Preparation and Uses of Nitrous Acid | Nitric(iii) acid | HNO2

Nitrous acid can be prepared by several steps in the laboratory. Nitrous acid cannot be stored in bottles as nitric acid because nitrous acid decomposes easily to nitric acid, nitric oxide (NO) and water. Solid sodium nitrate and cold dilute hydrochloric acid (HCl) are the chemicals you want to prepare nitrous acid. Nitrous acid is a weak monobasic acid.

In this tutorial, we will learn how to prepare nitrous acid and what are the uses of nitrous acid.

Nitrous acid | nitric(iii) acid | HNO2 preparing steps

As the main preparation method, we use solid NaNO3. In this method, there are three steps to prepare HNO2. Usually, we prepare nitous acid when we require it. Otherwise, we do not store it in laboratories because it readily decomposes.

Heating sodium nitrate as the first step of preparing nitrous acid

Solid sodium nitrate (NaNO3) is heated to decompose to solid sodium nitrite (NaNO2) and oxygen (O2) gas. This reaction is a redox reaction (oxidation - reduction reaction) because oxidation numbers of nitrogen and oxygen atoms are changed when reaction occurs.

sodium nitrate NaNO3 decompose when heating

Dissolving sodium nitrite in water to prepare aqueous sodium nitrite solution

Solid sodium nitrite is dissolved in water to prepare aqueous sodium nitrite. No reaction is occurred in this step. Only a dissolving occurs.

sodium nitrite dissolving

Cold dilute HCl and NaNO2 reaction

Add cold dilute HCl to aqueous sodium nitrite solution. It gives dilute nitrous acid and NaCl solution.

aqueous NaNO2 and cold dilute HCl reaction

Why dilute HCl is added to the aqueous NaNO2 solution?


If dolute HCl is added to solid NaNO2, NO2 brown gas is formed. Then, there is no way to prepare nitrous acid furthermore.

Why cold dilute HCl is added to the aqueous NaNO2

Nitrous acid is stable in cold state. If solution temperature is high, formed nitrous acid decomposes to nitric acid (HNO3), nitric oxide (NO), and water. Therefore we have to use cold HCl in preparation of nitrous acid.

Dinitrogen trioxide and water reaction | N2O3 + H2O

Dinitrogen trioxide reacts with water to give nitrous acid. It shows N2O3 has weak acid characteristics due to forming a weak acid. In this reaction, N2O3 reacts with water by protecting oxidation numbers of atoms.

Keeping the solution cold is important to protect produced HNO2 acid to prevent decomposition.

dinitrogen trioxide and water reaction| N2O3 + H2O

how to prepare nitrous acid HNO2

Oxidation number of nitrogen in nitrous acid

After drawing the lewis structure of HNO2, we can see oxidation number if nitrogen is +3.

From equation

Take oxidation number of nitrogen as x. Oxygen's and hydrogen atom's oxidation number is -2 and +1.

  • +1 + x + (-2)*2 = 0
  • x = 0

Uses of nitrous acid

Nitrous acid is used for so many applications in organic chemistry to produce so many chemicals. Now, we will learn, what are the those uses of nitrous acid.

To prepare alcohols

Nitrous acid reacts with primary aliphatic amines to give primary alcohol. But remember, for this reaction, NaNO2 and diute HCl is used to prepare HNO2 acid.

To prepare N-Nitrosamine

N-nitrosamine is a yellow colour oil. When a secondary amine compound is treated with nitrous acid, N-Nitrosamine is given as the product.

Preparing Benzenediazonium chloride

Benzenediazonium chloride is used to make pigments. When aniline is treated with NaNO2 and dilute HCl below 50C, Benzenediazonium chloride is given as the product.


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How do you identify aqueous NaNO2 and HNO2?


You and your friend were advised that prepare a nitrous acid solution from solid sodium nitrate by your class teacher to conduct more experiments. But you are unable to come to the preparing day and your friend said that he will do the preparation alone. When you come to the laboratory next day, you have to conduct the further experiments from prepared nitrous acid solution. When next day you went to the laboratory you see your friend has done some preparations and but he has forgotten to label the two solutions. So you want to know which solution contains the prepared nitrous acid. How do you identify nitrous acid from aqueous sodium nitrite solution. Give explanations.


When you prepare nitrous acid from solid sodium nitrate method, there are NaCl in the aqueous solution if distillation or other chemical separation method is not used to remove NaCl.

Add silver nitrate to NaNO2 and HNO2 solutions

  • Due to occurrence of chloride ion in the HNO2 solution, AgCl white precipitate forms in the solution while there is no change in aqueous NaNO2 solution.

pH value of NaNO2 and HNO2 solutions

HNO2 is a weak acid and NaNO2 has weak basic characteristics. So pH value of HNO2 solution is less than NaNO2 solution.

NaNO2 acid or base?

NaNO2 has weak basic characteristics. NaNO2 dissociates completely in water to sodium ion and NO2- ions. Sodium ion is stable in water and does not involve in hydrolysis of water.

Is nitrite ion stable or unstable in water?

Nitrite ion is not stable water and accept a proton from a water molecule and form nitrous acid and hydroxyl ion. Due to formation of hydroxyl ion, aqueous solution become basic.

According to the above explanations, aqueous NaNO2 solution is a basic solution.

Why we have to prepare nitrous acid in laboratory when it is required?

Nitrous acid is an unstable acid. So storing it long time is not possible because it decomposes easily.

What is nitric (iii) acid

Nitric(iii) acid is an oxo acid of nitrogen. This acid is a weak acid. In the brackets, oxidation number of nitrogen atom is mentioned as (iii). That is the way of naming inorganic acids according to the IUPAC nomenclature.

Monobasic acid

When acid has only one hydrogen atom to react with a base, we say it is a monobasic acid.

Can I make nitric acid from nitrous acid?

Yes, You can. Heat the nitrous acid solution. Due to instability, nitrous acid decomposes to nitric acid, NO and water.

What is the oxidation number of nitrogen in hno2

We can find the oxidation number of nitrogen by algebra equation. In most occasions, oxidation number of oxygen is -2 and hydrogen is +1. Take oxidation number of nitrogen as x.

HNO2 is a neutral molecule. So summation of oxidation numbers of each elements should be zero.

  • +1 + x + (-2)*2 = 0
  • x = +3

So, oxidation number of nitrogen in HNO3 is +3.

what is the oxidation state of nitrogen in nano2?

We can use algebra equation to find the oxidation number of nitrogen in NaNO2. Alkali metals always form only +1 oxidation state. So sodium's oxidation number is +1. Most times, oxygen form -2 oxidation state. Then unknown oxidation number of nitrogen is x.

+1 + x + (x)*2 = 0

x = +3

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