Water quality - Determination of total phosphorus - Ammonium molybdate spectrophotometric method
1 Subject content and applicable scope
This standard specifies ammonium molybdate spectrophotometric method of determining total phosphorus by digesting unfiltered water sample with potassium persulfate (or nitric acid-perchloric acid) as oxidizer.
Total phosphorus is in form of dissolved, granular, organic and inorganic phosphorus.
This standard is applicable to surface water, wastewater and industrial waste water.
For 25mL test portion taken, the minimum detectable concentration is 0.01mg/L, and the upper determination limit is 0.6mg/L in this standard.
Under acidic condition, interference determinations with arsenic, chromium and sulfur are carried out.
2 Principle
Under neutrality condition, potassium persulfate (or nitric acid-perchloric acid) is used to digest the specimen, and oxidize all the phosphorus contained into orthophosphate. In acid medium, after orthophosphate reacts with ammonium molybdate to generate phosphorus-molybdenum heteropoly acid in the presence of antimonic salt, it’s reduced immediately by ascorbic acid to generate a blue complex.
3 Reagent
Unless otherwise stated, the reagent in this standard shall be analytical reagent and distilled water or water with equivalent purity in accordance with the national standard or professional standard.
3.1 Sulfuric acid (H2SO4), 1.84g/mL in density.
3.2 Nitric acid (HNO3), 1.4g/mL in density.
3.3 Perchloric acid (HClO4), guaranteed reagent, 1.68g/mL in density.
3.4 Sulfuric acid (H2SO4), 1+1.
3.5 Sulfuric acid, about =1mol/L: adding 27mL sulfuric acid (3.1) into 973mL water.
3.6 Sodium hydroxide (NaOH), 1mol/L solution: dissolving 40g sodium hydroxide in water and diluting the solution to 1,000mL.
3.7 Sodium hydroxide (NaOH), 6mol/L solution: dissolving 240g sodium hydroxide in water and diluting the solution to 1,000mL.
3.8 Potassium persulfate, 50g/L solution: dissolving 5g potassium persulfate (K2S2O8) in water and diluting the solution to 100mL.
3.9 Ascorbic acid, 100g/L solution: dissolving 10g ascorbic acid (C6H8O6) in water and diluting the solution to 100mL.
Keep the solution in a brown reagent bottle, and it may be stable for weeks at cool place. It may be used after a long time if its color doesn't change.
3.10 Molybdate solution: dissolving 13g ammonium molybdate [(NH4)6Mo7O24·4H2O] and 0.35g antimony potassium tartrate [ ] in 100ml water respectively. Add ammonium molybdate solution slowly into 300mL sulfuric acid (3.4) with continuous stir, then add antimony potassium tartrate solution and mix them uniformly.
Keep the solution in a brown reagent bottle and it can be preserved for two months at cool place.
3.11 Turbidity-chroma compensation solution: mixing two volumes of sulfuric acid (3.4) with one volume of ascorbic acid solution (3.9).
Prepare the solution on the day of use.
3.12 Phosphorus standard stock solution: transfer 0.2197±0.001g potassium dihydrogen phosphate (KH2PO4), which is dried at 110℃ for 2h and cooled in a dryer, to a 1,000mL volumetric flask after being dissolved with water, add about 800mL water and 5mL sulfuric acid (3.4) and then dilute with water to the mark line before mixing them uniformly. 1.00mL of the standard solution contains 50.0μg phosphorus.
This solution may be preserved in glass bottle for at least six months.
3.13 Phosphorus standard application solution: transfer 10.0mL phosphonium standard solution (3.12) to a 250mL volumetric flask, dilute with water to the mark line before mixing them uniformly. 1.00mL of the standard solution contains 2.0μg phosphorus.
Prepare the solution on the day of use.
3.14 Phenolphthalein, 10g/L solution: dissolving 0.5g phenolphthalein in 50mL 95% ethanol.
4 Instruments
Common instruments and equipment in laboratory as well as the following instruments.
4.1 Medical portable steam sterilizer or general pressure cooker (1.1~1.4kg/cm2).
4.2 50mL graduated tube with stopper (ground).
4.3 Spectrophotometer.
Note: All glass wares shall be soaked in diluted hydrochloric acid or nitric acid.
5 Sampling and sample
5.1 After taking 500mL water sample, add 1mL sulfuric acid (3.1) to adjust the pH value of the sample to be less than or equal to 1, or keep the water sample at a cool place without adding any reagent.
Note: Water sample containing a little phosphorus shall not be sampled with plastic bottle, for phosphate tends to be absorbed on the wall of the plastic bottle.
5.2 Specimen preparation
Put 25mL sample (5.1) in the graduated tube with stopper (4.2). When preparing, the sample shall be shaken well in order to obtain typical specimens for both dissolved part and suspended part. If the sample has a relatively high phosphorus concentration, the specimen volume may be reduced.
1 Subject content and applicable scope
2 Principle
3 Reagent
4 Instruments
5 Sampling and sample
6 Analysis procedures
7 Expression of results
8 Precision and accuracy
Additional explanation
