How To Measure Ammonia Concentration In Water

Ammonia nitrogen refers to the combined nitrogen of ammonia or ammonium ions. In other words, it is free ammonia (NH3) and ammonium ions (NH4 +) in water. Ammonia nitrogen is an essential factor of water eutrophication and one of the important indicators to measure the level of water pollution. Too high concentration of ammonia nitrogen in water indicates that bacteria, sewage or animal faeces may pollute the water. Therefore, how do we measure ammonia nitrogen in water?

The determination methods of ammonia nitrogen in water include Nessler reagent spectrophotometry, salicylic acid spectrophotometry, gas phase molecular absorption method, electrode method and so on.

Among them, Nessler reagent spectrophotometry or salicylic acid spectrophotometry are commonly used.

Determination of ammonia nitrogen in water

Nessler reagent spectrophotometry pros and cons:

Simple operation and high sensitivity;

but toxic and harmful reagents are involved in the preparation of experimental solution;

Salicylic acid spectrophotometry pros and cons:

high sensitivity and good stability.

Some reagents need to be prepared on-site

Nessler reagent spectrophotometry

Ammonia nitrogen in the form of free ammonia or ammonium ions reacts with Nessler’s reagent to produce a light reddish brown complex. The absorbance of the complex is directly proportional to the ammonia nitrogen content. We can measure the absorbance with a spectrophotometer at the wavelength of 420nm.

Salicylic acid spectrophotometry

In the alkaline medium (pH = 11.7) and sodium nitroso ferricyanide, ammonia and ammonium ions in water react with salicylate and hypochlorite ions to produce a blue complex. We can measure the absorbance with a spectrophotometer at the wavelength of 697 nm.

Note:

1) we should take anti cross pollution in advance to o avoid the interference. For example, it should not be done simultaneously as nitrate nitrogen and ammonium compounds determination. The reason is that the ammonia in the nitrate-nitrogen test is highly volatile, and the Nessler reagent is very easy to absorb the volatile ammonia, which makes the ammonia nitrogen test result high.

2) store glassware and other experimental devices separately to avoid cross-contamination.

3) The experimental water must be ammonia-free water. Do not store it open to prevent ammonia in the air from dissolving into the testing water.

Step:

1) Sample collected

The samples should be analyzed as soon as possible after collection. If they need to be preserved, add sulfuric acid to adjust the water sample to pH < 2.

2) Sample pretreatment

Nessler reagent spectrophotometry:

The clean water sample should be pretreated by the flocculation sedimentation method. (The filter paper may contain ammonium salt, increasing blank value. The filter blank control experiment should be conducted to deduct the influence of filter paper. The pre-distillation method for wastewater can eliminate the interference of some organic substances, reducing substances and metal ions in the sample)

Salicylic acid spectrophotometry:

if the water sample is too dark or has too much salt or a high concentration of calcium, magnesium and chloride in the water sample, please pre distillation.

related:

How To Measure Chlorine In Water

3) Color development time

The best colour development time of Nessler reagent spectrophotometry is commonly 10min ~ 30min.

The colour development time of salicylic acid spectrophotometry should be at least 60min.

4) Colorimetry

Do not contact the optical surface of the cuvette with hard objects or dirt.

When containing the solution, the height is 2 / 3 of the cuvette. If there is residual liquid on the optical surface, it can gently adsorb with filter paper first and then be wiped with lens wiping paper in the same direction.

When testing, it should place the cuvette at the same position as the spectrophotometer to reduce the error.

Wash the cuvette with ammonia-free water immediately after use. If necessary, soak it in hydrochloric acid (1:1) and rinse it with ammonia-free water.

Determination of ammonia in water by phenol Spectrophotometry

The principle:

Under the hypochlorite ions, ammonia and phenol will react into dark blue compound-indophenol blue. We can get the ammonia concentration in water by absorbance measurement, but it is only suitable for an ammonia concentration of 0.06-4.5mg/l. In addition, when the content of copper and iron in the sample is more than 0.15mg/l, we can add EDTA to eliminate it.

Instruments and reagents

1. Wavelength range of spectrophotometer: 400-800nm

2. Temperature control accuracy of water bath pot ± 2 ℃

3. Volumetric flask: 50ml, 100ml, 1000ml

4. Pipette: 5ml, 10ml

5. Preparation of ammonia-free reagent water

a. Ion exchange method: make the secondary reagent water pass through hydrogen strong acid cation exchange resin.

b. Distillation method: add 0.10ml concentrated sulfuric acid into every 1000ml reagent water, re-distil in the glass instrument, discard the initial 50ml distilled water, collect the rest of the water in a glass bottle with a stopper, and add about 10g strong acid cation exchange resin (type H) into every litre of collected water.

6. EDTA solution (50g / L)

Weigh 5.0g disodium EDTA and dissolve it in 60ml sodium hydroxide solution (weigh 1g sodium hydroxide and dissolve it in 60ml ammonia-free reagent water), cool it to room temperature, and add ammonia-free reagent water to 100ml.

7. Phenol solution (630g / L)

Weigh 31.5g phenol and dissolve it in 5ml isopropanol and 10ml acetone. After dissolution, add isopropanol to 50ml. This solution should be prepared when used.

8. Sodium hydroxide solution (270g / L)

Weigh 27g of sodium hydroxide, dissolve it in a small amount of ammonia-free reagent water, dilute it to 100ml, and store it in a plastic bottle.

9. Sodium phenol solution

put 20ml phenol solution and 20ml sodium hydroxide solution into a 100ml volumetric flask, dilute to the scale with ammonia-free reagent water, shake well, and put them into a brown bottle. This solution should be prepared when used.

10. Acetic acid solution (1 + 1).

11. Starch solution (10g / L).

12. Sodium hypochlorite solution (available chlorine is about 10g / L)

Since commercial sodium hypochlorite solution is very unstable, its effective chlorine content needs to be measured quantitatively. After determining the concentration, dilute it into a 10g / L solution. This solution should be prepared when used.

The effective chlorine is determined as below:

Put 5ml sodium hypochlorite solution into a 100ml volumetric flask and dilute it to the scale with ammonia-free reagent water. Take 10ml of this solution into a 300ml iodine volumetric flask, add about 100ml of water, add 1 ~ 2g of potassium iodide, add 6ml of the acetic acid solution, and shake well.

Seal, place in the dark for 5min, titrate with 0.1mol/l sodium thiosulfate standard solution, add 2ml starch solution after the solution turns light yellow, continue to titrate until the blue disappears, record the volume of sodium thiosulfate standard solution consumed, conduct a blank test at the same time, and then get the effective chlorine content of sodium hypochlorite solution.

Preparation of ammonia standard solution

a. Ammonia stock solution (1ml containing 1mg NH3)

Weigh 3.1409g of high-grade pure ammonia chloride baked to constant weight at 110 ℃, dissolve it with a small amount of reagent water, put it in a 1000ml volumetric flask, dilute it to the scale, shake it well, and store it in the reagent bottle.

b. Ammonia working solution I (1ml containing 0.01mgnh3)

Take 10.00ml of the above ammonia stock solution into a 1000ml volumetric flask, dilute to the scale, and shake well. This solution is prepared when used

c. Ammonia working solution II (1ml containing 1ugnh3)

Put 10.00ml ammonia working solution I into a 100ml volumetric flask, dilute it with reagent water to the scale and shake it well. This solution is prepared when it is used.

Operation step of determining ammonia content in water by phenol Spectrophotometry

1. Water sample collection

Generally, it can sample it directly. When the water sample is turbid, it should be filtered with a 0.45um filter membrane, and discard the 50ml filtered out first; If it cannot measure immediately after sampling, adjust the pH value of the sample to 2 ~ 3 with hydrochloric acid,

Store in a dark place at 0 ~ 10 ℃ and determine as soon as possible.

2. Drawing of working curve

a. According to relevant operation regulations, take ammonia working solution, put it into a group of 50ml volumetric flasks, add 1.0mledta solution and 4.0mL sodium phenol solution, and shake well.

b. Add 3.0ml sodium hypochlorite solution, add ammonia-free reagent water to dilute to 50ml, and shake well. Please place it in a water bath with a temperature of 20 ~ 25 ℃ and keep it warm for 30min.

c. When the ammonia concentration is 60 ~ 600g / L, use a 50mm cuvette; When the ammonia concentration is 0.5 ~ 4.5mg/l, use a 10mm cuvette and take the reagent blank at the wavelength of 625nm as the reference to determine the absorbance.

d. Use the measured absorbance to draw the working curve for the corresponding ammonia content.

3. Water sample determination

a. put 10ml water sample to 50ml volumetric flask

b. Operate according to the drawing steps of the working curve.

c. According to the measured absorbance value, we can find the ammonia content from the working curve or calculate it from the regression equation.

Determination of trace ammonia in water by colourimetry

The principle:

The reaction of Nessler’s reagent with trace ammonia in an alkaline solution produces a yellowish-brown compound. It can determine the colour scale number corresponding to the colour of the solution by comparing the colour with the pre-prepared standard colour scale. Then, we can get the content of trace ammonia in a water sample by simple calculation.

The reaction formula :

Interference and elimination
Organic compounds such as aliphatic amines, aromatic amines, aldehydes, acetone and organic chloramines, as well as inorganic ions such as iron, manganese, magnesium and sulfur, will cause interference due to heterochromatic or turbidity, and the colour and turbidity of water also affect the colourimetry. Therefore, it must be pretreated by flocculation sedimentation filtration or distillation. Volatile interfering substances can also be heated under acidic conditions to remove them. It can eliminate the interference of metal ions by adding an appropriate amount of masking agent.
Range
The minimum determination concentration of this method is 0.025mg/l (photometric method), and the upper limit of determination is 2mg / L.
The minimum determination concentration is 0.02mg/l by colourimetry. It can be suitable for determining ammonia nitrogen in surface water, groundwater, industrial wastewater and domestic sewage after appropriate pretreatment.

Standard color scale of trace ammonia:

Preparation of Nessler’s reagent

There is free ammonia water used for preparing reagents.
1) Nessler reagent: we can choose one of the following preparation methods.
a)Weigh 60g of potassium iodide and dissolve it in 100ml water. While stirring, add a small amount of mercury dichloride (HgCl2) crystal powder (about 10g) several times. When vermilion precipitation is not easy to dissolve, add saturated mercury dichloride solution dropwise and stir it fully. Stop dropping mercuric chloride solution when a small amount of vermilion precipitation is not easy to dissolve.
In addition, take 60g of potassium hydroxide and dissolve it in water, dilute it to 250ml, fully cool it to room temperature, put the above solution into the potassium hydroxide solution slowly under stirring, dilute it to 400ml with water and mix well. Let it stand overnight. Move the last clear night into a polyethylene bottle and keep it tightly sealed.
B)Weigh 16g sodium hydroxide, dissolve it in 50ml water and cool it to room temperature.
Weigh 7g of potassium iodide and 10g of mercury iodide (HgI2) in water, slowly put the solution into sodium hydroxide solution under stirring, dilute it with water to 100ml, store it in a polyethylene bottle and keep it tightly sealed.
Potassium sodium tartrate solution: weigh 50g of potassium sodium tartrate (KNaC4H4O6 • 4H2O) and dissolve it in 100ml water, heat and boil it to remove ammonia, cool it and fix the volume to 100ml.
3) Ammonium standard stock solution: weigh 3.819g of superior pure ammonium chloride dried at 100 ℃ and dissolve in water, put it into a 1000 volumetric flask and dilute it to the scale. This solution contains 1.00mg ammonia nitrogen per ml.
4) Ammonium standard solution: put 5.00ml ammonium standard into a 500ml volumetric flask and dilute with water to the scale. This solution contains 0.010mg ammonia nitrogen per ml.
Steps:
Take 1ml of water sample with a pipette, put it into a 50ml vertical colourimetric tube, and add 1ml of Nessler reagent (use the dropper of a brown dropper and add it by counting drops). Dilute with distilled water to 50ml scribed line.
Pour it into the colour comparison tube for 10 minutes, then shake it to determine the colour. We can get the content of trace ammonia in a water sample by calculation.
Calculation:
Take 1ml of water sample and determine that the colour scale number with the same colour is No. 5 through colourimetry.

Ammonia content in water sample =

ammonia nitrogen tester

The ammonia nitrogen in the form of free ammonia or ammonium ions reacts with Nessler’s reagent to produce a light yellow-brown complex. The absorbance of the complex is directly proportional to the ammonia nitrogen content. The ammonia nitrogen tester displays the ammonia nitrogen content (mg / L).