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Buffer Action of Acidic and Basic Buffer

 

Investigatory project on Buffer Action of Acidic and Basic Buffer

Introduction


A buffer solution is a solution, where the pH of which does not change by the addition of small amounts of an acid or a base. The process by which a buffer solution resists the change in pH by the addition of an acid or a base is called buffer action. Two types of buffers are acid buffer and basic buffer.


Acid buffer


It is a mixture of a weak acid and its salt with a strong base. For example, a mixture of acetic acid and sodium acetate. Its buffer action can be explained as follows.


CH3—COOH + OH —> CH3—COO + H2O


Here pH is maintained the same by the neutralisation of acetic acid by OH— ions added.


CH3—COO + H3O+ —> CH3—COOH + H2O


In this case the pH is maintained by the reaction between acetate ion and hydronium ion.


Basic buffer


Basic buffer is a mixture of a weak base and its salt with a strong acid. For example, a mixture of ammonium hydroxide and ammonium chloride. Buffer action of this buffer solution can be explained as follows.


NH4OH + H3O+ —> NH4+ + 2H2O


Here the pH is maintained the same due to the neutralization of NH4OH by the acid added.


NH4+ + OH —> NH4OH


Here pH remains the same due to the reaction of OH ions from the base with NH4+ ions liberated from the strong electrolyte NH4Cl.


Requirements:


Acetic acid, sodium acetate, ammonium hydroxide, ammonium chloride, 0.1N hydrochloric acid, 0.1N NaOH, pH meter or pH paper, beakers etc


Aim


Aim of the investigatory project is to study the buffer actions of an acid buffer and a basic buffer.


Procedure


Preparation of Acid buffer solution


An acid buffer is prepared by dissolving 14.75 mL of N/5 acetic acid in 35.25 mL of N/5 sodium acetate solution. Note the pH of the solution


a) Buffer action of acid buffer


The acid buffer is taken in a 250mL beaker. Measure its pH using a pH meter or a pH paper. Now add 1 mL of 0.1N hydrochloric acid to the above solution. Stirr it well with a glass rod and note the pH of this solution. The experiment is repeated by using a buffer solution of acetic acid and sodium acetate as mentioned earlier and by adding 1mL of 0.1N NaOH solution. Note the pH of the solution before and after the addition of sodium hydroxide solution. Tabulate the results.


Preparation of Basic buffer solution


A basic buffer is prepared by mixing 1N solutions of NH4Cl and NH4OH. The pH of this solution is noted.


b) Buffer action of a basic buffer


The buffer solution is taken in a 250mL beaker. Measure the pH of this solution using a pH meter or a pH paper. Now add 1 mL of 0.IN HCI solution to the above buffer and note the pH again. Repeat the experiment by using the same quantity of the same buffer and then adding 1mL of 0.1N NaOH solution. Note the pH of this solution also and tabulate.


Observation


Buffer solution

pH of Buffer solution

pH of buffer solution after adding 0.1 NHCI

pH of buffer solution after adding 0.1N NaOH

Acid buffer (CH3COOH + CH3COONa)

 

 

 

Basic Buffer (NH4OH + NH4Cl)

 

 

 

 

Conclusion


The pH values of the buffer solutions remain the same after adding small amounts acid or base. This proves the buffer action of buffer solutions.


VIVA QUESTIONS WITH ANSWERS


1. What is a buffer solution?


Ans: A buffer solution is a solution the pH of which does not change by the addition of small amounts of an acid or a base.


2. How will you prepare an acid buffer?


Ans: An acid buffer is prepared by mixing a weak acid with its salt with a strong base.


3. Give the example of an acid buffer


Ans: A mixture of acetic acid and sodium acetate.


4. How does the pH of an acid buffer remain the same when acid is added to it?


Ans: The hydronium ions react with the salt of the acid to produce a weak acid.


5. What is buffer action?


Ans: The process by which a buffer solution resists the change in pH when an acid or a base is added to it is called buffer action.


6. Write equations which explain the buffer action of an acid buffer


Ans: Base reacts with weak acid as follows


CH3—COOH + OH —> CH3—COO + H20


Acid reacts with acetate ion as follows


CH3—COO + H3O+ —> CH3—COOH + H2O


7. Give the example of a basic buffer


Ans: A mixture of NH4OH and NH4CI


8. How does it act as a buffer?


Ans: When an acid is added to the buffer, it is neutralized by NH4OH


NH4OH + H+ —> NH4+ + H2O


When a base is added to the buffer, the OHions combine with NH4+ from NH4CI to form a weak base NH4OH.


NH4+ + OH —> NH4OH


9. What happens when excess acid is added to a buffer solution?


Ans: The pH value of the buffer solution decreases.


10. What happens when excess base is added to a buffer solution?


Ans: Its pH value increases.


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