(Monoglyme) from EMF Measurements - ACS Publications

13 Thermodynamic Study of Hydrobromic Acid in Water-1,2-Dimethoxyethane (Monoglyme) from E M F Measurements

Downloaded by UNIV OF ARIZONA on March 11, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0155.ch013

R. N. ROY, E. E. SWENSSON, G. LaCROSS, Jr., and C. W. KRUEGER Department of Chemistry, Drury College, Springfield, Mo. 65802

Electromotive force measurements of cells of the type: Pt; H (g, 1 atm) \ HBr(m) in Monoglyme + H O \ AgBr-Ag 2

2

at eleven different temperatures ranging from 278.15° to 328.15°K at intervals of 5°Κ were utilized to evaluate (a) the standard electrode potentials (E°)of the Ag-AgBr electrode in x = 10, 30, and 50 mass percent monoglyme, (b) the mean molal activity coefficients of hydrobromic acid for concentra tions ranging from 0.005-0.09 mol kg , (c) the relative partial molal enthalpy and heat capacity of HBr in x = 50, and (d) the thermodynamic functions (i.e., ∆G , ∆H , and ∆S ) for the transfer of one mole of HBr from the standard state in water to the standard state in x = 10,30, and 50 mass percent mono glyme. The standard emf was evaluated by using the extend ed terms of the Debye-Hückel theory with an ion-size parame ter of 0.6 nm. The dielectric constants for x = 10, 30, 50, 70, 90, and 100 mass percent monoglyme were measured at the temperatures under investigation. The significance of the re sults has been discussed in terms of ion-solvent interactions. -1

0

0

t

0

t

t

T h e behavior of electrolytes i n aqueous organic mixtures, p a r t i c u l a r l y those consisting of dipolar aprotic solvents (1,2,3,4,5,6) has long been a subject of considerable importance. Interest i n dipolar aprotic solvent-water mixtures arises, i n part, f r o m the recent studies of t e t r a h y d r o f u r a n - w a t e r mixtures (7), w h i c h i n v o l v e d ion-solvation a n d proton b o n d i n g . Because of the scarcity of 220

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

13.

ROY E T A L .

221

Hydrobromic Acid

experimental t h e r m o d y n a m i c data concerning the effects of m e d i u m changes on the t h e r m o d y n a m i c properties of h y d r o b r o m i c a c i d , three different compositions of the b i n a r y m o n o g l y m e - w a t e r mixtures were chosen, w h i c h contained 10, 30, a n d 50 mass percent m o n o g l y m e , respectively. O t h e r solvent systems of this type are sometimes used i n the investigation of the acid-base properties of compounds w h i c h are slightly soluble i n water, i n the spectrophotometric determination of the dissociation constant, p K , of m - n i t r o a n i l i n e (8), aniline, and substituted anilines (9), a n d i n studies of c h e m i c a l kinetics. T o meet these requirements, the standard electrode potential of the silver + silver b r o m i d e electrode must be k n o w n . T h e silver + silver b r o m i d e electrode is h i g h l y reproducible (JO) and, because of its low solubility, it performs better than the silver + silver chloride electrode, particularly for the determination of the dissociation constants of the nitrogen bases ( I I ) .


a

Measurements of the emf of cells of the type: Pt; H ( g , 1 atm) |HBr(m) i n m o n o g l y m e + H 0 | A g B r - A g 2

2

(I)

were made at eleven temperatures extending f r o m 278.15° to 3 3 3 . 1 5 ° K and for ten molalities of h y d r o b r o m i c a c i d i n the n o m i m a l range f r o m 0.01 to 0.1 m o l kg"

1

i n 10, 30, a n d 50 mass percent m o n o g l y m e . T h e standard e m f for cells of

type I was evaluated b y the use of the extended terms (12) of the D e b y e - H u c k e l theory.

This method of extrapolation is essentially the same as that recently used

by Roy, Robinson, and Bates (10) and Roy, Swensson, and LaCross (7).

Activity

coefficients and the relative partial molal enthalpy and heat capacity of H B r have been d e r i v e d .

T h e present investigation was undertaken w i t h the intent of

evaluating the standard electrode potential of the A g - A g B r electrode, w h i c h w i l l p e r m i t the calculation of the dissociation constant of glycine i n 50 mass percent m o n o g l y m e , and w i l l allow the determination of the standard t h e r m o d y n a m i c functions (Gibbs energy, enthalpy, a n d entropy) for the transfer of one mole of H B r f r o m the aqueous standard state to the standard state i n the m i x e d solvent. T h e densities a n d the vapor pressures for x = 10, 30, a n d 50 mass percent monoglyme, and the dielectric constants for x = 1 0 , 3 0 , 5 0 , 7 0 , 9 0 , and 100 percent m o n o g l y m e , were measured a n d are herein reported. Similar emf measurements on h y d r o c h l o r i c a c i d solutions i n m o n o g l y m e H 0 mixtures containing 8.68,17.81, 46.52, and 67.03 mass percent monoglyme at 2 7 8 . 1 5 ° , 2 8 8 . 1 5 ° , 2 9 8 . 1 5 ° , a n d 3 0 8 . 1 5 ° K were reported earlier (I), a n d are shown here for the purposes of comparison of both strong electrolytes. T h e results of the present study are discussed i n terms of h y d r o g e n b o n d i n g , as w e l l as preferential solvation of the two solutes (hydrogen ion a n d b r o m i d e ion) b y the molecules of the two solvent species, water a n d m o n o g l y m e . M o r e o v e r , the results are c o m p a r e d w i t h the similar parallel data (I) for h y d r o c h l o r i c a c i d i n 50 mass percent m o n o g l y m e . 2


222

THERMODYNAMIC BEHAVIOR

O F ELECTROLYTES

Experimental Procedure and Preparation of Solutions T h e stock solution, ca. 0.3 m o l d m , of h y d r o b r o m i c a c i d was prepared f r o m a twice-distilled sample of the hydrobromic acid. Its b r o m i d e content was determined gravimetrically as silver bromide. Triplicate runs agreed to w i t h i n ± 0 . 0 2 % . The silver + silver bromide electrode was of the thermal type, prepared by heating twice recrystallized silver bromate (10 mass percent) and silver oxide (90 mass percent) at a temperature of 820° K . T h e preparation of the silver oxide, the preparation of the hydrogen electrodes, the design of the cell, the purification of the hydrogen gas, a n d other experimental techniques, have been described earlier (13,14,15). T h e water bath i n w h i c h the cells were i m m e r s e d was controlled to w i t h i n 0.02°K.


- 3

M o n o g l y m e (Fisher C e r t i f i e d ) was p u r i f i e d i n the m a n n e r described b y W a l l a c e and Mathews (16). T h e m i d d l e fraction of the second distillate was subsequently used i n the preparation of the cell solutions. D r y nitrogen was bubbled through the distillation flask d u r i n g the distillation process. T h e purity of the middle fractions was verified by gas chromatography. T h e m i x e d solvents were prepared by weight dilution methods by diluting the aqueous stock solution of H B r with a k n o w n amount of monoglyme and doubly distilled water. V a c u u m corrections were applied to all weighings, and weight burets were employed when necessary. Dissolved air was always r e m o v e d b y b u b b l i n g p u r i f i e d h y d r o g e n gas into the solutions before the cells, w h i c h were fitted w i t h triple saturators, were filled. T h e molality of the a c i d i n a l l the solutions reported is correct to w i t h i n ± 0 . 0 3 % and the monoglyme content of the solutions is accurate to w i t h i n ±0.02%. Total vapor pressures (p) for the m i x e d solvents over the temperature range 2 7 8 . 1 5 ° - 3 2 8 . 1 5 ° K are essential i n order to correct the e m f data to a h y d r o g e n partial pressure of one atmosphere. T h e b o i l i n g temperature m e t h o d ( 1 , 1 7 ) , w h i c h utilizes a ballast bulb, a closed-tube manometer, a n d a triple-neck distillation flask, was employed, and the measurements of the vapor pressures for the m i x e d solvents under investigation were m a d e i n the temperature range of 308°-350°K. Extrapolation a n d interpolation procedures were based o n the linear plots of logio p as a f u n c t i o n of 1/T, where T is the t h e r m o d y n a m i c temperature. As a further verification, the vapor pressures of 50 mass percent m e t h a n o l - w a t e r mixtures were measured a n d were f o u n d to be i n good agreement w i t h the literature data (JO). T h e densities p° of the m i x e d solvents r e q u i r e d to calculate the parameters A and B of the D e b y e - H u c k e l equation were measured w i t h a pycnometer of about 2 5 - c m capacity. D u p l i c a t e determinations were always made, a n d the values agreed to w i t h i n ± 0 . 0 0 5 % . 3

T h e dielectric constants e, w h i c h are also needed to evaluate the D e b y e H u c k e l constants A and B, were measured f r o m 283.15° to 3 1 8 . 1 5 ° K at intervals of 5 ° K , using spectrograde acetone and freshly prepared, doubly distilled water as reference materials. T h e J a n z - M c l n t y r e bridge (18) w i t h Balsbaugh con-


13.

223

Hydrobromic Acid

ROY E T AL.

d u c t i v i t y cell (model 2 T N 5 0 ) h a v i n g a cell constant of 0.00177 c m " was used. T h e temperature of the bath containing the cells was regulated to w i t h i n 0.02° K . T h e values of 6 over the temperature range under investigation were obtained by the method of extrapolation and interpolation, w h i c h utilized the straight-line plots of logio e vs. 1/T. T h e values of the p, p°, e, A , and B are presented i n Tables I V , V , a n d V I for 10, 30, and 50 mass percent monoglyme, respectively, whereas 1


for x = 70, 90, a n d 100, the data for the dielectric constant are i n c l u d e d i n Table X . T h e data for the dielectric constant at 2 9 8 . 1 5 ° K are plotted i n F i g u r e 1 as a f u n c t i o n of the mass percent of m o n o g l y m e .

IVV

Q 20

o

20

40

60

80

Mass % Monoglyme

100

Figure 1. Plot of dielectric constant vs. mass percent monoglyme for 0,10, SO, 50, 70, 90, and 100 mass percent monoglyme-water mixtures at 298.15°K

T h e values of the e m f were corrected to a partial h y d r o g e n pressure of one atmosphere (101.325 k p ) a n d are g i v e n i n Tables I, II, a n d H I (see A p p e n d i x for all tables) for the respective solvent compositions. E a c h value i n the table represents the average of two duplicate cells, prepared f o r each m o l a l i t y of h y drobromic acid. Typically, the lowest and highest emf values for these duplicate cells d i f f e r e d b y no more than 0.14 m V . T h e e m f of the cells was always measured first at 2 9 8 . 1 5 ° K , then at descending temperatures to 2 7 8 . 1 5 ° K , again at 2 9 8 . 1 5 ° K , f i n a l l y ascending to 3 2 8 . 1 5 ° K , a n d e n d i n g w i t h a measurement at 298.15° K once more. T h e emf data are reliable a n d stable, as evident f r o m the data at 2 9 8 . 1 5 ° K , w h i c h were recorded a total of three different times; namely, at the start, the m i d d l e , a n d the e n d of each temperature r u n . O n the average, these three values agreed to w i t h i n 0.05 m V . T h e bias potential of the s i l v e r silver bromide electrode was always w i t h i n 0.05 m V . Measurements of the emf a

were made b y means of a Leeds a n d N o r t h r u p type K - 3 potentiometer, standardized against a n E p p l e y standard cell, a n d e q u i p p e d w i t h a Leeds a n d N o r t h r u p D . C . n u l l detector (model 9829), using a sensitivity of 25 uV. T h e vapor pressures of the solutions were assumed to be the same as those for the pure solvents at the experimental temperatures. As a f i n a l check on the r e l i a b i l i t y of the data a n d the performance of the cells, the emf's were measured i n 0.01 m o l - k g aqueous H B r solutions at _ 1


224

THERMODYNAMIC

298.15°K.

BEHAVIOR O F E L E C T R O L Y T E S

T h e pressure-corrected emf data is 0.31276 V , w h i c h is i n satisfactory

accord w i t h the literature value (9,11) of 0.31272 V .

Results T h e standard e m f E ° of the cell was d e t e r m i n e d b y means of a n extrapolation technique i n v o l v i n g a f u n c t i o n of the measured e m f E ( w h i c h was measured experimentally), taken to the l i m i t of zero ionic strength 7.

A linear

f u n c t i o n of I was observed w h e n the D e b y e - H u c k e l equation (in its extended form) (12) was i n t r o d u c e d for the activity coefficient of h y d r o b r o m i c a c i d over


the experimental range of molalities m .

W i t h this type of m a t h e m a t i c a l treat-

ment, the adjustable parameter became a , the ion-size parameter, a n d a slope 0

factor /J.

T h i s procedure is essentially the same as that used i n our earlier de-

terminations (7,10) although no corrections of E ° for i o n association were taken into account (e = 49.5 at 2 9 8 . 1 5 ° K ) . F r o m the Nernst equation for cells of type I, it is obvious that E ° = E + (2RT/F) in which m ° = 1 m o l - k g . - 1

log (m ±/m0) e

(1)

7

F u r t h e r m o r e , the extended terms i n d i c a t e d b y D

(12) can be w r i t t e n i n terms of — logio y±. - logio 7 ± = A m / / ( l + Ba°m ' ) 1

2

-

1 2

f3m/m°

+ logio (1 + 0.002m < M » - D / l o g 10 e

(2)

i n w h i c h A and B are the D e b y e - H u c k e l parameters, converted to a molality basis b y m u l t i p l i c a t i o n b y the square root of the solvent density.

T h e values of these

constants, w h i c h are listed i n Tables III, I V , a n d V , are functions of the thermod y n a m i c temperature T a n d the dielectric constant e, as w e l l as of the solvent density p°.

T h e m e a n m o l a r masses ( M ) f o r x = 10, 30, a n d 5 0 mass percent

m o n o g l y m e are 0.01958, 0.02370, a n d 0.03003 k g - m o l " , respectively.

The

1

contribution arising f r o m the extended terms towards the value of —logio 7 ± i n the case of a 1-1 electrolyte m a y be g i v e n b y = 1 0 - V / s W + 10-

(Monoglyme) from EMF Measurements - ACS Publications

Recommend Documents