Measurement of ammonia nitrogen in water

Summary

Ammonia nitrogen (NH3-N) is present in water as either free ammonia (NH3) or ammonium salts (NH4"), and the ratio of the two depends on the pH of the water and the water temperature. When the pH is high, the proportion of free ammonia is high, and vice versa for ammonium salts. The opposite is true for water temperature. Ammonia nitrogen determination methods, usually there are Nano reagent colorimetric method, gas phase molecular absorption method, phenol - hypochlorite (or salicylic acid - hypochlorite) colorimetric method, electrode method and so on. Nano reagent colorimetric method has the characteristics of easy operation, sensitivity and so on, it is a commonly used method. Calcium, magnesium and iron and other metal ions, sulfides, aldehydes and ketones, color and turbidity in the water interfere with the determination, and need to make the appropriate pretreatment. The minimum detection concentration of this method is 0.025 mg/L, and the upper limit of determination is 2.00 mg/L. It can be used for the determination of ammonia nitrogen in surface water, underwater, industrial wastewater and domestic sewage.

Operation method

Nano Reagent Colorimetric Method

Principle

Alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to produce light reddish-brown colloidal compounds, whose chromaticity is directly proportional to the content of ammonia nitrogen, and whose absorbance can usually be measured within the wavelength range of 410 nm~425 nm to calculate their content.

Materials and Instruments

Water samples
Potassium sodium tartrate Potassium iodide Mercury iodide Sodium hydroxide Ammonium chloride Zinc sulfate
Colorimetric tubes Tube holders Pipettes Conical flasks Funnels Filter paper Beakers Visible spectrophotometers

Move

I. Instrumentation
1. 10 50 mL colorimetric tubes and 1 tube rack.
2. 1 mL, 2 mL, 10 mL pipettes.
(3) 250 mL conical flask.
(4) Funnel, filter paper.
(5) 300 mL beaker.
(6) Visible spectrophotometer.
II. Pharmaceuticals
Potassium sodium tartrate, potassium iodide, mercury iodide, sodium hydroxide, ammonium chloride, zinc sulfate.
III. Reagents
1. Nano reagent

Weigh 16 g of sodium hydroxide, dissolved in 50 mL of water, cooled to room temperature; another 7 g of potassium iodide and 10 g of mercuric iodide ( _HgI2 ) dissolved in water, and then injected into the sodium hydroxide solution under stirring to dilute to 100 mL with water, and stored in polyethylene bottles, tightly closed for storage.
2. Potassium sodium tartrate solution

Weigh 50 g of potassium sodium tartrate and dissolve in 100 mL of water, heat and boil to remove ammonia, cool and then volume to 100 mL.
3. Ammonium standard stock solution (CNH4+ -N=100mg/mL): weigh 3.819g of superior pure ammonium chloride (NH4Cl) dried at 100°C, dissolve in water, transfer to a 1000mL volumetric flask, and set to volume. This solution is 1.00 mg of ammonium nitrogen per milliliter.
4. Ammonium standard use solution (CNH4+ -N = 1.00ug/mL): take the above solution and dilute it 100 times.
IV. Experimental steps
1. Preservation of water samples

Water samples collected in polyethylene bottles or glass bottles, and should be analyzed as soon as possible, if necessary, can add sulfuric acid to acidify water samples to pH < 2, stored at 2 ~ 5 ° C. Acidified samples should be careful to prevent the absorption of ammonia in the air and staining.
2. Pre-treatment of water samples

Water samples with color or turbidity and other interfering substances can affect the determination of ammonia nitrogen. For this reason, in the analysis of the need for appropriate pretreatment. For cleaner water, flocculation and precipitation method can be used; for seriously polluted water or industrial wastewater, the distillation method to eliminate interference.
(1) Flocculation and precipitation method: take 100 mL of water sample in a stoppered measuring cylinder or colorimeter tube, add 1 mL of 10% zinc sulfate solution and 0.1-0.2 mL of 25% sodium hydroxide solution, adjust the pH to 10.5 or so, mix well. Leave to precipitate, filter through filter paper washed with ammonia-free water at medium speed, and discard 20 mL of the primary filtrate.
(2) Distillation method: add 250 mL of water samples in a Kjeldahl flask, add 0.25 light magnesium oxide and a few glass beads, heat distillation until the distillate does not contain ammonia, discard the residue in the bottle.
3. Drawing of standard curves

Pipette 0, 0.50, 1.00, 3.00, 5.00, 7.00 and 10.00mL ammonium standard use solution (reagent 4) in 50 mL colorimetric tubes, add water to the mark, add 1 mL of sodium potassium tartrate solution (reagent 2), mix; add 1.5 mL of Nano reagent (reagent 1), mix, that is to say, the concentration of 0.00, 0.005, 0.01 respectively, 0.03, 0.05, 0.07, 0.10 mg/L standard series of solutions. After 10 min, the absorbance was measured at the wavelength of 420 nm with a 2 cm cuvette of light range, using ammonia-free water as the reference, and the standard curve was plotted.
4. Determination of water samples

Take an appropriate amount of water samples after flocculation pretreatment (so that the content of ammonia nitrogen content content does not exceed 0.1 mg), add 50 mL cuvette, dilute to the standard line, add 1 mL of potassium sodium tartrate solution (reagent 2), mix; add 1.5 mL of Nano reagent (reagent 1), mix well. After leaving it for 10 min, the sample was analyzed at the wavelength of 420 nm with an optical range of 2 cm cuvette, using an ammonia-freewater as reference, and measure the absorbance.
V. Calculation of results
Ammonia nitrogen (N, mg/L) = m x 1000/VEq:m - Ammonia nitrogen amount found from the standard curve, mg;V - volume of water sample, mL.

Caveat

1. The ratio of mercuric iodide to potassium iodide in the sodium reagent has a large effect on the sensitivity of the color development reaction, and the precipitate generated after standing should be removed.

2. Filter paper often contains traces of ammonium salts, and care should be taken when using it to wash it with ammonia-free water. The glassware used should be protected from ammonia contamination in the laboratory air.

3. ammonia-free water is used throughout the experiment.

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