🔬 Phosphates / Total Phosphorus in Water
In short
Spectrometric determination of phosphorus compounds in water with ammonium molybdate and ascorbic acid (molybdenum blue). The test is performed according to PN-EN ISO 6878:2006+Ap1:2010+Ap2:2010; Range without dilution: 0.005–0.8 mg/l P; after extraction detection limit about 0.0005 mg/l. The procedure comprises 8 steps; it is used for: Ground and surface waters — dissolved and undissolved phosphorus compounds (introduction of the standard), Seawater and effluents (clause 1), Waste water from treatment plants — total phosphorus 2 mg P/l (PE 15 000–99 999) and 1 mg P/l (from 100 000), in both cases alternatively 80 % reduction (Journal of Laws 2019, item 1311); PN-EN ISO 6878 is one of the reference methods (Annex 12).
At a glance
- Standard: PN-EN ISO 6878:2006+Ap1:2010+Ap2:2010
- Category: Physicochemistry
- Procedure steps: 8
- Range without dilution: 0.005–0.8 mg/l P; after extraction detection limit about 0.0005 mg/l
- Wavelength: 880 nm (most sensitive) or 700 nm (about 30 % lower sensitivity)
- Measurement: between 10 and 30 min after adding the reagents
Overview
STATUS (as of 28.10.2026). PN-EN ISO 6878:2006 — Polish version, published 04.09.2006, implements EN ISO 6878:2004 [IDT] and ISO 6878:2004 [IDT]; it replaced PN-EN ISO 6878:2005 (English and German versions). The standard is current — no withdrawal note. ISO 6878:2004 is the second edition, which replaced ISO 6878:1998.
NATIONAL CORRIGENDA. There are two corrigenda to the Polish version, whose text PKN makes available free of charge with the catalogue card of the standard. Ap1:2010 (approved 24.03.2010): in 8.1.1, instead of a sulfuric acid density of 1.94 g/ml it should read 1.84 g/ml. Ap2:2010 (approved 02.06.2010): in Annex B, Table B.3, in the relative reproducibility column, instead of mg/l it should read % — the numbers in the table do not change, only their unit does.
SCOPE (clause 1). The standard specifies the determination of orthophosphate (clause 4), orthophosphate after solvent extraction (clause 5), hydrolysable phosphate together with orthophosphate (clause 6) and total phosphorus after digestion with peroxodisulfate (clause 7) or with nitric and sulfuric acid (clause 8). The methods are applicable to all kinds of water including seawater and effluents; without dilution 0.005–0.8 mg/l of phosphorus is determined, and after extraction lower concentrations, with a detection limit of about 0.0005 mg/l. According to the introduction, the standard concerns phosphorus compounds in ground, surface and waste waters, in the dissolved and undissolved state.
FORMS OF PHOSPHORUS (clause 3). Polyphosphate and some organophosphorus compounds are determined after conversion into molybdate-reactive orthophosphate by sulfuric acid hydrolysis. Many organophosphorus compounds are converted into orthophosphate by digestion with peroxodisulfate; nitric acid–sulfuric acid digestion is used when a more vigorous treatment is required.
INTERFERENCES (clause 2, 4.4.4.1.1). Known interferences are given in Annex A (informative); the standard recommends verifying whether others are present and eliminating them. Arsenate is reduced to arsenite with thiosulfate in acidic medium — quantitatively for at least up to 2 mg/l As.
SAMPLES (4.3). Bottles of polyethylene, polyvinyl chloride or — preferably — glass; for low phosphate concentrations only glass; bottles with cap liners, which may contain phosphorus, are avoided. For the determination of orthophosphate the sample is filtered within 4 h of sampling through a 0.45 µm membrane filter washed with 200 ml of water at about 30–40 °C, the first 10 ml of filtrate are rejected and the determination is made immediately; filtration should not take longer than 10 min.
WASTE WATER — LEGAL REQUIREMENTS. Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 (Journal of Laws 2019, item 1311): the maximum permissible value of total phosphorus in waste water from treatment plants is 2 mg P/l for a population equivalent (PE) of 15 000 to 99 999 and 1 mg P/l for a PE of 100 000 or more, in both cases alternatively a minimum reduction of 80 %; the values for smaller plants depend on the type of plant and the place of discharge (Annexes 2 and 3 with explanatory notes; annual average). As reference methods for total phosphorus the regulation lists PN-EN ISO 6878, PN-EN ISO 11885 and PN-EN ISO 17294-2 (Annex 12, item 27).
WHAT WE DO NOT GIVE HERE. The procedure of extraction (clause 5), hydrolysis (clause 6) and digestion (clauses 7–8) — their reagents, temperatures and times are not covered by the available text of the standard. We do not reproduce the figures of the precision tables of Annex B.
Method principle
Orthophosphate ions react with an acid solution containing molybdate and antimony ions to form an antimony phosphomolybdate complex, which ascorbic acid reduces to a strongly coloured molybdenum blue complex. The absorbance is measured most sensitively at 880 nm or — with a loss of about 30 % of the sensitivity — at 700 nm.
Polyphosphate and some organophosphorus compounds are determined after hydrolysis with sulfuric acid, and total phosphorus after digestion with peroxodisulfate or — when a more vigorous treatment is required — with nitric and sulfuric acid.
Applications
- Ground and surface waters — dissolved and undissolved phosphorus compounds (introduction of the standard)
- Seawater and effluents (clause 1)
- Waste water from treatment plants — total phosphorus 2 mg P/l (PE 15 000–99 999) and 1 mg P/l (from 100 000), in both cases alternatively 80 % reduction (Journal of Laws 2019, item 1311); PN-EN ISO 6878 is one of the reference methods (Annex 12)
- Very low concentrations — orthophosphate after solvent extraction, detection limit about 0.0005 mg/l (clause 5)
Key parameters
| Parameter | Value |
|---|---|
| Range without dilution | 0.005–0.8 mg/l P; after extraction detection limit about 0.0005 mg/l |
| Wavelength | 880 nm (most sensitive) or 700 nm (about 30 % lower sensitivity) |
| Measurement | between 10 and 30 min after adding the reagents |
| Test portion | up to 40 ml for 0–0.8 mg/l (10 mm cell); 20, 10 and 5 ml for up to 1.6, 3.2 and 6.4 mg/l; 0–0.2 mg/l in a 40 or 50 mm cell |
| Filtration (orthophosphate) | within 4 h of sampling, 0.45 µm membrane filter |
| National corrigenda | Ap1:2010 (sulfuric acid density in 8.1.1: 1.84 g/ml) and Ap2:2010 (unit % in Table B.3) |
Standard
- Standard number
- PN-EN ISO 6878:2006+Ap1:2010+Ap2:2010
- Title (PL)
- Jakość wody — Oznaczanie fosforu — Metoda spektrometryczna z molibdenianem amonu
- Title (EN)
- Water quality — Determination of phosphorus — Ammonium molybdate spectrometric method
Step-by-step procedure
-
Sampling
Glass bottles (for low concentrations glass only), otherwise polyethylene or polyvinyl chloride; no cap liners that may contain phosphorus (4.3.1).
-
Filtration (orthophosphate)
Within 4 h of sampling (bring a cooled sample to room temperature): wash a 0.45 µm filter with 200 ml of water at about 30–40 °C, filter the sample, reject the first 10 ml, collect the rest in a clean, dry glass bottle and determine immediately; if the pH of the filtrate is outside 3–10, adjust it with sodium hydroxide solution or 2 mol/l sulfuric acid; filtration within 10 min (4.3.2).
-
Test portion
At most 40 ml in a 50 ml flask (0–0.8 mg/l in a 10 mm cell); for higher concentrations smaller volumes according to Table 1; if necessary dilute with water to about 40 ± 2 ml (4.4.1, 4.4.4.1.1).
-
Colour development
Add, while swirling, 1 ml of ascorbic acid solution, then 2 ml of acid molybdate solution I; make up to the mark and mix (4.4.3.2). If the sample contains arsenate: 0.4 ml of 4.5 mol/l sulfuric acid, 1 ml of ascorbic acid and 1 ml of thiosulfate solution, after 10 ± 1 min 2 ml of molybdate solution II (4.4.4.1.1).
-
Measurement
After 10–30 min measure the absorbance at 880 nm (or 700 nm) against water in the reference cell (4.4.3.3).
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Blank and calibration
Blank in parallel, with water instead of the sample and the same quantities of reagents (4.4.2). Calibration: 1.0–10.0 ml of the 2 mg/l working solution in 50 ml flasks (0.04–0.4 mg/l P), then as for the sample; absorbance is linear in concentration — determine the slope and check the linearity from time to time, especially after a change of reagent batches (4.4.3).
-
Other forms of phosphorus
Orthophosphate after extraction (clause 5), hydrolysable phosphate (clause 6) and total phosphorus after digestion (clauses 7–8) — we do not quote their conditions here; molybdate solution II is used for samples acidified with sulfuric acid (4.1.7).
-
Result
Phosphorus concentration in mg/l P; it is converted to orthophosphate ions with the factor 3.0665 (P → PO₄³⁻; conversion table in the textbook “Chemia wody”, Wrocław University of Science and Technology, 2024).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Spectrometer | prism, grating or filter type, for measurements in the visible and near-infrared regions, with 10–50 mm cells; with 100 mm cells the detection limit is lower (4.2.1) | — |
| Filter assembly | for a membrane filter of nominal pore size 0.45 µm (4.2.2) | — |
| Glassware | washed with 2.5 mol/l hydrochloric acid at about 40–50 °C and rinsed with water; no phosphate-containing detergents; preferably used only for phosphorus determination; glassware for colour development is rinsed occasionally with sodium hydroxide solution to remove deposits of the coloured complex (4.2.3) | — |
| 50 ml one-mark volumetric flasks | for calibration and test solutions (4.4.3.1, 4.4.4.1.1) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sulfuric acid about 9 mol/l | 7664-93-9 | add 500 ± 5 ml of acid (ρ = 1.84 g/ml) cautiously, with stirring and cooling, to 500 ± 5 ml of water; from it the 4.5 and 2 mol/l solutions (4.1.1–4.1.3) |
| Ascorbic acid, 100 g/l | 50-81-7 | 10 ± 0.5 g in 100 ± 5 ml of water; stable for 2 weeks in an amber glass bottle in a refrigerator — usable as long as it remains colourless (4.1.5) |
| Acid molybdate solution I | — | 13 ± 0.5 g of ammonium heptamolybdate tetrahydrate in 100 ± 5 ml of water added with stirring to 300 ± 5 ml of 9 mol/l sulfuric acid; add 0.35 ± 0.05 g of antimony potassium tartrate hemihydrate dissolved in 100 ± 5 ml of water; stable for at least 2 months in an amber glass bottle (4.1.6) |
| Acid molybdate solution II | — | for samples acidified with 4.5 mol/l sulfuric acid (clauses 6–8): 230 ml of 9 mol/l acid in 70 ml of water, molybdate and tartrate as in solution I; stable for at least 2 months (4.1.7) |
| Potassium dihydrogen phosphate — standard solutions | 7778-77-0 | stock 50 mg/l P: 0.2197 ± 0.0002 g of KH₂PO₄ dried to constant mass at 105 °C in about 800 ml of water, 10 ml of 4.5 mol/l sulfuric acid, made up to 1000 ml — stable for at least 3 months, storage at about 4 °C recommended; a commercial solution may be used. Working 2 mg/l — prepared on the day of use (4.1.10–4.1.11) |
| Sodium thiosulfate, 12.0 g/l | 10102-17-7 | 1.20 ± 0.05 g of Na₂S₂O₃·5H₂O in 100 ± 5 ml of water with 0.05 g of anhydrous sodium carbonate; for the reduction of arsenate; stable for at least 4 weeks (4.1.9) |
| Turbidity–colour compensation solution | — | 2 parts by volume of 4.5 mol/l sulfuric acid and 1 part of ascorbic acid solution; stable for several weeks in a refrigerator (4.1.8) |
Health and safety (OHS)
- The standard does not discuss hazards in detail — the user is responsible for safety; molybdate and antimony waste solutions should be disposed of properly (warning at the beginning of the standard)
- Sulfuric acid is diluted cautiously, adding the acid to water with stirring and cooling (4.1.1)
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 6878:2006+Ap1:2010+Ap2:2010 — “Water quality — Determination of phosphorus — Ammonium molybdate spectrometric method”.
How does this method work?
Orthophosphate ions react with an acid solution containing molybdate and antimony ions to form an antimony phosphomolybdate complex, which ascorbic acid reduces to a strongly coloured molybdenum blue complex. The absorbance is measured most sensitively at 880 nm or — with a loss of about 30 % of the sensitivity — at 700 nm.
What is the measuring range and accuracy?
Range without dilution: 0.005–0.8 mg/l P; after extraction detection limit about 0.0005 mg/l; Wavelength: 880 nm (most sensitive) or 700 nm (about 30 % lower sensitivity); Measurement: between 10 and 30 min after adding the reagents; Test portion: up to 40 ml for 0–0.8 mg/l (10 mm cell); 20, 10 and 5 ml for up to 1.6, 3.2 and 6.4 mg/l; 0–0.2 mg/l in a 40 or 50 mm cell.
How long does the test take?
The procedure comprises 8 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Spectrometer, Filter assembly, Glassware, 50 ml one-mark volumetric flasks.
Where is this test used?
Ground and surface waters — dissolved and undissolved phosphorus compounds (introduction of the standard); Seawater and effluents (clause 1); Waste water from treatment plants — total phosphorus 2 mg P/l (PE 15 000–99 999) and 1 mg P/l (from 100 000), in both cases alternatively 80 % reduction (Journal of Laws 2019, item 1311); PN-EN ISO 6878 is one of the reference methods (Annex 12); Very low concentrations — orthophosphate after solvent extraction, detection limit about 0.0005 mg/l (clause 5).
What safety precautions apply?
The standard does not discuss hazards in detail — the user is responsible for safety; molybdate and antimony waste solutions should be disposed of properly (warning at the beginning of the standard); Sulfuric acid is diluted cautiously, adding the acid to water with stirring and cooling (4.1.1).
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 6878.