How To Measure Nitrate Concentration In Water

Nitrate is a common nitrogen compound which is easily soluble in water. In general, the nitrate content in water is very low. However, when the water quality is bad, especially for the industrial wastewater and the tailwater of urban sewage that contain a high concentration of nitrate, which will be harmful to the surface water and groundwater if not treated. So, How to determine nitrate in water?

Determination of nitrate in water by UV Spectrophotometry

The principle:

Since nitrate and nitrite in water absorb ultraviolet light at the wavelength of 219.0nm, we can get the nitrate according to the absorption of ultraviolet light in the range of 0-40mg / L. For the interference of nitrite, we can remove it with amino sulfonic acid. Suppose there is other organic substances interference in the water sample. In that case, we can take two water samples and add a zinc copper particle reducing agent to the first one to remove all nitrate ions and nitrite ions as the blank control solution. In the second part, add sulfamic acid to destroy the nitrite ion and then measure the absorbance of the nitrate ion.

Instruments and reagents

Instruments

1. Ultraviolet-visible spectrophotometer.

2. Quartz cuvette 1cm.

3. Colorimetric tube with plug 25ml.

4.0.45 μ M filter membrane.

Reagent

1. sulfamic acid solution (10g/l)

Weigh 5g of sulfamic acid, dissolve it in 500ml of tertiary reagent water, shake it well, and store it in a reagent bottle (fresh preparation).

2. Copper sulfate solution (50g / L)

Weigh 25g copper sulfate (CuSO4 · 5H2O), dissolve it in 500ml reagent water, shake it well and store it in a reagent bottle.

3.2mol/l hydrochloric acid solution

Mix 17ml concentrated hydrochloric acid and 83ml tertiary reagent water.

4. Zinc copper reductant

Take 5g zinc particles with a particle size of 2 ~ 3mm, wash them twice with grade III reagent water, then wash them with 2mol / L hydrochloric acid solution, finally wash them twice with grade III reagent water, put them into 100ml beaker, add 100ml copper sulfate solution until a black film appears on the surface of zinc particles, discard the solution, wash them twice with grade III reagent water, dry the treated zinc copper particles and bottle them for standby.

5. Potassium nitrate standard solution (1ml containing 0.1mg NO3)

a. Potassium nitrate stock solution (1ml contains 0.4mg NO3): weigh 0.6523g of potassium nitrate dried at 105 ℃ for 2h, dissolve it in 20ml of tertiary reagent water, transfer it to a 1L volumetric flask, dilute it to the scale with tertiary reagent water, and shake it up.

b. Potassium nitrate standard solution (1ml contains 0.1mg NO3): accurately suck 25ml potassium nitrate stock solution into a 100ml volumetric flask, dilute it to the scale with grade III reagent water, and shake it well.

Steps

1. Draw the standard curve

According to the corresponding steps, accurately suck the standard solution into a 25ml colourimetric tube, dilute it with grade III reagent water to the scale, shake it, and measure its absorbance with a 1cm quartz cuvette at 219nm. The standard curve is drawn with absorbance as Y and nitrate ion content as X

2. Determination of water samples

a. Prepare to take two 10ml water samples filtered by 0.45um filter membrane and put them into a 25ml colourimetric tube respectively. Add 0.8g (about 3-4 tablets) of zinc copper reducing agent and 1ml of hydrochloric acid solution (2mol / L) to one water sample, place it for 5h, filter it into a 25ml colourimetric tube, wash it with a grade III reagent water and dilute it to the scale. Shake evenly as blank control.

b. Add 1ml of sulfamic acid solution to the other water sample, dilute it to the scale with tertiary reagent water, and shake it evenly. Then detect its absorbance and find out the content of nitrate ion from the standard curve. Finally, we can get the nitrate in water through the formula P = (M / V) X1000.

Determination of nitrate-nitrogen in water (phenol disulfonate colourimetry)

Nitrate reacts with phenol disulfonate in the absence of water to produce nitrophenol disulfonate. Molecular rearrangement occurs in alkaline solution to produce yellow compounds, which can be quantified by colourimetry.

If there is chloride, nitrite, and ammonium salt in water or coloured water sample, which will affect the determination, we must treat pretreatment appropriate.

Instrument

1.1 00ml porcelain evaporating dish

2.50ml colourimetric tube with a plug

3.250ml triangular bottle

4. Spectrophotometer

Reagent

1. Phenol disulfonate: weigh 15g of refined phenol, put it into a 250ml triangular flask, add 105ml of concentrated sulfuric acid to dissolve it, insert a small funnel at the mouth of the flask, and heat it in a boiling water bath for 6 hours, then get a light brown thick liquid, and store it tightly in a brown bottle.

Refining of phenol: heat the container containing phenol in hot water, pour out an appropriate amount after melting, put it in a distillation bottle with an air condensing tube, heat and distil, collect the evaporated part at 182 ~ 184 ℃, cool it to be colourless and pure crystal, put it in a brown bottle and store it in a cold and dark place.

2. Nitrate nitrogen standard stock solution: weigh 7.218g of potassium nitrate (KNO3) baked at 105 ~ 110 ℃ for 1 hour, dissolve it in pure water, fix the volume to 1000m1, mix it evenly, add 2M1 chlorine as preservation agent, and it can be used in at least 6 months. 1.00mL of this solution contains 1.00mg nitrate nitrogen.

3. Nitrate nitrogen standard application solution: suck 5.00ml of Nitrate Nitrogen Standard reserve solution, place it in an evaporating dish, add 0.1mol/l sodium hydroxide solution, adjust it to pH 8, and heat it in a water bath to evaporate dry. Then add 2ml of bisulfate phenol, quickly grind the inner wall of the evaporating dish with a glass rod to make the residue fully in contact with bisulfate phenol, place it for 30 minutes, add a small amount of pure water, transfer it into a 500m1 volumetric flask, rinse the evaporating dish with pure water, combine it in the volumetric flask, and finally dilute it to the standard line with pure water and mix it evenly. 1.00mL of this solution contains 10.0 μ G nitrate nitrogen.

4. Silver sulfate solution: weigh 4.397 g silver sulfate (Ag2SO4), dissolved in pure water, fix volume to 1000ml. It can remove 1.00mg of chloride ion (C1 -) by 1.00mL of this solution.

5. 0.5mol/l sulfuric acid solution: take 2.8ml concentrated sulfuric acid, add an appropriate amount of pure water, and dilute to 100ml.

6. 1mol / L sodium hydroxide solution: weigh 40g sodium hydroxide, dissolve in an appropriate amount of pure water and dilute to 1000ml.

7. 0.02mol/l potassium permanganate solution: dissolve 0.316g potassium permanganate in pure water and dilute to 100ml.

8. Concentrated ammonia water.

9. Disodium ethylenediamine tetraacetic acid solution: weighs 50g disodium ethylenediamine tetraacetic acid (EDTA-2na), mix it into a paste with 20ml pure water, add 60ml concentrated ammonia, thoroughly mix and dissolve it.

10. Aluminum hydroxide suspension.

Step:

1) Pretreatment of water sample: when calculating the volume of the water sample, we should deduct the volume of various solutions added for pretreatment.

1. Color removal: take 100m1 water sample into 100ml measuring cylinder with stopper, add 2ml aluminium hydroxide suspension, fully vibrate with stopper, stand for several minutes, filter after clarification, and discard 20ml of initial filtrate.

2. Turbidity removal: we can use the filter membrane with a pore diameter of 0.45μM by filtration to remove if there are suspended solids in the water sample.

3. Chloride removal: take 100 ml of water sample into a 250 ml triangular flask, add the right amount of silver sulfate solution according to the measured chloride content, then heat the flask in a water bath at about 80 ℃, shake it vigorously to make the silver chloride precipitation fully agglomerate, and filter it with slow filter paper after cooling.

4. Remove the influence of nitrate-nitrogen: if the content of nitrate-nitrogen in the water sample is more than 0.2mg/l, take 100ml water sample, add 1.0ml 0.5mol/l sulfuric acid solution, mix well, and then drop 0.02mol/l potassium permanganate solution until the light red unchanged for 15 minutes, so that the nitrate-nitrogen is oxidized to nitrate, and finally subtract this part of nitrite nitrogen from the measurement results.

2) Evaporation: take 25.0ml of raw water sample or pretreated clarified water sample, put it in an evaporation dish, check it with pH test paper, adjust it to slightly alkaline (pH ≈ 8) with sulfuric acid or sodium hydroxide solution if necessary, and evaporate it on a water bath to dryness.

3) Nitration: remove the evaporating dish, add 1.0ml of disulfonate phenol, grind with a broken rod to make the disulfonate phenol fully in contact with the residue in the evaporating dish, place it for a moment, grind it again, stand for 10 minutes, and add about 10ml of water.

4) Color development: add 3 ~ 4ml concentrated ammonia to the evaporating dish under stirring to make the solution appear the deepest yellow. If precipitation, it can be filtered, Or add EDTA-2Na solution dropwise until the precipitation dissolves. Transfer the solution into a 50m1 colourimetric tube, dilute it with pure water to the scale, and mix well.

5) Take another 10 pieces of 50ml colourimetric tubes and add nitrate-nitrogen standard application solutions 0, 0.10, 0.30, 0.50, 0.70, 1.00,3.00, 5.00，7.00 and 10.00ml respectively, each plus 1.0ml phenol disulfonate, add 10ml pure water respectively, add 3 ~ 4ml concentrated ammonia dropwise under stirring until the colour of the solution is the darkest, and add pure water to the scale.

6) At the wavelength of 420nm, the absorbance of the sample tube and the standard tube was measured with pure water as a reference. Take the standard series with the amount of standard solution of 0 ~ 1.50ml, use 3cm cuvette and 0 ~ 10.00ml cuvette for determination.

7) Draw a calibration curve and find out the content of nitrate-nitrogen in the sample tube on the curve.

Cadmium column reduction method

The principle:

Cadmium reductant can reduce nitrate in water to nitrite, Diazotize the original nitrate and p-aminobenzenesulfonamide in the water sample, and then couple with naphthalene ethylenediamine hydrochloride to produce a rose red dye. Subtract the nitrite measured by the same method in the water sample without reduction column by spectrophotometry to get the content of nitrate (calculated by N).

Thymol Spectrophotometry

The principle:

Nitrate and thymol produce nitrophenol compounds in concentrated sulfuric acid solution, and molecular rearrangement occurs in alkaline solution to produce yellow compounds for colourimetric determination.