The ammonium ion is a polyatomic ion formed as a result of the protonation of ammonia (NH3). It is a positively charged onium ion denoted by the chemical formula NH4+.
It is an inorganic ion usually formed as a waste product of the metabolic system of different animals. It is used in the production of fertilizers, explosives, etc. The excessive presence of ammonium ions in soil may cause acidification.
In this article, we will learn more about NH4+ and its different properties.
So let’s begin…
Is NH4+ an acid?
Yes, NH4+ is an acid. It has a pH of around 5.5 and is also capable of donating a proton in an aqueous solution. NH4+ ions do not dissociate completely in an aqueous solution and therefore NH4+ is considered as a weak acid. NH4+ is the conjugate acid of base Ammonia (NH3). The chemical equation is given below:
NH4+ + H2O —-> H2O + NH3 + H+
As per the above equation in an aqueous solution, the ammonium ion forms ammonia and water along with the release of a proton.
Is NH4+ an Alkali?
Alkalis or bases are substances that accept a proton or donate an electron. They have a pH of above 7 and turn red litmus blue. As none of these is the case with ammonium ions, it cannot be considered a base.
Also, unlike other alkalis, ammonium ions cannot produce OH– ions in an aqueous solution, nor are they able to donate lone pair of electrons to other atoms or molecules.
Therefore, NH4+ is not a base.
pH of NH4+
The pH of a substance provides a measure of its acidity or basicity. A pH above 7 indicates a substance to be basic, a pH below 7 is an indicator of acids, while a pH equal to 7 indicates a neutral substance.
In the case of ammonium ion, the protonation reaction mentioned above can also be written as follows:
NH4+ + H2O —–> NH4OH + H+
The equilibrium constant equation for the above reaction can be written as:
Ka = [NH4OH] [H+] / [NH4+]
Now, let us try to find out the pH of ammonium ion for 0.1 M solution.
The value of Ka for ammonium ion is 5.55 X 10-10.
Also, we know that the initial concentration of ammonium ions in the solution was 0.1.
Now, assuming that x moles of NH4OH and H+ is formed, let us put these values in the above reaction. Therefore,
5.55 X 10-10 = [x] [x]/[0.1 – x]
As the value of Ka is very small, we can consider 0.1 – x ≈ 0.1
Therefore, x2 = 55.5 X 10-12
Based on this, calculate the value of x from the above reaction,
We get x = 7.44 X 10-6
Hence, [H+] = 7.44 X 10-6
Now, pH = -log [H+]
= -log 7.44 X 10-6
As the pH value of ammonium ion solution is below 7, it is acidic in nature.
Also, as the pH is close to 7 and the value of Ka is less than 1, this means that NH4+ is mildly acidic.
Why is NH4+ Acidic?
Quite many theories have been given to differentiate an acid from a base. However, there are three most important theories that form the basis of the chemistry of acids and bases. They are:
• Bronsted-Lowry Theory: This theory states that an acid is a substance that readily donates proton in an aqueous solution. The ease with which the protons can be extracted provides the measure of the strength of an acid.
• Arrhenius Theory: This theory highlights that acids are capable of producing hydrogen ions in an aqueous solution.
• Lewis theory: This theory describes acids as the substances that readily accept a lone pair of electrons donated by any other molecule.
Now, considering all the above theories let us look at the ammonium ion.
As described in the above sections, the ammonium ions release protons in an aqueous solution. Hence, it is acid as per the Bronsted-Lowry theory.
Also, ammonium ion dissociates in the aqueous solution to form hydrogen atom and ammonia molecules. Therefore, it also stands up to the criteria of acids put forward by the Arrhenius theory.
Lastly, looking at the Lewis concept, the ammonium ion does not have any lone pair of electrons at its central atom and hence, it has the tendency to accept protons donated by other molecules. Hence, ammonium ion can be rightly classified as a Lewis acid.
Is NH4+ a Strong or Weak Acid?
To understand this let us first describe what we mean when we call an acid strong or weak.
Strong acids are those that readily and completely ionize in an aqueous solution. They have the tendency to donate their protons with ease.
Usually, they have a pH value quite lower than 7, although, it also depends upon the concentration of the acid in a solution. A few examples of strong acids are HCl, H2SO4, HBr, etc.
On the other hand, weak acids are those which only dissociate partially in an aqueous solution. They also donate proton but not with as much ease as strong acids. A few examples of weak acids are HCN, CH3COOH, H3PO4, etc.
Check out Is H3PO4 an acid or base.
Now, let us take a look at the ammonium ions.
The ammonium ions do not dissociate completely in an aqueous solution. This is also visible from the extremely low value of the acid dissociation constant (Ka).
Another reason for NH4+ being a weak acid is that NH3 which is formed as a conjugate base of this ion is a weak base and thus, readily reacts with water to reform NH4+ ions.
Therefore, most of the time the NH4+ ions remain unionized in the solution. Therefore, ammonium ions are weak acids.
Here, we must also understand the concept of acid dissociation constant which can be given as:
• For strong acids, Ka > 1
• For weak acids, Ka < 1
As already discussed in the previous sections, the value of the acid dissociation constant for NH4+ ions is always lower than 1. This further confirms that ammonium ions act as weak acids.
Which one is Stronger Acid NH4+ or NH3?
Ammonium ions i.e. NH4+ and ammonia i.e. NH3 are related to each other as conjugate acid-base pairs. The ammonium ion gives away one of its protons in the aqueous solution to form ammonia.
As we already know that the conjugate base of an acid is formed when the acid donates its proton, therefore, NH4+ acts as the Bronsted-Lowry acid while NH3 acts as its conjugate base.
The reaction is written as:
NH4+ (aq) —–> NH3 + H2O
Hence, NH4+ ions are definitely stronger as an acid in comparison with NH3 molecules.
Which is more stable NH4 or NH3?
The basic rule to determine the stability of a compound is that neutral molecules will always be more stable than any of their ions.
In the present case, we have ammonia, which is a neutral molecule, and on the other hand, we have ammonium ions that contain one positive charge. Hence, it is clear that NH3 will be more stable in comparison to NH4+ owing to the presence of charge on the latter.
However, one more point to be mentioned here is that ammonium ions being acidic in nature becomes relatively stable in acidic conditions, where ammonia molecules tend to be less stable.
Similarly, as the pH of ammonia lies towards the basic side of the scale, it will be more stable in an alkaline environment.
Is NH4 a Buffer?
In chemistry, a solution containing an almost equal concentration of acid as well as a base is referred to as a buffer.
These are used to safeguard a solution from any change in its pH that may occur due to the addition of a small amount of acid or base.
Usually, they are produced as a result of titration reactions. Every buffer solution has a pH range which is helpful i.e. if the pH of a solution gets altered beyond this range, the added buffer solution may not be helpful in moderating the pH of the solution.
To effectively stabilize the pH of a solution, a buffer solution is ideally made of a weak acid with its conjugate base. In the case of ammonium ions, a solution containing an optimum mix of NH4+ and NH3 acts as an efficient buffer solution.
Properties & Uses
• The molecular weight of ammonium ions is 18.039 gm.
• It is an inorganic compound that occurs as onium ions.
• The geometry of NH4+ ions is tetrahedral.
• It is a nitrogen hydride and behaves as monoprotic acid.
• It is the conjugate acid of ammonia.
• Under normal conditions, NH4+ ions behave as amalgam while at high pressure they act as metals.
Ammonium ions behave as weak acids and have a pH of around 5.5.
The ammonium ion behaves as a Bronsted acid by giving away a proton, as Arrhenius acid by donating hydrogen ion, and also as a Lewis acid by accepting a lone pair of electrons.
The ammonium ion and ammonia molecule are related as conjugate acid-base pair as the ammonium ion releases a proton in the aqueous solution to form ammonia, while the ammonia molecule accepts a proton to form ammonium ion in an aqueous solution.
Ammonium ions are weak acids as they do not dissociate completely in an aqueous solution. Also, it has a Ka value lower than 1, which is a characteristic of weak acids.
The ammonium ions are more stable in an acidic environment while their conjugate base ammonia is more stable in an alkaline environment.
A solution with an optimum concentration of ammonia and ammonium ions acts as a buffer, which can be used for moderating the pH of a solution.