Ammonium Chloride is denoted by the chemical formula NH4Cl. It naturally occurs in the form of a mineral called sal ammoniac.
It is an inorganic compound and a salt of ammonia. It is found in the form of white crystalline salt which is highly soluble in water (about 37%).
So, Is NH4Cl an acid or base? NH4Cl is an acidic salt. The pH value for NH4Cl lies between 4.5 and 6 and its pKa value is 9.24, therefore, it is mildly acidic. It is a salt of a strong acid and a weak base, which are hydrochloric acid and ammonia, respectively. As a salt acquires its pH based upon the acidic or basic strength of its constituent compounds, NH4Cl is acidic.
Let us read this out in more detail.
Why NH4Cl acts as Acidic Salt?
Neutralization is the reaction between an acid and a base that results in the formation of a salt that derives its properties from its constituent i.e. acid and base.
The acidic or basic nature of salt is determined by the strength of the acid and base that combine to form that salt.
By understanding the nature of its combining compounds the acidity or basicity of salt can also be estimated, viz,
Strong acid along with weak base are known to form acidic salt.
While basic salt is formed by the combination of weak acid along with a strong base.
Lastly, the reaction of a strong acid with a strong base gives neutral salts.
The reaction for the preparation of NH4Cl is as follows:
As clear from the above-mentioned chemical equation, NH4Cl is a neutralization product of hydrogen chloride, which is a strong acid and almost completely ionizes in the aqueous solution to form protons, and ammonia, which is known to be a weak base.
As discussed earlier, the combination of strong acid and weak base results in the formation of an acidic salt.
Therefore, NH4Cl is an acidic salt.
Why is an aqueous solution of NH4Cl Acidic?
When an acid or base is dissolved in an aqueous solution it results in dissociation of its molecules resulting in the formation of ions, therefore, the acidity or basicity of a substance in an aqueous solution can be understood by drawing its dissociation equation.
In the case of NH4Cl the dissociation equation can be written as:
NH4Cl (aq) —–> NH4+ + Cl –
Here, the NH4Cl hydrolysis to form an NH4+ ion, which is the conjugate acid of ammonia, while the Cl- ion which is the conjugate base of ammonia.
We shall recall at this point that:
A strong acid produces a weak conjugate base.
A strong base produces a weak conjugate acid.
A weak acid produces a strong conjugate base.
A weak base produces a strong conjugate acid.
Therefore, NH4+ is a strong conjugate acid while Cl- is a weak conjugate base. Therefore, ammonium chloride is an acidic salt.
This can also be justified by understanding further hydrolysis of these ions. As Cl- is a weak conjugate base it cannot further accept a proton.
On the other hand, the NH4+ ion gives away its proton to form a hydronium ion with the water molecule.
The reaction can be written as:
According to Arrhenius’s theory of acids and bases, acids are the compounds that release hydrogen or hydronium ions upon their dissociation in an aqueous solution.
As seen in the above equation on dissolving in aqueous solution NH4Cl releases hydronium ions due to the hydrolysis of ammonium ions.
These hydronium ions are responsible for the acidity of the aqueous solution of ammonium chloride.
pH of NH4Cl
The pH value of a substance is an indicator of the acidity or basicity of that substance in its aqueous solution. It is actually the concentration of hydrogen ions in a solution.
The value of pH for different substances ranges from 1 to 14 on the pH scale in which 7 is the value of pH for the neutral solution.
All the substances having a pH value below 7 are acidic while the substances having a pH value above 7 are basic.
In the case of NH4Cl, we have already learned that it is an ionic salt formed by the neutralization of a strong acid and a weak base.
Therefore, the pH of NH4Cl should be less than 7.
Calculating the pH for 1 M NH4Cl Solution
The concentration of hydrogen ions is calculated as:
[H+] = √Ka X M
As the value of Ka = NH4Cl is 5.6 X 10-10 at 25°C
Also, we are calculating the hydrogen ion concentration for 1 M NH4Cl solution
Therefore, [H+] = √5.6 X 10-10 X 1
= 2.3 X 10-5
The pH value for 1 M solution of NH4Cl can now be calculated as:
pH = – log [H+]
= – log [ 2.3 X 10-5 ]
= – [ log 2.3 – 5 log 10]
As the pH value of ammonium chloride is less than 7, therefore, NH4Cl is acidic.
Why is NH4Cl not a Base or Basic Salt?
There are three main theories given to distinguish an acid from a base. They are given below:
• Lewis theory: A molecule that gives away an unshared pair of electrons to another molecule is known as a base while the molecule that accepts those electrons is termed an acid
• Bronsted-Lowry theory: A molecule that readily gives away protons in an aqueous solution is an acid while the molecule that takes up those protons, given away by another molecule, is a base.
• Arrhenius theory: A molecule that produces hydroxide ion (OH-) in a solution is a base and the molecule which is unable to produce hydroxide ions is an acid.
Considering all the above-mentioned theories this can be easily deciphered that the properties of ammonium chloride molecule do not fit into the definition of the base, rendered by any of these theories, therefore, NH4Cl is not a base.
Preparation of NH4Cl
Ammonium Chloride naturally occurs as a mineral called sal ammoniac. It occurs near the volcanoes and forms volcanic rocks near fumaroles.
The crystals are formed as a result of the gaseous eruption, however, they do not last long as they are soluble in water.
Ammonium Chloride is majorly manufactured as a by-product of the Solvay process which is used for the production of Sodium Carbonate.
The process involves the reaction of Ammonia, Sodium Chloride, and Carbon dioxide in water.
Once Sodium bicarbonate precipitates it is filtered out from the solution. After this ammonium chloride is separated, washed, and dried from the precipitate. The reaction equation for the Solvay process is given below:
CO2 + 2NH3 + 2NaCl + H2O —–> 2NH4Cl + Na2CO3
Ammonium Chloride is commercially prepared by a reaction between ammonia and hydrogen chloride also known as hydrochloric acid when present in an aqueous solution.
The chemical equation for the same is written as follows:
NH3 + HCl —–> NH4Cl
Double decomposition of ammonium sulfate on reaction with sodium chloride also results in the formation of ammonium chloride.
When hydrogen chloride is readily available, a direct neutralization reaction may be used for the production of ammonium chloride.
However, as ammonium chloride is easily available as a by-product in double decomposition reactions, therefore, being cost-effective they are more favored.
• The Molecular mass of NH4Cl is 53.49 gm/mol
• It appears as a hygroscopic white solid.
• It is odorless with a density of 1.519 gm/cm3
• It has a pH value between 4.5 and 6 and its pKa value is 9.24
• It has a refractive index of 1.642 at 20°C.
• It is soluble in liquid ammonia, hydrazine, and slightly soluble in acetone.
• The boiling point of ammonium chloride is 520°C.
• NH4Cl has a melting point of 338°C.
Uses of NH4Cl
• The major use of ammonium chloride is in nitrogen-based fertilizers. Almost 90% of the total global production of NH4Cl is used in the production of fertilizers.
• In the preparation of metals, it is used for tin coating, where it is used for cleansing the metal surfaces of metal oxides.
• NH4Cl is used as a urinary acidifying salt as it helps in maintaining the pH and exhibits a diuretic effect.
• It is also used for eliminating cough as it has an expectorant effect i.e. it causes irritation in the mucous membrane.
• Ammonium Chloride is also used as a food additive under E number E510 as an acidity regulator.
• It is also used as a feed supplement for cattle.
• It is also used as a ferroptosis inhibitor.
• It is used for producing lower temperatures in cooling baths.
• Some aquatic animals utilize ammonium chloride to maintain their buoyancy in seawater.
• In Leclanché cells, the aqueous solution of ammonium chloride was used as an electrolyte.
Ammonium Chloride is an acidic salt. The pH value for NH4Cl lies between 4.5 and 6 and its pKa value is 9.24.
HCl is a strong acid while NH3 is a weak base and NH4Cl is formed as the product of their neutralization reaction. Therefore, it is an acidic salt.
Ammonium chloride in its aqueous solution is acidic as it releases hydronium upon its dissociation in a solution.
NH4Cl is not a base as it does not fit into the definition of base given by any of the acid-base theory viz. Lewis theory, Arrhenius theory, or Bronsted-Lowry theory.