Ammonia gas sensitive electrode method
Nessler reagent colorimetry
principle
tool
reagent
Determination step
calculate
Matters needing attention
2 Edit the comparison of two main measurement methods in this paragraph.
Definition and editing of 1 ammonia nitrogen online monitor
Ammonia nitrogen refers to nitrogen in the form of free ammonia (NH3) and ammonium ion (NH4) in water. The nitrogen content of animal organic matter is generally higher than that of plant organic matter. At the same time, the nitrogen-containing organic matter in human and animal feces is unstable and easily decomposed into ammonia. Therefore, when the content of ammonia nitrogen in water increases, it refers to the combined ammonia in the form of ammonia or ammonium ions. On-line ammonia nitrogen monitor is an instrument installed in a specific location to continuously analyze ammonia nitrogen for 24 hours. Several main testing methods and comparison of methods for ammonia nitrogen on-line monitor
Ammonia gas sensitive electrode method
1 principle
When the pH value is greater than 1 1, ammonium ions are converted into ammonia, and ammonia is transferred through the hydrophobic membrane of the ammonia sensitive electrode, resulting in the change of electromotive force of the ammonia sensitive electrode. The instrument measures the concentration of ammonia nitrogen according to the change of electromotive force.
2 detection steps
Rinse the container for measuring the volume of water sample and reagent and the electrode installation tube with new water sample.
Use a peristaltic pump to inject the sample. The water sample is not in direct contact with the peristaltic pump tube-there is an air buffer zone. The sample volume is controlled by the visual measurement system.
Similar to injection, the auxiliary reagent is also added by peristaltic pump, and the dose volume is controlled by visual system.
Mix water samples and reagents by bubbling.
The reflection time is automatically controlled by the measuring system.
Residual liquid is discharged by peristaltic pump.
In the user-defined measurement cycle, the analyzer will automatically use the built-in calibration standard solution and cleaning solution for calibration and cleaning.
3 mainstream instrument brand ammonia gas sensitive electrode method
Imported brands: Germany WTW, Britain Ruijin.
Domestic brand: Ruiquan
How to distinguish the performance of ammonia gas sensitive electrode instrument
1. range: the range specification of ammonia nitrogen by electrode method is: 0-1200; 0-2000; 0-3000; 0- 10000. And the range can be switched freely. The larger the measuring range, the stronger the adaptability of the electrodes used in the instrument.
2. Minimum detection limit: The lower the minimum detection limit of the instrument, the better the quality of the representative electrode, generally 0.05 mg/L. ..
3. Accuracy: Accuracy is the most basic requirement of online monitoring instruments. The smaller the error between the measured value and the real value (generally required to be 10%), the better the performance of the instrument.
4. Repeatability: Repeatability is also the basic requirement of online monitoring instruments. Repeated determination of the same quality control sample, under the premise of meeting the accuracy error, the data deviation of each determination should not exceed 5%. 10% is normal.
Nessler reagent colorimetry
principle
The alkaline solution of mercuric iodide and potassium iodide reacts with ammonia to produce reddish-brown colloidal compounds, and its chromaticity is proportional to the content of ammonia nitrogen. Usually, their absorbance can be measured and their content can be calculated in the wavelength range of 4 10~425nm.
The minimum detection concentration of this method is 0.025mg/L (photometry), the upper limit is 2mg/L, and the minimum detection concentration of visual colorimetry is 0.02mg/L.. After proper pretreatment, this method can be used to determine ammonia nitrogen in surface water, groundwater, industrial wastewater and domestic sewage.
tool
2. 1 nitrogen-fixing distillation unit with nitrogen ball: 500mL Kjeldahl flask, nitrogen ball, direct condenser tube and conduit.
2.2 spectrophotometer
2.3 Acidimeter
reagent
The water used to prepare reagents should be free of ammonia.
3. 1 ammonia-free can be prepared by one of the following methods:
3. 1. 1 distillation method: add 0. 1mL sulfuric acid to every liter of distilled water, redistill it in an all-glass distiller, discard 50mL of the initial distillate, put the remaining distillate into a glass bottle with a stopper, grind it, and store it in a dense plug.
3. 1.2 ion exchange method: pass distilled water through strongly acidic cation exchange resin column.
3.2 1mol/L hydrochloric acid solution.
3.3 1mol/L sodium hydroxide solution.
3.4 Light magnesium oxide (MgO): Magnesium oxide is heated at 500℃ to remove carbonate.
3.5 0.05% bromothymol blue indicating solution: pH6.0~7.6.
3.6 Defoamer, such as paraffin chips.
3.7 absorption liquid:
3.7. 1 boric acid solution: dissolve 20g boric acid in water and dilute it to1L.
3.7.2 0.0 1mol/L sulfuric acid solution.
3.8 Nessler's reagent: One of the following methods can be selected for preparation:
3.8. 1 Weigh 20g of potassium iodide and dissolve it in about 100mL of water, add a small amount of mercuric dichloride (HgCl2) crystal powder (about 10g) while stirring, rewrite the dripping saturated mercuric dichloride solution until cinnabar precipitates insoluble, and fully stir. When a small amount of cinnabar precipitate is no longer dissolved, the dropping of mercuric dichloride is stopped.
Another 60g of potassium hydroxide is weighed, dissolved in water and diluted to 250mL. After cooling to room temperature, slowly inject the above solution into potassium hydroxide solution, dilute it to 400mL with water, and mix well. After standing overnight, the supernatant was transferred to a polyethylene bottle and stored in a dense plug.
3.8.2 Weigh 16g sodium hydroxide, dissolve it in 50mL water, and fully cool it to room temperature.
In addition, 7g of potassium iodide and mercury iodide (HgI2) were dissolved in water, and then the solution was slowly injected into sodium hydroxide solution with stirring, diluted to 100mL with water, stored in a polyethylene bottle and sealed.
3.9 Sodium and potassium tartrate solution: dissolve 50g of sodium and potassium tartrate (KNaC4H4O6 4H2O) in 100mL water, heat and boil to remove ammonia, and cool to 100Ml constant volume.
3. 10 ammonium standard stock solution: weigh 3.8 19g of superior grade pure ammonium chloride (NH4Cl) dried at 100℃, dissolve it in water, transfer it to 1000mL volumetric flask and dilute it to the calibration line. This solution contains 65438 0.00 mg of ammonia nitrogen per milliliter.
3. 1 1 ammonium standard use solution: transfer 5.00mL of ammonium standard reserve solution into a 500mL volumetric flask and dilute it with water to the calibration line. The solution contains 0.0 10 mg of ammonia nitrogen per ml.
Determination step
4. 1 Pretreatment of water sample: Take 250mL of water sample (if the ammonia nitrogen content is high, add an appropriate amount of water to 250mL, so that the ammonia nitrogen content does not exceed 2.5mg), transfer it to a Kjeldahl flask, add a few drops of bromothymol blue indicator, and adjust it to pH7 with sodium hydroxide solution or calculus solution. Add 0.25g of light magnesium oxide and some glass beads, immediately connect the nitrogen ball with a condenser tube and guide it.
The lower end of the tube is inserted below the liquid level of the absorption liquid. When the distillate reaches 200mL, stop the distillation and fix the volume to 250mL.
When acid titration or Nessler's colorimetry is used, 50 ml boric acid solution is used as absorption liquid; In salicylic acid-hypochlorite colorimetry, 50 ml of 0.01mol/l sulfuric acid solution is used as the absorption solution.
4.2 Drawing of standard curve: Absorb 0, 0.50, 1.00, 3.00, 7.00 and 1.0 ml ammonium standard solution with a 50 ml colorimetric tube, add water to the calibration line and mix well. Add 1.5 ml Nessler's reagent. Measure the absorbance with a cuvette with an optical path of 20 mm and water as a reference. Subtract the absorbance of the blank tube with zero concentration from the measured absorbance to obtain the corrected absorbance, and draw the standard curve of ammonia nitrogen content (mg) versus the corrected absorbance.
4.3 Determination of water sample:
4.3. 1 Take a proper amount of water sample pretreated by flocculation and precipitation (so that the ammonia nitrogen content does not exceed 0. 1mg), add it into a 50mL colorimetric tube, dilute it to the marking line, and prepare a 0. 1mL potassium sodium tartrate solution. The following standard curves are drawn together.
4.3.2 After distillation pretreatment, divide an appropriate amount of distillate, add it into a 50mL colorimetric tube, add a certain amount of 1mol/L sodium hydroxide solution to neutralize boric acid, and dilute it to the marking. Add 1.5mL Nessler's reagent and mix well. After standing for 65438 00 minutes, measure the absorbance according to the standard curve procedure.
4.4 Blank experiment: replace the water sample with ammonia-free water, and do the whole procedure blank determination.
calculate
After subtracting the absorbance of the blank experiment from the absorbance measured in the water sample, the amount of ammonia nitrogen (mg) is calculated from the standard curve.
Calculated according to the following formula:
Ammonia nitrogen (n, mg/L)=m/V× 1000.
Where: m—— the amount of ammonia nitrogen detected by standard curve, mg;
V-volume of water sample, mL.
Matters needing attention
：
6. The ratio of mercury iodide to potassium iodide in1Nessler's reagent has great influence on the sensitivity of color reaction. Sediments produced after standing should be removed.
6.2 Filter paper often contains trace ammonium salts, and it should be washed without ammonia water. The glass dishes used should avoid ammonia pollution in the laboratory air.
2. Edit the comparison of the two main measurement methods in this paragraph.
Comparison between colorimetric method and ammonia gas sensitive electrode method
Comparison project
Electrode method
colorimetry
Response time
Fast, continuous detection can be realized, the fastest time is only 3 minutes, and the longest time is 1 mg/L for low-range precision measurement 10 minutes.
Slow, can only be detected in batches, and can only be detected after the color reaction is completed. The measurement takes at least 30 minutes.
Test range
Wide, from 0.00- 10000 mg/l NH4-N, the full range test can be realized only by using 1 electrode, and the instrument can automatically switch the range and adjust the resolution.
Small in scope or segmented. When changing the measuring range, a new instrument is needed (the measuring range is determined by the colorimetric cell), and the resolution is low.
Minimum detection limit
0.05 mg/l
5.0 mg/l
hinder
Strong anti-interference ability, free from chromaticity and turbidity interference, without additional compensation.
It is easily disturbed by the chromaticity and turbidity of the sample, and the spectrophotometry is easily affected by the changes of environmental temperature and humidity.
Sampling requirements
No special requirements
Strict requirements, so as not to pollute the optical components and affect the absorbance test.
Reagent operating cost
Low, the electrode method does not need chromogenic agent, the service life of the electrode is long, the reagent formula is open, and the domestic reagent is adopted, which is convenient and cheap to buy.
High chromogenic reagents must be imported, and other reagents are recommended to be imported, which has high maintenance cost.
consumables
The service life of the electrode is long, and the cost of replacing the electrode is low.
The light source is aging, and the cost of replacing the light source is high, so the colorimetric cell should be replaced regularly.
conclusion
Electrode method is more suitable for on-line testing and analysis. For on-line analysis of nitrogen and phosphorus, electrode method is usually preferred, followed by colorimetric method. At present, the technology of measuring other nutrients (such as nitrate nitrogen, nitrite nitrogen, phosphate, total phosphorus, COD, etc.). ) the method of passing through the electrode is not mature, and a durable electrode has not been developed, so colorimetry is used to replace it temporarily. At present, the technology of measuring ammonia nitrogen by electrode method is very mature, and many well-known professional manufacturers choose electrode method to detect ammonia nitrogen, gradually replacing the old colorimetric method.