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TOPIC- ACIDS, BASES AND SALTS

                  SUB TOPIC- BUFFER SOLUTION

Objectives of subtopic- By the end of this subtopic you have to understand the following,

1. To know the concept of buffer solutions,

2. To describe the properties of buffer solution,

3. To know how to prepare buffer solution and

4. To know how to carry out calculation based on buffer solution

Before discussing the third objective, try to answer the following questions

i) By using examples, define the following terms

ii) What are the main components of buffer solution?

iii) Mention any two properties of buffer solution

iv) Identify buffer solutions from the list below.

a) Conjugate acid    b) Conjugate base   c) Weak acid    d)Weak base

1. 0.13 M sodium hydroxide + 0.27 M sodium bromide

2. 0.13 M nitrous acid + 0.14 M sodium nitrite

3. 0.24 M nitric acid + 0.17 M sodium nitrate

4. 0.31 M calcium chloride + 0.25 M calcium bromide

5. 0.34 M ammonia + 0.38 M ammonium bromide

PREPARATION OF BUFFER SOLUTION

The preparation of a buffer solution with a known pH is a two-step process.

1. A weak acid/conjugate base pair is chosen for which the weak acid pKa is within about

1 pH unit of the desired pH. The buffer is most effective when the ratio of component

concentration is close to 1, in which case pH: pKa of the acid.

2. The desired pH and the weak acid pKa are used to determine the relative concentrations of weak acid and conjugate base needed to give the desired pH.

Once the desired weak acid and conjugate base concentrations are known, the solution is

prepared in one of two ways,

1. Direct addition, where the correct amounts of the weak acid and conjugate base are added to water.

2. Acid-base reaction, where, for example, a conjugate base is created by reacting a weak

acid with enough strong base to produce a solution containing the correct weak acid and

conjugate base concentrations.

 EXAMPLE

1: Preparation of Buffers by Direct Addition

Describe how to prepare 500. mL of a buffer solution with pH = 9.85 using one of the weak acid/conjugate base systems shown below.

Solution

Step 1. Choose a weak acid/conjugate base pair. The bicarbonate ion/carbonate ion buffer

system is the best choice here because the desired pH is close to the pKa of the weak acid(Desired pH is 9.85 which is close to 10.32, pKa  of a weak acid)

Write the balanced equation for the acid hydrolysis reaction as follows

Step 2. Determine the necessary weak acid/conjugate base ratio using the rearranged  Ka

expression for the weak acid.)

Ka= [H3O+ to[H3O+(aq) ]= Ka[HCO3

expression

[H3O+(aq) ]/Ka =[HCO3   ratio of [HCO3

From the data given, pH =9.85 and Ka= 4.8 ×10

from this expression  pH =  -log[H3O+

Finding the ratio can be worked out as shown below.

(aq) ][ CO32-(aq]/ [HCO3-(aq)], then make [H3O+(aq) ]

 as the subject, this will results

(aq)]/ [ CO32-(aq], divide by Ka both sides, this will results to this

2-(aq], therefore the ratio of [H3O+(aq)]/ [ CO3-(aq)]/ [ CO3-2-(aq]–11

 the value of [H3O+

(aq) ]/Ka is equal to the (aq) ] can be obtained (aq) ]

Notice that the volume of buffer is cancelled in the ratio. The required amounts of weak acid

and conjugate base are independent of the solution volume, so the volume of a buffer has no effect on the buffer pH.

Step 3.

 Determine the amount of weak acid and conjugate base that must be combined to produce the buffer solution. Mixing 2.9 mol of HCO3–of this ratio) will result in a buffer with a pH of 9.85.and 1.0 mol of CO32– (or any multiple

Exercise

How would you prepare 10mL of a 0.01M phosphate buffer, pH 7.40, from stock solutions of  0.10M KH and 0.25M KPO24HPO? pKa of KH24PO= 7.20.24

                                                I WISH YOU ALL THE BEST IRENE

                                        CHEMISTRY FORM FIVE/SIX

BUFFER SOLUTION

Is a solution which has ability to maintain its PH value constant on either addition of acid or base provided that its buffering capacity is not exceeded.

         

                      TYPES OF BUFFER SOLUTION

1 Acidic buffer solution PH>7

                                                  2 Basic buffer solutions PH<7

                                                         PREPARATION OF BUFFER SOLUTION

                            Acidic buffer solution

Ø  Is prepared from reaction btn weak acid and  strong salts of the same acid,for example ,The mixture containing acetic acid(CH3COOH)  and sodium acetate(CH3COONa),is an example of acidic buffer solution

                                                                     Basic buffer solution

Ø  Is prepared from the reaction btn weak base and strong salt of the same base,forexample A mixture containing ammonium chloride(NH4Cl),is a basic buffer solution.

MECHANISM OF HOW BUFFER SOLUTIONS WORK TO MAINTAIN THEIR PH VALUE

Ø  At first Acetic acid in its aqueous solution  dissociates partially to give small amount of H ions.

CH3COOH    --------    CH3COO ions and H Ions

Ø  When the sodium acetate is added into Acetic acid solution 

                                                         CH3COONa   -------   CH3COO ions and Na ions

                                 Ionization of CH3COONa increases the acetate ions (CH3COOions) in the solution.

                Therefore the increase in CH3COO ions by CH3COOH will cause the equilibrium of Acetic acid to shift more backward hence lead to formation of CH3COOH, Because CH3COO ions will react with Ions of H, being formed from dissociation of Acetic acid originally.

Ø  The new equilibrium will be re established which H ions that remain after establishment of new equilibrium will be the one which determines, the PH of the resulting buffer solution and must remain constant since the PH value of the buffer solution has to remain un changed.

  

              Worked examples on buffer solution

1.       What will happen to the PH of buffer solution if acidic solution like HCL,is added into acidic buffer solution.

                                                                            solution

                                               CH3COOH   ---------   CH3COOions and Hions

                                             CH3COONa    ----------    CH3COOions and Na ions

                                                HCL   ------------                  H ions and CL ions

Ø  The H ions being released by Hcl will disturb the dynamic equilibrium position of CH3COOH .In this case  the equilibrium has to move backward to form more acid,since H ions become combined with CH3COO ions of salts to form acetic acid.

     The PH value of acidic solution will remain fairly constant due to the new re established equilibrium.  

2.       What happens when NaoH becomes added into Acetic buffer solution?

solution

                                                  CH3COOH      --------           CH3COO ions and H ions

                                                     CH3COONa   ---------            CH3COO ions and Na ions

                                                  NaOH      ------------                       Na ions and OH ions

Ø  OH ions released by NaOH,combine with H ions from original CH3COOH to form water,hense causing more acid to dissociate therefore make the equilibrium to move this will result  into decrease of concentration of acetic acid.

                                              Attempt this question below

3.       What is the role of weak acid like CH3COOH in Acidic buffer solution or weak base(NH4OH) In basic buffer solution?

                                                     PH     EQUATION FOR BUFFER SOLUTION

           ACIDIC BUFFER SOLUTION

Ø  Consider dissociation of acetic acid

                                                                        

CH3COOH------------CH3COO ions and H ion

                          NOTE we shall continue with this part after you understand the first part.

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