⚗️ Total phosphorus in sewage sludge — mineralisation with a mixture of sulfuric and nitric acid, determination of the orthophosphates by the molybdate method
In short
The standard describes the complete mineralisation of raw or digested sludge, after which all the forms of phosphorus pass into orthophosphates in an aqueous solution; the final determination itself is carried out according to EN ISO 6878 (the molybdate method), EN ISO 10304-2 or another validated method. The test is performed according to PN-EN 14672:2006 (EN 14672:2005 [IDT]); Range: up to 50 g P/kg with a suitable dilution of the acid digest at the measuring step. The procedure comprises 7 steps; it is used for: Sewage sludge for agricultural use — phosphorus as a fertilising component in the documentation of the handing over of the sludge, The phosphorus balance in a treatment plant — control of the effectiveness of chemical and biological phosphorus removal, Sludge before the recovery of phosphorus (struvite, ashes) — assessment of the richness of the raw material.
At a glance
- Standard: PN-EN 14672:2006 (EN 14672:2005 [IDT])
- Category: Physicochemistry
- Procedure steps: 7
- What the result covers: all the forms of phosphorus converted into orthophosphates — total phosphorus
- Range: up to 50 g P/kg with a suitable dilution of the acid digest at the measuring step
- Mineralisation: a mixture of sulfuric and nitric acid; a moist or a dry sample
Overview
PN-EN 14672 concerns the preparation of sludge for the determination of phosphorus and not the measurement itself. The standard specifies the procedure for the complete mineralisation of raw or digested sludge, adapted for the subsequent determination of phosphorus; this way of pretreatment makes it possible to determine all the forms of phosphorus — that is, total phosphorus — by a quantitative oxidation giving an aqueous solution of orthophosphates, which is analysed for phosphates according to EN ISO 6878, EN ISO 10304-2 or an equivalent validated method. The range of the method reaches 50 g P/kg with a suitable dilution of the acid digest at the stage of the final measurement.
The comparative report HORIZONTAL points out that prEN 14672 describes a digestion with a mixture of sulfuric and nitric acid, on a moist or a dry sample, and leaves the choice of the method of the final determination open — referring to the measuring standards. This arrangement is convenient (the laboratory uses the method it already has validated), but it requires a careful description in the procedure: the test report should make clear what route of mineralisation and what method of measurement were used.
The method of the final determination (EN ISO 6878) consists in the reaction of orthophosphate ions with an acid solution containing molybdate and antimony ions, as a result of which an antimony-phosphomolybdate complex is formed; the complex is reduced with ascorbic acid to intensely coloured molybdenum blue, whose absorbance is measured spectrophotometrically. The standard notes that polyphosphates and part of the organophosphorus compounds are determined if they are converted into a form reactive towards the molybdate — after the mineralisation according to EN 14672 that is precisely what happens.
Practice: the phosphorus in sludge is for the most part bound to the solid phase, and for that reason the result is decided by the effectiveness of the digestion — the standard requires a complete mineralisation giving a quantitative oxidation of all the forms of phosphorus to orthophosphates. The weighed portion is taken from well homogenised sludge, and the result is related to the dry matter determined in parallel. The waste after the mineralisation contains concentrated acids and requires neutralisation.
Method principle
A weighed portion of sludge (moist or dried, depending on the variant adopted) is digested with a mixture of concentrated sulfuric and nitric acid at a raised temperature, until a clear acid digest is obtained — all the forms of phosphorus (orthophosphates, polyphosphates, organic phosphorus, phosphates bound to iron and aluminium) then pass into orthophosphates in an aqueous solution. The acid digest is transferred quantitatively, diluted to a known volume and the orthophosphates in it are determined by a validated method: according to EN ISO 6878 by the reaction with an acid solution of molybdate and antimony to an antimony-phosphomolybdate complex, reduction with ascorbic acid (solution 100 g/l — 10 g ± 0.5 g of ascorbic acid in 100 ml ± 5 ml of water) to molybdenum blue and measurement of the absorbance, or according to EN ISO 10304-2 by the method of ion chromatography. The range up to 50 g P/kg is achieved by a suitable dilution of the digest before the final measurement. In parallel a blank is carried out through the whole course of the procedure and the dry matter is determined, to which the result is related.
Applications
- Sewage sludge for agricultural use — phosphorus as a fertilising component in the documentation of the handing over of the sludge
- The phosphorus balance in a treatment plant — control of the effectiveness of chemical and biological phosphorus removal
- Sludge before the recovery of phosphorus (struvite, ashes) — assessment of the richness of the raw material
- Digestate and biomass — determination of total phosphorus alongside Kjeldahl nitrogen
- Laboratories which have a validated method of determining phosphates and need a standardised route of mineralising sludge
Key parameters
| Parameter | Value |
|---|---|
| What the result covers | all the forms of phosphorus converted into orthophosphates — total phosphorus |
| Range | up to 50 g P/kg with a suitable dilution of the acid digest at the measuring step |
| Mineralisation | a mixture of sulfuric and nitric acid; a moist or a dry sample |
| Final determination | EN ISO 6878 (molybdate + antimony, reduction with ascorbic acid), EN ISO 10304-2 (ion chromatography) or another validated method |
| Reference of the result | to the dry matter determined in parallel (PN-EN 12880 / PN-EN 15934) |
| Note | the standard does not impose a method of measurement — the test report has to give the route of mineralisation and the method of determination |
Standard
- Standard number
- PN-EN 14672:2006 (EN 14672:2005 [IDT])
- Title (PL)
- Charakterystyka osadów ściekowych — Oznaczanie fosforu ogólnego
- Title (EN)
- Characterization of sludges — Determination of total phosphorus
Step-by-step procedure
-
Homogenisation and weighed portion
Homogenise the sludge thoroughly and take a representative weighed portion (moist or dried, in accordance with the variant adopted). In parallel determine the dry matter, so as to be able to relate the result to the dry matter of the sludge.
-
Mineralisation
Digest the weighed portion with a mixture of concentrated sulfuric and nitric acid at a raised temperature, until a clear acid digest is obtained. Carry it out under a fume hood; where there is heavy foaming, heat gently at first.
-
Transfer and dilution
Transfer the cooled acid digest quantitatively into a volumetric flask, make it up with water to the mark and mix. Choose the dilution so that the concentration of phosphorus in the solution falls within the range of the measuring method (the range of the standard reaches 50 g P/kg precisely thanks to the dilution).
-
Final determination — the spectrophotometric variant
In a portion of the acid digest determine the orthophosphates according to EN ISO 6878: add the acid solution of molybdate with antimony ions (an antimony-phosphomolybdate complex is formed), reduce it with ascorbic acid to molybdenum blue and measure the absorbance against a blank.
-
Final determination — the chromatographic variant
Alternatively determine the orthophosphates by the method of ion chromatography according to EN ISO 10304-2 or by another validated method — describe the choice in the procedure and in the test report.
-
Calibration and blank
Prepare a calibration curve from a solution of potassium dihydrogen phosphate in the range corresponding to the diluted acid digests. Carry out a blank through the whole course of the procedure (mineralisation without the sample) and subtract its result.
-
Calculation and test report
Calculate the content of total phosphorus and relate it to the dry matter of the sludge. In the test report give the route of mineralisation (EN 14672) and the method of the final determination, because the standard leaves that choice to the laboratory.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Digestion block or wet digestion set with acid-resistant flasks | work under a fume hood with extraction of the nitrous fumes | — |
| Spectrophotometer for measuring molybdenum blue | the final determination according to EN ISO 6878 | — |
| Ion chromatograph (the alternative variant) | determination of the orthophosphates according to EN ISO 10304-2 | — |
| Homogeniser for the sludge, analytical balance | a weighed portion representative of inhomogeneous material | — |
| Drying oven at 105 °C | the parallel determination of the dry matter for the conversion of the result | — |
| Class A volumetric flasks and pipettes | quantitative transfer and dilution of the digest | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Concentrated sulfuric acid | 7664-93-9 | a component of the mineralising mixture |
| Concentrated nitric acid | 7697-37-2 | the oxidant in the mineralising mixture; nitrous fumes — work under a fume hood |
| Ammonium molybdate | — | formation of the phosphomolybdate complex in the determination according to EN ISO 6878 |
| Potassium antimonyl tartrate | 28300-74-5 | the antimony ions in the molybdate reagent — they accelerate the formation of the complex |
| Ascorbic acid, solution 100 g/l | 50-81-7 | 10 g ± 0.5 g in 100 ml ± 5 ml of water; reduction of the complex to molybdenum blue |
| Potassium dihydrogen phosphate (the phosphate standard) | 7778-77-0 | preparation of the calibration solutions for the final determination |
Health and safety (OHS)
- The mixture of concentrated sulfuric and nitric acid is highly corrosive and oxidising; carry out the digestion only under a fume hood, in goggles and acid-resistant gloves.
- The nitrous fumes arising during the digestion are toxic — it is not permitted to work without efficient ventilation.
- Do not mix the acid digest with organic materials (the risk of a violent reaction); cool it before dilution, add the acid digest to the water, not the other way round.
- The molybdate and antimony reagents are harmful — collect the waste separately, do not introduce it into the drains.
- Sewage sludge is infectious material — gloves, avoidance of aerosols, disinfection of the working place after the work.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN 14672:2006 (EN 14672:2005 [IDT]) — “Characterization of sludges — Determination of total phosphorus”.
How does this method work?
A weighed portion of sludge (moist or dried, depending on the variant adopted) is digested with a mixture of concentrated sulfuric and nitric acid at a raised temperature, until a clear acid digest is obtained — all the forms of phosphorus (orthophosphates, polyphosphates, organic phosphorus, phosphates bound to iron and aluminium) then pass into orthophosphates in an aqueous solution. The acid digest is transferred quantitatively, diluted to a known volume and the orthophosphates in it are determined by a validated method: according to EN ISO 6878 by the reaction with an acid solution of molybdate and antimony to an antimony-phosphomolybdate complex, reduction with ascorbic acid (solution 100 g/l — 10 g ± 0.
What is the measuring range and accuracy?
What the result covers: all the forms of phosphorus converted into orthophosphates — total phosphorus; Range: up to 50 g P/kg with a suitable dilution of the acid digest at the measuring step; Mineralisation: a mixture of sulfuric and nitric acid; a moist or a dry sample; Final determination: EN ISO 6878 (molybdate + antimony, reduction with ascorbic acid), EN ISO 10304-2 (ion chromatography) or another validated method.
How long does the test take?
The procedure comprises 7 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Digestion block or wet digestion set with acid-resistant flasks, Spectrophotometer for measuring molybdenum blue, Ion chromatograph (the alternative variant), Homogeniser for the sludge, analytical balance, Drying oven at 105 °C, Class A volumetric flasks and pipettes.
Where is this test used?
Sewage sludge for agricultural use — phosphorus as a fertilising component in the documentation of the handing over of the sludge; The phosphorus balance in a treatment plant — control of the effectiveness of chemical and biological phosphorus removal; Sludge before the recovery of phosphorus (struvite, ashes) — assessment of the richness of the raw material; Digestate and biomass — determination of total phosphorus alongside Kjeldahl nitrogen; Laboratories which have a validated method of determining phosphates and need a standardised route of mineralising sludge.
What safety precautions apply?
The mixture of concentrated sulfuric and nitric acid is highly corrosive and oxidising; carry out the digestion only under a fume hood, in goggles and acid-resistant gloves.; The nitrous fumes arising during the digestion are toxic — it is not permitted to work without efficient ventilation.; Do not mix the acid digest with organic materials (the risk of a violent reaction); cool it before dilution, add the acid digest to the water, not the other way round.; The molybdate and antimony reagents are harmful — collect the waste separately, do not introduce it into the drains.; Sewage sludge is infectious material — gloves, avoidance of aerosols, disinfection of the working place after the work.
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 14672.