Nitrite

inorganic compound

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Nitric acid is a kind of inorganic compound , the molecular formula is HNO two , no free state, can only exist in dilute aqueous solution, bright blue weak acid ， Ionization equilibrium constant 4.6x10 -4 。 Nitrite solution decomposes when it encounters slight heat, Acid anhydride Is N two O three 。 Nitrite existing Oxidizability , and Reducibility 。 Its oxidizability is stronger than its reducibility, and it is more obvious in acidic medium. Most nitrite salts are colorless crystals, easily soluble in water and toxic.^[1-2]

Chinese name: Nitrite
Foreign name: Nitrous acid
chemical formula: HNO two
molecular weight: forty-seven point zero one three four

CAS login number: 7782-77-6
EINECS login number: 231-963-7
Water solubility: Easily soluble in water
Density: 1.54 g/cm³ （±0.1 g/cm3(Predicted)）
Appearance: Bright blue (aqueous solution)

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Physical and chemical properties

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physical property

Appearance and properties: bright blue (aqueous solution).^[1]

Smell: pungent smell.

Molecular weight: 47.0134

Density: 1.54 ± 0.1g/cm three (Predicted)^[3]

chemical property

1. Weak acidity

Nitric acid is a weak acid slightly stronger than acetic acid, and its ionization equilibrium constant is 4.6x10 -4 。

HNO two ↔H + +NO two -

Nitrite HNO two It is an unstable acid and can only exist in dilute aqueous solution. Before use, use NaNO two Cold dilute solution of and H two SO four It is produced by the action of:

NaNO two +H two SO four =NaHSO four +HNO two

Nitrite solution decomposes when exposed to slight heat:

2HNO two =NO+NO two +H two O^[2]

2. Oxidizability

The N atom in nitrous acid and its salt has an intermediate oxidation state of+3. Although they are both oxidizing and reducing, they are mainly oxidizing, and their oxidation capacity in dilute solution is higher than that of NO three - The ions are also strong, which can be seen from their standard electrode potential values in acid solution:

HNO in acid solution two ψ of θ Value:

HNO two +H + +e - ↔NO+H two Oψ θ =+0.99V

HNO in acid solution three ψθ value of:

NO three - +4H + +3e - ↔NO+2H two Oψ θ =+0.95V

In acid solution, HNO two ψ of θ Value ratio HNO three Is high, so NO in dilute solution two - I can be changed - Oxidation to simple substance I two , and NO three - But cannot oxidize I - , this is NO two - And NO three - One of the important differences between ions. This reaction can be used to identify NO two - Ions.

2HNO two +2I - +2H + →2NO+I two +2H two O^[5]

3. Reducibility

Although HNO in acid solution two It is a stronger oxidant, but KMnO is more oxidizing than it four ，Cl two When the oxidant is of equal strength, it can also show reducibility and be oxidized to nitrate:

5NO two - +2MnO four - +6H + →5NO three - +2Mn 2+ +3H two O

NO two - +Cl two +H two O→NO three - +2H + +2Cl -

NO in alkaline solution two - The reducibility is main, and the oxygen in the air can convert NO two - Oxidation to NO three - 。

NO in alkaline solution two - And O two ψ of θ The values are as follows:

NO three - +H two O+2e - ↔NO two - +2OH - ψ θ =+0.01V

NO two - +H two O+e - ↔NO+2OH - ψ θ =-0.46V

O two +2H two O+4e - ↔ 4OH - ψ θ =+0.401V

2NO two - +O two →2NO three -^[5]

4. Easily generate coordination compounds

At NO two - In the ion, both N atom and O atom have lone electron pairs, which can form coordination bonds with many transition metal ions to form coordination compounds, such as [Co (NO two ) four ] three - And [Co (NO two )(NH three ) five ] two + And so on, which is related to K + Ions generate yellow K three [Co(NO two ) six ]Precipitation, this method can be used to identify K + The presence of ions:

3K + +[Co(NO two ) six ] 3- →K three [Co(NO two ) six ]↓ (yellow)^[5]

5. Interaction with fat and aromatic primary, secondary and tertiary amines

Aliphatic primary amine reacts with nitrite to form extremely unstable aliphatic diazonium salt through diazotization reaction. Its diazonium salt will automatically decompose immediately even at low temperature, releasing N quantitatively two And carbon positive ions are generated. The generated carbonium ions can undergo different reactions to generate olefins, alcohols, halohydrocarbons and other mixtures.

CH three CH two CH two CH two →〔CH three CH two CH two —N≡N〕 + Cl - →N two ↑+Cl - +CH three CH two CH two +

It can release N quantitatively two In analysis, N can be released according to two To quantitatively determine the primary fatty amine. This reaction is often used for quantitative analysis of amino acids and peptides.

Aromatic primary amine reacts with nitrite in strong acidic solution to form aromatic diazonium salt at low temperature.

Benzene diazonium chloride

Fats and aromatic secondary amines react with nitrous acid to form yellow oily or solid N-nitroso compounds.

R two NH+HNO two →R two N—N═O+H two O

N-Nitrosodiphenylamine (yellow solid)

N-methyl-N-nitrosoaniline (brown oily liquid)

Fatty tertiary amine reacts with nitrite as a weak base to receive protons and generate unstable and easily hydrolyzed salts. The salt reacts with alkali to dissociate tertiary amine again.

R three N+HNO two ↔R three NH + NO two -

If there is no substituent in the para position of aromatic tertiary amine, the amino group activates the aromatic ring and electrophilically replaces it with nitrous acid to generate p-nitrosamine.

N. N-Dimethyl-4-nitrosoaniline (green crystal)

To sum up, amine is identified by the difference in the products generated by the reaction of nitrite with fat and aromatic primary, secondary and tertiary amines.^[8]

6. Properties of nitrite

Nitrite has high thermal stability, and can be prepared by reducing nitrate with metal at high temperature:

Pb (powder)+NaNO three →PbO+NaNO two

Nitrite to remove yellow AgNO two Insoluble in water, generally soluble in water.^[5]

structure

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At HNO two In the molecule, N atom adopts sp two Hybrid to generate two σ bonds, one π bond, and a pair of lone electrons, NO two - The ion has a plane triangle structure, and the oxidation number of N is+3. HNO two There are two structures: cis and trans. Generally speaking, the trans structure is more stable than the cis structure.^[7]

preparation

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Prepared by reaction of nitrite and acid:

Ba(NO two ) two +H two SO four →2HNO two +BaSO four

AgNO two +HCl→HNO two +AgCl↓

Cool Ba (NO) with ice water two ) two (or AgNO two ）Dissolve in water, add dilute sulfuric acid (or dilute hydrochloric acid). Filter to remove precipitated BaSO four (or AgCl) precipitation, and nitrite remains in the solution. If there is salt in the solution, it does not matter. At lower than 0 ℃, acidify sodium nitrite with hydrochloric acid.^[3]

purpose

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1. Nitric acid is used in industrial organic synthesis to convert amines into diazo compounds to prepare azo dyes.^[1]

2. The amine is identified by the difference in the products produced by the reaction of nitrite with fat and aromatic primary, secondary and tertiary amines.^[8]

3. Nitrite is a kind of food preservative, which can prevent food from spoilage and has significant bactericidal or bacteriostatic effects, especially in the curing industry.^[6]

test

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test method

Nitrite ion It can be qualitatively detected by reacting with iron (II) to produce brown ring in acidic medium.

Fe 2+ +NO two - +2H + =Fe 3+ +NO↑+H two O

Fe 2+ +SO four 2- +NO=[Fe（NO）]SO four

Produce brown ferrous nitrite sulfate (I) (it will react when the acid is acetic acid)^[4]

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