THE R E L I A B I L I T Y O F THE DISSOCIATION CONS T A N T AS A MEANS OF DETERMINING THE I D E N T I T Y A N D P U R I T Y O F ORGANIC COMPOUNDS
-BY HEYWARD SCUDDER
Since the publication of the work of Ostwald’ on the dissociation constants2 of organic acids it has been assumed that this constant furnishes a reliable guide to the identity, strength and purity of organic compounds. Ostwalds states that the constant gives a satisfactory guide to the purity of analytically pure compounds showing, by a steady diminution of the value found at different dilutions, the presence of isomers or of acids of similar composition which could not be detected by analysis. Nernst4 repeats practically the same idea in a note on the experimental work of Wakemans on isohydric solutions, stating that in the case of a mixture of two acids. the variation of the constant at different dilutions gives, as Ostwald had previously pointed out, a criterion of purity. T h e assumption that it is a suitable means of identification is not explicitly stated in text-books but will become obvious if the articles relating to conductivity measurements are studied. Unfortunately a study of the literature has shown that the dissociation constant is of no greater reliability than the other physical constants. It is an additional and welcome aid, but can only be used as an aid and not as a crucial test for determining either identity or purity. Note.-Since almost all the references to Ostwald’s work are taken from one series of articles, unless otherwise stated they will be found in Vol. 3 of Zeit. phys. Chem. and only the page number will be given. Zeit. phys. Chem. 3, I73 (18891. The dissociation constant is also called the affinity constant. ’ 1. C . 173, Theoret. Chemie (3 Aufl.) 473 (1900). Zeit. phys. Chem. 15, 162 (1894).
2 70
Heywasd Sczcdder
T h e object of this paper is to point out the causes of the errors that may affect the value of the constant, to draw conclusions as to its accuracy as a means of identification and to show that it is not reliable as a guide to purity. T h e following symbols for the Ostwald formula will be used; K = rook = mz . m =- k~. room = percent dissociated. (I--m)ZJ
11.43
‘u = volume in liters. conductivity. T h e temperature a t which the measurements quoted were made is 25’ unless otherwise stated. T h e solvent used in all cases was water. For the sake of brevity complete tables will not always be given. T h e condensed forms will readily be understood. N o essential data will be omitted. T h e expression limits of K ” means the maximum and minimum values found for K at the different dilutions measured. It does not indicate that the variations were regular, giving a steady increase or decrease of K with increasing dilution.
p = molar
((
Causes of error Measurement of conductivity.- OstwaldI states that on account of the difficulty of making accurate measurements, and since an error of one percent in the conductivity makes a n error of at least two percent i n the value of K, there is an allowable error of five percent in the value of K. All subsequent observers have accepted this as the allowable error. VaZue of pm.-Until within a few years the molar conductivities were expressed in terms of the Siemens unit. A t the present time they are frequently expressed in reciprocal ohms. It is apparent that this change will not affect the value of K, for m simply expresses the ratio & . P43
Since C
L equals ~ the
sum of the equivalent conductivities of the anion and kation a change in the equivalent conductivity of the hydrogen or hydroxyl ion will affect the value of K only when the equivalent conductivity of the other ion (kation or anion as the case may
Identity and P u d y of OYganic Comjounds
271
be) remains the same. For instance, Ostwald in his measurement of the dissociation constant of acetic acid used different from those given later by him,I but values for H and C2H301 since remains the same the constant is not changed. H.
1. c. 2.
+
C,H,O,
.
Pffl
K.
o.oor8o 320.5 43.5 = 364 0.00180 325 38.4 = 363 Lehrb. d. allgemeinen Chemie vol. I1 [ I ] 675, 677
+
172.
Aufl.
But if the value for the equivalent conductivity of H is changed while that of CZH302 remains the same, the difference in K may be considerable. C*H,O,
H
(a) 320.5 (b) 325 (c, 345
$-
++
Pa
43 5 == 364 43.5 = 368 43.5 = 389
K
0.00180
0.00175 0.00158
T h e value of H given i n (c) is calculated to Siemens units from the value in reciprocal ohms found by Noyes and Sammet.* It will be seen that a variation in the value of H of about one and one-half percent as in (a) and @)doesnot change K beyond the limits of allowable error, but that a variation of about six percent as i n (bj and (cj changes I< about six percent. T h e equivalent conductivity of H as given at present is,
H 325 345
i
I
1
I
346.4 365 346
~
I
Authority
Ostwald 1. c. ( I n Siemens unit. ) ', Calculated to reciprocal ohms by h'oyes and Sammet. Calculated to reciprocal ohms by M ~ l l e r . ~ Bredig. Calculated to reciprocal ohms by Wegscheider.' Noyes and Sammet ( I n reciprocal ohms). ( 1
The change in the values of the equivalent conductivity of the anions was due to a better determination of the equivalent conductivity of the sodium ion. Jour. Am. Chem. SOC.24, 968 ( ~ g o z ) . Bull. SOC.chim. Paris, ( 3 ) 27, 1012 ( ~ g o z ) . Monatshefte 23, 613 (1902).
Heyward Scudder
272
o-Toluic acid
1-28
256 512 1024
i I
0.0135 0.0134 0.0130 0.0130
K
i ~
I
0.0119 0.0118
o.0115 0.0115
1 ~
~
0.0107
1
0.0105
0.0103 0.0103
~
Guinchard' gives the following measurements. Zeit. phys. Chem. 13, 293 (1894). Ibid. 25, 517 (1898). Ber. chem. Ges. Berlin, 32, 1728, 1741 (1899).
0.00672 0.00660 0.00652
0.~0652
Ideiztity and Purity of Organic Compounds I
Levulinic acid
O0
2S0 3 5 5 O
Ii
I
0.00229
Violuric acid
0.00 144
0.0021 I 0.00228’
273
I1
0.00273
0.00333
K =o 00255 at 25’ Ostwald. H e ascribes the considerable change i n the value of K in the case of violuric acid to the fact that it is a pseudo acid, since true acids, like levulinic, show no such great differences. It will be noted that in Schaller’s table the change in value of K for a change of ten degrees in temperature is about ten percent, though the values found at different dilutions at each teinperature agree well. I n the case of other acids the change for ten degrees was in many cases as great as this, in other cases much less. Decomposiiion. -Under this head will be described what is commonly known as decomposition, that is, change in molecular composition. Some compounds are decomposed quite rapidly when in solution. T h e decomposition may be hastened greatly by the influence of the platinized electrodes, though the weight that should be ascribed to this influence is at times a matter of dispute. Bader2 states that, partly on account of oxidation the constants of the phenols are in doubt. Hantzsch,3 on the other hand, states that the phenols are not oxidized if pure, and that Bader’s trouble was due to the use of impure material. Bader’s table for phenol will be given later under the head of Degree of Dissociation. H e found that K increased with increasing dilution. Angeli4 found that acetonedicarboxylic acid is decomposed by electrolysis, to which fact is due the diminution of the constant with increasing dilution. Hantzsch and Miolatij in their work on the oximes of ketone-acids found a number of cases of decomposition due to 0.00239 is the value given o n p. 1736. Zeit. phys. Chem. 6, 289 (1890). Ber. chem. Ges. Berlin, 32, 3068 (1899). Gazz. chim. Ital. 22, II., 31 (1892). Zeit. p h j s. Chem. 10, 19 (1892).
274
Heyward Scuddev
different causes. their work.
T h e next three sets of tables are taken from I
71
21.46 171.68 686.72
I ~
0.0743 0.0671 K = 0.079
I O O ~
~
i _ _
64 128 256 512 1024
K diminishes steadily
0.0787
i ,
a-Oxirnino-succinic acid (Syn-)
32
i
K
I
K I
0.103 0.113
.-
p-Oximino-succinic acid (Anti-)
K
I
__ -
I/
0. I 14 0.127 43.02 0.141 56.20 0. I59 69.93 K=o.I~o
32 64 I 28 256 512 1024 K can not
-
-
49.19 59.60 71.37 81.90 be found
I
0.3861 0.3874 0.3749 0.3470 0.3518 0.3860
They attribute the increase of K in the case of the a-acid to the effect of the second carboxyl, not to decomposition. T h e &acid is rapidly decomposed in water to carbon dioxide, water and cyanacetic acid (K = 0 , 3 7 2 Ostwald). They state that under the conditions of electrolysis the acid is almost instantly decomposed, as shown by the approach of K to the value of K for cyanacetic acid. They found that the monethyl esters of these acids were deconiposed in solution, but in different ways. Ester of a-acid
K
ZI
32 64 I 28 256 512 1024
Ester of +acid
-
0.0188 0.0195 0.0196 0.0194 0.0193 35.27 0.0188 K = 0.0192
-I
!'
0.5330 0.5554 0.5107 0.4391 0.3427 0.3220
Identity and Purity oj' Organic Compounds
275
T h e ester of the a-acid is slowly decomposed by saponification, as shown after standing by the increase of conductivity and by titration.
' V
32
I-
Fresh sol
I
After I
P
,
1I
P
1
26.59
hour
I
I
27.68
'
After 5 hours U
33.0
T h e ester of the P-acid is not changed to the isomer, as was shown by measurements of conductivity and by titration of solutions that had been allowed to stand. It is rapidly saponified to the P-acid, which at once breaks down to cyanacetic acid as just shown. I t will be noted that in most of the illustrations given, except in the case of the phenols where other factors are of influence, K decreases on dilution, because of the effect of the decotnposition products i n the solution. But there is 110 regularity in the case of P-oximino-succinic acid. Change o f intramoZecuZar comtitzctiorz. - T h i s is the change from one isomer to another. I t produces in thesolution a second compound that may have a value for K and a degree of dissociation quite different from the first, so that a change of K as i n the case of impurities might be expected. Optical isomers, however. have the same value for K, so that if a change from one to another occurred, it would not be shown by conductivity measuremen ts. I
I
M.p.
I
d-Tartaric acid /-Tartaric acid Racemic acid
~
1
K
0.097 0.097
0.097
1
I ~I
170'
1
(From Walden)'
170'
205'
1
I n the case of all other isomers the change may affect K greatly. T h e next two tables are taken from Hantzsch and Miolati.z Zeit. phys. Chem. 8, 465 (1891). 1. c. 24.
Phenyloximino-propionic acid
/I
Syn- or stable
64
I 28
256 512
-
-
0.00175 0.00182 0.00196 0.00224
Anti- or labile
K can not be got. Comparison of p shows that it is much stronger than the syn-acid. Syn-
v=
101.6
p = 10.76
Anti-
v=
100.6
p = 17.69
They state that the change in K o n dilution in the case of the syn-acid is due to transforniation to the anti-acid. Phenyloximino-acetic acid I/
V
I
si-
16 32 64 I 28 2 56
-
32.73 50.00
1.53 1.56
59.58
1-37
-
0.861
-
82.29 I K = 1.55
1.15
0.638 0.373
They state that the diminution in K in the case of the anti-acid is due to transforniation to the syn-acid and not to impurity. I t will be seen from these examples that the change from one isomer to another may cause K either to increase or to diminish with increasing dilution. If the measurement for a-oximino-succinic acid given under the last head (Deconiposition) is examined it will be seen that K increases steadily and shows no sudden great increase after fifty or sixty percent dissociation. This might suggest a change to a n isomer.
Ide&ity and PuriL'y of Organic Comjounds
277 .
Great degree of dissociatioiz. - For some reason still in doubt the Ostwald formula is satisfactory only for the so-called half electrolytes, which are but moderately dissociated at ordinary dilutions. T h i s group includes most organic electrolytes. For strongly dissociated compounds the constant varies with varying dilution. Rudolphi and van 't Hoff have proposed formulae for such cases, but these have been so little used in measurements of organic compounds that they will not be considered. T h e constants calculated by their use may vary widely from those obtained by the Ostwald formula. I n the following tables the names of some of the compoiinds have been slightly altered, so that they correspond to the names given i n Beilstein's Organische Cheniie (3d Aufl.) and Erganzungsband. _____
Papaveric acid Ostwald'
Kirpa12
I
512 1024
~
'
87.1
-
.05
- ,
'1I
256 512
1
- ] -
K = 0.9 (approx.)
78.0 87.7
! '
I .08 1.20
K not given
-
4,6-Dibrornanilin-2-sulphonic acid
109.8 439.2
~
95.5
97.5
18 9
I1
I1
1'
4,5-Dibron1anilin-z-sulphonic acid
'
556
'
1oollz
97.8
Both these were measured by Ostwald.3 given. 1. c. 398.
* Monatshefte, 1. c. 408.
18, 466 (1897).
I
'K
7.8
No constant is
2 78
Heywavd Scudder 3-Chlor-6-nitro-benzoic acid
1
Bethmann'
Holleman and d e B r u p i Z
70.4 1 62.92 140.8 ' 73.29 281.6 I 81.88 563.2 89.04 1126.4 94.66 K = 1.52
81.7 89.3 93.5 K = 1.42
1
1.44 1.44 1.36
Bethiiiann ascribes the fall of K to extreme dissociation. 2-Brom-5-nitrobenzoic acid ~-
~~
V
256 512
1024
I
100 m
75.4 85.0
91.8 K = 0.91
~
~
K
0.89 0.93
HOllenl3ll and de Bruyn3 'I
I' '
1.01
I t will be noted that K falls, rises or remains constant on dilution. Ostwald's constant for papaveric acid is an approximate value obtained by extrapolation. Since the acid is dibasic and is more than seventy percent dissociated at the first dilution measured, this value is of course very doubtful, because the effect of the second carboxyl is unknown. When K has a value much greater than 1.5 it varies greatly. Bethmann does not use a n extrapolated value for K or his value would be still higher. Holleman and de Bruyn, measuring the same acid, get constant values. If the acid were as much dissociated as the dibromanilin-sulphonic acids such an agreement would be attributed to chance. SmaZZ degree of dissociation.- When a conipound is very
* Zeit.
phys. Chem. 5, 393 (1890). Recueil Trav. Pays-Bas, 2 0 , 361 ( I ~ O J ) . 1. c. 361.
Identity a d Puyity qf Ovganic Conzpouizds
279
slightly dissociated the constant is so small that its value is affected greatly by small amounts of impurity or by small errors in measurement. ~-
-~
p-Nitro-phenol
~I
Bader'
P
21
i
Hantzsch2
K
I
K
~-
0.75
35.6 71.2 142.4 284.8 j69.6
1.04 ._
I
I
o.oooo12 0.000012
0.000013 3.01 I 0.000013 =355 -
PW
K
n
~
o.ooooI2
~
-512
11
1 I I
1.28
-
I
11
--
2.??
I
~
O.OOOOOg8
0.0000102 l0.0000100
1
( K of water used not subtracted) U
n n n n T 3
n nnnnnnh
Hantzsch had this measurement repeated on other samples of the phenol with the same result. His criticism of Bader's work has already been given under the head of Decomposition. In the case of compounds that have a smaller constant the difference is still more marked. _____
Phenol 11
Bader3
Observer
11
v I
/ - I
K ,
'~
K
I
_ L _ _
25
1
50 IOO
1
0.14 1 0.000000j6 I 0.0000005 Hantzsch' ( K of aq. not subtr.) 0.23 o.oooooo77 ~~0.~0000042 Van Laar ( ~ a l c u l a t e d ) ~ 0.41 ' 0.0000012 1 1 0.000000013 Walker and Cormack'
Walker and Cormack state that values as small as these can be '1. C. 297 1. C. 3070 1. c. 291.
' 6 6
1. c. 3069. Zeit. phys. Chem. 12, 748 (1893). Jour. Chem. SOC.77, 5 ('goo).
Heyward Scudder
280
measured if the conductivity of the water used is not greater than 0.7 X IO-^. In a revision of this article Walkerr criticises unfavorably the values given by the other observers which agree in magnitude among themselves but not with the value found by Cormack and himself. H e also regards the agreement in magnitude of the value of K for hydrocyanic acid as found by Ostwald, and by Morgan, and calculated by Van Laar as pnrely accidental. I n this group of cases if K has a value less that 10-5 the only possible comparison of values found by different observers is the coniparison of the magnitude. Isohydric solutions. Wakeman2 gives tables of measurements of isohydric mixtures of acids. T h e following tables are from his paper.
-
-
___-
Acetic acid Propionic acid
K = 0.0018 K = o 00134
I.
Acetic acid Propionic acid
50 parts 1
1
part
I1~
11.
1 Acetic acid Propionicacid
~
J
IOO 1
parts part
K (calc.)
0.00179 64.8 129.6 259.2 518.4
o.ooI8j 0.00186
