410
E. B. HUGHES, H. H. G. JELLINEK, AND B. A. AMBROSE
PYRIDINE ULTRAVIOLET ABSORPTION SPECTRUM AND DISSOCIATION CONSTANT E. B. HUGHES, H . H . G . JELLINEK,
AND
(RIIRS.) B. A . AMBROSE
Thc L a b o r a t o r i e s , J . L y o n s and Cornpanu, Lid.,L o n d o n , W.14, E n g l a n d
Received Junuar!j 56, 1948
The ultraviolet absorption spectrum of pyridine was investigated by Spiers and Wibaut (3) in heptane and by Marchleivslti and Wyrobek (2) in aqueous solution. In this work the spectrum of pyridine has been determined at different pH values; this has not been reported so far by other workers. The ultraviolet absorption spectrum of pyridine was measured in 0.1 N sulfuric acid, in 0.1 N sodium hydroxide, and in an acetate buffer at pH 5.13. Figure t, shows the spectra in which the molecular extinctions are plotted against the wave lengths in Angstrom units. The absorption spectrum shows a peak a t 2560 8. ( 6 = 5.32 X lo3) in acid solution; as the pH increases the peak value becomes emaller, until in alkaline solution the molecular extinction is 2.G X lo3 and two additional smaller peaks at 2510 and 2630 8. make their appearance. There are two isobestic points, one in the region of 2G75 8.,the other a t 2350 A. The existence of three peaks was also reported by Marchlewski and IVyrobek (2). At pH 5.13 experiments over a concentration range of 0.612-3.67 X moles per liter have shown that pyridine obeys Beer’s law (figure 2), so that the thermodynamic dissociation constant can be calculated from the expression pK;
=
pK, - pk’; = pH
+ log -.1--a: ff
YA+ YA
where pKi and pK, are the negative logarithms of the thermodynamic dissociation constants for pyridine and water, respectively, a is the degree of dissociation, and yA+and yA are the activity coefficients of the ionic and undissociated forms of pyridine. Further,
where €2 is the molecular extinction coefficient of pyridine a t pH 5.13, and el and e3 are the molecular extinction coefficient,s of the fully dissociated and the undissociated forms of pyridine, respectively. yAis, as usual, taken to have unit value. Log yA+can be evaluated according to the Debye-Huckel equation:
where h = - . 12.303
“9
2DkT
and y
=
J
D3.
NA 103
411
ULTRhVIOLET ABSORPTION SPECTRUM O F P Y R I D I S E
Here ( J is the chaige on the electron (4.803 X lO-'"~.u.),1, is Boltzmann's conD is the dielectric constant of water at 20°C. (80.57), and stant (1.3803 X .VI is A\-ogadro's number ((3.023 X loz3). The value of 9 amounts t o 3.27 X 10'. Further, 1' is the average distance ions can appi,onc.h, talien izs 3 X IO-' 6S
4.(
2.c
I
2300
I
I
2400
2500
h
in
I
I
2LOO
2700
I
2800
i.
FIG.1. Ultraviolet absorption spectrum of pyridine in aqueous solution a t different pH values. I, 0.1 IV sulfuric acid; 11, buffer a t pH 5.13; 111, 0.1 Ai sodium hydroxide.
em., which is a reasonable value (l),z is the valency of the ion, and the concentration of acetate, 0.028 mole per liter. Hence
p
is equal to
The experimental results for a number of wave lengths are given in table 1, whence pK; = 8.88
or
I
