VOL.
xu.
No. 7.
THE JOURNAL OF THE
American Chemical Society with which has been incorporated the
American Chemical Journal (Founded by Ira Renuen)
[CONTRIBUTION FROM THE NEWYORKSTATECOLLEGE OF FORESTRY AND SYRACUSE UNIVERSITY. 1
ON THE QUINONEPHENOLATE THEORY OF 1NDICATORS.l A SPECTROPHOTOMETRIC METHOD FOR MEASURING THE CONCENTRATIONS OF THE QUINOIDAL AND LACTOIDAL SALTS AND THE EQUILIBRIUM AND AFFINITY CONSTANTS OF THE P H E NOLPHTHALEINS AND PHENOLSULFONPHTHALEINS. BY R. T. BIRGEAND S. F. ACREE. Received August 26, 1918.
The quinone-phenolate theory2 postulates that phenolphthalein and phenolsulfonphthaleinindicators, with both phenol groups alike, exist in solutions in the following equilibrium. The equations for indicators with unlike phenols, and with a hydrated form, HOOCCeH4C(OH)(COH~OH)~, will be given in the general theoretical article now completed. 1 This article is one of a series which we are publishing kom the New York State College of Forestry at Syracuse University in co6peration with Dr. Haven Metcalf, in charge Forest Pathology, Bureau of Plant Industry, Dept. of Agriculture, on quantitative studies of the various chemical and physical factors governing the growth of fungi on culture media and trees. Am. Chem. J.,37, 71 (1907); 38, I (1907);39, 528 (1908);42, 115 (1909); THIS JOURNAL, 38, 2772 (1916); 39, 648 (1917); 40, 1092 (1918);Science, 42, IOI (191.5).
1032
R. T. BIRGE AND S. F. ACRICE.
+
(CsH4COO)C(CsHdlH)2 ++ (C.SH~COO)(C~H~OH)(C~H~O-) H+ colorless: HgI, colorless : HI,I4 (CsH4COO)C(GHdO-)n R1 colorless : I,-C ( :CsHd :0)(C&O -) H +
t
+
C(: C6H4:O)(C6H4OH).
I
GHiCOOH
yellow: H$Q
K1
red: HQ-‘
t
>
+
C(:C~H~:O)(CBHIO-) 2H+
K2
\C6K,cOO-
-1.
C( :CsH, :0)(CsItOH)
I
t
+ 2Hf
red: Q--
+
CaHiC00H+ yellow: HQ-
We have found that HQ-’ and Q-- give a strong absorption band in the green at about I/X = 1800. In other words an intense red color is transmitted by HQ-’ and Q--,a faint yellow color is transmitted by solutions of H2Q and HQ-, while H2L, HL- and L-- are colorless. From the ionization and equilibrium equations for such tautomeric’ compounds,
By simple algebraic combination of two or more of these equations, we may obtain the equilibrium ratio between any one (or several) of the above forms (ions or molecules) and any other (or several other) forms. Certain of the more interesting of these ratios are given below, the important ratios being indicated for convenience by a new constant.
QUINONE-PHENOLATE THEORY OF INDICATORS.
1033
The three Equations 12u, 12b, 12c give the “effective” ionization constantsl (MI and M z ) obtained when one ignores the tautomerism, and considers simply the molecular, monovalent and divalent forms. Thus M1 corresponds to KlKl’ or K3, while M2 corresponds to Kz, Kz’ or K4. Similarly M I M Zcorresponds to KIKz = K1’K2’ or to K3K81. Equations 8 to 126 all involve a ratio between different steps of ionization, and so (H) or (H)2 occurs in each equation. The following equations give some of the more interesting ratios where only one step of ionization is involved :
HI,- - -- R1K3= R2 or R2/R1= K3/K1 HQKI
HQ’-/Q-- = HKi‘/K1K2. (18) The general method of measuring the absorption ut diferent frequencies in the visible and ultraviolet regions, and a t successive stages of neutralization t o learn the amounts of the several salts present, was proposed in lectures and articles a t various times from 1908 to the present. The plan was suggested for use with colored indicators and also with such tautomeric compounds as the colorless urazoles whose absorption spectra would be studied in the ultraviolet.2 Schaeffer, Paulus, Hutchinson and Jones3 measured the absorption a t different frequencies by means of an improved suspended thermocouple method developed by Professor A. H. Pfund and Dr. J. S. Guy. Their studies substantiated the applicability of the theory in the best work yet published on rosolic acid, phenolphthalein and methyl orange. Pfund, Uhler, Anderson, Jones and Wood4 had also used the photographic methods in the visible and ultraviolet in other studies, and Uhler and Wood photographed the spectra of a number of dyes. Hildebrandts had used a spectrophotometer with phenolphthalein but made measurements a t just one frequency in the red. Dr. H. A. Lubs prepared urazole esters and salts in 1914 for the
* The method of derivation of IZU,I z b , I Z C and other similar complex equations will be discussed in the long theoretical article now ready for publication. * See Am. C h . J.,42, 123 (1909)and all of uur more recent articles. a THISJOURNAL, 37, 776, 1694 (191s). 4 Carnegie Publications. 6 2.Elektrochem., 14,351 ( 1 ~ 8 ) .
10.34
R. T, RIKCE AND S. F. ACRJ3E.
investigations as a part of his dissertation but did not have time to undertake the spectrophotometric measurements. These studies in the urazole series will now be continued. In the quantitative use of indicators for measuring the acidity of solutions, as in studying reaction velocities or the growth of fungi on culture media, it becomes imperative to know the magnitude of the above constants and to vary them at will by introducing substituent groups which will make the indicators extremely sensitive and useful over a wide range of hydrogen ion concentration. In this article, therefore] we shall discuss especially the equilibrium and affinity constants and their relation to the intensity of the colors of various indicators at different hydrogen ion concentrations. It is obvious that these constants and ratios can vary widely with Werent substituent groups and we therefore have a means of preparing different series of indicators that will (a) give varying intensities for definite equivalents of alkali and, ( b ) give their color changes a t the different hydrogen ion concentrations or PHvalues. Up to this time it has not been possible for us to calculate with any certainty the relative concentrations of the colorless lactoidal, yellow quinoidal, and deeply colored quinone-phenolate salts in the acid or alkaline solutions of the phenolphthalein series. We did know from White’s synthetic, conductivity and spectroscopic work’ and I,ubs’* PH values that the phenolsulfonphthaleins exist practically only as quinone-phenolates in alkaline solutions and consequently give brilliant color changes. But our own, and Guy’s unpublished (finished I 9 I 6 ) spectrophotometric measurements and those of Howe and Gibson3 now give us sufficient insight into the phenomena involved to enable us to begin to measure or calculate (a) the concentrations of the lactoidal (H&) and of the quinoidal (HzQ) free indicator, ( b ) the colorless lactoidal mono- (HL-) and dibasic (I,--) salts, (6)the yellow quinoidal mono-basic salt (HQ-), (d) the deeply colored mono-basic quinone-phenolate salt (HQ’-) , and (e) the deeply colored dibasic quinone-phenolate salt (Q--) of all indicators of this general type. This is such a very important advance in the history of the study of the indicators that the results will now be outlined, together with some plans for future work on these tautomeric compounds. Discussion of the Phenolsulfonphthalein Series. In order to aid the reader in following the discussion, it may be well to summarize the meaning and probable values of the various constants defined by Equations I to 7. 1 White and Acree, New Orleans Address, Science, 42, I O I ( 1 9 1 5 ) : T ~ I I JOURNAL, S 39, 648 (1917); 40, 1092 (1918). Lubs and Acree, Ibid., 38, 2772 (1916); Lub5 arid Clarke, J . Wash .4cud .Vci.. 5r 609 (1916),6,483 ( x g r i ) ; .J. Duct., 2, I , 10% 137 (1917). 3 P h y s Rei , IO, 767, 7 7 9 (1qr8).
QUINONE-PHENOLATE THEORY OF INDICATORS.
1035
(a) R1 gives the equilibrium between the lactoidal and quinoidal forms. Our work seems to indicate a value of about 1000 for phenolphthalein and of about unity for phenolsulfonphthalein. (b) K I and K2‘ refer to the ionization constants of the sulfonic acid residue in phenolsulfonphthalein, or to the phthalic acid residue in phenolphthalein. They are very large, about 1 0 - l ) for phenolsulfonphthalein, and about IO+ or IO-^ for phenolphthalein. (c) K1’ and K3 refer to a primary phenol ionization. They are both very small, and probably of about the same order of magnitude, say IO-* to I O - ~ O , for phenols with no negative or positive substituent groups. The introduction of positive or negative groups into the phenol radical . decreases or increases KI’ and K3. ( d ) K2 and K4 refer to a secondary phenol ionization. They are therefore still smaller than K1’ and K3. Equation 13 shows that in a series of indicators having the same absorption index for unit equivalent of quinone-phenolate ions, those indicators having the largest values of K1K2 and smallest values of R1K3K4 should give the largest value Q--/I,--, and the greatest absorption in the green and consequently the most intense transmitted red color. For that reason we chose the sulfonphthalein series with a large value for K1,about IO-^, for the sulfonic acid group, with .a small value for R1, about IO or less, and with K2, Ka and K4 variable at will, as being one of the most promising groups of indicators for practical use as well as for the study of the general theory. The small ratio R1 enables us to study the properties of the actual yellow quinoidal form, H2Q (which is not present in appreciable quantities in the phenolphthalein series), and of the monobasic yellow salts corresponding, KHQ, which have been made and described by White’ (and which have not been isolated in the phenolphthalein series but which we shall measure spectrophotometrically in the alkaline solutions). The value of K1 is so large in comparison with RlK3, K2 and K4 in the phenolsulfonphthalein that well over 90% of the indicator is titrated as a yellow quinoidal monobasic acid before the “turning point” or intense color change appears. To illustrate, to state that K1 is large in comparison with R1K3 means from Equation 15 that little of the monovalent ions are lactoidal and colorless (HI,-). When K4 is small in comparison with K1, and RlK3 is likewise small, Equation 13 shows that little of the divalent ions are lactoidal and colorless (I,--). When K2/K1 is small, and RlK3 and K1 are also small, Equations I and 3 give by division K2/KI = ( H 2 Q )
(’--), which shows that no appre(HQ--) ciable quantity of (Q--), the red salt, can be formed while there is appreciably any unchanged indicator (H2Q) remaining in solution with the 1
Reference to article now in editor’s hands.
1036
R. T. BIRGE AND S. F. ACWE.
large amount of (HQ-) (over 90% at the first appearance of the red color). It is therefore certain that in alkaline solution practically only the quinonephenolate dibasic salt (Q--) is present, with no appreciable amount of the colorless lactoidal dibasic salt (I,--); i. e , , X3 is very small. The alkaline solutions therefore exhibit unusually brilliant color changes because such a large per cent is in the quinone-phenolate form. The exact per cent. can be calculated from our theory when all the constants are known of the indicator salt for the sulfonand shown to be practically 1007~ phthalein series. We shall make full use of this high per cent. of quinone-phenolate form to study the influence of all kinds of negative, basic and alkyl groups on the values of Kl and K z and on the specific absorption index and hydration rate for the quinone-phenol, -C( C6H4 :O)(CaH40H), and quinonephenolate groups, -C( :CsH4:0)(C6H40-),of each “practically completely” quinoidal sulfonphthalein indicator. The mono-phenolicethers and the sulfonic esters will also be investigated for the same purpose. Similar studies will be made with other simpler purely quinoidal groups, such as the aurines, sulfonated aurines, and mono-alkyl aurines, (ROC&&)C(C6H4:O)(C6H40H),and the esters of the phenolphthalein series, such as ROOCC6H4C(:C6H4:0)(C6H40H)and their substitution products and salts, in which various factors can be controlled and measured. These constants, taken together, will give us an extensive set of data by which to estimate what influence the substitution of such groups in the phenolic, and in the sulfonic (-CeH4SOaH) and phthalic (-CBH~COOH) residues will have on the I
