KINETICS OF THE BROMOACETATE-THIOSULFATE REACTION
429
T H E ELECTROLYTE EFFECT UPON T H E KINETICS OF T H E BROMOACETATE-THIOSULFATE REACTION I N %-PROPYL ALCOHOL-WATER SOLUTIONS' F. G. CIAPETTAP A N D H. M. TOMLINSON Department of Chemistry, Temple University, Philadelphia, Penanylvania Received March $9, 1960
The kinetics of the bromoacetate-thiosulfate reaction has been studied by BrCH2COO-
+ SzOa-- -+ S2OaCH2C00-+ Br-
a number of investigators (6, 10, 11, 13, 15, 16, 18, 19). The rate data exhibit in all cases positive salt effects, as predicted for a reaction between ions of the same sign. Excellent agreement exists with the quantitative forms of the Br9nsted rate theory obtained by introducing the Debye-Hiickel first-approximation equations for the activity coefficients. This agreement, however, is maintained only when the cations present are monovalent and the dielectric constant of the medium is greater than 60, since the critical complex is a trivalent negative ion. For example, in very dilute aqueous solutions the presence of calcium, magnesium, and lanthanum ions produces a positive salt catalysis which far exceeds and renders quite invalid any predictions from the simple principle of ionic strength, as has been shown by La Mer and Fessenden (15). Tomlinson (19) found these abnormal effects to be progressively increased when the dielectric constant was reduced by adding dioxane to water. In solvents of low dielectric constant the breakdown of the ionic strength principle and the simple forms of the'Debye-Huckel theory appear, even in the case of monovalent cations, in conformity with the abnormal behavior of equilibrium properties (solubility, etc.) under similar conditions (12). Since equilibrium studies have shown that this abnormal behavior is often dependent upon some specific characteristic of the solvent, in addition to being a function of the product of the charges of the ions of opposite sign divided by the dielectric constant, the investigation has been extended to n-propyl alcohol to compare that solvent with dioxane. EXPERIMENTAL
Reagents Eastman n-propyl alcohol (b.p. 96.0-98.0"C.) was fractionally distilled over calcium oxide (2, 5), using an all-glass apparatus. Freshly distilled alcohol (b.p. 96.9-97.2"C.) was used for each experiment. The bromoacetic acid was purified by distilling the chemically pure acid twice under vacuum. It was stored in a desiccator over calcium chloride. All salts were of reagent grade. The lanthanum and calcium salts were ana1 Presented at the Third Meeting-in-Miniature of the Philadelphia Section of the American Chemical Society, Philadelphia, Pennsylvania, January 20, 1949. 2 Present address: The Atlantic Refining Company, Philadelphia, Pennsylvania.
430
F. Q. CIAPETTA AND H. M. TOMLINSON
lysed by heating to the oxide. The purity of the strontium salt was determined by precipitation 88 the sulfate. The cesium chloride was analyzed by precipitation with silver nitrate. Resublimed iodine (8) was used to prepare the primary standard iodine solutions. Sodium hydroxide solutions were prepared and stored according to the method of Clark (4). They were standardized against potassium acid phthalate obtained from the Bureau of Standards. Sodium thiosulfate solutions were prepared using the freshly recrystallized salt and/or the anhydrous salt (20). The thiosulfate concentrations were determined by titration with standard iodine solutions. “he bromoacetate solutions were prepared by neutralization of the calculated weights of bromoacetic acid with standard sodium hydroxide solution. All solutions of reactants were prepared in volumetric flasks, using the weights of n-propyl alcohol and water, as reported by Wkerlof ( l ) , required to give a solvent of dielectric constant 30. Each solution of iodine, sodium bromoacetate, and sodium thiosulfate was prepared independently without the use of “master” solutions. As previously reported (19), the starch-iodide end point in solvents of low dielectric constant is yellow. Correction blanks were determined for each solution of iodine. No effect of n-propyl alcohol-water solution on the titer of sodium thiosulfate was observed over a rsriod of 24 hr. The added salts were dissolved in the bromoacetate solutione.
Apparatus The constant-temperature bath was maintained at 25°C. f O.0lo. The temperature was checked against a Beckmann thermometer which had been calibrated against a certified Bureau of Standards resistance thermometer. Determination of rate The technique of La Mer and Kamner (16) was employed to determine the reaction rate. One hour was allowed for temperature equilibrium before mixing the contents of the reaction vessels. The molar bimolecular velocity constant k was calculated from the equation
k =
X
at(a’
- 2)
where t = time in minutes, a = initial molar concentration of thiosulfate = initial molar concentration of bromoacetate, a’ = initial concentration of thiosulfate in terms of milliliters of iodine solution required a t t = 0, (a’- x) = milliliters of same iodine solution required at time t, and x = thiosulfate reacted in terms of milliliters of iodine solution.
KINETICS OF THE BROMOACETATE-THIOSULFATE REACTION
431
TABLE 1 Bromoacetate-thiosuljate reaction
pa cmi
tnVl.
1
1058 1256 1430 2669
,
’
1 ~
17.7 24.4 33.4 42.4 51.3
1 1
i 1
21.48 20.29 19.41 14.67
13.97 11.47 9.24 7.66 6.79
~
1 ’
9.53 10.73 11.60 16.34
30.7 34.6 37.4 52.7
18.94 21.44 23.67 25.25 26.12
76.7 79.4
0.419 0.421 0.418 0.417
15.3 15.6 15.3
1
Average
15.4 f 0.1
TABLE 2 Bromoacetate-thiosuljate reaction i n the absence of added salts Solvent: n-propyl alcohol-water ( D = 30)
mles/nir.
Y
0.001 0.001 0.003 0.003 0.005 0.005
0.0633 0.0633 0.1095 0.1095 0.1414 0.1114
0.419 f 0.002 0.417 f 0.005 0.606 f 0.001 0.605 f 0.002 0.755 f 0.005 0.754 f 0.003
RESULTS
Table 1 contains data typical of the experimental precision obtained in determining the velocity constants for the reactants alone and in the presence of lanthanum nitrate. The bimolecular molar rate constants for three concentrations of the reactants are given in table 2. Column 1 gives the molar concentration of sodium thiosulfate ( a ) and sodium brornoacetate ( b ) ; column 2, the square root of the ionic
432
F.
GI. CIAPEWA AND H.
M. TOMLIXSON
strength; and column 3, the velocity constant in moles per minute. Check runs were made a t each reactant concentration. As shown in table 2, the experimental technique employed in preparing solutions of the reactants gave reproducible results. Concentrations of the reactants lower than 0.001 M could not be studied, owing to the difficulty experienced in determining the starch-iodide end point at higher dilutions in this solvent. At reactant concentrations greater than 0.005 TABLE 3 The efect of sodium and cesium chlorides on the kinetics of the bromoacetale-thiosulfate reaction i n n-propyl alcohol-water solutions ( D = SO) [S*Oa--] = [BrAc-] = 0.005 ADDZD SALT C O N C E m A T I O N
I
I
6
k
Sodium chloride molrs/min.
Y
0.01106 0.02512 0.04649 0.09678 0.12791
0.1762 0.2124 0.2578 0.3418 0.3845
1.02 f 0.01 1.23 f 0.00 1.44 f 0.01 1.83 f 0.01 2.13 f 0.01
0.00255 0.00685 0.01585 0.02708 0.07648
0.1502 0.1638 0.1893 0.2170 0.3106
0.91 f 0.003 1.17 f 0.01 1.55 f 0.01 1.96 f 0.01 2.94 f 0.02
ADDED SALT C O N C E m A T I O N Y
1
6
0.02053
0.04317 0.06979 0.08458 0.13060
I
k wWlrs/min.
,
0.0998
.
0.1483 0.1804 0.2349 0.2860 0.3108 0.3776
0.885 f 0.001 1.11 f 0.01 1.31 f 0.02 1.60 f 0.02 1.64 f 0.01 2.00 0.02
*
KINETICS OF THE BROMOACETATE-THIOSULFATE REACTION
433
In the case of the monovalent cations sodium and cesium, the velocity constant increased slowly with increasing salt concentration. The greater effect of the cesium ion at the same ionic strength is in agreement with the order of electrolyte effects of alkali metal chlorides on the chloroacetate-thiosulfate reaction TABLE 4 The effect of calcium and strontium nitrates on the kinetics of the bromacetate-thiosuljafate reaction i n n-propyl alwholwater solutions ( D = S O ) [SIOS--] = [BrAc-] = 0.005
Calcium nitrate moleslmin.
Y
0.00236 0.00580 0.01452 0.01885 0.02320 0.04352
0.1646 0.1934 0.2521 0.2767 0.2993 0.3880
1.98 3.17 3.82 4.01 3.85 3.45
f 0.03 f 0.05 f 0.03 f 0.05 zt 0.01 f 0.02
Strontium nitrate
0.00204 0.00602 0.01206 0.01917 0.02893 0.03624 0.04826
0.1616 0.1951 0.2370 0.2775 0.3268 0.3588 0.4059
1.62 f 0.02 3.04 f 0.01 3.77 zt 0.03 4.09 0.04 3.82 zt 0.01 3.82 f 0.03 3.92 0.04
* *
[SIOS--] = [BrAc-I = 0.003 Calcium nitrate ADDED BAL1 CONCEN1PATION
4-
0.00293 0.00597 0.01019 0.01451 0.02327 0.03480
I
k molrs/min.
Y
0.1442 0.1730 0.W3 0.2357 0.2860 0.3412
2.79 3.60 3.70 3.89 3.54 3.48
f 0.02 f 0.01 f 0.02 f 0.02
f 0.02 i 0.02
observed by Kappana (9). This investigator found that the electrolyte effect increased in the order of increasing atomic weight of the alkali metal; i.e., Li+
