⚗️ Calcium in water — complexometric titration with EDTA at pH 12–13 against the HSN indicator (calconcarboxylic acid)
In short
To 50. 0 ml of the sample 2 ml of NaOH 2 mol/l and about 0. The test is performed according to PN-ISO 6058:1999 (ISO 6058:1984); Range: 2–100 mg/l Ca (0.05–2.5 mmol/l); above that — dilution (factor F). The procedure comprises 6 steps; it is used for: Drinking water, groundwater and surface water — calcium 2–100 mg/l (above that after dilution), Municipal and industrial raw waters without interfering amounts of heavy metals, Calculation of the calcium hardness and — from the total hardness — of the magnesium content.
At a glance
- Standard: PN-ISO 6058:1999 (ISO 6058:1984)
- Category: Physicochemistry
- Procedure steps: 6
- Range: 2–100 mg/l Ca (0.05–2.5 mmol/l); above that — dilution (factor F)
- pH of the titration: 12–13 (2 ml of NaOH 2 mol/l per 50 ml of sample); Mg precipitated as Mg(OH)₂
- Titrant: EDTA 10 mmol/l (3.725 g of Na₂EDTA·2H₂O/l), standardised against CaCO₃ 10 mmol/l
Overview
ISO 6058:1984 specifies the titrimetric method with ethylenediaminetetraacetic acid (EDTA) for determining the calcium content of groundwater, surface water and drinking water; it can also be used for municipal and industrial raw waters, provided they do not contain interfering amounts of heavy metals. It is not suitable for sea water or other waters of high salinity. It applies to waters with a calcium content of 2–100 mg/l (0.05–2.5 mmol/l); above 100 mg/l a diluted sample is used. Precision: the results obtained for the same solution in different laboratories should agree to within ± 5 mg/l in the range 30–100 mg/l. Interferences come from the ions of aluminium, barium, lead, iron, cobalt, copper, manganese, tin and zinc (they are titrated like calcium or they blur the end point); orthophosphate above 1 mg/l precipitates calcium at the pH of the titration; calcium carbonate may precipitate if the titration is too slow or the calcium content is high. Iron up to 30 mg/l is masked by adding 250 mg of sodium cyanide or a few ml of triethanolamine immediately before the titration; the cyanide also limits the effect of zinc, copper and cobalt, and the triethanolamine that of aluminium (the solution must be alkaline before the cyanide is added). Where the interferences cannot be removed, atomic absorption spectrometry is used. In Poland the standard is in force as PN-ISO 6058:1999.
In the PWr textbook "Chemia wody" [Water chemistry] the determination of calcium with sodium versenate (based on PN-81/C-04551-01 and PN-ISO 6058:1999) is described in the variant with murexide: at about pH 13 murexide forms a red-pink complex with calcium, and once the calcium has been bound by the versenate the colour changes to violet; to 100 cm³ of the sample a pinch of murexide and 2 cm³ of NaOH 2.5 mol/dm³ are added and it is titrated with versenate (6.65 g of Na₂EDTA·2H₂O per 1000 cm³; 1 cm³ ≡ 0.1 °dH), and in the same sample, after acidification and alkalisation with ammonia water, the magnesium is determined against eriochrome black T.
Method principle
Complexometric titration of the calcium ions with an aqueous solution of the disodium salt of EDTA at pH 12–13. The indicator is HSN [2-hydroxy-1-(2-hydroxy-4-sulfo-1-naphthylazo)-3-naphthoic acid, also called calconcarboxylic acid; 0.2 g of HSN ground with 100 g of NaCl], which forms a red complex with calcium; calcein may also be used. The magnesium is precipitated as the hydroxide and does not interfere. During the titration the EDTA first reacts with the free calcium ions and then with the calcium bound to the indicator, which changes colour from red to light blue. The 10 mmol/l EDTA solution is prepared from 3.725 g of the disodium salt of EDTA dihydrate dried for 2 h at 80 °C and made up to 1000 ml (it is kept in a polyethylene bottle and its concentration is checked regularly) and is standardised against a standard calcium solution of 10 mmol/l (1.001 g of CaCO₃ dried for 2 h at 150 °C dissolved in HCl 4 mol/l, the CO₂ boiled off and the solution neutralised with ammonia 3 mol/l against methyl red to an orange colour, made up to 1000 ml; 1 ml contains 0.4008 mg of Ca); the concentration of the EDTA is c₁ = c₂·V₁/V₂. The test portion should contain 2–100 mg/l Ca; at a higher concentration a known volume is diluted and the factor F is recorded; samples acidified for preservation are neutralised with the calculated amount of NaOH 2 mol/l (8 g of NaOH in 100 ml of freshly distilled water, in a polyethylene bottle, protected from CO₂). The calcium content is c_Ca = c₁·V₃/V₀ (mmol/l) or ρ_Ca = c₁·V₃/V₀·40.08 (mg/l), where V₀ is the volume of the portion and V₃ the volume of EDTA used in the titration; the result is reported to 1 mg/l or 0.02 mmol/l.
Applications
- Drinking water, groundwater and surface water — calcium 2–100 mg/l (above that after dilution)
- Municipal and industrial raw waters without interfering amounts of heavy metals
- Calculation of the calcium hardness and — from the total hardness — of the magnesium content
- Control of softening, decarbonisation and reverse osmosis processes
- Laboratories without atomic spectrometry — the classical titrimetric method with a 25 ml burette (graduated in 0.05 ml)
Key parameters
| Parameter | Value |
|---|---|
| Range | 2–100 mg/l Ca (0.05–2.5 mmol/l); above that — dilution (factor F) |
| pH of the titration | 12–13 (2 ml of NaOH 2 mol/l per 50 ml of sample); Mg precipitated as Mg(OH)₂ |
| Titrant | EDTA 10 mmol/l (3.725 g of Na₂EDTA·2H₂O/l), standardised against CaCO₃ 10 mmol/l |
| Indicator | HSN (calconcarboxylic acid) 0.2 g per 100 g of NaCl, about 0.2 g per determination; red → blue; alternatively calcein |
| Precision | ± 5 mg/l between laboratories in the range 30–100 mg/l |
| Result | to 1 mg/l or 0.02 mmol/l; ρ_Ca = c₁·V₃/V₀·40.08 |
Standard
- Standard number
- PN-ISO 6058:1999 (ISO 6058:1984)
- Title (PL)
- Jakość wody — Oznaczanie zawartości wapnia — Metoda miareczkowa z EDTA
- Title (EN)
- Water quality — Determination of calcium content — EDTA titrimetric method
Step-by-step procedure
-
Standardisation of the EDTA
Dry Na₂EDTA·2H₂O for 2 h at 80 °C and dissolve 3.725 g in 1000 ml. Standardise it against 20.0 ml of the standard calcium solution of 10 mmol/l diluted to 50 ml, proceeding as in the determination; calculate c₁ = c₂·V₁/V₂.
-
Test portion
Take the samples in accordance with ISO 5667. The portion should contain 2–100 mg/l Ca — at a higher concentration dilute a known volume and record F; neutralise samples preserved with acid using the calculated amount of NaOH 2 mol/l (allow for the dilution).
-
Alkalisation and indicator
Transfer 50.0 ml of the solution with a pipette into a 250 ml conical flask, add 2 ml of NaOH 2 mol/l and about 0.2 g of the HSN indicator; mix. Where there are interferences add 250 mg of NaCN immediately before the titration (the solution must be alkaline) or a few ml of triethanolamine.
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Titration
Titrate at once with the EDTA solution from the burette while stirring continuously — faster at the beginning, slowly towards the end — until the colour changes to a distinct blue that does not change when another drop is added. Do not titrate too slowly (precipitation of CaCO₃).
-
Calculation
c_Ca = c₁·V₃/V₀ (mmol/l); ρ_Ca = c₁·V₃/V₀·40.08 (mg/l); where the sample was diluted multiply by F. Report the result to 1 mg/l or 0.02 mmol/l.
-
Test report
Give the reference to the standard, the identification of the sample, the result, the preparation of the portion (dilution, neutralisation) and any deviations from the procedure.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Burette of 25 ml graduated in 0.05 ml (ISO 385/1, class A) | titration with EDTA | — |
| Conical flasks of 250 ml and 500 ml, pipette 50 ml, volumetric flask 1000 ml | test portions and standard solutions | — |
| Drying oven (80 °C, 150 °C) and desiccator | drying of Na₂EDTA·2H₂O and CaCO₃ | — |
| Magnetic stirrer | continuous stirring during the titration | — |
| Polyethylene bottles | storage of the NaOH and the EDTA | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Disodium salt of EDTA dihydrate — solution of 10 mmol/l | — | titrant (standardised against the calcium standard) |
| Calcium carbonate (standard), HCl 4 mol/l, ammonia 3 mol/l, methyl red | — | standard calcium solution of 10 mmol/l |
| Sodium hydroxide 2 mol/l | — | alkalisation to pH 12–13; neutralisation of acidified samples |
| HSN indicator with NaCl (or calcein) | — | indicator of the end point |
| Sodium cyanide or triethanolamine | — | masking of Fe (≤ 30 mg/l), Zn, Cu, Co (cyanide) and Al (triethanolamine) |
Health and safety (OHS)
- Sodium cyanide is a poison — solutions containing cyanide must never be acidified (HCN); add it only to an alkaline solution and dispose of it in accordance with the regulations
- Sodium hydroxide 2 mol/l and hydrochloric acid 4 mol/l — goggles, gloves
- Triethanolamine — avoid contact with the skin
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 6058:1999 (ISO 6058:1984) — “Water quality — Determination of calcium content — EDTA titrimetric method”.
How does this method work?
Complexometric titration of the calcium ions with an aqueous solution of the disodium salt of EDTA at pH 12–13. The indicator is HSN [2-hydroxy-1-(2-hydroxy-4-sulfo-1-naphthylazo)-3-naphthoic acid, also called calconcarboxylic acid; 0.
What is the measuring range and accuracy?
Range: 2–100 mg/l Ca (0.05–2.5 mmol/l); above that — dilution (factor F); pH of the titration: 12–13 (2 ml of NaOH 2 mol/l per 50 ml of sample); Mg precipitated as Mg(OH)₂; Titrant: EDTA 10 mmol/l (3.725 g of Na₂EDTA·2H₂O/l), standardised against CaCO₃ 10 mmol/l; Indicator: HSN (calconcarboxylic acid) 0.2 g per 100 g of NaCl, about 0.2 g per determination; red → blue; alternatively calcein.
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 of 25 ml graduated in 0.05 ml (ISO 385/1, class A), Conical flasks of 250 ml and 500 ml, pipette 50 ml, volumetric flask 1000 ml, Drying oven (80 °C, 150 °C) and desiccator, Magnetic stirrer, Polyethylene bottles.
Where is this test used?
Drinking water, groundwater and surface water — calcium 2–100 mg/l (above that after dilution); Municipal and industrial raw waters without interfering amounts of heavy metals; Calculation of the calcium hardness and — from the total hardness — of the magnesium content; Control of softening, decarbonisation and reverse osmosis processes; Laboratories without atomic spectrometry — the classical titrimetric method with a 25 ml burette (graduated in 0.05 ml).
What safety precautions apply?
Sodium cyanide is a poison — solutions containing cyanide must never be acidified (HCN); add it only to an alkaline solution and dispose of it in accordance with the regulations; Sodium hydroxide 2 mol/l and hydrochloric acid 4 mol/l — goggles, gloves; Triethanolamine — avoid contact with the skin.
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 6058.