Nitrite ion is an ion consisting of one nitrogen atom and two oxygen atoms. The nitrogen in this ion has a charge of +3, so the charge of the entire ion is -1. The particle is univalent. The formula of the nitrite ion is NO2-. The anion has a nonlinear configuration. Compounds containing this particle are called nitrites, for example sodium nitrite - NaNO2, silver nitrite - AgNO2.
Physical and chemical properties
Alkali, alkaline earth and ammonium nitrites are colorless or slightly yellowish crystalline substances. Potassium, sodium, barium nitrites dissolve well in water, silver, mercury, copper nitrites - poorly. As the temperature increases, the solubility increases. Almost all nitrites are poorly soluble in ethers, alcohols and low-polarity solvents.
Table. Physical characteristics of some nitrites.
| Characteristic | Potassium nitrite | Silver nitrite | Calcium nitrite | Barium nitrite |
|
Tpl, °С |
440 |
120 (decomposed) |
220 (decomposed) |
277 |
|
∆H0rev, kJ/mol |
- 380, 0 | - 40, 0 | -766, 0 | - 785, 5 |
| S0298, J/(molK) | 117, 2 | 128, 0 | 175, 0 | 183, 0 |
| Solution in water, g in 100 g |
306, 7 (200C) |
0, 41 (250C) |
84, 5 (180C) |
67, 5 (200C) |
Nitrites are not very resistant to heat: only alkali metal nitrites melt without decomposition. As a result of decomposition, gaseous products are released - O2 , NO, N2, NO2, and solid substances - metal oxide or the metal itself. For example, the decomposition of silver nitrite (already at 40 ° C) is accompanied by the release of elemental silver and nitrogen oxide (II):
2AgNO2=AgNO3 + Ag + NO↑
Because decomposition occurs with the release of a large amount of gases, the reaction may be explosive, for example, in the case of ammonium nitrite.
Redox properties
The nitrogen atom in the nitrite ion has an intermediate charge of +3, which is why nitrites are characterized by both oxidizing and reducing properties. For example, nitrites will decolorize a solution of potassium permanganate in an acidic environment, showing propertiesoxidizer:
5KNO2 + 2KMnO4 +3H2SO4 =3H2O + 5KNO3 + 2MnSO4 + K 2SO4
Nitrite ions exhibit the properties of a reducing agent, for example, in a reaction with a strong solution of hydrogen peroxide:
NO2- + H2O2=NO3- + H2O
The reducing agent is nitrite when interacting with silver bromate (acidified solution). This reaction is used in chemical analysis:
2NO2- + Ag+ + BrO2 -=2NO3- + AgBr↓
Another example of reducing properties is a qualitative reaction to the nitrite ion - the interaction of colorless solutions [Fe(H2O)6] 2+ with acidified sodium nitrite solution with brown coloration.
Theoretical foundations of NO2 detection¯
Nitrous acid, when heated, disproportionates to form nitric oxide (II) and nitric acid:
HNO2 + 2HNO2=NO3- + H2O + 2NO↑ + H+
Therefore, nitrous acid cannot be separated from nitric acid by boiling. As can be seen from the equation, nitrous acid, decomposing, partially turns into nitric acid, which will lead to errors in determining the content of nitrates.
Almost all nitrites dissolve in water, the least soluble of these compounds is silver nitrite.
Nitrite ion itselfit is colorless, therefore it is detected by reactions of formation of other colored compounds. The nitrites of uncolored cations are also colorless.
Quality reactions
There are several qualitative ways to determine nitrite ions.
1. Reaction forming K3[Co(NO2)6].
In a test tube put 5 drops of the test solution containing nitrite, 3 drops of cob alt nitrate solution, 2 drops of acetic acid (diluted), 3 drops of potassium chloride solution. Hexanitrocob altate (III) K3[Co(NO2)6] is formed - a yellow crystalline precipitate. The nitrate ion in the test solution does not interfere with the detection of nitrites.
2. Iodide oxidation reaction.
Nitrite ions oxidize iodide ions in an acidic environment.
2HNO2 + 2I- + 2H+ =2NO↑ + I 2↓ + 2H2O
During the reaction, elemental iodine is formed, which is easily detected by starch staining. To do this, the reaction can be carried out on filter paper previously impregnated with starch. The response is very sensitive. The blue color appears even in the presence of traces of nitrites: the opening minimum is 0.005 mcg.
Filter paper is impregnated with a starch solution, 1 drop of 2N acetic acid solution, 1 drop of experimental solution, 1 drop of 0.1N potassium iodide solution are added to it. In the presence of nitrite, a blue ring or spot appears. Detection is interfered with by other oxidants leading to the formation of iodine.
3. Reaction with permanganatepotassium.
Place 3 drops of potassium permanganate solution, 2 drops of sulfuric acid (diluted) in a test tube. The mixture must be heated to 50-60 ° C. Add carefully a few drops of sodium or potassium nitrite. The permanganate solution becomes colorless. Other reducing agents present in the test solution, capable of oxidizing the permanganate ion, will interfere with the detection of NO2-.
4. Reaction with iron sulfate (II).
Ferrous sulfate reduces nitrite to nitrate in an acidic environment (dilute sulfuric acid):
2KNO2 (TV) + 2H2SO4 (diff.) + 2FeSO4 (solid)=2NO↑ + K2SO4 + Fe 2(SO4)3 + 2H2O
The resulting nitric oxide (II) forms with an excess of Fe2+ (which have not yet reacted) brown complex ions:
NO + Fe2+=[FeNO]2+
NO + FeSO4=[FeNO]SO4
It should be noted that nitrites will react with dilute sulfuric acid, and nitrates will react with concentrated sulfuric acid. Therefore, it is dilute acid that is needed to detect the nitrite ion.
5. Reaction with antipyrine.
NO2- with antipyrine in an acid medium gives a green solution.
6. Reaction with rivanol.
NO2-- with rivanol, or ethacridine (I) in an acid medium gives a red solution.
Quantitative determination of nitrite content in water
According to GOSTthe quantitative content of nitrite ions in water is determined by two photometric methods: using sulfanilic acid and using 4-aminobenzenesulfonamide. The first one is arbitrage.
Due to the instability of nitrites, they must be determined immediately after sampling, or samples can be preserved by adding 1 ml of sulfuric acid (concentrated) or 2-4 ml of chloroform to 1 liter of water; you can cool the sample down to 4 °C.
Turbid or colored water is cleaned with aluminum hydroxide by adding 2-3 ml of suspension to 250-300 ml of water. The mixture is shaken, a transparent layer is taken for analysis after clarification.
Determination of nitrite content with sulfanilic acid
The essence of the method: nitrites of the analyzed sample interact with sulfanilic acid, the resulting s alt reacts with 1-naphthylamine with the release of a red-violet azo dye, its amount is determined photometrically, then the concentration of nitrites in the water sample is calculated. 1-naphthylamine and sulfanilic acid and are part of the Griess reagent.
Determination of nitrite ions: technique
To 50 ml of a water sample, add 2 ml of a solution of Griess reagent in acetic acid. Mix and incubate for 40 minutes at normal temperature or 10 minutes at 50-60 ° C in a water bath. The optical density of the mixture is then measured. As a blank sample, distilled water is used, which is prepared similarly to the sample of the analyzed water. The concentration of nitrites is calculated by the formula:
X=K∙A∙50∙f / V, where: K is the coefficientcalibration characteristic, A is the set value of the optical density of the analyzed water sample minus the set value of the optical density of the blank sample, 50 – volume of volumetric flask, f – dilution factor (if the sample was not diluted, f=1), V is the volume of the aliquot taken for analysis.
Nitrites in water
Where do nitrite ions come from in wastewater? Nitrites are always present in small amounts in rainwater, surface and groundwater. Nitrites are an intermediate step in the transformations of nitrogen-containing substances carried out by bacteria. These ions are formed during the oxidation of the ammonium cation to nitrates (in the presence of oxygen) and in the opposite reactions - the reduction of nitrates to ammonia or nitrogen (in the absence of oxygen). All these reactions are carried out by bacteria, and organic matter is the source of nitrogen-containing substances. Therefore, the quantitative content of nitrites in water is an important sanitary indicator. Exceeding the norms of nitrite content indicates fecal pollution of water. The ingress of runoff from livestock farms, factories, industrial enterprises, pollution of water bodies with water from fields where nitrogen fertilizers were used are the main reasons for the high content of nitrites in water.
Receive
In industry, sodium nitrite is obtained by absorption of nitrous gas (a mixture of NO and NO2) with NaOH or Na2CO solutions 3 followed by sodium nitrite crystallization:
NO +NO2 + 2NaOH (cold)=2NaNO2 + H2O
The reaction in the presence of oxygen proceeds with the formation of sodium nitrate, so anoxic conditions must be provided.
Potassium nitrite is produced by the same method in industry. In addition, sodium and potassium nitrite can be obtained by oxidizing lead with nitrate:
KNO3 (conc) + Pb (sponge) + H2O=KNO2+ Pb(OH)2↓
KNO3 + Pb=KNO2 + PbO
The last reaction takes place at a temperature of 350-400 °C.
