Comparative Study of Kinetics of Saponification of Ethyl Acetate by pH Method and Titration Method

 

Ulhas Balkrishna Hadkar1*, Asavari Sameer Hadkar2

1Director/Professor, Mumbai Educational Trust Institute of Pharmacy, Bandra (West), Mumbai – 400050.

2Research Assistant, Mumbai Educational Trust Institute of Pharmacy, Bandra (West), Mumbai – 400050.

*Corresponding Author E-mail: ulhashadkar@yahoo.com, asavarihadkar@gmail.com

 

ABSTRACT:

The hydrolysis of ethyl acetate in presence of NaOH has been studied by pH method and by titration method. In the pH method the pH of the reaction mixture was recorded at various time intervals and the data was used to calculate the specific reaction rate constant of the reaction at room temperature (28oC). In the pH method,“alkaline error” correction was used to get the “true pH” of the reaction mixture and was used to calculate the specific reaction rate constant.In the titration method excess acetic acid was used to arrest the reaction. The pH method gave more reliable value of specific reaction rate constant of saponification of ethyl acetate than the titration method.

 

KEYWORDS: Saponification, Specific reaction rate constant, pH method, titration method, alkaline error, glass combination pH electrode.

 

 


INTRODUCTION:

The hydrolysis of ethyl acetate in presence of alkali (saponification) has been reported using a pH meter1. However the experimental details are not available. The saponification reaction has also been reported by using titration method in which HClwas used to arrest the reaction2or acetic acid was used to arrest the reaction.The reported values of specific reaction rate constant k determined by conductometric method3-6by different investigators are significantly different. The authors of the present articles have studied the kinetics of saponification of ethyl acetate using pH meter and titration method.

 

In the solution of high Na+ ion concentration the observed pH using glass combination pH electrode is less than the true pH of the solution and this is termed as alkaline error.7,8 In the pH method reported here the true pH of the reaction mixture was calculated by considering the “alkaline error”. The pH method has so far not been reported by using the “true pH” for the calculation of H+ ion concentration and OH- ion concentration of the reaction mixture. A significant difference in the reaction rate constant k calculated by using the alkaline error correction and that calculated without using the correction was observed. For comparison purpose the saponification of ethyl acetate was also investigate at room temperature using the titration method and is reported here.

 

MATERIAL AND METHODS:

Instrument:

pH meter: ELICO LI 120.

Electrode: Glass combination pH electrode.

 

Materials:

Ethyl acetate, Sodium hydroxide, Acetic acid, Potassium chloride used were of s d fine-chem limited.

All the solutions were prepared in deionized distilled water.

 

METHOD:

pH Method–I:

The pH meter was standardized by dipping the glass combination pH electrode into Buffer solution9 of pH 12. The saponification reaction was started by mixing 50ml 0.04M Ethyl acetate solution with 50ml 0.04M NaOH solution at room temperature (28oC). The initial concentration of the ester and that of the alkali in the reaction mixture is 0.02M. In other words the reaction is started by mixing equimolar quantities of the ester“b” moles/litre and the alkali“a” moles/litre (a =b).

 

Alkaline Error:- To determine the “alkaline error” 0.02M NaOH solution was prepared and its pH was read with the standardized pH meter and was found to be 12. The theoretical value of pH of 0.02M NaOH was calculated assuming complete dissociation of NaOH and taking pKw value as 14 at the room temperature (28oC). The calculated theoretical value of pH of 0.02M NaOH is 12.30. The difference in the calculated pH of 12.30 and the observed pH reading at the start of the reaction (t = 0min) was added to the observed pH readings at different time intervals from the start of the saponification reaction. The correction was added to the observed pH since observed pH was less than the true pH (12.30). The specific reaction rate constant, k was calculated using the “true pH” (Table 1). The experiment was repeated thrice and the pH (observed) in Table 1 are the average of three readings.

 

pH Method–II:

The pH method-II is the same as the pH method-I. The pH data from the pH method-I was used to calculate the OH- ion concentration in the solution at various time intervals from the start of the reaction by using the observed pH and not the true pH. The specific reaction rate constant, k for the saponification was calculated. The data is given in Table 2.The initial concentration of alkali a = 0.02moles/litre was considered for the calculation of k.

 

Titration Method:

The saponification of ethyl acetate was studied by mixing 50 ml of 0.04M ethyl acetate with 50ml 0.04M NaOH at room temperature(28oC) and 10ml of the reaction mixture at various time intervals from the start of the reaction was added to 25ml of 0.01M acetic acid solution taken in a conical flask containing crushed ice and titrated against 0.01N NaOH solution using phenolphthalein as the indicator. The data was used to calculate the specific reaction rate constant k of the reaction (Table 3).The experiment was repeated thrice and the titre values (Tt) in Table 3 are the average of three readings.

 

RESULT AND DISCUSSION:

The specific reaction rate constant of saponification of ethyl acetate has been reported by various methods such as conductivity method, pH method, titration method. The specific reaction rate constant reported in the literature vary considerably. (Table 4) The values of the k determine by conductivity method are 9.72 litre     mole-1min-1and

 

4.09litre mole-1min-1in the temperature range26oC to 30oC. The values of the k reported by titration method in some cases are not reliable sincethe units of the k stated are not proper (Table 4). If time is measured in minutes then the units of k should be litre mole-1min-1 since, for the second order reaction (a = b), k = x/t.a.(a-x) where x is the change in the concentration of the reactants and  (a-x) is the concentration of reactants remaining in the reaction mixture at time t from the start of the reaction.

 

The pH method has been reported in the literature to determine the rate constant of saponification reaction, however, the authors have not considered the “alkaline error” correction. The reported value of k of the saponification reaction by pH method is 8.4 litre         mole-1min-1 (Table 4).

 

The authors of the present article have determined rate constant of the saponification reaction by pH method by considering the alkaline error correction.The values of k determinedby considering the alkaline errorcorrection are different from the reported values of k by pH method. The mean value of specific reaction rate constant by pH method by considering the alkaline error correction was found to be 4.190litre mole-1min-1         (Table 1).

 

Titration method: Acetic acid is used to arrest the reaction. Acetic acid being weak acid the excess acetic acid ionizes to negligible extent and hence the acid hydrolysisof the ester is extremely slow and the titre reading obtained is not significantly increased, consequently the rate constant of saponification determined by using acetic acid to arrest the reaction should be practically the same as determined by pH method by considering the true pH and this infact is observed. The reaction rate constant observed by pH method I is practically the same as that determined by the titration method. The value of k by pH method is 4.190litre mole-1min-1 (Table 1) and that determined by titration method is 4.108litre mole-1min-1 (Table 4).

 

The plot of 1/(a-x) vs t for the pH method-I and for the titration method are given in Figure 1 and Figure 2 respectively. The correlation coefficient for the plot by pH method and for the plot by titration method were 0.9825 and 0.9433 respectively which shows that pH method is more accurate than the titration method.


 

 

Table 1: Kinetic data using  pH Method-I (“alkaline error” correction is used).

t

min

pH (observed)

True pH

(corrected)

(14-pH) = pOH =

– log [OH-]

[OH-] = [NaOH] = (a-x) mole/l

1/(a-x)

       1        1            1

k =  --- [------    -    ----]  

         t       a-x         a     

litre mole-1min-1

0

12.00

12.30

1.70

1.99 x 10-2

 

 

3

11.90

12.20

1.80

1.58 x 10-2

63.09

4.36

6

11.80

12.10

1.90

1.258 x 10-2

79.43

4.90

10

11.73

12.03

1.93

1.175 x 10-2

85.11

3.51

15

11.63

11.93

2.07

8.51 x 10-3

117.48

4.49

20

11.57

11.87

2.13

7.41 x 10-3

134.89

4.24

30

11.52

11.82

2.18

6.60 x 10-3

151.35

4.05

40

11.36

11.66

2.34

4.57 x 10-3

218.77

4.22

50

11.30

11.60

2.40

3.98 x 10-3

251.18

4.02

60

11.24

11.54

2.46

3.467 x 10-3

288.40

3.966

 

 

 

 

 

 

Mean k = 4.190 litre mole-1min-1

The standard deviation is 0.3852 and k =4.190±0.3852 litre mole-1min-1

Note: Alkaline error correction: True pH – Observed pH at t = 0   = 12.30 – 12.00 = 0.3unit

At 28oC pKw = 14 and pOH = 14 – pH

 

 

Table 2: Kinetic data using  pH Method-II(without “alkaline error” correction).

t min

pH (observed)

(14-pH) = POH =

– log [OH-]

[OH-] = [NaOH] = (a-x) mole/l

1/(a-x)

       1        1            1

k =  --- [------    -    ----]  

         t       a-x         a     

litre mole-1min-1

0

12.00

 

 

 

 

3

11.90

2.10

7.943 x 10-3

125.89

25.39

6

11.80

2.20

6.309 x 10-3

158.49

18.08

10

11.73

2.27

5.370 x 10-3

186.21

13.62

15

11.63

2.37

4.266 x 10-3

234.42

12.29

20

11.57

2.43

3.715 x 10-3

269.15

10.96

30

11.52

2.48

3.311 x 10-3

301.99

10.08

40

11.36

2.64

2.291 x 10-3

436.52

9.66

50

11.30

2.70

1.995 x 10-3

501.19

9.03

60

11.24

2.76

1.738 x 10-3

575.44

8.76

 

 

 

 

 

Mean k = 13.096 litre mole-1min-1

Note: Initial concentration of alkali a = 0.02moles/litre was considered for the calculation of k. The values of k obtained

without alkaline error correction are not constant.

 

 

Table 3: Kinetic data for the Titration Method using Acetic acid solution to arrest the reaction

t min

Tt ml

(a-x) mol/l = (25-Tt) (10-3)

1/(a-x)

       1        1            1

k =  --- [------    -    ----]  

         t       a-x         a     

litre mole-1min-1

0

-

-

-

-

3

9.3

0.0157

63.69

4.56

6

12.5

0.0125

80.00

5.00

10

15.3

0.0097

103.09

5.30

15

16.0

0.0090

111.11

4.074

20

16.2

0.0088

113.63

3.18

25

17.7

0.0073

136.98

3.48

30

18.1

0.0069

144.92

3.16

 

 

 

 

Mean k = 4.108litre mole-1min-1

The standard deviation is 0.0.8733 andk = 4.108±0.8733 litre mole-1min-1

 

 


Note:  The infinity reading is the reading when the ester is completely hydrolyzed. On complete hydrolysis acetic acid formed will completely neutralize the alkali. Since 25ml of 0.01M acetic acid is used to arrest the reaction the infinity reading would be 25ml of 0.01M NaOH i.e.

T = 25ml

 

Calculation for (a-x) values

The saponification reaction is the second order reaction10. The saponification reaction of ethyl acetate is:

 

CH3COOC2H5OH + NaOH         CH3COONa + C2H5OH

 

It can be seen from the reaction that x moles of the ester reacts with x moles of NaOH

 

On mixing 50ml 0.04 M NaOH with 50 ml 0.04 M ester the concentration of NaOH ,a = 0.02 moles/litre at t = 0 and of ester, b = 0.02 moles/litreat t = 0

 

Let the titre reading at time t be Tt ml of 0.01 N NaOH ≡ Tt ml 0.01 N acetic acid unreacted

 

Acetic acid pipetted out = 25 ml 0.01N acetic acid

.ˑ. ml of acetic acid reacted per 10 ml reaction mixture = added - unreacted

= (25 – Tt) ml 0.01N acetic acid

= (25 –Tt) ml 0.01 N NaOH remain unreacted in 10 ml reaction mixture at t = t min

= (25 –Tt)/2 ml of 0.02NNaOH remain unreacted per 10 ml reaction mixture.

= 50 (25 –Tt) ml of 0.02 N NaOH remain unreacted per litre of reaction mixture at time t

 

Now, 1000 ml 1N NaOH ≡ 1 mole

ˑ. 50(25-Tt) 0.02N NaOH ≡ (25-Tt) x 10-3 mole

ˑ. Amount of NaOH remaining unreacted

≡ (25- Tt) 10-3 mole/l

 

ˑ. (a-x) = (25- Tt) x 10-3 mole/l

 

Table 4: Literature values of specific reaction rate constant k, of the saponification of ethyl acetate.

Method

Specific reaction rate constantk

Reference

pH Method

8.4 litre mole-1min-1

1

Titration Method

0.003min-1cm-3 *

2

Conductometric Method

9.6 litre mole-1min-1

3

9.720litre mole-1min-1

4

4.092litre mole-1min-1

5

4.938litre mole-1min-1

6

* The units of k reported by titration method are incorrect

 

 

Figure 1: Plot of 1/(a-x) vs t (pH Method using “alkaline error” correction)

 

Figure2: Plot of 1/(a-x) vs t (Titration Method)

 

CONCLUSION:

In the pH method the specific reaction rate constant k determined by considering the true pH is significantly different from rate constant determined by using observed pH (without “alkaline error” correction). The k value in fact determined by pH method without “alkaline error”  correction are not constant and vary significantly. Thus it is necessary to calculate k using alkaline error correction. In the titration method practically constant value of specific reaction rate constant is obtained when weak acid (acetic acid) is used to arrest the reaction. The rate constant k determined by pH method (using true pH) and by the titration method by using acetic acid to arrest the reaction are in close agreement. It also agrees with the reported value 4.092 litre mole-1min-1 by conductivity method. It may be concluded that it is necessary to use the value of true pH in the study of kinetics of saponification reaction using glass combination pH electrode (pH method).

 

ACKNOWLEDGEMENT:

The authors wish to thank trustees of Mumbai Educational Trust for the laboratory facilities provided to carry out the research work.

 

REFERENCES:

1.     Estimation of Ethyl acetate kinetic parameters from Dynamic Data. A review. Available from: URL: apmonitor.com/me575/index.php/Main/Ethyacetate.

2.     Ikhazuangbe, Prosper Monday Ohien and Oni, AisosaBabalola. Reaction rate and rate constant of the hydrolysis of ethyl acetate with sodium hydroxide. A review. Available from: URL: www.scihub.org/AJSIR Volume 6, Issue 1 February 2015 pp. 1-4.

3.     Smith H.A., Leuenson H.S., J.Am. C Hem. Soc.1939,61,1172-1175

4.     International Journal of Chemical Kinetics” by Kulaeli Das, P. Sahoo, M.SaiBaba, N. Murali, P Swaminathan Volume 43, Issue 11, Nov. 2011, pp. 648-656.

5.     Determination of reaction rate and reaction rate constant. A review. Available from: URL: https://dissa4ict.wordpress.com/2013/01/09

6.     Kinetic Studies on Saponification of Ethyl acetate by the Conductance Method – Physical Chemistry. A review. Available from: URL: LuLeLaboratory.Blogspot.in/2014/04/kinetic-studies-on-saponification-of.ht.ml

7.     Quantitative Chemical Analysis, Daniel C. Harris 7th Edition, p.31

8.     Under Graduate Instrumental Analysis by James W. Robinson, Eileen M. Skelly Frame and George M. Frame II, 6thedition.2009,pp.939

9.     Lange’s Handbook of Chemistry, Editor: John A. Dean, McGraw Hill Book Company, 12th edition, pp. 5-78.

10.  Organic Chemistry Francis A. Carey, The McGraw Hill Companies, INC, 3rd edition, 1996 pp. 828.

 

 

 

 

 

 

 

Received on 11.12.2017          Accepted on 15.02.2018        

© Asian Pharma Press All Right Reserved

Asian J. Pharm. Ana. 2018; 8(1):20-24. 

DOI: 10.5958/2231-5675.2018.00004.2