726
ANALYTICAL CHEMISTRY
!I
SH,
SH*
\
2 C-S-
6
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+ Hg,++
+NH*
+
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9
juH? \\
+SH,
ISH*
Sartori and Liberti ( 3 ) have also described the reaction of thiourea at the dropping inercury electrode as being a complex or insoluble compound formation rather than an oxidation. They did not, however, give any details of t'he reaction or enough esperiment,al evideiice t o allow a derFvation of possible reactions. I
0
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Figure 4.
t.06
+.08
ACKYOWLEDGMEIVT
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in VOLTS
Reversibility Check
T h e only other possibility t h a t seems t o fit the experimental d a t a is t h a t a soluble conipIe\: is formed having the composition
The author gratefully acl;rio\\-ledgcs t'he help and advice of Sidney Siggia, H. J. Ptolten, and I,. J. Frauenfelder. Thanks are also expressed t o I,. T. Hallett, under whose direction this work \\-as performed. LITERATURE CITED (1) Degering, E. F., "Organic Sitrogen Compounds," PD. 1367-87,
Judgirig from known complexes and reactions of thiourea, the first assumption is more reasonable, and the reaction is probably as indicated herewith.
Ypsilanti, Llich., L-niversity Lithoprinters, 1943. (2) Edsberg, R. L., Eichlin, Dale, and Garis, J. J., ANAL.CHEM.,25, 798 (1953). (3) Sartori. G., and Liberti, .I.,Proc. Intern. Polarog. Congr. Prague, I d COILQT.,1951, Pt. I, PP, 260-4. (4) Sidgwick, X. V.,"The Organic ('hemistry of Sitrogen," pp. 27592, Oxford, England, Oxford Press, 1942. K E C L . I Y Efor D review October 8, 1953.
Accepted Sovember 2 1 , 1953.
Comparison of pK; Values Determined by Electrometric Titration and Ultraviolet Absorption Methods 1. M. VANDENBELT, CAROLA HENRICH, and SUZANNE G. VANDEN BERG The Research Laboratories, Parke, Davis & Co., Detroit 32, M i c h .
T
HE dissociation constants of organic compounds are receiving
increased attention as the possibilities of practical application t o problems of structure and identity and t o the determination of absorption spectra, are more generally appreciated. In view of this importance, i t is of interest t o compare pKA values obtained by two methods of determination, electrometric titration and spectrum absorption, as a partial evaluation of t h e relative quality of d a t a and the merits of the tlvo methods in routine laboratory operations. DISSOCIATION COhSTAhT \ ALUES
I n connection with extensive observations of the absorption spectra of purified benzenoid compounds (6, 7 ) , the pK: values of those derivatives with ionizable groups were determined b y the absorption technique of Sager, Schooley, Carr, and Acree ( 1 4 ) . T h e potentiometric values of these compounds were obtained subsequently, using the method of Parke and Davis (IS) and equipment identical x i t h theirs. Results. Table I summarizes p K ' values obtained in routine application of the two methods to substituted phenols, anilines, and benzoic acids, at a room temperature of 25" C. T h e absorption figures given are the means of values from all the intermediate curves obtained for each ionization; the titration figures are t o the nearest 0.05 unit read off the plotted curve through the transparent plastic overlay ( I S ) . Agreement with Literature Values. It is of course importafit t h a t values obtained by both absorption and titration methods, in addition to agreeing with each other, be of acceptable accuracy. A comparison with representative constants available in the literature shows t h a t either method gives satisfactory values. Methods.
The pK; of phenol, for example. iq reported as 9.94 (8) and 9.95 ( 9 ) ; aniline as 4.60 ( 1 1 ) and 4.636 ( 1 0 ) ; benzoic acid a s 4.18 ( 2 ) ,4.21 (4),4.30 ( f ) , and 4.198 ( I O ) . Values for disubstituted compounds are equally succesPfu1: p-Hydrox!-benzoic acid, first anion, is given as 4.5:3 ( 4 ) and 4.54 (2); second anion, as 9.53 ( 1 4 ) and 9.39 ( a t 20" C . ) ( 1 2 ) ; p-t'oluidine, as 5.12 ( I I ) , 5 . l i (3),and 5.098 ( 1 0 ) : tu-toluidine as 4.77 (10); o-toluidine, as 4.46 ( 1 0 ) ; p-anisic acid, as 4.43 (41, 4.44 (2) and 4.47 ( , 5 ) . A number of the pK', values of the compounds in Table I have not been reported heretofore; some are given in mixed rolvents or a t tempei'atures tiiffe!,ent from t h a t used in this study. In gerieral, the values by ab-orption and titration are in good :tgreement with literat,ure value? obtained under comparable contiitionr. DISCUSSION
I t is evident from inspection of the table t h a t values obtained by the two methods are in good agreement. CompariPon would indicate t h a t either is satisfactoyy, and that, in r e d t s obtained, there is little advantage i n one over the other. However, titration is usually a bit more rapid than the absorption method if the equipment is conveniently set up and manipulative expei,ience has been acquired. This arises from the automatic nature of the procedure; it is not necessary t o knoiv or discover from conr;ecutive curves the solution p H of the limiting species, nor to learn b y trial the intermediate pH a t which ionization takes plarc. T h e titration method has a great advantage when there are two or more ionizable groups in a molecule. Usually time is prohibitive, and execution is fairly difficult', t o run a sufficient number of curves t o eqtab1i.h adequately the limiting species when
V O L U M E 25, N O . 4, A P R I L 1 9 5 4
727
_ _ _ _ ~~____ __ ____~ sevei al groups are modified by pFI change. If the ioniraTable I. Comparison of ph;: Values by Ultra\iolet ibsorption and Electrometric Titration hlethods tion= happen to take place at Substituted Plynols ___ Substituted Anilines Subqtitiite 1 ' p-i c c lcids adjoining-~pH units. the indivi 1-, 1, p I
