Doubts about the determination of total nitrogen, the method of determination of total nitrogen nati

Total nitrogen is measured at 220 and 275 nm This is the ultraviolet region, not the visible region, so it has nothing to do with whether you can see the color or not.

I do total nitrogen, the effect has not been very good, there are many reasons here, no ammonia, drug purity (generally domestic potassium persulfate ammonia nitrogen content is high can not be used), digestion pressure control (time and gas prevention should be strictly in accordance with the requirements), instrument (two wavelengths you are measured at one time or divided into two 220 at a time 275 measurement, some instruments can measure two wavelengths at a time, he has to switch in the middle, a switching wavelength must have an error, of course, it is also related to the quality of the instrument, anyway, the total nitrogen is not easy to do, You can find out what you know on the Internet).

Spectrophotometry to measure the blank value, so what to do as a reference is used to zero, you are measuring the blank and the sample is not supposed to be zeroed, the error may be the lamp for a long time The energy is not enough, the deuterium lamp used externally is not a tungsten lamp, the life is very limited, or you are a dual-path spectrophotometer cuvette transmittance is not the same, you have to change two clean, one manufacturer and one batch of cuvettes, the others do not remember for the time being, my instrument is still in the repair .........

Colorless and transparent is right, there is color to indicate that there is a problem, cuvette to use quartz, spectrophotometer to ultraviolet.

In addition, the value of the blank should be lower than that of distilled water, and if it is too large, the purity of potassium persulfate should be considered.

When switching between different wavelengths, it will definitely go to zero.

I've measured it, but it doesn't seem to have any color.

Of course, with a spectrophotometer, you have to reset it to zero

You should also use potassium persulfate digestion first, and then measure; Then you should test it once at 275nm and then measure it at 220, so that the error is smaller. And at 220nm, sometimes it's negative.

The first time I did the total nitrogen correction absorbance a=a220-2a275 The result was negative Have you encountered this situation What is the reason for this Do you know Please advise.

What is the concentration of what do you do? If it's too low, it can't be measured.

The national standard method for the determination of total nitrogen is as follows:

The total nitrogen determination method usually uses potassium persulfate oxidation to convert organic nitrogen and inorganic nitrogen compounds into nitrate, and then determines it by ultraviolet method, azo colorimetry, ion chromatography or gas phase separation absorption method.

1. Sample preservation.

After the water sample was collected, it was acidified to pH<2 with sulfuric acid and measured within 24 hours.

Potassium persulfate oxide ultraviolet spectrophotometry (GB-11849-89).

Second, the principle of methodology.

In the aqueous solution above 60, potassium persulfate decomposes according to the following reaction formula, and it is suspected that it will become hydrogen ions and oxygen early.

k2s2o8+h2o→2khso4+1/2o2

khso4→k-1+hso4-

hso4-→h++so42-

Sodium hydroxide is added to neutralize hydrogen ions and completely decompose potassium persulfate.

Under the alkaline medium condition of 120 124, potassium persulfate can be used as an oxidant, which can not only oxidize ammonia nitrogen and nitrite nitrogen in water samples to nitrate, but also oxidize most of the organic nitrogen compounds in water samples to nitrate.

Then, the absorbance was measured by ultraviolet spectrophotometry at the wavelengths of 220 nm and 275 nm, and the absorbance value of nitrate nitrogen was calculated according to a=a220-2a275, so as to calculate the content of total nitrogen. Its molar absorbance coefficient is.

3. Interference and Elimination.

1. When the water sample contains hexavalent chromium ions and ferric ions, 1 2ml of 5% hydroxylamine hydrochloride solution can be added to eliminate its influence on the determination.

2. Iodine ions and bromine ions interfere with the determination. When 20ug nitrate nitrogen was determined, there was no interference when the iodine ion content was compared to the multiple of the total nitrogen content; There is no interference with the bromine ion content relative to the multiple of the total nitrogen content.

3. The influence of carbonate and bicarbonate on the determination can be eliminated after adding a certain amount of hydrochloric acid.

4. Sulfate and chloride have no effect on the determination.

Notes:

1. When the absorbance ratio of A275 A220 100% is greater than 20%, it should be identified.

2. The tightness of the glass plug colorimetric tube should be good. When using a pressure steam sterilizer, it is necessary to deflate slowly after cooling; When using a civilian pressure cooker, it is necessary to cool it well before removing the lid to prevent the colorimetric tube plug from popping out.

3. Glassware can be soaked with 10% hydrochloric acid, rinsed with distilled water, and then rinsed with ammonia-free water.

4. When using the high-pressure steam sterilizer, the pressure gauge should be checked regularly: when using a civil pressure cooker, the rubber sealing ring should be checked to reduce the pressure without air leakage.

5. When measuring water samples with more suspended solids, precipitation may occur after oxidation of potassium persulfate. In this case, the oxidized supernatant can be aspirated for ultraviolet spectrophotometric determination.

High-temperature catalytic oxidation.

The high-temperature catalytic oxidation method uses a high-temperature combustion tube or a high-temperature combustion reactor to digest the water sample, and the nitrogen-containing compounds in the sample are converted into NO-gas under the combined action of 850 high-temperature, high-purity oxygen and catalyst. Total nitrogen is determined by continuous flow analysis.

, the sample in the alkaline medium was exposed to potassium persulfate under the condition of 107 110 ultraviolet light.

Oxidized to NO3-.

In the process of measuring total nitrogen by ultraviolet and spectrophotometry, ozone is measured in ultraviolet light.

The free oxygen radicals produced under irradiation react with water to form hydroxyl radicals.

It has strong oxidation ability to organic matter, which can oxidize nitrogen-containing organic matter in water samples to NO3-.

The list of total nitrogen blind concentrations in 2023 is as follows:

The standard values for the total nitrogen blind sample should be known and stable values, usually derived from national or local standard methods or reference materials. The exact values can be determined according to different analysis methods and criteria. <>

For example, the Chinese national standard "Code for Water Quality Monitoring" (GB T 11901-2019) stipulates that for the determination of total nitrogen content in water samples by the KJELDAHL method, a blind sample of total nitrogen should be added during the determination process, and the total nitrogen content of the blind sample should be between and should be similar to the total nitrogen content of the water sample to be measured. Therefore, the standard values of the total nitrogen blind sample need to be determined according to the specific analytical method and standard. Ammonia nitrogen is one of the components of total nitrogen.

Total nitrogen is the total amount of inorganic and organic nitrogen in various forms in water. It includes inorganic nitrogen such as nitrate NO3-, nitrous NO2- and ammonia nitrogen NH4+, and organic nitrogen such as proteins, amino acids, and organic amines. As for the proportion of ammonia nitrogen to total nitrogen, it is difficult to determine how much. Ammonia nitrogen refers to the nitrogen present in water in the form of free ammonia (NH3) and ammonium ions (NH4+).

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 manure is very unstable and easily decomposes into ammonia. <>

The structure of ammonia nitrogen resin T-42H is good for hail characteristics:

It is composed of two parts: a skeleton and an active group. The skeleton, also known as the matrix, is the structural body that forms the ion exchange resin. It is based on a linear structure of polymer organic compounds (polystyrene), plus a certain amount of cross-linking agent, through the cross-key bridging action to form a spatial network structure.

The active group consists of an immobilized ion (-SO3-sulfonic acid group) and an active ion (H+). The immobilized ions are fixed on the resin skeleton, and the active ions (or exchange ions) are combined with the immobilized ions by electrostatic attraction.

Summary. Hello, the absorbance of total nitrogen 275 is negative: one is that the blank reference system is different, that is, there is a significant difference between the two media; The second is the normal instrument deviation, but generally this deviation is relatively small, will not exceed the absorbance of this item generally corresponds to 1mg l, so the detection limit is very high, with 4 times the detection limit as the lower detection limit, the lower limit is, theoretically the following data are not very accurate, unless the method deviates if the negative value is not large, the impact on the result is not large, it is generally the cause of the blank, but it is not ruled out that the reason for the drift of the instrument is not ruled out, and it may be caused by pure water containing organic matter.

Note that the absorbance of the blank should not be greater than the absorbance of the place, which is mainly caused by the absorption of organic matter in the water, and the absorbance value of the reticle here is negative, and the curve obtained after 220-2*275 is very good.

Hello, total nitrogen blank is negative? What's going on.

Hello, the absorbance of total nitrogen 275 is negative: one is that the blank reference system is different, that is, there is a significant difference between the two media; The second test brigade is a normal instrument deviation, but generally this deviation is relatively small, will not exceed the absorbance of this item generally corresponds to 1mg l, so the detection limit is very high, with 4 times the detection limit as the detection of the lower Sun Lu orange limit, the lower limit is, theoretically the following data are not very accurate, unless the method deviates if the negative value of the group is not large, the impact on the result is not large, it is generally the reason for the blank, but it is not excluded that the reason for the drift of the instrument is not excluded, and the pure water may be caused by organic matter. Note that the absorbance of the blank should not be greater than the absorbance of the place, which is mainly caused by the absorption of organic matter in the water, and the absorbance value of the reticle here is negative, and the curve obtained after 220-2*275 is very good.

Except for pure water? There is no other impact.

Without.

In an aqueous solution of more than 60, potassium persulfate can be decomposed to produce potassium bisulfate and atomic oxygen, and potassium bisulfate is dissociated in the solution to produce hydrogen ions, so the decomposition process tends to be complete in the alkaline medium of sodium hydroxide.

The decomposed atomic oxygen can convert the nitrogen containing nitride in the water sample into nitrate under the condition of 120 124. And in this process, the organic matter is oxidized and decomposed at the same time. The absorbance A220 and A275 can be measured by ultraviolet spectrophotometry at the wavelengths of 220 nm and 275 nm, respectively, to correct the interference of the absorbance of organic matter at 220 nm.

This method can determine the sum of nitrite nitrogen, nitrate nitrogen, inorganic ammonium salt, dissolved ammonia, and most organic nitrogen-containing compounds in water. The detection range is:

The molar absorbance coefficient of this method is:

The main methods for determining total nitrogen in water quality are:

1. Alkaline potassium persulfate ultraviolet spectrophotometry (Shishan Jin HJ 636-2012: Nowadays, the main methods of water quality monitoring and search, such as British RAIKING, China Ruiquan and other brands are the mainstream monitoring products optimized on the basis of this standard.

2. Gas phase molecular absorption spectrometry: This method is mainly used in laboratories.

3. Ammonia nitrogen, nitrate, and nitrite are also used for measurement, and then the cumulative value of the results is used as the measurement result of total nitrogen. Typical applications such as the German WTW. In the field of environmental surface water and water quality monitoring, alkaline potassium persulfate ultraviolet spectrophotometry and optimization methods are the main methods at present.