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To Study The pH of Weak Acid and Weak Base - Common Ion Effect

Last updated date: 28th Feb 2024
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pH Change by Common Ion in Case of Weak Acids and Weak Bases Experiment

With the addition of soluble compounds in the solution having a common ion with precipitate, the ionic precipitate solubility decreases. This is the concept of the common ion effect. In other terms, we refer to the shift in equilibrium due to the introduction of common ions in a solution is the common ion effect.

This effect is seen with the ionization of weak acid and weak base, which is also reversible. The reactions involve the formation of the precipitate, with the addition of chemicals having the same ions, is common with residue. You’ll understand ph change by common ion in case of weak acids and weak bases experiment with examples.

The pH of Weak Acid and Weak Base

The weak acid and weak base study is experimental-based observation and an important concept that deals with forming a salt precipitate. We will understand this concept in detail with examples:

Let us take one weak acid and one weak base as HA and BOH, respectively. 

HA is H+ and A-

BOH is B+ and OH-

CH3COOH + H2O ⇋ CH3COO- + H3O+

CH3COONa → CH3COO- + Na+

In the first reaction above, H+ ions concentration increases. Thus reaction takes a reverse change and shifts equilibrium to the left-hand side. The second reaction states the shift in equilibrium.

CH3COOH is a weak acetic acid, and NH4OH is Ammonium hydroxide, a weak base. 

The reaction depicts the shift is equilibrium with a decrease in H3O+ ions, and the pH of the entire solution increases.

Weak bases get partially dissociated. Thus ammonium hydroxide dissociates partially, and when we add Ammonium chloride to it, the concentration of NH4+ ions increases. The shift in equilibrium is noted in the below reaction:

NH4OH ⇋ NH4+ + OH-

NH4Cl → NH4+ + Cl-

Let us learn more about weak acid and weak base reaction.

Experiment on the pH of Weak Acid and Weak Base

The Common Ion effect deals with forming residue with a decrease in the concentration of ions. As studied under Le Chatelier's principle, a reaction’s equilibrium does not change unless you apply any external force. This external force can be in the form of tempurature, pressure, or the addition of some ions in the compound.

If a force is applied, it will help is re-establishing equilibrium. In all, the effect of solubility of solute decreases.


To study pH change by common ion in case of weak acids and weak bases experiment.

Things Required:

You’ll require pH paper, test tubes, glass rod, Test tube stand, sodium acetate, acetic acid, ammonium chloride, and ammonium hydroxide for the experiment. 

The Change in pH Level of a Solution for Weak Acid with Common Ion Effect:

  • The first step is to note the pH value of 10 ml acetic acid in a test tube.

  • To get the exact pH value, compare the shade with the standard pH chart.

  • With the help of a glass rod, mix 1g of sodium acetate with acetic acid in a test tube.

  • Now note down the final pH value considering the standard pH chart as a reference.

  • Now again, add 1g of sodium acetate to the solution and note down the solution’s pH value.

  • Repeat the experiment and keep noting your observations for the same.

Your Observations

The noticed changes show the solution’s pH value increases with the addition of common ions to weak acid.

S. NO. 

Sample Solution Taken

Colour as on pH Paper

pH Value 









The Change in pH Level of a Solution for Weak Base with Common Ion

  • In a test tube, add 10 ml of ammonium hydroxide, note down its pH value using pH paper, and consider the standard pH chart as a reference.

  • Now keep adding 1.5g of ammonium chloride in the test tube using a glass rod and keep mixing.

  • Take pH paper and note down pH value considering the pH chart as a reference.

  • Again add 1.5g of ammonium chloride to the test tube and mix well.

  • Note down the pH value of the final solution noting the pH chart as a reference.

  • Repeat the experiment and note down the final pH value in the observation table.

Your Observations

Change in pH of a weak base with the addition of conjugate acid is noted in the table below:

Sl. No. 

Sample Solution Taken

Colour as on pH Paper

pH Value 




1.5g NH4Cl +  NH4OH


3g NH4Cl +  NH4OH


4.5g NH4Cl +  NH4OH

Experimental Precautions to be Taken:

  • Be careful while noting your observations after matching them with the standard pH chart.

  • Keep noting your observations simultaneously.

FAQs on To Study The pH of Weak Acid and Weak Base - Common Ion Effect

1. What Happens to the pH Value of Weak Acid and Weak Base Reaction?

Ans: When a weak base reacts with weak acid, there can be certain observations to make. However, if a weak acid’s strength is more than base, the solution will be acidic with a pH value of less than 7. If the weak base’s strength is more than weak acid, then the solution will be basic, and the pH value will be more than 7. of both acid and base strength is equal, then the solution pH value will be neutral. Hence the same will be visible through changing pH paper colour. Generally, we do not titrate weak acid weak base because noted changes are minute.

2. Why is the pH of Weak Acid Higher?

Ans: Weak acids do not entirely dissociate ions in their aqueous form. However, strong acids and strong bases dissociate completely in their aqueous form. Also, this affects the concentration of weak acids and strong acids. Weak acids are highly concentrated as compared to strong acids. The difference is due to their pH value. However, both of them release H+ ions but the difference in the amount released. The pH value of both strong and weak acids is less than 7. The difference corresponds to the polarity of acids in their aqueous form. Polarity is the electrons present in the chemical bond.