Nitrite ion is an ion consisting of one nitrogen atom and two oxygen atoms. Nitrogen in this ion has a charge of +3, hence the charge of the entire ion is -1. The particle is monovalent. The formula of nitrite ion is NO 2 - . Anion has a non-linear configuration. Compounds containing this particle are called nitrites, for example sodium nitrite - NaNO 2 , silver nitrite - AgNO 2 .
Physical and chemical properties
Nitrites of alkali, alkaline earth metals and ammonium are colorless or slightly yellowish crystalline substances. Potassium, sodium, barium nitrites dissolve well in water, silver, mercury, copper nitrites - poorly. With increasing temperature, solubility increases. Almost all nitrites are poorly soluble in ethers, alcohols and low-polar solvents.
Table. Physical characteristics of some nitrites.
| Characteristic | Potassium nitrite | Silver nitrite | Calcium Nitrite | Barium Nitrite |
T pl , ° C | 440 | 120 (with decomposition.) | 220 (with decomposition.) | 277 |
∆H 0 arr., KJ / mol | - 380.0 | - 40.0 | -766.0 | - 785.5 |
| S 0 298 , J / (mol * K) | 117.2 | 128.0 | 175.0 | 183.0 |
| Dissolve in water, g per 100 g | 306.7 (20 0 ) | 0.41 (25 0 C) | 84.5 (18 0 C) | 67.5 (20 0 ) |
Nitrites are unstable to heat: only alkali metal nitrites melt without decomposition. As a result of decomposition, gaseous products - O 2 , NO, N 2 , NO 2 , and solids - metal oxide or the metal itself are released. For example, the decomposition of silver nitrite (already occurring at 40 ° C) is accompanied by the release of elemental silver and nitric oxide (II):
2AgNO 2 = AgNO 3 + Ag + NO ↑
Since decomposition involves the release of a large amount of gases, the reaction can occur with an explosion, 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 both oxidizing and reducing properties are characteristic of nitrites. For example, nitrites will decolorize a solution of potassium permanganate in an acidic environment, exhibiting the properties of an oxidizing agent:
5KNO 2 + 2KMnO 4 + 3H 2 SO 4 = 3H 2 O + 5KNO 3 + 2MnSO 4 + K 2 SO 4
The properties of the reducing agent nitrite ions show, for example, in reaction with a strong solution of hydrogen peroxide:
NO 2 - + H 2 O 2 = NO 3 - + H 2 O
The reducing agent is nitrite when reacted with silver bromate (acidified solution). This reaction is used in chemical analysis:
2NO 2 - + Ag + + BrO 2 - = 2NO 3 - + AgBr ↓
Another example of the reducing properties is a qualitative reaction to a nitrite ion - the interaction of colorless [Fe (H 2 O) 6 ] 2+ solutions with an acidified sodium nitrite solution with the appearance of brown staining.
Theoretical Foundations of NO2¯ Detection
Nitric acid disproportionates when heated to form nitric oxide (II) and nitric acid:
HNO 2 + 2HNO 2 = NO 3 - + H 2 O + 2NO ↑ + H +
Therefore, nitrous acid cannot be separated from nitric acid by boiling. As can be seen from the equation, nitrous acid, when decomposed, partially turns into nitric acid, which will lead to errors in determining the content of nitrates.
Almost all nitrites are soluble in water, the least soluble of these compounds is silver nitrite.
The nitrite ion itself is colorless, so it is detected by the reactions of the formation of other colored compounds. The nitrites of unpainted cations are also colorless.
Qualitative reactions
There are several qualitative methods for determining nitrite ions.
1. The reaction of the formation of K 3 [Co (NO 2 ) 6 ].
5 drops of a test solution containing nitrite, 3 drops of a cobalt nitrate solution, 2 drops of acetic acid (diluted), 3 drops of a solution of potassium chloride are placed in a test tube. Formed hexanitrocobaltate (III) K 3 [Co (NO 2 ) 6 ] - a yellow crystalline precipitate. The nitrate ion in the test solution does not interfere with the detection of nitrites.
2. The oxidation reaction of iodides.
Nitrite ions oxidize iodide ions in an acidic environment.
2HNO 2 + 2I - + 2H + = 2NO ↑ + I 2 ↓ + 2H 2 O
During the reaction, elemental iodine is formed, which is easily detected by staining of starch. For this, the reaction can be carried out on filter paper pre-impregnated with starch. The reaction is very sensitive. A blue color appears even in the presence of traces of nitrites: the opening minimum is 0.005 mcg.
The filter paper is impregnated with starch solution, add 1 drop of 2N acetic acid solution, 1 drop of test solution, 1 drop of 0.1N potassium iodide solution. In the presence of nitrite, a blue ring or spot appears. Detection is prevented by other oxidizing agents leading to the formation of iodine.
3. The reaction with potassium permanganate.
3 drops of a solution of potassium permanganate, 2 drops of sulfuric acid (diluted) are placed in a test tube. The mixture must be heated to 50-60 ° C. Gently add a few drops of sodium or potassium nitrite. The permanganate solution is discolored. Other reducing agents present in the test solution capable of oxidizing the permanganate ion will interfere with the detection of NO 2 - .
4. The reaction with iron sulfate (II).
Ferrous sulfate reduces nitrite to nitrate in an acidic environment (dilute sulfuric acid):
2KNO 2 (tv) + 2H 2 SO 4 (t) + 2FeSO 4 (tv) = 2NO ↑ + K 2 SO 4 + Fe 2 (SO 4 ) 3 + 2H 2 O
The resulting nitric oxide (II) forms with an excess of Fe 2+ (not yet had time to enter into the reaction) complex ions of brown color:
NO + Fe 2+ = [FeNO] 2+
NO + FeSO 4 = [FeNO] SO 4
It should be noted that nitrites react with dilute sulfuric acid, and nitrates will react with concentrated sulfuric acid. Therefore, dilute acid is needed to detect the nitrite ion.
5. Reaction with antipyrine.
NO 2 - with antipyrine in an acidic environment gives a green solution.
6. The reaction with rivanol.
NO 2 - with rivanol, or ethacridine (I) in an acidic medium gives a red solution.
Quantitative determination of nitrite content in water
According to GOST, the quantitative content of nitrite ions in water is determined by two photometric methods: using sulfanilic acid and using 4-aminobenzenesulfonamide. The first of these is arbitration.
Due to the instability of nitrites, they must be determined immediately after sampling or canned samples by adding 1 ml of sulfuric acid (concentrated) or 2-4 ml of chloroform to 1 liter of water; you can cool the sample to 4 ° C.
Turbid or colored water is purified with aluminum hydroxide by adding 2-3 ml of suspension to 250-300 ml of water. The mixture is shaken, a transparent layer is selected for analysis after clarification.
Determination of nitrite content using sulfanilic acid
The essence of the method: the nitrites of the analyzed sample interact with sulfanilic acid, the obtained salt reacts with 1-naphthylamine with the release of 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 Griss reagent.
Determination of nitrite ions: method
2 ml of a solution of Griss reagent in acetic acid are added to 50 ml of a water sample. Stirred and maintained for 40 minutes at normal temperature or 10 minutes at 50-60 ° C in a water bath. Then measure the optical density of the mixture. Distilled water is used as a blank sample, which is prepared similarly to a sample of analyzed water. The concentration of nitrite is calculated by the formula:
X = K ∙ A ∙ 50 ∙ f / V,
where: K is the coefficient of the calibration 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 - volumetric volumetric flask,
f is the dilution coefficient (if the sample was not diluted, f = 1),
V is the volume of an aliquot taken for analysis.
Nitrites in water
Where does nitrite ions come from in wastewater? A small amount of nitrite is always present in rainwater, surface water and groundwater. Nitrites are an intermediate step in the transformation 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 (with a lack of oxygen). All these reactions are carried out by bacteria, and the source of nitrogen-containing substances is organic. Therefore, the quantitative content of nitrites in water is an important sanitary indicator. Exceeding the nitrite content indicates fecal contamination of water. The runoff of livestock farms, factories, industrial enterprises, water pollution from the fields where nitrogen fertilizers were used are the main reasons for the high nitrite content in the water.
Getting
In industry, sodium nitrite is obtained by absorption of nitrous gas (a mixture of NO and NO 2 ) with solutions of NaOH or Na 2 CO 3 followed by crystallization of sodium nitrite:
NO + NO 2 + 2NaOH (cold) = 2NaNO 2 + H 2 O
The reaction in the presence of oxygen proceeds with the formation of sodium nitrate; therefore, oxygen-free conditions must be ensured.
In the same way, potassium nitrite is also obtained in industry. In addition, sodium and potassium nitrites can be obtained by oxidation of lead with nitrate:
KNO 3 (conc) + Pb (sponge) + H 2 O = KNO 2 + Pb (OH) 2 ↓
KNO 3 + Pb = KNO 2 + PbO
The last reaction is at a temperature of 350-400 ° C.