How To Measure Phosphorus In Water

Phosphorus is an essential element that causes water eutrophication. Total phosphorus includes water dissolved, suspended solids, organic phosphorus and inorganic phosphorus. Too much phosphate can cause the crazy growth of plants, such as algae.

The increase of algae not only affects the light scattering level of water but also increases the growth of organisms that feed on algae, resulting in the decrease of oxygen content in water, which easily causes the death of biological hypoxia.

For phosphorus pollution, we must control the phosphorus content in water when the wastewater is discharged. So how to measure the content of phosphorus in water?

Determination of total phosphorus in water – nitric acid sulfuric acid digestion method

1. Instruments

Electric furnace or electric heating plate with adjustable temperature. 125ml Kjeldahl flask.

2. Reagent

Nitric acid（ ρ= 1.40g/ml）; (1 + 1) sulfuric acid; Sulfuric acid (1 / 2h2so4): 1mol / L; Sodium hydroxide solution: 1mol / L, 6mol / L;1% phenolphthalein ethanol indicator solution.

3. Steps

Take a 25.0ml water sample and put it into a Kjeldahl bottle. Add several glass beads, 2 ml (1 + 1) sulfuric acid and 2 ~ 5ml nitric acid. Heat it on an electric heating plate or an adjustable electric furnace until white smoke emerges. If the liquid is not clear and transparent, cool it, add 5ml nitric acid, heat it until white smoke comes out, and make a transparent liquid.

After cooling it:

1. Add about 30ml sulfuric acid and heat it to boil for 5 minutes.
2. After cooling, add a drop of phenolphthalein indicator and low-cost sodium hydroxide solution to make it slightly red.
3. Add 1mol / L sulfuric acid solution to make the slightly red just fade and mix well.
4. Move it into a 50ml colourimetric tube. If the solution is turbid, filter it with filter paper.
5. Wash the Kjeldahl bottle and filter paper with ice and water, and move them into the colourimetric tube together.
6. Dilute them to the mark for analysis.

4. Measure

Take an appropriate amount of colourless and transparent sample (standard solution containing phosphorus less than 50%) μg) in a 50ml colourimetric tube and dilute it to the scale with distilled water. Add 1ml 10% ascorbic acid, add 2m molybdates after the 30s, and shake well. After standing in the dark for 15min, take it out. At the wavelength of 700nm, use a 30mm cuvette with water as the reference to measure the absorbance (the measurement temperature is more than 13 ℃). After deducting the absorbance measured by the blank test, check the phosphate content from the calibration curve.

5. Standard curve drawing

Add 0, 0.20, 0.50, 1.00, 2.00, 4.00, 6.00, 8.00 and 10.0ml phosphorus standard solution to a series of 50ml colourimetric tubes respectively, and dilute them with water to the mark. Measuring the sample according to the same step.

After subtracting the absorbance of the blank test from the measured absorbance, draw the curve of the amount of hexavalent chromium to the absorbance.

Determination of total phosphorus in water- Ammonium molybdate spectrophotometric method

The principle

Under neutral conditions, potassium persulfate solution is heated above 120 ℃ in the autoclave to produce the below reactions:

K2S2O4+H2O→2KHSO4+[O]

Thus, the organic phosphorus, inorganic phosphorus and phosphorus in suspended solids in water are oxidizedoxidized into orthophosphoric acid.

In an acidic medium, orthophosphoric acid reacts with ammonium molybdate. In the presence of antimony salt, phosphomolybdenum heteropoly acid is immediately reduced by ascorbic acid to produce a blue complex with maximum absorption at 880nm and 700nm.

Reagent

Sulfuric acid (H2SO4, A.R.) ρ= 1.84

(1 + 1) sulfuric acid: mix sulfuric acid (H2SO4, A.R.) ρ= 1.84

with water in equal volume

potassium persulfate, 50g / L solution:

Dissolve 5g potassium persulfate (K2S2O8, A.R.) in water and dilute to 100ml.

Ascorbic acid, 100g / L solution: dissolve 10g of ascorbic acid (c6h8o6, C.P.) in water and dilute to 100ml. The solution is stored in a brown reagent bottle in a cold place for several weeks. It can be used for a long time if it does not change colour.

Molybdate solution:

Dissolve 13g ammonium molybdate [(NH4) 6mo7024 · 4h20] in 100ml water. Dissolve 0.35g of potassium antimony tartrate [ksbc4h4o7 · 1 / 2H2O, A.R] in 100ml water, slowly add ammonium molybdate solution to 300ml (1 + 1) sulfuric acid under continuous stirring, and then add potassium antimony tartrate solution and mix evenly. The solution is stored in a brown bottle in a cold place for two months.

Phosphorus standard stock solution: weigh 0.2179g of potassium dihydrogen phosphate (KH2PO4, A.R.) dried at 110 ℃ for 2 hours, cool it in a dryer, dissolve it with water and transfer it to a 1000ml volumetric flask. Add about 800ml water, add 5ml sulfuric acid, dilute with water to the mark, and shake well. The concentration is 50.0 µ g / ml (calculated as P).

Phosphorus standard solution: put 10.00ml of standard phosphorus solution into a 250ml volumetric flask, dilute with water to the mark, and mix well. The concentration is 2.00 µ g / ml (calculated by P).

Instrument

medical portable steam sterilizer or general pressure cooker (1.1-1.4kg / cm2).

50ml colourimetric tube with plug (ground mouth).

gauze and cotton thread

spectrophotometer and 10mm or 30mm cuvette

Steps

1)take 25.00ml of sample into the colourimetric tube with a stopper (shake the sample evenly during sampling to make the suspension or precipitation evenly. If the phosphorus content of the sample is high, reduce the sampling amount accordingly and supplement it to 25ml with water).

Add 4ml of potassium persulfate (if the test solution is acidified and stored, it should be neutralized first). After the colourimetric tube is plugged tightly, the glass is plugged tightly with gauze and cotton thread, placed in a large beaker in a high-pressure steam sterilizer, heated, and stopped heating after the pressure reaches 1.1kg/cm2 and maintained for 30 minutes. After the pressure returns to zero, take it out for cooling and dilute it to 40ml with water.

2) color change: add 2ml molybdate solution to each digestion solution and shake well. After 30 seconds, add 1ml ascorbic acid solution and then add water to a 50ml scale. Mix well. After 15 minutes, measure with 10mm or 30mm cuvette.

3)blank test solution

Replace the sample with water and do the blank test according to steps (1) and (2).

Determination

According to the operation steps of spectrophotometry, adjust the wavelength to 700nm and measure the absorbance with water as the reference. After deducting the absorbance of the blank test, we can get the phosphorus content from the working curve or the counter of correlation regression statistics.

Preparation of working curve

Take 7 pieces of 50ml graduated tubes with plugs and add 0.00, 0.50, 1.00, 2.50, 5.00, 10.00, and 15.00ml phosphate standard solution respectively. Add water to 50ml. colour change according to step (2). Determine the absorbance with water as the reference. After deducting the absorbance of the blank test, we can get the corrected absorbance corresponding to the corresponding phosphorus content statistical regression calibration curve.

Calculation

Total phosphate (P, mg / L) = m / V

Where:

m — phosphorus (P) content measured by the sample, µ g; Calculated from the calibration curve.

V — the volume of sample for measurement, ml.

Note:

Arsenic in water will hardly affect the determination and make the determination result high.

It will produce Cl2 during digestion if water samples with High Cl compounds have negative interference with the determination. A large amount of organic substance without phosphorus in water will affect the digestion and conversion of organic phosphorus into orthophosphoric acid. In case of this, we can use other digestion methods. For example, the sample is digested by the HNO3-HClO4 method.

Since potassium persulfate is difficult to dissolve, we can heat and dissolve it in a water bath at about 40 ℃, but avoid directly heating the beaker on the electric furnace. Otherwise, it will cause the potassium persulfate to decompose failure when the local temperature reaches 60 ℃.

Determination of total phosphorus-Molybdenum antimony resistance Spectrophotometry

Under acidic conditions, orthophosphate reacts with ammonium molybdate and potassium antimony oxide tartrate to produce phosphomolybdenum heteropoly acid, which the ascorbic acid reducing agent reduces to a blue complex, which is also called phosphomolybdenum blue. At the wavelength of 700 mm and the optical path of 10 mm, the level of light absorption is directly proportional to the concentration of phosphorus molybdenum blue.

Calculation method:

phosphate (P, mg / L) = m / V

Where,

M – phosphorus amount obtained from the calibration curve (g);

V – the volume of the water sample (ML).

The detection limit of this method is 0 01~ 0. 6 mg /L。

Main instruments and reagents:

Instrument:

spectrophotometer

Reagent:

blank solution: experimental water with conductivity < 1 . s / cm.

Natural water sample: take a representative water sample and place it for a certain time to stabilize the composition.

Natural water sample 2: add a certain concentration of the tested substance to the natural water sample 2 to make its concentration more than that of the natural water sample, but not more than 0.9 C.

Standard sample: total phosphorus standard sample, true value = (0. 320! 0. 014) mg / L.

For the above solutions, except for blank, the relative deviation of parallel determination should be less than 20% and 10% respectively in the analysis results of 0.01 mg /L、0. 1mg / L.

Malachite green phosphomolybdenum heteropoly acid spectrophotometry

Under neutral conditions, digest the sample with potassium persulfate, oxidize all the phosphorus contained into orthophosphate. While orthophosphate reacts with ammonium molybdate in acidic medium. The substance produced after a series of reactions reacts with the chromogenic agent. we can measure the absorbance value by spectrophotometer and then calculate the total phosphorus content from the standard curve.

Range: the total phosphorus content in the sample digestion water sample is less than 2 μ g.

Instrument

50.0ml colourimetric tube with stopper; Pressure cooker (capable of heating up to 120 ℃); Spectrophotometer (3 cm glass cuvette); Pipettes of various specifications.

reagent

1. Ammonium molybdate (analytical purity) solution: dissolve 176.5g ammonium molybdate ((NH4) 6mo7o24 • 4H2O) in water and dilute to 1000ml;

2. Malachite green (analytical pure) solution: heat and dissolve 1.12g malachite green in water and dilute to 100ml;

3. Chromogenic agent: add 30ml concentrated sulfuric acid and 36ml malachite green solution to 40ml ammonium molybdate solution orderly, mix well, stand for 30 minutes, and then pass through 0.45 μ M filter membrane filtration ;

4. Polyvinyl alcohol (PVA) solution: take 1g of PVA (level of polymerizationpolymerization 1750 ± 50) and dissolve it in 100ml hot water, filter it with filter paper and use it;

5. Phosphate stock solution: dry potassium dihydrogen phosphate at 110 ℃ for 2 hours and cool it in a dryer. Weigh 0.2197g of dissolved water, put it quantitatively into a 1000ml volumetric flask, add 5ml of (1 + 1) sulfuric acid, dilute it with water to the mark, and the mass concentration of phosphorus in this solution is 50.00 μ g/mL；

6. Phosphate standard solution: weigh 1ml of the stock solution into a 250ml volumetric flask, dilute with water to the scale, and the phosphorus mass concentration of this solution is 0.2 μg / ml phosphorus ;

7.5% potassium persulfate solution: dissolve 5g of potassium persulfate in water and dilute to 100ml;

8. Concentrated sulfuric acid (analytical purity);

Steps

Pretreatment of water sample: take 25.0ml of mixed water sample into a colourimetric tube with a plug, add 4ml of 5% potassium persulfate solution, plug and wrap it with gauze, put it in a pressure cooker, digest it at 120 ℃ for 30min, take it out and cool it, and determine the total phosphorus in the water.

Drawing of the standard curve

add 0.00, 0.50, 1.00, 2.00, 3.00, 5.00 and 10.00ml of standard phosphate solution respectively into 25ml colour comparison tube with a stopper, add water to 25ml, then add 3ml of reagent and 1ml of 1% PVA solution, mix well, place it for 30 minutes, measure the absorbance at the wavelength of 650 nm with reagent white as the reference, and get the absorbance phosphorus content standard curve.

Determination: take an appropriate amount of pretreated water sample (the phosphorus content is less than 2μ g), dilute it with water to the scale line, measure colour change according to the steps of drawing the standard curve, and calculate the phosphorus content of the water sample.

Range: the total phosphorus in the digested water sample should be less than 2ug. It is suitable for determining trace phosphorus in groundwater such as rivers and lakes in the range of 1-10ug phosphorus.

Gas chromatography

Since the boiling point of phosphorus is relatively low and volatile, we can directly determine the phosphorus in water samples extracted by organic solvents through gas chromatography. Addison et use toluene as an extractant to extract phosphorus from water samples. After separation by chromatographic column, elemental phosphorus was oxidized and burned in flame photometric detector to produce phosphorus oxide, and then reduced to fragment PHO * by h of hydrogen-rich flame. The fragment PHO * excited by the high temperature of the flame releases the energy of the characteristic spectrum, and its maximum measurement wavelength is 526 nm. Determine the emission spectrum’s intensity to get the total phosphorus content.

Range:

It is suitable for determining multi-component such as groundwater, surface water, soil medium speed phosphorus elimination, phorate, etc. The minimum detection concentration is 0.0001-0.0029mg/kg

Ion chromatography

A: special ion exchange resin with very low exchange capacity is used as the stationary phase;

b. After the separation column, another inhibition column is used to eliminate the high background of the eluent;

c. Since the conductivity detector eliminates the high background conductivity of the eluent, we can sensitively measure various inorganic ion components.

Pros:

Fast, convenient, high sensitivity, good selectivity, good stability of separation column and high capacity. It is suitable for the determination of phosphate fertilizer.

Rhodamine 6G fluorescence spectrometry

In an acidic medium, phosphorus and platinum salt can produce phosphoplatinum salt that can produce a stable complex with rhodamine 6G to quench the fluorescence of Rhodamine 6G.

It is suitable for the determination of trace phosphorus by spectrophotometer.