🔬 Total iron in water
In short
Spectrometric determination of iron in water and waste water with 1,10-phenanthroline — total iron (directly or after digestion), total dissolved iron and dissolved iron(II). The test is performed according to PN-ISO 6332:2001; Range: 0.01–5 mg/l; higher concentrations after dilution (clause 1). The procedure comprises 9 steps; it is used for: Water intended for human consumption — iron 200 µg/l (Journal of Laws 2026, item 748), Waste water — one of the reference methods for iron (Journal of Laws 2019, item 1311, Annex 12), Monitoring of surface and ground waters — total iron (Journal of Laws 2021, item 1576).
At a glance
- Standard: PN-ISO 6332:2001
- Category: Physicochemistry
- Procedure steps: 9
- Range: 0.01–5 mg/l; higher concentrations after dilution (clause 1)
- Wavelength: about 510 nm; measured against water in the reference cell
- pH for complex formation: 3.5–5.5, preferably 4.5 (acetate buffer added last)
Overview
STATUS (as of 29.10.2026). PN-ISO 6332:2001 — Polish version, published 26.10.2001, implements ISO 6332:1988 [IDT]; it replaced PN-C-04586-03:1973 and PN-C-04586-04:1990. The standard is current — no withdrawal note; it has the national amendment PN-ISO 6332:2001/Ap1:2016-06. ISO 6332:1988 (second edition, which replaced the 1982 edition) is current in the ISO catalogue, with no newer edition or draft.
NATIONAL AMENDMENT Ap1:2016-06 (approved 10.06.2016). It corrects an error in the original ISO text in 4.11: standard solution II (1 mg/l) is prepared from 5 ml of the iron stock solution (0.10 g/l), not from 5 ml of standard solution I — this is how we give it below.
SCOPE (clause 1). Iron in water and waste water: total iron (sum of dissolved and undissolved iron) — determined directly or after digestion, total dissolved iron (sum of iron(II) and iron(III)) and dissolved iron(II). Range 0.01–5 mg/l; higher concentrations after suitable dilution of the sample.
IRON IN WATER. Iron compounds come from soil and rocks or from waste water and, as corrosion products, also from components of the water supply network. In ground waters mainly iron(II) compounds occur, in surface waters iron(III) compounds; iron(II) in contact with atmospheric oxygen is oxidized to iron(III). Iron in tap water does not endanger health, but causes rusty-brown deposits and stains.
INTERFERENCES (clause 10). The 1,10-phenanthroline method is relatively free from interferences. Copper, cobalt, chromium and zinc interfere at concentrations ten times that of iron, nickel above 2 mg/l; these interferences are avoided by adjusting the pH to 3.5–5.5. Bismuth, silver and mercury interfere above 1 mg/l, cadmium above 50 mg/l. Cyanides are usually removed by acidification of the sample; for some complex cyanides decomposition (7.1.2) is recommended. Acidification converts pyrophosphates and polyphosphates into orthophosphates, which do not interfere up to ten times the iron concentration; at higher concentrations decomposition is recommended.
PRECISION (clause 9). The repeatability table gives means of 30 results in several laboratories, e.g. for 0.100 mg/l standard deviations of 0.004–0.015 mg/l (five laboratories), for 1.000 mg/l 0.004–0.05 mg/l.
LEGAL REQUIREMENTS. Water intended for human consumption: iron 200 µg/l (Regulation of the Minister of Health of 22 May 2026, Journal of Laws 2026, item 748, Annex 1 Part C Table 2 item 15); measurement uncertainty up to 30 % of the parametric value (Annex 4 Part B). PN-ISO 6332 is one of the reference methods for iron in waste water — molecular absorption spectrophotometry, alongside PN-EN ISO 15586 and PN-EN ISO 11885 (Journal of Laws 2019, item 1311, Annex 12 item 75) — and the spectrophotometric method for total iron in the monitoring of surface and ground waters (Journal of Laws 2021, item 1576, Annex 8, indicator 4.3.1).
Method principle
A 1,10-phenanthroline solution is added to the test portion; iron(II) forms an orange-red complex whose absorbance is measured at about 510 nm — at the absorption maximum (molar absorption coefficient 11 × 10³ l/(mol·cm)). For total iron and total dissolved iron, hydroxylammonium chloride reduces iron(III) to iron(II); undissolved iron, iron oxides and iron complexes must first be brought into solution (7.1.2). The complex is stable in the pH range 2.5–9, the colour intensity is proportional to the amount of iron(II), and the relationship between absorbance and concentration is linear up to 5.0 mg/l (clause 3).
Applications
- Water intended for human consumption — iron 200 µg/l (Journal of Laws 2026, item 748)
- Waste water — one of the reference methods for iron (Journal of Laws 2019, item 1311, Annex 12)
- Monitoring of surface and ground waters — total iron (Journal of Laws 2021, item 1576)
- Distinguishing the forms: total iron, total dissolved iron and dissolved iron(II) (clause 1)
Key parameters
| Parameter | Value |
|---|---|
| Range | 0.01–5 mg/l; higher concentrations after dilution (clause 1) |
| Wavelength | about 510 nm; measured against water in the reference cell |
| pH for complex formation | 3.5–5.5, preferably 4.5 (acetate buffer added last) |
| Colour development | 15 min in the dark after adding phenanthroline |
| Test portion | 50.0 ml of sample acidified to pH 1 |
| Result | to the nearest 0.001 mg/l (0.010–0.100 mg/l), 0.01 mg/l (up to 10 mg/l), 0.1 mg/l (above 10 mg/l) |
Standard
- Standard number
- PN-ISO 6332:2001
- Title (PL)
- Jakość wody — Oznaczanie żelaza — Metoda spektrometryczna z 1,10-fenantroliną
- Title (EN)
- Water quality — Determination of iron — Spectrometric method using 1,10-phenanthroline
Step-by-step procedure
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Sampling and preservation
Containers such as polyethylene (ISO 5667-1). Total iron: acidify the sample to pH 1 immediately after collection — generally 1 ml of concentrated sulfuric acid per 100 ml is sufficient. Total dissolved iron: filter the sample immediately after sampling and acidify the filtrate to pH 1. Iron(II): 1 ml of concentrated sulfuric acid in an oxygen flask, fill the flask completely with the sample, avoiding unnecessary contact with air (clause 6).
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Undissolved iron — oxidation
If undissolved iron, iron oxides or iron complexes are present: boil 50.0 ml of the acidified sample in a 100 ml flask gently with 5 ml of peroxodisulfate solution for about 40 min, ensuring that the volume does not fall below about 20 ml; cool, transfer to a 50 ml volumetric flask and make up to the mark (filter a turbid solution through a membrane filter). Alternatively an autoclave: 30 min in a closed 100 ml bottle, dilution to 100 ml and a factor of 2 in the calculation (7.1.1, 7.1.1.1).
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Total iron after digestion
Heat 50.0 ml of the acidified sample in a 100 ml beaker with 5 ml of nitric acid and 10 ml of hydrochloric acid at 70–80 °C until complete solution; after about 30 min add 2 ml of concentrated sulfuric acid and evaporate to the appearance of white sulfur trioxide fumes, not to dryness; cool, add 20 ml of water, transfer to a 50 ml volumetric flask and make up to the mark (7.1.2).
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Reduction to iron(II)
Solution from step 2 or 3 (for direct determination and dissolved iron — 50.0 ml of sample) in a 100 ml flask: add 1 ml of hydroxylammonium chloride solution and mix thoroughly, then 2 ml of acetate buffer — pH 3.5–5.5, preferably 4.5. The reduction proceeds most effectively at pH 1, so the buffer is added last (7.1.1.2, 7.2).
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Colour development and measurement
Add 2 ml of 1,10-phenanthroline solution and place in the dark for 15 min; measure the absorbance at 510 nm against water in the reference cell (7.1.1.3–7.1.1.4).
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Iron(II)
50.0 ml of the sample from the oxygen flask in a 100 ml flask: as in step 4 but without hydroxylammonium chloride, then step 5; avoid contact with air as far as possible (7.3).
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Blank test
The same procedure with 50 ml of water instead of the test portion (7.4).
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Calibration
A series of reference solutions from standard solutions I and II in 50 ml volumetric flasks with 0.5 ml of 4.5 mol/l sulfuric acid, appropriate to the expected concentration and treated like the samples for the form of iron to be determined. A separate curve for each form of iron, each photometric instrument and each optical path length; the calibration is checked periodically, especially for each new batch of reagents (7.5).
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Calculation
ρ = f · (A₁ − A₀) mg/l, where f is the slope of the appropriate calibration graph (7.5.2), A₁ is the absorbance of the test solution and A₀ that of the blank; the volume of acid added to the sample is taken into account. The result is reported indicating the form of iron determined (clause 8).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Spectrometer | prism or grating type, suitable for measurements at 510 nm (5.1) | — |
| Photometric cells | optical path length of at least 10 mm, appropriate to the expected absorbance; longer cells are preferable below 1.0 mg/l (5.2) | — |
| Membrane filters | average pore size 0.45 µm (5.3) | — |
| Oxygen (Winkler) flask, 100 ml | for the sample for iron(II), filled completely (5.4, 6.4) | — |
| Laboratory glassware | all glassware, including sample containers, washed with hydrochloric acid and rinsed with water; 100 ml flasks, 50 ml and 500 ml volumetric flasks, 100 ml beakers (clauses 5, 7) | — |
| Heating | gentle boiling for about 40 min (oxidation) or 70–80 °C and evaporation to SO₃ fumes (digestion); an autoclave may be used instead of boiling (7.1.1.1, 7.1.2) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sulfuric acid, concentrated, ρ = 1.84 g/ml | 7664-93-9 | for preserving samples (generally 1 ml per 100 ml) and for digestion (4.1) |
| Sulfuric acid, c(½ H₂SO₄) = 4.5 mol/l | — | add 1 volume of concentrated acid slowly, with vigorous stirring and cooling, to 3 volumes of water (4.2) |
| Nitric acid, concentrated, ρ = 1.40 g/ml | 7697-37-2 | for digestion (4.3) |
| Hydrochloric acid, ρ = 1.12 g/ml, c(HCl) = 7.7 mol/l | 7647-01-0 | for washing glassware, dissolving the iron wire and digestion (4.4) |
| Acetate buffer | — | dissolve 40 g of ammonium acetate and 50 ml of glacial acetic acid (ρ = 1.06 g/ml) in water and dilute to 100 ml (4.5) |
| Hydroxylammonium chloride, 100 g/l | 5470-11-1 | dissolve 10 g of NH₂OH·HCl in water and dilute to 100 ml; stable for at least one week (4.6) |
| 1,10-Phenanthroline solution | — | dissolve 0.5 g of 1,10-phenanthroline chloride (monohydrate) in water and dilute to 100 ml, or dissolve 0.42 g of 1,10-phenanthroline monohydrate in 100 ml of water containing 2 drops of hydrochloric acid; stable for one week if stored in the dark (4.7) |
| Potassium peroxodisulfate, 40 g/l | 7727-21-1 | dissolve 4 g of K₂S₂O₈ in water and dilute to 100 ml; stable for several weeks at room temperature in a dark glass bottle (4.8) |
| Iron stock solution, 0.10 g/l | — | dissolve 50.0 mg of iron wire (purity 99.99 %) in a 500 ml volumetric flask with 20 ml of water and 5 ml of 7.7 mol/l hydrochloric acid, warming gently; cool and make up to the mark; stable for at least one month in a bottle of resistant glass or plastic; commercial stock solutions may be used (4.9) |
| Iron standard solutions: I — 20 mg/l, II — 1 mg/l | — | I: 100 ml of stock solution to 500 ml; II: 5 ml of stock solution to 500 ml (according to amendment Ap1:2016-06 — the original ISO text wrongly says “standard solution I” here); both are prepared on the day of use (4.10–4.11) |
| Water | — | with as low an iron concentration as possible; a measurable iron concentration in the reagents is permissible provided the lowest concentration to be determined is at least three times the standard deviation of the blank results; deionized water or water distilled in an all-glass apparatus is suitable (clause 4) |
Health and safety (OHS)
- Toxic gases may be released when samples are acidified — take appropriate safety precautions (clause 6)
- Samples containing cyanide or sulfide are acidified with care because of the formation of highly toxic vapours (clause 10)
- Concentrated sulfuric acid is added to water slowly, with vigorous stirring and cooling (4.2)
- Handle nitric acid, hydrochloric acid, hydroxylammonium chloride and potassium peroxodisulfate in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-ISO 6332:2001 — “Water quality — Determination of iron — Spectrometric method using 1,10-phenanthroline”.
How does this method work?
A 1,10-phenanthroline solution is added to the test portion; iron(II) forms an orange-red complex whose absorbance is measured at about 510 nm — at the absorption maximum (molar absorption coefficient 11 × 10³ l/(mol·cm)). For total iron and total dissolved iron, hydroxylammonium chloride reduces iron(III) to iron(II); undissolved iron, iron oxides and iron complexes must first be brought into solution (7.
What is the measuring range and accuracy?
Range: 0.01–5 mg/l; higher concentrations after dilution (clause 1); Wavelength: about 510 nm; measured against water in the reference cell; pH for complex formation: 3.5–5.5, preferably 4.5 (acetate buffer added last); Colour development: 15 min in the dark after adding phenanthroline.
How long does the test take?
The procedure comprises 9 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Spectrometer, Photometric cells, Membrane filters, Oxygen (Winkler) flask, 100 ml, Laboratory glassware, Heating.
Where is this test used?
Water intended for human consumption — iron 200 µg/l (Journal of Laws 2026, item 748); Waste water — one of the reference methods for iron (Journal of Laws 2019, item 1311, Annex 12); Monitoring of surface and ground waters — total iron (Journal of Laws 2021, item 1576); Distinguishing the forms: total iron, total dissolved iron and dissolved iron(II) (clause 1).
What safety precautions apply?
Toxic gases may be released when samples are acidified — take appropriate safety precautions (clause 6); Samples containing cyanide or sulfide are acidified with care because of the formation of highly toxic vapours (clause 10); Concentrated sulfuric acid is added to water slowly, with vigorous stirring and cooling (4.2); Handle nitric acid, hydrochloric acid, hydroxylammonium chloride and potassium peroxodisulfate 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 6332.