Spectrophotometric Method for Studying the Kinetics of Saponification of Alkyl Esters R. S. Roy and H. N. AI-Jallo Faculty of Science, University of Mosul, Mosul, Iraq
THESAPONIFICATION of alkyl esters can be studied by a spectrophotometric method developed by Roy ( I ) . The method may be called the same absorbance method as both the absorbing species, ester and acid, absorb at the same region, and the spectrophotometric analysis is carried out with absorbances at that region. Reactions between esters and an alkali are of the second order. The simultaneous disappearance of the ester and appearance of the acid with time can be conveniently followed at a region where there is absorption caused by both. Results obtained for the saponification of ethyl acetate by sodium hydroxide are reported here. Of the three methods so far proposed [titrimetric, conductimetric (2, 3), and spectrophotometric ( I ) ] the spectrophotometric method is the most convenient as it is simple, rapid, and quantitative. Theory. The velocity constant of hydrolysis, k , of a first order reaction, can be estimated with the expression mentioned in the previous note ( I ) . For a second order reaction, introduction of the e, and e, values ( I ) into the kinetic equation ( 4 ) when both reactants are of the same concentration gives:
Aqueous solutions of 0.03M were freshly prepared in 25ml portions from the distilled ester solution and a shock solution of alkali. A 1.5-ml amount of the alkali was added to the cell containing 1.5 ml of the ester solution; both were regulated thermostatically (*3 “C) at the cell holder and mixed thoroughly and quickly. The spectrometric scans were made at known time intervals at 217 mM with a 1-mm slit width. Reactions were thus carried out in solutions containing initially 0.03M with respect to both ethyl acetate and alkali in the ratio of 1 :1 and 3 :1. The ultraviolet absorbance was measured against a reference solution containing the same amount of sodium hydroxide as the sample solution in each case.
2.0
1.6
When both reactants are of different concentrations, the equation becomes : k = -t(a
1
- 6) In
a(be,
b(A
- ae,)
- be,
- at,
+ A)
(2)
where e, is the molar absorptivity of the ester at, , , ,A e, is the molar absorptivity of the acid at, , ,A, A is the absorbance of the ester at, , ,A at any time t , and a and b are the initial concentration of the reactants. The velocity constant of hydrolysis, k , of a second order can be estimated with either Equation 1 or Equation 2, and can also be evaluated graphically from the plot of the ratio of (ace - &/(A - ae,) US. t. Also, In
b(A a(&,
- be,
- ae,) - ae,
1.2
P
2
f< 0.8
+ A)
can be plotted os. t yielding a straight line with a slope of k(a - b).
0.4
EXPERIMENTAL
Analar grade ethyl acetate after purification and Analar grade sodium hydroxide were used in this work. Spectrometric measurements were recorded with a double beam Unicam SP 800 ultraviolet spectrophotometer fitted with a thermostat with silica cell of 1-cm thickness.
0.0
Po0
PP5
250
WAVELENGTH, mp
(1) R. S. Roy, ANAL.CHEM.,40, 1724 (1968). (2) W. G. Leighton and G. S. Forbes, J. Amer. Chem. SOC.,52, 3139 (1930). (3) G. S. Forbes and L. J. Heidt, ibid., 56,2363 (1934). (4) E. A. Moelwyn Hughes, “Physical Chemistry”, 3rd ed., Pergamon Press, 1964, pp 1117-18.
Figure 1. Spectra for ethyl acetate saponification at different time intervals
+
A: 0.03M ethyl acetate. B: 0.03M ester 0.03M alkali, (1) 46 sec; (2) 118 sec; (3) 171 sec; (4) 218 sec; (5) 285 sec; (6) 363 SBC
VOL 40, NO. 1 1 , SEPTEMBER 1968
1725
Table I. Values of A and k at Different Time Intervals at 30 “C
X Time, sec A k
+ 0.06 X 10-1 (mole-’ sec-1)
46 0.995
= 217.0
ea = 38
118 0.850
1.50
171 0.775
1.44
Average k
= 9.3
Ea
218 0.725
1.44
1.42
= 1.44 X 10-1 mole-’
RESULTS AND DISCUSSION The spectra of ester solution (0.03M) (A) appears in Figure 1 with that of ester solution (B) for the 1 :1 ester-alkali solution at different time intervals during hydrolysis. Referring to spectra B in Figure 1, the maximum absorbance of the ester shifts somewhat toward the longer wavelength with time during hydrolysis because of the production of acetate ion. The values of E ~ , B , - , and A at wavelength X = 217 have been determined from many experimental observations of the ester, acetate, and ester alkali mixture of 1 :1 composition at 30 OC. The k value has been calculated at different time intervals with Equation 1. The data are summarized in Table I. A plot of the ratio (ace - A ) / ( A - a€=-)US. t gives a straight line which passes through the origin; the linearity of the plot gives further indication that the equation is appropriate. The slope of the line yields a velocity constant, k = 1.42 X 10-1 mole-’ sec-’ at 3o OC in good agreement with the average value of 1.44 x 10-1 mole-“ sec-’ obtained in Table I.
285 0.670 1.41
363 0.615 1.44
sec-1
The average velocity constant estimated for the O.O3M, 3 :1 mixture at 20 “C with Equation 2 was 0.68 x lo-’ mole-’ sec-1 which is in reasonable agreement with the reported value of 0.58 X 10-l mole-’ sec-l at 16 “C obtained by the usual titration method (5). It may be mentioned that recently Yates and McClelland (6) studied the hydrolysis of alkyl and aryl esters in the ultraviolet region. ACKNOWLEDGMENT
The authors thank Miss S. Sharif for preparing the solutions used in this research, M. AI-Tahafi for recording the ultraviolet spectra, and M. Shindalla for his interest. Received February 29, 1968. Accepted May 15, 1968 (5) S. Glasstone, “Text Book of Physical Chemistry”, 2nd ed., Macmillan, New York, 1956, p 1058. ( 6 ) K. Yates and R. A. McClelland, J. Amer. Chem. SOC.,89, 2686 (1967).
Rapid Electrophoretic Separation of RheniuNm(lll), Rhenium(lV), and Rhenium(Vl1) Juan F. Facetti’ and Marcelina VBlez de Santiago2 Puerto Rico Nuclear Center, Mayaguez, P . R .
00708
SEVERAL METHODS have been reported for the separation of rhenium in different oxidation states. Re7+ is usually separated by solvent extraction ( I , 2), or by preparation of the tetraphenylarsonium complex and extraction with chloroform (3). Re’+ can also be precipitated with nitron (4). Re4+ is also precipitated by tetron (5). Other methods of separation involve changes in the oxidation state ( I , 6, 7). 1 Present address, lnstituto de Ciencias, Universidad de Asunci6n, Paraguay. 2 Present address, Department of Chemistry, Catholic University of Puerto Rico, Ponce, P.R.
(1) P. Pascal, “Noveau Traite de ChCmie Minerale,” Vol. XVI 1097, Maison, Pans, 1956. (2) J. B. Gerlit, Pvoc. Intern. Con$ Peaceful Uses At. Energy (Geneoa),7,145 (1956). (3) H . N. Willard and G. M. Smith, Ind. Eng. Chem., 11, 186, 269 (1939). (4) W. Geilmann and A. Voigt, Z . Anorg. Allgem. Chem., 193, 311 (1930). ( 5 ) W. Herr, 2.Elekfrochem.,59,911 (1952). (6) G. Leddicotte, “The Radiochemistry of Rhenium,” NAS-NS3028 (1961). (7) S. Tribalat, “Rhenium,” B. Gonser, Ed., section 6, p 191 Elsevier, New York, 1956.
1726
0
ANALYTICAL CHEMISTRY
For the study of nuclear transformations in rhenium systems, it is essential to use a rapid method, which does not result in coprecipitation and is effective for small amounts of substance. Electrophoresis offers a rapid method of analysis. However, the only published data on rhenium concern the behavior of perrhenate using sodium chloride and nitrate (8) or dilute hydrochloric acid (0.3-OSN) (9),and chlorobromo complexes of rhenium(1V) in the same electrolyte (IO). It has now been found that Re3+, Re4+,and Re7+ can be readily separated by electrophoresis using 0.005M hydrochloric acid as electrolyte. EXPERIMENTAL
Compounds. ReCL, ReCh, and NH4Re04 were purchased from Alfa Inorganics, Inc., and Reoz was prepared according to Sidgwick (11). ReC1, was dissolved in 12M HCl, Reoz in 10% NaCl in 4 M HCl, Re’+ in 2-4M HCI, (8) V. P. Shvedov and K. V. Kotegov, Radiokhimiya, 5,374 (1963). (9) P.-H. Ying and T.-T. Cheng, Hua Huseh Tung PRO,1964, p 315. (10) E. Blasius and W. Preetz, 2.Anorg. Allgem. Chem., 355, 16 (1965). (11) N. V. Sidgwick, “The Chemical Elements and Their Compounds,” 11, Clarendon Press, Oxford, 1950, p 1304.
