Aim of the experiment: To determine the dissociation constant of an indicator such as methyl red spectrophotometrically

Theory: Dissociation or ionization constant of a substance may be determined by spectrophotometric method provided there is appreciable difference between the absorption of molecular and ionic forms of the substance. Acid-base indicators, by definition, having differing colors (absorption vs. wavelength patterns) for the molecular and ionic forms, the dissociation constants (indicator constants) of such indicators can be determined in the  visible region of the spectrum.

dissociation of a weak acid HA can be represented as :

                        HA =====  H+ + A-

                              1-a                    a        a

The classical dissociation constant Ka is given by:

                        Ka = ( CH+ . CA- ) / C HA

Taking logarithm and rearranging the terms :

                       – log CH+ =  – log Ka + log ( CA-  / C HA)

                    or pH  = pKa + log (CA- / CHA) 

                          or  pH  = pKa + log {a/(1-a)}  ------------------------------ ( 1 )

If DHA ,DA- and D are the absorbance's (all at the same wavelength) of the acidic solution, the alkaline solution and of a solution of intermediate pH (for all three equimolar solutions) respectively, then :

                           D = (1-a)D HA + a DA-

                        or a = ( D HA– D ) / ( D HA– DA- )  ---------------------( 2 )

Combining equation (1) and (2), we get :

                             pH  = pKa + log [( D HA– D ) / ( D – DA- )]  ------------( 3 )

A plot of  pH vs. log [( D HA– D ) / ( D – DA- )] ( abscissa ) is a straight line having intercept pKa .

Hence from the intercept of the plot Ka can be calculated .

                                               

Requirements:   1) Spectrophotometeter
                            2) pH meter with glass electrode assembly
                            3) methyl red
                            4) 0.04 M CH3COONa solution
                            5) 0.02 M CH3COOH solution
                            6) 0.1 M HCl solution
                            7) 95% ethanol solution

Procedure:

1)Preparation of stock solution: A stock solution of methyl red is prepared by dissolving 0.1 g of pure crystalline methyl red in 30 ml of 95% ethanol and made up to 50 ml with distilled water. After this the following solutions are made as follows :

Solution A : 5 ml of stock solution of the indicator and 45 ml of 95% ethanol are mixed and made up to 100 ml with water in a standard flask. 

Solution B :  10 ml of solution A, 10 ml of 0.1 M HCl are mixed and diluted to 100 ml with water .pH of this solution is about 2 and the indicators entirely in the molecular form (HA).

Solution C : 10 ml of solution A, 25 ml of 0.04 M CH3COONa made up to 100 ml  .The pH of this solution is about 8 and the indicator is entirely in the ionic form (A–).

2) Now the preparation of the following buffer solutions will be done:    

      No. Volume of 0.04 M CH3COONa (ml) Volume of 0.02 M CH3COOH Volume of indicator solution A (ml) Water to fill up to  mark (ml)
      1 25.0 50.0 10.0 15
      2  25.0 25.0 10.0 40
      3  25.0 10.0 10.0 55
      4 25.0 5.0 10.0 60

       After the preparation of these buffer solutions the pH of these solutions is measured with a pH-meter.

3) The absorbance of each of the solutions B and C at the wavelengths ( l1and l2) of maximum absorption is to be determined. Also we have to determine the absorbances D1  and D2 of each buffer solutions at these wavelengths ( l1and l2) . 

Observations:

1) pH determination table:

         Buffer Solution                                     pH
           1
           2
           3
           4

 2) Calculation of maximum absorption of solution B( l1), HA

           Wavelength ( nm )            Absorbance 
             400

    ( A no. of wavelengths are to be observed to get the maximum)

3) Calculation of maximum absorption of solutions C (l2), A

                            [Similar as in 2]

4) Measurement of absorbance of the buffers and solutions B and C at maximum wavelengths l1 and l2 :

Wavelength Buffer sol.1 Buffer sol.2 Buffer sol.3 Buffer sol.4 Sol. A Sol. C
l1
l2

5) Table of pH values and log [(D HA – D ) / ( D – DA- )] values for the four buffer solutions ( using the data obtained from solution B):

Buffer Solutions pH (D HA – D) /
( D – DA-) 		log
[(DHA–D) /(D–DA-)]
1      
2      
3      
4      

Calculations: A plot of pH vs. log [(DHA–D) /(D–DA-)] is drawn using the above data from the table. The plot yields a straight line whose intercept is the measure of pKa value according to the equation (iii) .

From the graph, it is found that

Intercept = pKa = – log Ka = .....

Or Ka = ......

Result: Hence the dissociation constant of the indicator is found to be .................