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Introduction to the working principle of the online analyzer for measuring total phosphorus
Date: 2024-08-20Read: 4
What is total phosphorus? Phosphorus in water can be elemental phosphorus. Orthophosphate. Condensed phosphates. Pyrophosphate. Partial phosphates and organic phosphates. The main source of total phosphorus is domestic sewage. Fertilizer. Organophosphorus pesticides and phosphate cleaning agents used in modern detergents. Total phosphorus is the result of measuring the phosphorus content in milligrams per liter of water sample after converting various forms of phosphorus into orthophosphate after digestion.
1. Method principle.
Under acidic conditions, orthophosphate reacts with ammonium molybdate and potassium antimony oxide tartrate to produce phosphomolybdic acid, which is reduced by a reducing agent to form a blue complex, usually integrating phosphomolybdic blue.
The minimum detection concentration of this method is 0.01mg/L (the concentration corresponding to absorbance a=0.01); The upper limit of measurement is 0.6mg/L. It can be used for the analysis of orthophosphate in industrial wastewater such as surface water, domestic sewage, daily chemical products, phosphate fertilizers, surface phosphating treatment of processed metals, pesticides, steel, coking, etc.
2. Instruments.
Spectrophotometer.
3. Reagents.
(1) 1+1 sulfuric acid.
(2) 10% (m/V) ascorbic acid solution: Dissolve 10 grams of ascorbic acid in water and dilute to 100 milliliters. This solution is stored in brown glass bottles and can be stable for several weeks in cold places. If the color turns yellow, give up on remastering.
(3) Molybdate solution: Dissolve 13g of ammonium molybdate [(NH4) 6mo7o24.4H2O] in 100ml of water. Dissolve 0.35g of potassium antimony oxide tartrate (Sbo) C4H4 · 1/2H2O in 100ml of water. Slowly add ammonium molybdate solution to 300ml (1+1) sulfuric acid, then add potassium antimony tartrate solution and mix well. The reagents are stored in brown glass bottles and kept in a cold place. At least 2 months.
(4) Turbidity color compensation solution: Mix volume of 1+1 sulfuric acid and 10% (m/V) ascorbic acid solution.
(5) Phosphate storage solution: Dry potassium dihydrogen phosphate (KH2PO4) at 110 ° C for 2 hours, then cool in a dryer. Weigh 0.217g and dissolve it in water, then transfer it into a 1000ml volumetric flask. Add 5ml of (1+1) sulfuric acid and dilute with water to the mark.
(6) Phosphate standard solution: Absorb 10.00ml of phosphate storage solution in a 250ml volumetric flask and dilute with water to the mark. This solution contains 2.0ug of phosphorus per milliliter.
4. Measurement steps (taking inlet and outlet water samples as an example only)
(1) Retrieve the incoming water sample, shake the outgoing water sample well (shake the water sample taken from the biochemical pool and let it sit for a period of time to collect the supernatant).
(2) Take 3 stoppered graduated tubes, add distilled water to the first stoppered graduated tube and add it to the upper scale line; Add 5ml of inlet sample to the second stoppered graduated tube, and then add distilled water to the upper graduated line; Soak the third stoppered graduated tube in hydrochloric acid for 2 hours, or scrub it with a phosphate free detergent.
(3) After use, soak in dilute nitric acid or chromic acid solution for a moment to remove the adsorbed molybdenum blue substance.
The working principle of the total phosphorus monitor is that under acidic conditions of high temperature and high pressure, phosphorus containing compounds such as polyphosphates in water hydrolyze to form phosphates; For other difficult to oxidize phosphorus compounds, they are oxidized to phosphate by the strong oxidizing agent sodium sulfate. Then, according to the measurement range, total phosphorus monitoring instruments include molybdenum blue method and molybdenum alum method.
1. Molybdenum blue method: Phosphate ions react in a strong acid solution containing molybdate to form a yellow phosphomolybdate complex, which is reduced to blue phosphomolybdate by ascorbic acid. Measure the absorbance of phosphomolybdate at a wavelength of 850nm and obtain the total phosphorus content of the sample compared to the standard.
2. Molybdenum alum method: Phosphate ions react in a strong acid solution containing molybdenum salts to form yellow phosphomolybdic acid complexes, and alum reacts with this compound to form yellow alum phosphomolybdic acid. Measure the absorbance of alum phosphomolybdic acid at a wavelength of 430nm and compare it with the standard to obtain the total phosphorus content of the sample.
The working principle of the Botu Total Nitrogen Analyzer: The instrument uses ultraviolet spectroscopy (UV254); When organic compounds containing conjugated double bonds or polycyclic aromatic hydrocarbons dissolve in water, they absorb ultraviolet radiation. Therefore, by measuring the absorption degree of these organic compounds towards 254nm ultraviolet light, the measurement results are represented by the special absorption coefficient of SAC254, which serves as a physical quantity for measuring the total amount of organic pollutants in water.
According to Bilombo's law, the measurement principle is based on the ultraviolet absorption of unsaturated organic molecules with a wavelength of 254nm. [C] =k log (Iin/Iout)/[C]: sample concentration; K: Linear correlation coefficient between UV absorption and COD. Iin: incident light intensity/Iout: transmitted light intensity/instrument automatically compensates for turbidity through a dual beam system (with another detector at 550nm).
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