Phosphorus (total) in Effluents
Photometric determination using the PMB method after decomposition in a thermoreactor

Introduction

Phosphorous is a key nutrient that can have adverse environmental effects when it is present at high levels. In excess, it can upset marine ecosystems by causing an excessive growth of algae and plants in a process termed eutrophication.¹ As a result, testing phosphorous in effluents is critical to maintaining the environment. In this application note, we describe a photometric determination of total phosphorous using Crack-Sets for sample digestion in a thermoreactor followed Spectroquant^® Phosphate Test Kits.

Experimental Method

In sulfuric solution orthophosphate ions react with molybdate ions to form molybdophosphoric acid. Ascorbic acid reduces this to phosphomolybdenum blue (PMB) that is determined photometrically. The method is analogous to EPA 365.2+3, APHA 4500-P E, and DIN EN ISO 6878.

Reagents, Instruments and Materials

Test Kits

One of the following test kits is necessary for this analysis:

• Phosphate Cell Test (o-phosphate and total phosphorous) Method: photometric, PMB 0.05 - 5.00 mg/l PO₄-P; 0.2 - 15.3 mg/l PO₄^3-; 0.11 - 11.46 mg/l P₂O₅ Spectroquant^® (1.14543) or
• Phosphate Cell Test (o-phosphate and total phosphorous) Method: photometric, PMB 0.5 - 25.0 mg/l PO₄-P; 1.5 - 76.7 mg/l PO₄^3-; 1.1 - 57.3 mg/l P₂O₅ Spectroquant^® (1.14729), or
• Phosphate Test (o-phosphate) Method: photometric, PMB 0.0025 - 5.00 mg/l PO₄-P; 0.0077 - 15.3 mg/l PO₄^3-; 0.0057 - 11.46 mg/l P₂O₅ Spectroquant^® (1.14848)
• Crack Set 10C for the digestion of lead, cadmium, iron, copper, nickel, phosphorus (total) and zinc 25 digestions Spectroquant^® (1.14688)
• Crack Set 10 for the digestion of lead, cadmium, iron, copper, nickel, phosphorus (total) and zinc 100 digestions Spectroquant^® (1.14687)
• Sodium hydroxide solution c(NaOH) = 1 mol/l (1 N) Titripur^® Reag. Ph Eur, Reag. USP (1.09137)
• Sulfuric acid c(H₂SO₄) = 0.5 mol/l (1 N) Titripur^® Reag. Ph Eur, Reag. USP (1.09072)

Instruments

One of the following instruments is necessary for this analysis:

• Spectroquant^® VIS Spectrophotometer Prove 100 (173016)
• Spectroquant^® UV/VIS Spectrophotometer Prove 300 (173017)
• Spectroquant^® UV/VIS Spectrophotometer Prove 600 (173018)
• Spectroquant^® Photometer NOVA 30 (1.09748)
• Spectroquant^® Photometer NOVA 60 (1.09751)
• Spectroquant^® Photometer NOVA 60A (1.09752)
• Spectroquant^® Colorimeter Move 100 (1.73632)

Materials

• Rectangular cells 10 mm (1.14946) and/or
• Rectangular cells 20 mm (1.14947) and/or
• Rectangular cells 50 mm (1.14944)
• Empty cells with screw caps 16 mm Spectroquant^® (1.14724)
• pH-indicator strips pH 0 - 14 Universal indicator non-bleeding pH 0 - 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12 - 13 - 14 MQuant^® (1.09535)

Analytical Approach

Variant 1:

• Cat.No. 1.14543 Spectroquant^® Phosphate Cell Test
• Cat.No. 1.14729 Spectroquant^® Phosphate Cell Test

Variant 2:

• Cat.No. 1.14848 Spectroquant^® Phosphate Test
• Cat.No. 1.14688 Crack-Set 10C

Variant 3:

• Cat.No. 1.14848 Spectroquant^® Phosphate Test
• Cat.No. 1.14687 Crack-Set 10
• Cat.No. 1.14724 Empty round cells 16 mm with screw caps

Note

Sewage water containing complexing agents like EDTA or Cyanide must be digested with potassium peroxodisulfate (= K₂S₂O₈). Digestion frees the PO₄-P from these complexes. When the COD concentration is >300 mg/l to 500 mg/l double the quantity of potassium peroxodisulfate.

Sample preparation using Variant 1

1. Pipette 5.0 ml sample into a reaction cell
2. Add 1 dose of reagent P-1K (= K₂S₂O₈), close the cell tightly, and mix
3. Heat the cell at 120 °C in the preheated thermoreactor for 30 min
4. Allow the closed cell to cool to room temperature in a test-tube rack. Do not cool with cold water!
5. Check the pH of the solution with Universal indicator strips
6. Adjust the pH with sodium hydroxide solution or sulfuric acid within the range 0 - 10

Sample preparation using Variant 2

1. Pipette 10.0 ml sample into a digestion cell
2. Add 1 dose reagent R-1K (= K₂S₂O₈), close the cell tightly, and mix
3. Heat the cell at 120 °C in the preheated thermoreactor for 30 min
4. Allow the closed cell to cool to room temperature in the cell rack. Do not cool with cold water!
5. Add 3 drops of reagent R-2K to the cool cell and mix
6. Check the pH of the solution with Universal indicator strips
7. Adjust the pH with sodium hydroxide solution or sulfuric acid within the range 0 - 10.

Sample preparation using Variant 3

1. Pipette 10.0 ml pretreated sample into an empty cell
2. Add 1 drop reagent R-1 and mix
3. Add 1 dose reagent R-2 (= K₂S₂O₈), close the cell tightly, and mix
4. Heat the cell at 120 °C in the preheated thermoreactor for 30 min
5. Allow the closed cell to cool to room temperature in the cell rack. Do not cool with cold water!
6. Add 3 drops reagent R-3 to the cool cell and mix
7. Check the pH of the solution with Universal indicator strips
8. Adjust the pH with sodium hydroxide solution or sulfuric acid within the range 0 - 10.

Anaylsis

Determine with the above-mentioned test kits.

Calculation

Phosphorus (total) content in mg/l P = analysis result in mg/l P