🔬 Phosphates / Total Phosphorus in Water

Physicochemistry PN-EN ISO 6878

Overview

Phosphorus is a key biogenic element responsible for eutrophication of surface waters — excessive algal growth, cyanobacterial blooms, oxygen deficiency, and aquatic ecosystem degradation. In natural waters phosphorus occurs as: orthophosphates (PO₄³⁻, reactive, directly assimilable by organisms), polyphosphates (hydrolyze to orthophosphates), and organic phosphorus (bound to organic matter).

Total phosphorus = orthophosphates + polyphosphates + organic phosphorus (after mineralization).

Standards: drinking water ≤0.4 mg PO₄³⁻/L, treated wastewater ≤2 mg P/L (>10 000 PE), surface waters class I ≤0.2 mg P/L. Urban Wastewater Directive requires phosphorus reduction in treatment plants above 10 000 PE.

ISO 6878 method (molybdenum blue with ascorbic acid) is reference method — highly sensitive, range 0.005–0.8 mg P/L.

ONE KEY, FOUR DIFFERENT DETERMINATIONS — WHAT THE TAB REALLY SHOWS HERE (23.09.2026). The key "PN-EN ISO 6878" is matched by the beginning of the value and covers 327 entries at 158 laboratories. BUT IN THE SCOPES THIS STANDARD IS BROKEN DOWN INTO CLAUSES, AND EACH CLAUSE IS A DIFFERENT DETERMINATION: the measurement (23.09, copy of the scopes) gives 67 laboratories with a record without a clause reference, 53 with "pkt 7" [clause 7], 50 with "pkt 4", 45 with "pkt 8" and one with "pkt 5", while 91 laboratories have ONLY a record referring to a specific clause and none without. SO THE LIST OF 158 LABORATORIES IS NOT A LIST OF LABORATORIES PERFORMING THE SAME TEST. Someone looking for the determination of orthophosphates and someone looking for total phosphorus after mineralization will get the same list — because the key distinguishes documents, while the laboratories distinguish clauses within a document. THIS IS A SEPARATE VARIETY OF OVER-INCLUSION: it is not a foreign standard pulled in by a common prefix, nor another part of the series, but a DIFFERENT PARAMETER DETERMINED ACCORDING TO THE SAME DOCUMENT. Across the whole set there are 4892 records with a reference to a clause of a standard, at 640 laboratories, so this is not a peculiarity of this one standard.

THE SECOND CORRECTION IS LESS VISIBLE THAN THE FIRST. After Ap2 the figures in Table B.3 stay the same; only their meaning changes — a reproducibility read as mg/l at a value of 5 means something different from 5 %. Anyone comparing a result with the table from before the correction will see no difference in the digits.

Method principle

Orthophosphate ions react with ammonium molybdate and antimony potassium tartrate in acidic environment (H₂SO₄), forming heteropolyacid phosphomolybdic complex (antimony-phosphomolybdate). Complex is reduced by ascorbic acid to intensely blue complex (molybdenum blue).

PO₄³⁻ + Mo(VI) + Sb³⁺ → (H⁺) → phosphomolybdate complex Complex + ascorbic acid → molybdenum blue (intensely blue)

Absorbance is measured at 880 nm (maximum sensitivity) or 700 nm (with lower sensitivity). Color develops in 10–30 minutes and is stable for 24 hours.

For total phosphorus: before analysis sample is mineralized (K₂S₂O₈ + H₂SO₄, 120°C/30 min) to convert all phosphorus forms to orthophosphates.

Applications

• Surface water eutrophication monitoring
• Treated wastewater control (limit ≤1–2 mg P/L)
• Drinking water quality control
• Chemical phosphorus removal process monitoring (coagulation)
• Enhanced biological phosphorus removal (EBPR) process control
• Groundwater testing (agricultural impact)

Key parameters

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

1. Mixed reagent preparation

   Mix in sequence: 50 mL H₂SO₄ 4.5M + 5 mL antimony potassium tartrate solution + 15 mL ammonium molybdate solution + 30 mL ascorbic acid solution. Prepare on day of use!

   ⏱ Time: 10 min

2. Mineralization (total phosphorus only)

   To 40 mL sample add 5 mL K₂S₂O₈ solution and 1 mL H₂SO₄ 4.5M. Mineralize in autoclave 120°C/30 min or thermoreactor. Cool to room temperature.

   ⏱ Time: 60 min • 🌡 Temperature: 120°C

3. Sample preparation (orthophosphates)

   Measure 40 mL sample into 50 mL volumetric flask. For turbid samples — filter through 0.45 μm filter. Sample must be at room temperature.

4. Reagent addition

   Add 1 mL ascorbic acid, then 2 mL acidic molybdate solution (mixed reagent). Fill to 50 mL with water. Mix thoroughly.

   ⏱ Time: 2 min

5. Color development

   Wait 10–30 minutes for full blue color development. Color stable up to 24 hours.

   ⏱ Time: 10–30 min

6. Absorbance measurement

   Measure absorbance at 880 nm (or 700 nm) against blank. Read P concentration from calibration curve. Cuvette 10–40 mm (select for concentration).

   ⏱ Time: 5 min

7. Calculations

   P concentration [mg/L] = curve reading × dilution factor. Conversion: PO₄³⁻ [mg/L] = P × 3.066.

8. Quality control

   Control standard (e.g., 0.5 mg P/L) — deviation max ±10%. Blank — absorbance <0.01. Duplicates — RPD <10%.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sulfuric acid — highly corrosive, causes severe burns. Use goggles and gloves.
• Antimony potassium tartrate — contains antimony (Sb), toxic. Avoid inhalation and ingestion.
• Potassium persulfate — strong oxidizer, irritating. Store away from combustibles.
• Safety goggles, nitrile gloves, and coat mandatory.
• Molybdenum and antimony-containing waste collect separately.

Frequently asked questions