium chloride (nitrous oxide–acetylene flame) to reduce interferences. The test is performed according to PN-EN ISO 7980:2002; Range without dilution: up to 50 mg/l of calcium and up to 5 mg/l of magnesium (optimum 3–50 and 0.9–5 mg/l with the air–acetylene flame). The procedure comprises 6 steps; it is used for: Raw and drinking waters — dissolved calcium and magnesium (clause 1), Monitoring of surface and ground waters — calcium and magnesium, and total hardness calculated from them (Journal of Laws 2021, item 1576), Water intended for human consumption — magnesium 7–125 mg/l and hardness 60–500 mg/l CaCO₃ as recommended values (Journal of Laws 2026, item 748).
At a glance
- Standard: PN-EN ISO 7980:2002
- Category: Physicochemistry
- Procedure steps: 6
- Wavelengths: calcium 422.7 nm; magnesium 285.2 nm
- Range without dilution: up to 50 mg/l of calcium and up to 5 mg/l of magnesium (optimum 3–50 and 0.9–5 mg/l with the air–acetylene flame)
- Flame: air–acetylene with lanthanum chloride or nitrous oxide–acetylene with caesium chloride
Overview
STATUS (as of 29.10.2026). PN-EN ISO 7980:2002 — Polish version, published 21.03.2002, implements EN ISO 7980:2000 and ISO 7980:1986 [IDT]. The standard is current — no withdrawal note. ISO 7980:1986 and EN ISO 7980:2000 are current in the ISO and CEN catalogues, with no newer edition.
SCOPE (clause 1). Dissolved calcium and magnesium in raw and drinking waters with a calcium content of up to 50 mg/l and a magnesium content of up to 5 mg/l; for higher concentrations a smaller volume of sample is taken for analysis. With the air–acetylene flame and a dilution of 1 : 10 (6.1), the optimum range is 3–50 mg/l of calcium and 0.9–5 mg/l of magnesium; with the nitrous oxide–acetylene flame, 2–20 mg/l of calcium and 0.2–2 mg/l of magnesium (Table 2).
CALCIUM AND MAGNESIUM IN WATER. Calcium and magnesium ions occur in all natural waters; there is usually several to more than ten times more calcium than magnesium. They come from weathering of rocks and leaching of soil, and also from industrial waste water. The sum of their concentrations is the total hardness of the water.
CHOICE OF FLAME (clause 4, note 2). The choice of flame is left to the user of the standard. The nitrous oxide–acetylene flame is considered preferable for samples with a high content of dissolved matter or containing phosphate, sulfate, aluminium or silica; it is generally recommended when the composition of the sample is complex or unknown. With the air–acetylene flame, lanthanum chloride minimizes interference from refractory compounds containing phosphate, sulfate, aluminium or silica; with the nitrous oxide–acetylene flame, caesium chloride minimizes ionization effects (6.4, note 2).
PRECISION (7.2). In a single laboratory, for distilled water with added calcium at 9.0 and 36 mg/l, the standard deviations were 0.3 and 0.6 mg/l and the recovery 99 %. In an interlaboratory study (more than 30 laboratories; calcium 100–300 mg/l, magnesium 7–85 mg/l), the coefficients of variation were 3.5–4.6 % for calcium and 2.9–6.9 % for magnesium.
LEGAL REQUIREMENTS. Water intended for human consumption: magnesium 7–125 mg/l (not more than 30 mg/l when sulfate is 250 mg/l or more) and hardness 60–500 mg/l expressed as calcium carbonate — values recommended for health reasons that do not oblige the supplier to supplement; calcium has no parametric value (Regulation of the Minister of Health of 22 May 2026, Journal of Laws 2026, item 748, Annex 1 Part D Table 2 items 1 and 3); measurement uncertainty for magnesium and hardness up to 20 % (Annex 4 Part B). In the monitoring of surface and ground waters, PN-EN ISO 7980 is the method for calcium and magnesium (FAAS), and total hardness may be calculated from their results (Journal of Laws 2021, item 1576, Annex 8, indicators 3.3.6–3.3.8).
Method principle
Calcium and magnesium are measured by flame atomic absorption spectrometry after adding lanthanum chloride (air–acetylene flame) or caesium chloride (nitrous oxide–acetylene flame), which reduce interferences; the absorbance of calcium is measured at 422.7 nm and that of magnesium at 285.2 nm (clause 2). The sample is aspirated into the flame and atomized, and a beam of radiation from a hollow cathode lamp passes through the flame into a monochromator and onto a detector, which measures the amount of radiation absorbed by the atoms of the element determined. Within the stated ranges the calibration graph must be linear (6.4).
Applications
- Raw and drinking waters — dissolved calcium and magnesium (clause 1)
- Monitoring of surface and ground waters — calcium and magnesium, and total hardness calculated from them (Journal of Laws 2021, item 1576)
- Water intended for human consumption — magnesium 7–125 mg/l and hardness 60–500 mg/l CaCO₃ as recommended values (Journal of Laws 2026, item 748)
- Higher concentrations — a smaller volume of sample (clause 1, 6.1)
Key parameters
| Parameter | Value |
|---|---|
| Wavelengths | calcium 422.7 nm; magnesium 285.2 nm |
| Range without dilution | up to 50 mg/l of calcium and up to 5 mg/l of magnesium (optimum 3–50 and 0.9–5 mg/l with the air–acetylene flame) |
| Flame | air–acetylene with lanthanum chloride or nitrous oxide–acetylene with caesium chloride |
| Measuring solution | 10.0 ml of sample + 10 ml of lanthanum (or caesium) solution, made up to 100 ml with 0.1 mol/l hydrochloric acid |
| Calibration | 7 solutions: calcium 0–5 mg/l, magnesium 0–0.5 mg/l (air–acetylene flame) |
| Result | to the nearest 1 mg/l or 0.02 mmol/l |
Standard
- Standard number
- PN-EN ISO 7980:2002
- Title (PL)
- Jakość wody — Oznaczanie wapnia i magnezu — Metoda atomowej spektrometrii absorpcyjnej
- Title (EN)
- Water quality — Determination of calcium and magnesium — Atomic absorption spectrometric method
Step-by-step procedure
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Sampling and preservation
In clean polyethylene or polypropylene bottles; as soon as possible after collection, acidify with 8 ml of hydrochloric acid (ρ = 1.18 g/ml) per litre of sample — this reduces the pH and prevents precipitation of calcium carbonate; analyse as soon as possible (clause 5).
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Measuring solution
Samples containing particulate material after acidification are filtered to prevent clogging of the nebulizer and burner. Into a 100 ml volumetric flask: 10 ml of lanthanum chloride solution (air–acetylene flame) or caesium chloride solution (nitrous oxide–acetylene), 10.0 ml of sample by pipette, made up with 0.1 mol/l hydrochloric acid. If the concentrations are above the ranges in Table 2, an appropriately smaller volume of sample is used (6.1).
-
Blank test
Simultaneously with the determination, using the same reagents in the same quantities, with water instead of the sample (6.2).
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Calibration solutions
Into seven 100 ml volumetric flasks put 10 ml of lanthanum (or caesium) solution each and, with pipettes, 0; 2.5; 5; 10; 15; 20 and 25 ml of calcium-magnesium standard solution, and make up with 0.1 mol/l acid: calcium 0; 0.5; 1; 2; 3; 4 and 5 mg/l, magnesium 0; 0.05; 0.1; 0.2; 0.3; 0.4 and 0.5 mg/l. These concentrations apply to the air–acetylene flame; with nitrous oxide, other concentrations may be appropriate (6.3, Table 1).
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Calibration and measurement
At 422.7 nm (calcium) and 285.2 nm (magnesium), aspirate the calibration and blank solutions in random order, with 0.1 mol/l hydrochloric acid between each. The calibration graph must be linear; if it is not, find and eliminate the source of error and repeat the calibration. Then aspirate the test solutions, also separated by acid. It is good practice to check the slope of the graph at regular intervals, e.g. every 10 samples (6.4, note 1).
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Calculation
ρ₁ = ρ₂ · f · V₁ / V₀, where ρ₂ is the concentration read from the graph after taking the blank into account, f the dilution factor from the acid added for preservation (normally 1.008), V₀ the sample volume (normally 10 ml) and V₁ the flask volume (100 ml). Amount-of-substance concentrations: cCa = ρCa / 40.1, cMg = ρMg / 24.3 [mmol/l]. The result is expressed to the nearest 1 mg/l or 0.02 mmol/l (7.1).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Atomic absorption spectrometer | set up and operated according to the manufacturer’s instructions, with a burner for the air–acetylene or nitrous oxide–acetylene flame and hollow cathode lamps for calcium and magnesium (clause 4) | — |
| Volumetric flasks, 100 ml and 1000 ml, pipettes | for test, calibration and standard solutions (clauses 3, 6) | — |
| Polyethylene or polypropylene bottles | for samples and stock solutions (clauses 3, 5) | — |
| Glassware | cleaned with warm dilute (1 + 1) hydrochloric acid and rinsed with water (clause 4, note 1) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Hydrochloric acid, ρ = 1.18 g/ml | 7647-01-0 | for preserving samples (8 ml per litre) and for the lanthanum chloride solution (3.1) |
| Hydrochloric acid, 0.1 mol/l | — | dilute 8 ml of hydrochloric acid ρ = 1.18 g/ml to 1 l; solvent for the standard solutions and for making up the flasks (3.2) |
| Lanthanum chloride solution, 20 g La/l | — | put 24 g of lanthanum oxide La₂O₃ (atomic absorption spectrometry grade) into a 1 l volumetric flask, add slowly and cautiously, while stirring, 50 ml of hydrochloric acid ρ = 1.18 g/ml, and after dissolution make up to the mark with water (3.3) |
| Caesium chloride solution, 20 g Cs/l | — | dissolve 25 g of CsCl in 1 l of 0.1 mol/l hydrochloric acid (3.4) |
| Calcium stock solution, 1000 mg/l | — | dry calcium carbonate at 180 °C for 1 h and cool in a desiccator; suspend (2.50 ± 0.01) g in 100 ml of water, add slowly the minimum amount of 0.1 mol/l hydrochloric acid needed to dissolve it (about 250 ml), boil briefly (expelling CO₂), cool and make up to 1000 ml with 0.1 mol/l acid; store in a polyethylene or polypropylene bottle (3.5) |
| Magnesium stock solution, 1000 mg/l | — | dry magnesium oxide at 180 °C for 1 h; dissolve (1.66 ± 0.01) g in 0.1 mol/l hydrochloric acid and dilute with the same acid to 1000 ml; store in a polyethylene bottle (3.6) |
| Calcium-magnesium standard solution (20 mg/l Ca, 2 mg/l Mg) | — | with pipettes, 20.0 ml of calcium stock solution and 2.0 ml of magnesium stock solution, made up to 1000 ml with 0.1 mol/l acid; commercially available ready-made solutions may be used (3.7) |
| Water | — | distilled water or water of equivalent purity; reagents of recognized analytical grade (clause 3) |
Health and safety (OHS)
- Hydrochloric acid is added to lanthanum oxide slowly and cautiously, while stirring (3.3)
- Spectrometer, burner and gases (acetylene, nitrous oxide) — according to the manufacturer’s instructions (clause 4); handle the reagents in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7980:2002 — “Water quality — Determination of calcium and magnesium — Atomic absorption spectrometric method”.
How does this method work?
Calcium and magnesium are measured by flame atomic absorption spectrometry after adding lanthanum chloride (air–acetylene flame) or caesium chloride (nitrous oxide–acetylene flame), which reduce interferences; the absorbance of calcium is measured at 422.7 nm and that of magnesium at 285.
What is the measuring range and accuracy?
Wavelengths: calcium 422.7 nm; magnesium 285.2 nm; Range without dilution: up to 50 mg/l of calcium and up to 5 mg/l of magnesium (optimum 3–50 and 0.9–5 mg/l with the air–acetylene flame); Flame: air–acetylene with lanthanum chloride or nitrous oxide–acetylene with caesium chloride; Measuring solution: 10.0 ml of sample + 10 ml of lanthanum (or caesium) solution, made up to 100 ml with 0.1 mol/l hydrochloric acid.
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?
Atomic absorption spectrometer, Volumetric flasks, 100 ml and 1000 ml, pipettes, Polyethylene or polypropylene bottles, Glassware.
Where is this test used?
Raw and drinking waters — dissolved calcium and magnesium (clause 1); Monitoring of surface and ground waters — calcium and magnesium, and total hardness calculated from them (Journal of Laws 2021, item 1576); Water intended for human consumption — magnesium 7–125 mg/l and hardness 60–500 mg/l CaCO₃ as recommended values (Journal of Laws 2026, item 748); Higher concentrations — a smaller volume of sample (clause 1, 6.1).
What safety precautions apply?
Hydrochloric acid is added to lanthanum oxide slowly and cautiously, while stirring (3.3); Spectrometer, burner and gases (acetylene, nitrous oxide) — according to the manufacturer’s instructions (clause 4); handle the reagents 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-EN ISO 7980.