Thermodynamic Study of Glycine in Different TetrahydrofuranWater

18 Thermodynamic Study of Glycine in Different Tetrahydrofuran-Water Mixtures at Several

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Temperatures from 278.15 to 328.15 Κ R. N. ROY, J. J. GIBBONS, J. L. PADRON, K. BUECHTER, and S. FASZHOLZ Department of Chemistry, Drury College, Springfield, MO 65802

The first and second dissociation constants of glycine have been determined by precise emf methods in 10, 30, and 50 mass % tetrahydrofuran-water mixtures at eleven different temperatures (ranging from 278.15 to 328.15 Κ at intervals of 5 K). The thermodynamic quantities (ΔG°, ΔS°, ΔΗ°, and ΔCp°) were derived from the variation of these pK's with temperature. For glycine in 10 mass % THF-H O, pK = 2715.7/Τ - 15.031 + 0.02805T, whereas that for pK = 4622.3/T - 13.521 + 0.02607T. For glycine in 30 mass % THF-H O, pK = 2084.1/T - 10.677 + 0.02126T, while that for pK = 3561.7/T - 6.9091 + 0.01577T. For glycine in 50 mass % THF-H O, pK = 703.6/T - 1.2781 + 0.006203T, whereas that for pK = 3880.8/T - 9.3273 + 0.020258T. The results have been discussed in terms of the solute-solvent interactions and were compared with those in water, as well as in 50 mass % methanol-water and in 50 mass % monoglyme-water. 2

1

2

2

1

2

2

1

2

/ C o n s i d e r a b l e a t t e n t i o n has b e e n d e v o t e d to t h e n a t u r e o f t h e s o l v e n t ^

effects ( as d e t e r m i n e d i n w a t e r a n d i n v a r i o u s m i x e d solvents ) o n t h e

i o n i c dissociations ( a n d r e l a t e d t h e r m o d y n a m i c q u a n t i t i e s )

a n d other

acid-base properties of aliphatic zwitterionic compounds. S u c h investi gations i n c l u d e studies o f t r i c i n e i n 5 0 mass

% methanol-water ( J ) ,

" B e s " i n p u r e w a t e r a n d i n 5 0 mass % m e t h a n o l - w a t e r ( 2 , 3 ) , g l y c i n e i n 50 mass % m o n o g l y m e - w a t e r (4), a n d g l y c i n e i n p u r e w a t e r a n d i n 5 0 mass % m e t h a n o l - w a t e r (5,6,7).

T h e n u m e r o u s factors (8,9,10)

0-8412-0428-4/79/33-177-277$05.25/l © 1979 American Chemical Society Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

which

278

T H E R M O D Y N A M I C BEHAVIOR O F E L E C T R O L Y T E S

d e s c r i b e t h e p r e f e r e n t i a l ( o r selective or specific)

II

solute-solvent inter

actions, a n d are u s e f u l i n a c c o u n t i n g f o r t h e e q u i h b r i u m b e h a v i o r

of

electrolytes i n s o l u t i o n , i n c l u d e t h e h y d r o g e n - b o n d i n g c a p a b i l i t i e s of t h e solute a n d solvent species

a n d t h e d i e l e c t r i c p r o p e r t i e s of

(11)

m e d i u m a n d the ionic charge (12). the p r o p e r t i e s of t h e solvent dielectric constant)

the

T h e changes i n p K as f u n c t i o n s of

( w h i c h a l l o w f o r a w i d e r a n g e i n the

m a y p r o v i d e u s e f u l i n f o r m a t i o n i n r e g a r d to the

i d e n t i f i c a t i o n of the n a t u r e of these s o l u t e - s o l v e n t i n t e r a c t i o n patterns i n b i n a r y solvent systems.

H o w e v e r , d i r e c t m e a s u r e m e n t s of t h i s k i n d of

i n t e r a c t i o n are difficult to p e r f o r m . A s a p a r t of t h e c o n t i n u i n g studies of t h e effect of d i p o l a r a p r o t i c Downloaded by COLUMBIA UNIV on July 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch018

solvent p l u s w a t e r m i x t u r e s o n these specific s o l u t e - s o l v e n t i n t e r a c t i o n s , w e h a v e e x a m i n e d the first a n d s e c o n d d i s s o c i a t i o n steps of g l y c i n e a n d computed

t h e i r associated

tetrahydrofuran-water

thermodynamic

q u a n t i t i e s i n 10 mass

( T H F - H 0 ) solvents

71.8 at 298.15 K ) , 30 mass % T H F - H 0 ( 2

mass % T H F - H 0 (c = 2

€

%

( d i e l e c t r i c constant, c

2

=

— 56.6 at 298.15 K ) , a n d 50

40.0 at 298.15 K ) f r o m 278.15 to 328.15 K .

T h e s e specific m i x e d solvents w e r e c h o s e n f o r this s t u d y , s i n c e solvent systems of this t y p e h a v e b e e n u s e d i n s i m i l a r studies s u c h as the s t u d y of glucose m u t a r o t a t i o n k i n e t i c s a n d e q u i l i b r i a i n 50 mass %

THF-H 0 2

( 1 3 ) , t h e i n v e s t i g a t i o n of i o n i z a t i o n constants of w e a k acids ( s u c h as p h e n o l ) , t h e d e t e r m i n a t i o n of s o l u b i l i t y p r o d u c t constants, K

a p

s o l u b l e ionogens ( s u c h as A g C l ) i n 10, 30, 50, a n d 70 mass %

, of s l i g h t l y THF-H 0 2

(14) , a n d the s t u d y of the s t a n d a r d G i b b s energy of transfer of n - ( B u ) 4

N B r f r o m w a t e r to T H F - H 0 m i x t u r e s ( 1 5 ) . 2

O t h e r studies h a v e i n c l u d e d

t h e d e t e r m i n a t i o n of the d i s s o c i a t i o n constant of m - n i t r o a n i l i n i u m i o n i n v a r i o u s T H F - H 0 m i x t u r e s (16), 2

i n T H F - H 0 m i x t u r e s (17), 2

mass % T H F - H 0 2

t h e s o l v a t i o n s t u d y of K C l a n d K C 1 0

4

a n d the c o n d u c t a n c e s t u d y of C s B r i n 50

(18).

M e a s u r e m e n t s w e r e m a d e of the e m f of cells of the t y p e ( w i t h o u t l i q u i d j u n c t i o n ) : for the first d i s s o c i a t i o n step, Pt;H (g,latm) 2

IGHBrirox),

G ( m ) i n χ mass % T H F - H 0 | A g B r , A g (I) ±

2

2

a n d f o r the s e c o n d d i s s o c i a t i o n stage: P t ; H ( g , l a t m ) | G*(roi), N a G ^ ) , 2

K B r ( m ) i n χ mass % T H F - H 0 | A g B r , A g ( I I ) 3

where χ =

10, 30, a n d 50 mass %

2

T H F - H 0 , a n d m represents the 2

respective molality.

Furter; Thermodynamic Behavior of Electrolytes in Mixed Solvents—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

18.

ROY E T AL.

Experimental

279

Glycine

Procedure

and Preparation

of

Solutions

T h e precise emf m e t h o d o r i g i n a l l y i n t r o d u c e d b y H a r n e d a n d E h l e r s ( 1 9 ) , a n d m o d i f i e d b y Bates a n d R o b i n s o n , w a s u s e d i n the present s t u d y . T h e values of t h e e m f w e r e c o r r e c t e d to a p a r t i a l pressure of one a t m o s p h e r e (101.325 k p ) of h y d r o g e n . a

T h e v a p o r pressure d a t a r e q u i r e d f o r

e m f c o r r e c t i o n of 10, 30, a n d 50 mass % T H F - H 0 at different t e m p e r a 2

tures are g i v e n elsewhere ( 2 0 ) .

T h e c o r r e c t e d e m f d a t a at the r e s p e c t i v e

solvent c o m p o s i t i o n s are g i v e n i n T a b l e s I , I I , a n d I I I ( f o r C e l l I ) , a n d i n Tables I V , V , a n d V I (for C e l l I I ) , respectively.

T h e emf data were

stable, as is e v i d e n t f r o m the d a t a at 298.15 K , w h i c h w e r e r e c o r d e d at


t h e b e g i n n i n g , i n t h e m i d d l e , a n d at t h e e n d of e a c h r u n . T h e m e a n difference b e t w e e n the first a n d the last r e a d i n g s w a s n e v e r m o r e t h a n 0.08 m V . T h e values of the e m f w e r e m e a s u r e d at

five-degree

f r o m 273.15 to 298.15 K , u s i n g a p r e c i s i o n p o t e n t i o m e t e r

intervals

(Leeds

and

N o r t h r u p T y p e K - 5 ) , w h i c h w a s s t a n d a r d i z e d against a n E p p l e y s t a n d a r d c e l l , a n d e q u i p p e d w i t h a L e e d s a n d N o r t h r u p D . C . n u l l detector 9 8 2 9 ) , u s i n g a s e n s i t i v i t y of 25 /*V. T h e t e m p e r a t u r e of the

(model constant

t e m p e r a t u r e b a t h w a s k n o w n to w i t h i n 0.02 K . T h e d e s i g n of the cells, the p r e p a r a t i o n s of t h e s i l v e r + s i l v e r b r o m i d e electrodes (of the t h e r m a l t y p e ) , a n d the h y d r o g e n electrodes, h a v e b e e n described elsewhere (21, 2 2 ) .

T h e bias p o t e n t i a l of the s i l v e r - s i l v e r b r o

m i d e electrodes w a s a l w a y s w i t h i n 0.05 m V . T h e c e l l solutions for the d e t e r m i n a t i o n of the p K i w e r e p r e p a r e d b y d i l u t i o n w i t h T H F - H 0 solvent of a stock s o l u t i o n m a d e b y d i s s o l v i n g 2

g l y c i n e i n a p o r t i o n of t h e h y d r o b r o m i c a c i d s o l u t i o n a n d a d d i n g t h e p r o p e r a m o u n t s of w a t e r a n d p u r e T H F . F o r the d e t e r m i n a t i o n of the p K , stock solutions w e r e p r e p a r e d b y m i x i n g a c c u r a t e l y w e i g h e d p o r t i o n s 2

of g l y c i n e , s o d i u m h y d r o x i d e solutions, a n d the r e q u i r e d amounts of w a t e r a n d T H F . T h e s e stock solutions t h e n w e r e b r o u g h t to t h e p r o p e r m o l a l i ties b y d i l u t i o n w i t h d i s t i l l e d w a t e r a n d T H F . M o l a l i t i e s are e s t i m a t e d to b e a c c u r a t e to w i t h i n 0 . 0 5 % .

Dissolved air was removed 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 t h r o u g h t h e solutions b e f o r e t h e e m f cells filled.

were

V a c u u m corrections w e r e a p p l i e d to a l l w e i g h i n g s . A

c o m m e r c i a l s a m p l e of g l y c i n e

(Sigma Chemical Co.)

crystallized twice from low-conductivity water ( κ =

was

1 Χ 10" o h m ' 6

1

recm"

1

at 298.15 K , as d e t e r m i n e d u s i n g a Y S I M o d e l 31 c o n d u c t i v i t y b r i d g e ) . It assayed at 9 9 . 9 6 %

( s t a n d a r d d e v i a t i o n — 0.12) w h e n t i t r a t e d w i t h a

s t a n d a r d s o l u t i o n of N a O H t o the p h e n o l p h t h a l e i n e n d p o i n t . T h e d e tailed procedure

to c h e c k this p u r i t y has b e e n

Recrystallized K B r (free from contamination b y

given elsewhere chloride)

was

(7). used.

T e t r a h y d r o f u r a n w a s r e f l u x e d over a n h y d r o u s s o d i u m sulfate for 3 days and then vacuum-distilled

(23,24).


280


Table I.

Electromotive Force ( £ ) of the C e l l : P t / H ( g ) / G H B r ( i » i ) , Temperatures ( T ) from 2

mjmol kg'

m /mol kg' 2

1

1

0.097950 0.077198 0.040002 0.068372 0.030001 0.019992 0.009997 Downloaded by COLUMBIA UNIV on July 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch018

Π

Table II.

0.302699 0.324519 0.120139 0.328423 0.089629 0.060077 0.029635

5

10

15

20

0.31383 0.32609 0.33109 0.33121 0.33686 0.34551 0.36129

0.31509 0.32745 0.33257 0.33265 0.33734 0.34820 0.36346

0.31623 0.32881 0.33388 0.33413 0.33936 0.34983 0.36564

0.31722 0.33001 0.33522 0.33555 0.34109 0.35143 0.36767

Electromotive Force ( £ ) of the C e l l : P t / H ( g ) / G H B r ( * * i ) , Temperatures ( T ) from 2

T/K

-

273.15

mj/mol kg'

m /mol kg'

5

10

15

20

0.100010 0.086069 0.040004 0.069656 0.030001 0.019991 0.010000

0.301070 0.350453 0.121078 0.330622 0.091001 0.060511 0.029677

0.32387 0.33403 0.34242 0.34190 0.34847 0.35670 0.37110

0.32433 0.33461 0.34478 0.34275 0.34946 0.35785 0.37250

0.32464 0.33419 0.34405 0.34421 0.34976 0.35951 0.37378

0.32489 0.33483 0.34459 0.34473 0.35060 0.36042 0.37498

1

Table III.

t

1

Electromotive Force ( £ ) of the C e l l : P t / H ( g ) / G H B r ( i » i ) , Temperatures ( Γ ) from 2

Ύ/Κ -

273.15

mt/mol kg'

/ 7 mi/moi, kg'

5

10

15

20

0.098864 0.080142 0.070149 0.060113 0.039814 0.029997 0.020000 0.009998

0.148651 0.171518 0.182134 0.191735 0.119902 0.090539 0.060533 0.030410

0.31565 0.32778 0.33574 0.34333 0.35053 0.35573 0.36359 0.37704

0.31412 0.32662 0.33441 0.34198 0.34996 0.35523 0.36325 0.37695

0.31253 0.32499 0.33308 0.34101 0.34931 0.35458 0.36286 0.37684

0.30995 0.32326 0.33121 0.33974 0.34797 0.35390 0.36233 0.37656

1

1


18.

ROY E T AL.

281

Glycine

G*(m?) i n Water + 1 0 278.15 to 328.15 Κ

Mass Percent T e t r a h y d r o f u r a n / A g B r / A g at


T/K

-

273.15

25

80

85

40

45

50

55

0.31837 0.33130 0.33630 0.33697 0.34268 0.35306 0.36978

0.31922 0.33255 0.33809 0.33841 0.34425 0.35514 0.37192

0.32012 0.33300 0.33882 0.33934 0.34580 0.35521 0.37300

0.32090 0.33487 0.34074 0.34091 0.34731 0.35824 0.37562

0.32190 0.33604 0.34214 0.34222 0.34888 0.36042 0.37756

0.32312 0.33727 0.34367 0.35053 0.36135 0.36343 0.37962

0.32644 0.34067 0.34539 0.34743 0.35272 0.36309 0.38221

G (m ) i n Water + 278.15 to 328.15 Κ ±

2

30 Mass Percent T e t r a h y d r o f u r a n / A g B r / A g at T/K

-

273.15

25

30

85

40

45

50

55

0.32575 0.33675 0.34516 0.34517 0.35123 0.36040 0.37592

0.32547 0.33676 0.34560 0.34586 0.35204 0.36185 0.37710

0.32633 0.33772 0.34635 0.34680 0.35295 0.36262 0.37783

0.32650 0.33916 0.34690 0.34711 0.35362 0.36327 0.37886

0.32606 0.33946 0.34681 0.34749 0.35479 0.36391 0.37998

0.32651 0.33980 0.34709 0.34772 0.35581 0.36430 0.38059

0.32476 0.33795 0.34704 0.34733 0.35504 0.36442 0.38131

G*(m ) i n Water + 5 0 278.15 to 328.15 Κ 2

Mass Percent T e t r a h y d r o f u r a n / A g B r / A g at T/K

-

273.15

25

80

85

40

45

50

55

0.30747 0.32194 0.33096 0.33917 0.34713 0.35296 0.36169 0.37617

0.30764 0.32109 0.32983 0.33839 0.34651 0.35267 0.36123 0.37606

0.30651 0.32038 0.32895 0.33770 0.34567 0.35185 0.36057 0.37575

0.30476 0.31874 0.32732 0.33630 0.34489 0.35117 0.36023 0.37548

0.30203 0.31627 0.32529 0.33466 0.34379 0.35040 0.35945 0.37505

0.29846 0.31383 0.32312 0.33283 0.34212 0.34935 0.35840 0.37485

0.29607 0.31102 0.33096 0.33978 0.34218 0.34004 0.34753 0.37401


282


Table I V .

Electromotive Force (E)

II

of the C e l l : P t / H ( g ) / G ( i » i ) , ±

2

A g B r / A g at Temperatures τα,/mol kg'

ra /mol kg'

m /mol kg'

0.100440 0.081639 0.069554 0.060209 0.031846 0.019601 0.010286

0.099993 0.079977 0.069978 0.059933 0.029965 0.019925 0.009993

0.100250 0.079431 0.068937 0.060679 0.030237 0.019662 0.011249

t


1

Table V .

1

m /mol kg'

0.099301 0.079570 0.070927 0.060574 0.040763 0.031108 0.022737 0.011232

0.099977 0.080009 0.069994 0.059995 0.039976 0.029932 0.019997 0.009282

Table V I .

0.70652 0.71100 0.71501 0.71731 0.73103 0.74291 0.75455

20

l5~

0.70731 0.71195 0.71599 0.71838 0.73250 0.74453 0.75638

Ύ/Κ-

uis/mol kg'

t

1

0.70809 0.71282 0.71679 0.71935 0.73378 0.74601 0.75807

0.70872 0.71350 0.71749 0.72012 0.73482 0.74729 0.75955

0.101190 0.079156 0.070684 0.060940 0.041685 0.031125 0.020702 0.009836

0.70447 0.70891 0.71037 0.71355 0.72099 0.72660 0.73420 0.74927

Electromotive Force (E)

_7 m / / moi kg'

0.10810 0.07232 0.06850 0.04090 0.03285 0.02432 0.00935 0.00437

0.10020 0.07978 0.07001 0.06000 0.05000 0.04000 0.03000 0.01000

2

1

273.15

10

15

20

0.70466 0.70900 0.71070 0.71391 0.72158 0.72722 0.73495 0.75046

0.70467 0.70911 0.71090 0.71411 0.72211 0.72768 0.73556 0.75144

0.70460 0.70906 0.71099 0.71420 0.72240 0.72792 0.73598 0.75224

1

vcit/mol kg 1

To

~6

Electromotive Force (E) of the C e l l : P t / H s C g V G ' O w i ) , A g B r / A g at Temperatures

ταχ/mol kg1

Ύ/Κ - 278.15

s

1

of the C e l l : P t / H 2 ( g ) / G ( f » i ) , A g B r / A g at Temperatures

Ύ/Κ - 273.15

/ 7 kg-

5

10

15

0.10200 0.08058 0.07464 0.04860 0.04112 0.02999 0.02190 0.01006

O.7O082 0.70675 0.70496 0.72055 0.72370 0.73132 0.75340 0.76091

0.70034 0.70602 0.70436 0.72055 0.72340 0.73096 0.75359 0.76123

0.69909 0.70518 0.70359 0.71983 0.72313 0.73083 0.75363 0.76181

1


18.

ROY E T AL.

283

Glycine

N a G ( f » 2 ) , K B r ( i » ) in Water + ( Γ ) from 278.15 to 328.15 Κ

10 Mass Percent T e t r a h y d r o f u r a n /

3


T/K

-

278.15

25

30

35

40

45

50

55

0.70927 0.71422 0.71821 0.72099 0.73600 0.74863 0.76116

0.70964 0.71472 0.71880 0.72167 0.73693 0.74980 0.76256

0.70996 0.71520 0.71928 0.72226 0.73786 0.75086 0.76396

0.71027 0.71570 0.71968 0.72286 0.73873 0.75192 0.76533

0 .71061 0 .71621 0.,72029 0 .72345 0,.73964 0 .75307 0 .76684

0.71084 0.71684 0.72106 0.72409 0.74055 0.75442 0.76875

0.71767 0.72277 0.72526 0.74186 0.75689 0.77074

N a G ( w 2 ) , K B r ( * » ) i n Water + ( T ) from 278.15 to 328.15 Κ


3

T/K

-

273.15

25

80

35

40

45

50

55

0.70449 0.70878 0.71096 0.71411 0.72257 0.72827 0.73598 0.75246

0.70413 0.70897 0.71093 0.71436 0.72279 0.72904 0.73746 0.75354

0.70370 0.70864 0.71068 0.71421 0.72273 0.72917 0.73705 0.75387

0.70323 0.70822 0.71030 0.71404 0.72271 0.72920 0.73720 0.75406

0.70242 0.70755 0.70990 0.71269 0.72228 0.72907 0.73712 0.75424

0.70154 0.70663 0.70944 0.71222 0.72208 0.72884 0.73717 0.75452

0.70548 0.70789 0.71141 0.72130 0.72830 0.73703 0.75561

N a G ( » * 2 ) , K B r ( i » ) i n Water + ( Γ ) from 278.15 to 323.15 Κ


3

Ύ/Κ 20

25

0.69769 0.70368 0.70199 0.71833 0.72161 0.72933 0.75253 0.76045

0.69707 0.70271 0.70091 0.71701 0.72027 0.72805 0.75119 0.75883

.30 0.69467 Ο.70Ο61 0.69888 0.71571 0.71911 0.72652 0.74999 0.75809

273.15

85

40

45

50

0.69422 0.70035 0.69865 0.71504 0.71909 0.72742 0.75100 0.75913

0.69360 0.69959 0.69796 0.71499 0.71845 0.72656 0.75051 0.75857

0.69412 0.69981 0.69813 0.71525 0.71872 0.72679 0.75071 0.75885

0.69627 0.70159 0.69945 0.71605 0.71932 0.72693 0.75157 0.75884


284


Methods

and

II

Results

G l y c i n e is n o r m a l l y w r i t t e n i n the f o r m N H - C H - C O O ~ . 3

I f this

2

z w i t t e r i o n is r e p r e s e n t e d b y G , t h e n t h e first d i s s o c i a t i o n process is t h e ±

d e p r o t o n a t i o n of the c a r b o x y l a t e g r o u p of the species H G : +

Ki HG

*± H

+

+

+ G*

(1)

w h i l e the s e c o n d d i s s o c i a t i o n step is the e q u i h b r i u m g i v e n b y K τ±


2

G w h e r e Κχ a n d K

2

±

H

+ G"

+

(2)

are the t h e r m o d y n a m i c e q u i l i b r i u m constants, w h i c h

w e r e d e t e r m i n e d i n 10, 30, a n d 50 mass % T H F - H 0 at e l e v e n t e m p e r a 2

tures f r o m 278.15 to 328.15 K . T h e p K i w a s d e t e r m i n e d b y e x t r a p o l a t i n g the v a l u e s of the p K / function (the apparent dissociation constant)

to i n f i n i t e d i l u t i o n (i.e.,

zero i o n i c s t r e n g t h , I ). T h e f o l l o w i n g t w o equations are i m p o r t a n t :

„ p

K

Ε — E° l

=

=

m + ^Br-

THG+yBr-

HG

( R T l n l O ) / F

+

I

°

g

+

1

°

V

g

( 3 )

and π , ΎΓ ρΚχ - p K i - ^ -

Ε — E°

o T

(

R

r

i

n

l

0

)

/

F

+log

m {m 1

(

2

m ')

-

1

m

+

H

m

s

0

-

1 +

2A7* BaJ* (4)

E q u a t i o n 3 w a s o b t a i n e d b y c o m b i n i n g the N e r n s t e q u a t i o n f o r the emf of C e l l I w i t h the e q u i l i b r i u m constant of the a c i d i c d i s s o c i a t i o n

of

g l y c i n e . I n E q u a t i o n s 2 a n d 3, E° is t h e s t a n d a r d e m f of t h e c e l l i n t h e r e s p e c t i v e solvent c o m p o s i t i o n a n d these values w e r e o b t a i n e d f r o m a n earlier w o r k (20).

I n E q u a t i o n 4, β is the l i n e a r slope p a r a m e t e r f o r the

p l o t of p K / vs. 7, a

0

H u c k e l constants

is the i o n - s i z e p a r a m e t e r , A a n d Β are t h e

o n the m o l a l scale

(20)

for

Debye-

the respective

mixed

s o l v e n t systems, a n d ί is t h e i o n i c s t r e n g t h g i v e n b y m i . B e c a u s e of t h e a p p r e c i a b l e d i s s o c i a t i o n of the species, H G , at l o w +

p H values of t h e buffer solutions, m G H

+

=

— ra ', a n d m ± =

m

1

H

G

m

2

+

m ' . This apparent hydrogen i o n molality, m ' , was estimated according H

H

to t h e e q u a t i o n ,


18.

ROY E T AL.

285

Glycine 2£°

β -

l 0 g

W

h

' -

2Α7*

(RrinlO)/F +

l 0 g

m

"

i

T + B ^ F

( 5 )

T h e least-squares estimate of the i n t e r c e p t for t h e l i n e a r regression of p K / vs. I f r o m E q u a t i o n 4 ( u s i n g the p r o p e r v a l u e of a )

was termed the

0

a c t u a l p K i , the values of w h i c h are p r e s e n t e d i n T a b l e s V I I , V I I I , a n d I X , for 10, 30, a n d 50 mass % T H F - H 0 , r e s p e c t i v e l y . 2

the best l i n e a r fit of t h e d a t a , several values of a

0

I n o r d e r to o b t a i n

( f r o m 0.1 to 0.8 n m )

w e r e i n s e r t e d i n t o this e q u a t i o n . T h e true v a l u e s of p K i w e r e those w h i c h gave the smallest s t a n d a r d d e v i a t i o n of regression. T h e s e e x p e r i m e n t a l values of p K i w e r e fitted i n t o a n e q u a t i o n selected Downloaded by COLUMBIA UNIV on July 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0177.ch018

b y H a r n e d a n d R o b i n s o n ( 2 5 ) , b y the least-squares m e t h o d , w i t h the f o l l o w i n g results: p K i ( i n 10 mass % T H F - H 0 ) = 2

2715.7/7

7

-

15.031 + 0.02805Γ

(6)

10.677 + 0.02126Γ

(7)

1.2781 + 0.006203Γ

(8)

w h e r e Τ is the t h e r m o d y n a m i c t e m p e r a t u r e i n K . p K i ( i n 30 mass % T H F - H 0 ) = 2

2084.1/T -

and p K i ( i n 50 mass % T H F - H 0 ) — 7 0 3 . 6 / Γ 2

I n o r d e r to get a satisfactory fit of the d a t a , i t was necessary to use d o u b l e p r e c i s i o n a r i t h m e t i c (16 d i g i t s ) . T h e average deviations of t h e fit f o r E q u a t i o n s 6, 7, a n d 8 w e r e 0.007, 0.003, a n d 0.003 p K u n i t s , r e s p e c t i v e l y . T h e s e c o n d d i s s o c i a t i o n constant, p K

( w h i c h involves the removal

2

of the p r o t o n f r o m the n i t r o g e n a t o m i n t h e process G * τ± H

+

+

G"),

c a n b e o b t a i n e d b y e x t r a p o l a t i o n of p K ' values ( t h e a p p a r e n t

second

2

d i s s o c i a t i o n constants) to zero i o n i c strength I, w h e r e I =

+ m .

m

2

3

The

p e r t i n e n t e q u a t i o n is

p

K

2

= ( R T l n l O ) / F +

l

o

g

(m -moH-')

+

2

1

°

g

_

(

9

)

I n E q u a t i o n 9, y ± is a s s u m e d to b e e q u a l to u n i t y ; hence, t h e last t e r m is G

d i r e c t l y p r o p o r t i o n a l to the i o n i c s t r e n g t h I . S i n c e t h e a n i o n G " undergoes h y d r o l y s i s at h i g h p H values, t h e h y d r o x y l i o n c o n c e n t r a t i o n , m H - ' , w h i c h 0

is v e r y s m a l l i n c o m p a r i s o n w i t h πΐχ a n d m , c a n b e e v a l u a t e d w i t h suffi 2

c i e n t a c c u r a c y b y the e q u a t i o n — l o g ra H-' = p K 0

w

+ log m y H

H


(10)

286


T h e c o n t r i b u t i o n o f mon-' to t h e v a l u e o f p K

Π

( f o r e x a m p l e , a t 298.15 K )

2

lies w i t h i n 0.0005 p K u n i t , w h i c h is w e l l w i t h i n t h e e x p e r i m e n t a l error. T h u s , t h e s i m p l i f i e d v e r s i o n of E q u a t i o n 9 ( i g n o r i n g these n e g l i g i b l e corrections) was used:

j7- , p K z

— E° , m^g =(imnio)/F+ -^T Ε

.

.

l o g

T h e values of p K l i s t e d i n T a b l e s V I I , V I I I , a n d I X w e r e o b t a i n e d f r o m 2

the l i n e a r p l o t s of p K ' vs. I ( w h e r e I = m + m ) , w h e n e x t r a p o l a t e d to 2

2

3

zero. T h e v a r i a t i o n i n t h e o b s e r v e d values o f t h e p K w i t h t e m p e r a t u r e 2


can be w e l l represented b y H a r n e d a n d Robinson's equation ( 2 5 ) . T h e final forms r e s u l t i n g f r o m these e q u a t i o n s a r e : pK

2

( i n 10 mass % T H F - H 0 ) = 4 6 2 2 . 3 / Γ -

13.521 + 0.02607T

(12)

pK

2

( i n 30 mass % T H F - H 0 ) = 3 5 6 1 . 7 / Γ -

6.9091 + 0.01577Γ

(13)

pK

2

( i n 50 mass % T H F - H 0 ) = 3 8 8 0 . 8 / Γ -

9.3273 + 0.020258Γ (14)

2

2

2

The

average d e v i a t i o n s b e t w e e n t h e e x p e r i m e n t a l v a l u e s a n d those

c a l c u l a t e d f r o m E q u a t i o n s 12, 13, a n d 14 are 0.008, 0.003, a n d 0.007, respectively. T h e t h e r m o d y n a m i c f u n c t i o n s f o r t h e first a n d s e c o n d i o n i z a t i o n processes w e r e e v a l u a t e d f r o m E q u a t i o n s 6, 7, a n d 8 ( f o r p K i ) a n d 12, 13, a n d 14 ( f o r p K ) i n 10, 30, a n d 50 mass % T H F - H 0 , r e s p e c t i v e l y , 2

2

u s i n g t h e c u s t o m a r y t h e r m o d y n a m i c f o r m u l a e . T h e values o f t h e s t a n d a r d G i b b s energy ( A G ° ) , enthalpy ( Δ Η ° ) , entropy ( A S ) , a n d heat capacity 0

Table V I I . T h e p K i , ρ Κ , σ, and a of Protonated Glycine ( G H G ) in 10 Mass Percent T e t r a h y d r o f u r a n - W a t e r from 278.15 to 328.15 Κ on the Molal Scale 0

2

+

and

±

T/K — 273.15 5 10 15 20 25 30 35 40 45 50 55

pKi (obs)

pKj (calc)

(pK )

2.527 2.502 2.481 2.463 2.447 2.440 2.411 2.423 2.424 2.429 2.462

2.535 2.503 2.476 2.456 2.441 2.431 2.426 2.425 2.429 2.437 2.450

0.002 0.008 0.006 0.005 0.005 0.005 0.002 0.004 0.006 0.008 0.006

σ

pK (obs)

pK (calc)

10.338 10.186 10.038 9.895 9.762 9.634 9.512 9.396 9.288 9.195 9.138

10.348 10.185 10.032 9.889 9.755 9.629 9.512 9.403 9.302 9.207 9.119

2

t

Ά /ΠΜ 0

0.25 1.50 1.50 1.50 1.50 1.50 0.50 0.25 0.50 0.25 1.50

2


σ

(pK ) 2

0.003 0.003 0.003 0.002 0.002 0.002 0.002 0.002 0.002 0.003 0.005

18.

BOY E T AL.

287

Glycine

Table V I I I . T h e p K i , p K , σ, and a of Protonated Glycine ( G H * and G*) i n 30 Mass Percent Tetrahydrofuran—Water from 278.15 to 318.15 Κ on the Molal Scale 0

2


T/K-

pK

pK

278.15

(obs)

(calc)

(pKJ

a /nm

5 10 15 20 25 30 35 40 45

2.723 2.705 2.681 2.663 2.651 2.641 2.632 2.629 2.640

2.727 2.701 2.680 2.663 2.650 2.641 2.635 2.634 2.635

0.002 0.008 0.008 0.007 0.002 0.003 0.004 0.006 0.008

0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

t

t

pK

σ

pK,

2

0

σ

(obs)

(calc)

(pKJ

10.282 10.136 9.996 9.860 9.731 9.628 9.510 9.402 9.301

10.282 10.135 9.995 9.863 9.738 9.620 9.508 9.402 9.303

0.003 0.003 0.003 0.003 0.003 0.004 0.002 0.004 0.004

Table I X . T h e pK p K ,

Thermodynamic Study of Glycine in Different TetrahydrofuranWater

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