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Investigatory Project on Determination of pH Value

 

INVESTIGATORY PROJECT ON STUDY OF THE CHANGE IN pH VALUES OF ACETIC ACID AND AMMONIUM HYDROXIDE BY THE ADDITION OF SALTS HAVING COMMON IONS

 

Introduction

pH value is a convenient method for expressing the hydronium ion concentration (H3O+) of a solution. The negative logarithm of hydronium ion concentration in terms of moles per litre is called as pH of a solution.


pH = - log [H3O+]


In general, at room temperature all neutral solutions would have a pH equal to 7, all acidic solutions would have a pH less than 7 and all basic solutions would have a pH more than 7.


Common ion effect


Weak acids and weak bases are ionised only to small extent in their aqueous solutions. The degree of ionisation of a weak electrolyte (may be a weak acid or a weak base) is more suppressed if a strong electrolyte which can provide several ion common with the ions provided by weak electrolyte, is added to its solution. This effect is called common ion effect.


For example, degree of ionisation of acetic acid (a weak acid) is suppressed by the addition of sodium acetate (a strong electrolyte). The ionisation of acetic acid and sodium acetate in solution is given below.


CH3COOH CH3COO- + H+

CH3COONa  CH3COO- + Na+


Due to the presence of excess acetate ions furnished by sodium acetate, the equilibrium for the ionisation of acetic acid shifts in the backward direction. i.e., more acetate ions combine with hydrogen ions to form unionised acetic acid. This reduces the concentration of hydrogen ion (or more correctly hydronium ion) in solution. Therefore the pH of the solution increases. Similarly, the degree of ionisation of NH4OH (a weak base) is suppressed by the addition of NH4Cl (a strong electrolyte).


NH4OH NH4+ + OH-

NH4CI NH4+ + Cl-


So the concentration of OH- decreases in solution or the concentration of H3O+ increases. Thus the pH of the solution is lowered.


Determination of pH


pH paper can be used to determine the approximate pH value of any solution. A pH paper is a narrow piece of paper that is prepared by dipping the piece in the solutions of various indicators and then drying it. It is available commercially.


Aim of the project


The aim of the project is


(a) to study the change in pH of acetic acid by the addition of sodium acetate, and

(b) to study the change in pH of ammonium hydroxide by the addition of ammonium chloride.


Apparatus and chemicals


1. Test tubes and test tube stand

2. Measuring cylinders

3. Glass tubes

4. pH paper

5. Dilute acetic acid solution

6. Sodium acetate

7. Ammonium hydroxide solution

8. Ammonium chloride


Procedure


(a) Take 10 mL. of dilute acetic acid in a clean test tube Place a drop of this solution on a strip of pH paper and note its colour. Compare the colour with the colour on chart paper and note the pH of the acid. Now weigh 1 g of solid sodium acetate and dissolve it in the acetic acid taken. Verify the pH of the solution using pH paper. Weigh again 1g of sodium acetate and dissolve it in the same solution. Determine the pH again. Record the observations.


(b) Take 10 mL of dilute ammonium hydroxide solution in a clean test tube. Place a drop of this solution on a strip of pH paper and determine its pH. Now weigh 1g of solid ammonium chloride and dissolve it in the ammonium hydroxide solution. Verify the pH of the solution using pH paper. Weigh again 1g of ammonium chloride and dissolve it in the same solution, Determine the pH of the solution. Record the observations.


OBSERVATIONS

(a)

Exp No:

Solution

Shade of colour

pH Value

1

Acetic acid

 

 

2

Acetic acid + 1g sodium acetate

 

 

3

Acetic acid+ 2g sodium acetate

 

 

 

(b)

Exp No:

Solution

Shade of colour

pH Value

1

Ammonium hydroxide

 

 

2

Ammonium hydroxide + 1g ammonium chloride

 

 

3

Ammonium hydroxide + 2g ammonium chloride

 

 

 

Result:


1. pH value of acetic acid goes on increasing on adding more and more of sodium acetate to it.

2. pH value of ammonium hydroxide goes on decreasing on adding more and more ammonium chloride to it.

 

Viva Questions and Answers

 

1. What is pH?

Ans: See the 'introduction' of this experiment.


2. Give the pH of pure water at a temperature of 25°C ?

Ans: 7


3. The pH of a solution is 3. What is its hydrogen ion concentration? Is the solution acidic or basic ?

Ans: Its hydrogen ion concentration is 10-3 molL-1. The solution is acidic.


4. The pH of two solutions A and B are 4 and 5 respectively. Which is more acidic ?

Ans: Solution A is more acidic.


5. Explain why the addition of NH4Cl decreases the pH of a solution of NH4OH.

Ans: See the 'introduction' of this experiment.


6. What is common ion effect ?

Ans: See the 'introduction' of this experiment.


7. What is an acid base indicator ? Give one example

Ans: An acid base indicator is an organic compound which changes its colour within a certain pH range in solution. Methyl orange is an example. It is golden yellow in alkaline medium and rose red in acid medium.


8. What do you mean by universal indicator ?

Ans: It is an indicator made by mixing a number of acid base indicators. It exhibits various colour changes over a wide range of pH. Each colour corresponds to a particular value of pH. Universal indicator is used to determine the pH of a solution.


9. What is the connection between the pH and pOH of an aqueous solution ?

Ans: pH + pOH = 14 (at 298K)


10. What is a buffer solution ?

Ans: A buffer solution is one which resists changes in its pH value even when small quantity of acid or base is added to it.

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