How To Measure Nitrogen In Water

The determination methods of total nitrogen in water

Nitrogen is one of the causes of water eutrophication, so total nitrogen (TN) is an important index to measure water quality. With the development of industrialization, lots of industrial wastewater, agricultural wastewater and domestic sewage contain various nutrients such as organic substances and nitrogen, making the water quality poor daily. So how to measure the total nitrogen in water?

There are mainly ultraviolet spectrophotometry, high-temperature oxidation chemiluminescence detection, potassium persulfate oxidation ion chromatography, microwave digestion electrode method and online digestion flow injection spectrophotometry for the determination methods of total nitrogen.

Ultraviolet spectrophotometryAdd potassium persulfate solution and sodium hydroxide solution to the water sample, heat it at 120 ℃ for oxidation and decomposition for 30min, and the nitrogen compound in the water sample is decomposed into nitrate. After cooling the digested water sample to a certain temperature:

• Take part in the sample.
• Add hydrochloric acid to adjust the pH value to 2 ~ 3.
• Measure the absorbance value of nitrate-nitrogen with an ultraviolet spectrophotometer to calculate total nitrogen content.

Pros:

Relative low lost

Cons:

Poor stability due to using pressure steam sterilizer or pressure cooker for digestion

Time-consuming

The principle:

Total nitrogen in water commonly includes organic nitrogen, ammonia nitrogen, and nitrate nitrogen. Since nitrate nitrogen in water samples can be reduced to ammonium sulfate with sulfuric zinc acid, organic nitrogen is converted into ammonium sulfate after being digested by sulfuric acid with copper sulfate as a catalyst.

Ammonia is distilled under alkaline conditions, absorbed in boric acid solution, and then reacts with Nessler’s reagent to produce a yellowish-brown complex. The colour depth of the complex is directly proportional to the nitrogen content. So, we can measure the total nitrogen in the water.

Reagents and instruments

1. Nessler reagent

Weigh 16g of sodium hydroxide, dissolve it in 50ml of water, cool it to room temperature, weigh 7g of potassium iodide and 10g of mercuric iodide, dissolve it in the pure water, and then slowly add the solution to the sodium hydroxide solution under stirring, dilute it to 100ml, store it in a brown bottle, plug it tightly with a rubber stopper and store it in the dark.

2. Potassium sodium tartrate solution

Weigh 50g potassium sodium tartrate, dissolve it in 100ml water, heat and boil it to remove ammonia, and dilute it to 100ml after sufficient cooling.

3. Total Nitrogen Standard Solution

Prepare 20mg / L with commercially available total nitrogen standard solution.

4. Ammonia-free distilled water

Add 0.1ml concentrated sulfuric acid to each litre of distillation for re-distillation.

5. Copper sulfate

Weigh 10g of copper sulfate and dissolve it in 100ml pure water.

6. Boric acid solution

Weigh 20g of boric acid and dissolve it in 1000ml pure water.

7. Sodium hydroxide solution

Weigh 500g sodium hydroxide and dissolve it in 1000ml pure water

8. Zinc powder

9. Zinc particles

10. Kjeldahl flask 500ml or 100ml

11. All Glass Distiller 1000ml

12. Electric furnace

13. Colorimetric tube with 50ml

14. Volumetric flask with 200ml

15. Spectrophotometer

16. Anhydrous sodium sulfate

17. Sulfuric acid

Steps

1. Take an appropriate amount of collected water sample into a Kjeldahl flask, add 1g of zinc powder, 10ml of 10% copper sulfate solution and 0.4g of anhydrous sodium sulfate. After the zinc powder reaction ends, heat and digest until the digestion solution is transparent blue-green, and continue to digest for 20min-30min.

2. After cooling the digestion solution, transfer it to the distillation flask, add water to make the volume of about 200ml, add 50ml boric acid solution to the 200ml volumetric flask to absorb the distillate, and insert the liquid guide pipe below the absorption liquid level.

3. Put 2 pieces of zinc particles into the distillation flask. Immediately add 40ml sodium hydroxide solution through the separating funnel, blow the separating funnel with the washing bottle, and close the piston to heat and distil. When the distillate is about 200ml, stop the distillation and fix the volume of the distillate to the scale.

4. Take an appropriate amount of distillate into a 50ml colourimetric tube as the test part, add 1ml potassium sodium tartrate solution, mix evenly, then add 1ml Nessler reagent, shake well, place it for 10min, use a 10mm cuvette at the wavelength of 420nm, and measure the absorbance with pure water as the reference.

Draw curve

Add 0ml, 1.00mL, 2.00ml, 5.00ml, 8.00ml, 10.00ml, 15.00ml and 20.00ml total nitrogen standard solutions into 8 piece of Kjeldahl flasks respectively. Operate according to the measurement steps. Deduct the absorbance of the blank test from the measured absorbance of the above series of standard solutions to get the corrected absorbance.

Take the corrected absorbance as the Y and the total nitrogen concentration as the X, draw the curve and use 100ml pure water as the blank test sample.

Finally, calculate the total nitrogen concentration according to the relevant calculation formula.

Affecting factors:

1. Reagent Purity:

Potassium persulfate and sodium hydroxide

It will get the high blank experimental value due to the high nitrogen content in some reagents, failing regular colourimetric determination.

2. Digestion temperature and pressure

The digestion process of the water sample is essential in determining total nitrogen. Whether the alkaline potassium persulfate is completely decomposed affects the accuracy of the determination results.

The maximum working pressure of a high-pressure steam sterilizer must be more than 1.1 ~ 1.4 kg / cm2, and the maximum working temperature is less than 120 ~ 124 ℃. In the actual measurement, we should strictly control the digestion temperature and pressure, regularly calibrate the pressure gauge of the high-pressure steam sterilizer and check the sealing condition of the rubber sealing ring to avoid decompression due to air leakage.

3. Digestion time

There is strong absorption at 220 nm for Alkaline potassium persulfate. This absorption gradually weakens with the digestion process’s continuous decomposition of alkaline potassium persulfate. In other words, the absorption of alkaline potassium persulfate decreases with the increase of heating time. It is suggested that the digestion time is within 45 min to reduce the blank experimental value and keep stability.

4. Experimental water

Experimental water is also an essential factor affecting the blank absorbance value. Generally, It requires ammonia-free water or newly prepared deionized water.

5. Environmental conditions

The determination of total nitrogen is required in a laboratory environment without ammonia to avoid cross pollution on the determination results. All water and reagents that can contain nitrogen cannot be used in the total nitrogen laboratory, or the analysis project of ammonia, nitric acid and other ammonia salt reagents, for example, measure the total hardness, volatile phenol and nitrate nitrogen in the same lab, which will affect the determination of total nitrogen, resulting in high determination results.

6. Appliance pollution

After each measurement, all glassware must be soaked in an acid tank of (1 + 9) hydrochloric acid or (1 + 35) sulfuric acid solution for one night. Take it out and rinse it with tap water, wash it with deionized water or ammonia-free water several times, then dry it naturally for standby.

Wipe the quartz cuvette with (1 + 9) hydrochloric acid or absolute ethanol, wash with deionized water or ammonia-free water, dry with mirror wiping paper and put into a special cuvette box for standby. Clean the high-pressure steam sterilizer with deionized water or ammonia-free water before and after use.

1. Online digestion Flow Injection Spectrophotometry

Using the alkaline persulfate ultraviolet oxidation method, the nitrogen in nitrogen compounds is oxidized to nitrate at 90 ℃. Then the generated nitrate is quantitatively reduced to nitrite through a copper-plated cadmium column. Under acidic conditions, nitrite reacts with a sulfonamide to produce a diazonium ion. Diazonium ion combines with naphthalene ethylenediamine hydrochloride to produce a purple substance with maximum absorption at 540nm. The concentration of this substance is directly proportional to the total nitrogen concentration in water.

Pros：

Fast analysis speed, low detection limit, high precision and accuracy.

The high automation level of analysis in the mass analysis of environmental water samples.

1. High-temperature oxidation chemiluminescence detection

The carrier gas brings the sample into the high-temperature furnace for oxidation. The sample is wholly vaporised and oxidised at a high temperature of more than 950 ℃. The reaction product is brought into the reaction chamber by the carrier gas and finally generates NO2 * in the excited state. When the NO2 * in the excited state transitions to the ground state, it emits photons, and its intensity is directly proportional to the total nitrogen content in the sample. Therefore, we can measure the sample’s total nitrogen content by measuring chemiluminescence’s intensity.

There is no significant difference in the determination results compared with distillation spectrophotometry.

Pros：

high automation operation,

accurate, sensitive and rapid determination

Cons:

The water sample must be filtered in advance.

The test temperature is high.

3) Potassium persulfate oxidation ion chromatography

The digested sample passes the ion chromatograph sampling tube through the microporous membrane’s filter joint for automatic analysis.

Pros:

Little interference and strong specificity

Cons:

Expensive instruments

1. Microwave digestion electrode method

Add water sample and potassium persulfate into the digestion tank, tighten the cover to seal it, and put it evenly into the microwave digestion device for digestion at the appropriate time and power. After cooling the sample, add ionic strength buffer, measure the electromotive E1, and then accurately add nitrate-nitrogen standard solution, and measure the electromotive E2. So, we can calculate the mass concentration of nitrate-nitrogen in water samples by the corresponding formula.

Wrap up

Determining total nitrogen in water commonly uses alkaline potassium persulfate to convert the sample’s organic nitrogen and inorganic nitrogen compounds into nitrate. It then uses ultraviolet spectrophotometry, azo colourimetry, ion chromatography and gas-phase molecular absorption method. Among them, alkaline potassium persulfate digestion ultraviolet spectrophotometry is preferred due to its low cost and simple operation. However, some disadvantages exist, such as abnormally high blank experimental value and significant fluctuation of sample measured value.