PHENOLPHTHALEIN and METHYL ORANGE CHARLES A. PETERS AND BRYAN C. REDMON' 'Massachusetts State College, Amherst, Massachusetts
v
We assume, as did Kober and Marshall (9), that the ARIOUS authors are not in agreement on the action of the common indicators. In discussing iirst equivalent of base acts on a phenolic hydrogen methyl orange Willard and Fnrman (1) put the leaving a charge on the oxygen as shown in (la). It hydrogen on the dimethyl nitrogen, while Kolthoff and would appear that the phenolic form of one phenolic Sandell (2) place it, in the red form, on the azo group. group was in equilibrium with the quinoid of the same In phenolphthalein the doubt concerns the action of the group. Such a change, however, calls for the breakfirst equivalent of base. Does i t break the lactoue ing of the lactone ring, and the shifting of the charge to the carboxyl as is shown in equation (1). ring or neutralize one of the phenolic hydrogens? The second equivalent of base acts on the second The statement of Bury (4) that resonance should be responsible for color in these indicators makes certain phenolic hydrogen, leaving a charge on the oxygen as of the forms ordinarily used a t once electronically pos- pictured in equation (2). sible, and others equally impossible. The paper of Wheland and Pauling (3) showing mathematically that action on any one point of a molecule changes the electronic condition a t other points is stimulating. Supported by these papers we are encouraged to set forth what seems to us a logical explanation of the action of the common indicators.
It is frequently stated that the quinoid chromophor is responsible for the color of phenolphthalein; however, Willsatter and Piccard in 1908 (5) gave examples of quinoid compounds that were colorless. Acree (6) also in 1908 insisted that the phenolate group in addition to the quinoid is necessary for color in phenolphthalein and like substances. Kober and Marshall in 1911 and 1912 (7) outlined the actions of phenolphthalein and we find our electronic presentation follows their work rather closely. F
'
Present address: The American Cyanamid Company, Stam. ford, Connecticut.
The part of the compound containing the two phenolic groups is now symmetrical and resonance is possible. The resonance forms are given in (3) and show a charge
alternately appearing on each phenolic oxygen. The swingingof electron pairs around six atoms, five carbons, and one oxygen, is all that is necessary to make the change. Because these reactions (1 and 2 ) are written separately i t must not be presumed that reaction (1) is complete before reaction (2) sets in; rather think of a small fraction of the phenolphthalein undergoing reaction ( 1 ) and reaction ( 2 ) setting in while most of the phenolphthalein is still present as such. No one seems to have been able to prepare either of the salts in equation ( 1 ) as would have been the case if reaction (1) was complete before (2) stbxted. Further, as will be mentioned later, Kober and Marshall (10) support Green and Perkin in the statement that when the tribasic salt (4b) is slowly neutralized, cold, by acetic acid it is necessary to replace two hydrogens before an endpoint can be detected. This might be stated in a different way by saying that the two pK's of the two phenolic hydrogens were nearly the same value and, consequently, both hydrogens, to all practical purposes, act alike. When acid is added to the red colored substance the reverse changes set in. One hydrogen ion is attached to the oxygen not in qninoid formation (2b), immediately (2a) is formed, resonance is stopped, and no color is possible. As the lactone ring closes the quinoid form is destroyed and the form ( l a )is present which can take a second equivalent of the hydrogen ion. The opening of the lactone ring would be rapid, the closing slow. Naturally all indicator color changes must be from rapid actions. The third equivalent of base adds a hydroxyl ion to the central carbon atom forming carbinol and causing the disappearance of quinoid form and the loss of color. Resonance wonld be prevented because there is no longer the flexible pair of electrons in the double bond. The action would be expected to be slow. The change is pictured in equation (43.
Lnnd (13) in measuring the rate of fading of phenolphtbalein concludes that an excess of hydroxyl ions shows a salt effect and also that the rate is constant
in the presence of sodium chloride, the action being what would be expected from the Brgnsted theory. In another article Lnnd (14), measuring the rates of fading and color formation in the presence of a hydroxyl ion, finds the rate of fading to be constant while the formation of the colorless compound is shown to be subject to the salt effect. Assuming equation (4) represents what is happening when the hydroxyl ion combines to form the tribasic salt, it is evident that this eqnation is supported by Lund's work. To form the colorless substance two ions must find each other, and salts naturally would get in the way, while the production of color requires only the breaking down of one substance. The constant of equation ( 4 ) to the right, according to Lund, is several times greater than that to the left, which is what wonld be expected. The carbinol form resulting from the combination of the hydroxyl ion from the third equivalent of base is quite stable, for Green and Perkin (8)report that if the solntion is carefully cooled and slowly neutralized with acetic acid it may be rendered neutral without the return of color. If, on the other hand, the neutralized solution is left for some time or heated the color returns. Green and Perkin further state that tribasic phenolphthalate when titrated with acid is &basic. Kober and Marshall (10) concur when they say that the tripotassium phenolphthalate reacts quickly with two equivalents of acetic acid and decolorizes slowly with the third. To discuss what happens when acid is added to the colorless tribasic salt it is necessary to keep in mind that the carboxyl is a strong weak-acid; stronger than acetic.acid as Kober and Marshall (7) point 6ut. The first two equivalents of acid then, would replace hydrogen ions on the two phenolic groups, making the compound dibasic; a rapid reaction producing no colored substance. The third eqnivalent of a hydrogen ion would unite with the hydroxyl group of the carbinol provided the carboxyl ion was in a position to furnish an electron pair and a hydrogen ion was sufficiently near; such a reaction would be expected to be slow, and is shown in eqnation (5).
It might be profitable to discuss whether it is the ionized or unionized carboxyl that takes part in the lactone formation. The ionized form is less plentiful, its length is shorter, and a hydrogen ion must be in the immediate vicinity in order that the closing may take place. On the other hand, the unionized form is more plentiful, its length is greater, and no outside elements are needed to effectclosure. In equation ( 5 ) i t was assumed that the ionized form does the closing. In the remainder of this paragraph i t will be assumed that the unionized form does the closing. The facts are that Kober and Marshall (7) made the colorless mono-salt from the colorless tripotassium phthalate. In water this mono-salt gave a precipitate of phenolphthalein and the solution became colored. The theory says that the dry mono-salt would contain the hydroxyl ion and the sodium ion would be on the carboxyl. Hydrolysis of the mono-salt would replace hydrogen on some carboxyl ions; slowly the lactone ring would close with the elimination of water and phenolphthalein would be formed. The hydroxyl ion produced by hydrolysis of the carboxyl would act in the regular way as pictured in equations (I), ( Z ) , and (3) with the formation of the resonating colored forms. The color of the esters of phenolphthalein support the explanation offered in this paper. When the ester is on the carboxyl the compound is colored as soon as the second equivalent of base acts, while the alkaline solutions of the phenolic esters are colorless as Acree. and Slagle (11)and others have pointed out. When the ester is on the carboxyl the remainder of the compound is symmetrical after the phenolic hydroxyl ions are gone and resonance is possible, but with the ester on the phenolic group, as with the hydrogen on one phenolic group, the compound is not symmetrical, the electrons, apparently, are not free to move, no resonance results, and consequently no color is seen. The red color of phenolphthalein fades. Kober and Marshall (12) found it impossible'to use this substance as a color standard. Certainly the fading of the indicator color a t an endpoint is of no analytical significance, but it is interesting that a calculation shows that with as little as 0.01 ml. of 0.1 N base present in excess with three drops of 0.1 per cent phenolphthalein solution there are over three times as much hydroxyl ion present as is necessary for the fading change shown in equation (4). Kober and Marshall (9) say, "Our figures show that hydration begins with the smallest amount of alkali." METHYL ORANGE
The two colors of methyl orange would seem to call for two resonating mechanisms. These we have been unable to fi.nd by juggling electrons with pencil and paper so we conclude that the two forms usually pictured are the resonance mechanism for the red color, and that the yellow color is due to some other phenomena; further, that the electronic arrangements necessary for resonance require a hydrogen on the nitrogen of the azo group nearest the sulfonic acid radical, and none
of the dimethyl nitrogen exactly as outlined by Bury (4). The resonating forms responsible for the red color are, then, the changes usually given for the yellow and red forms and are shown in equation (6).
The simultaneous electronic changes of the azo and the dimethyl nitrogens are pictured during resonance in equation (7).
The introduction of an equivalent of base will withdraw the hydrogen from the azo nitrogen, making it neutral, consequently upsetting the resonance mechanism and resulting in the loss of the red colored form. The electronic condition of the azo nitrogen is pictured in equation (8) which shows that while the requisite eight electrons are a t all times present
the addition of H+ reduces the electronic charge of the nitrogen from five to four. The question arises, has the sulfonic group any part in the resonance system? From the fact that aminobenzene has the same color changes and no sulfonic group the answer would be, no. On the other hand, the fact that methyl orange is yellow in strong sulfuric acid, becoming red on dilution, is interesting. One line of reasoning, to explain this action, would be to say that the unionized form only existed in the sulfuric acid, and consequently the ionized form was necessary for resonance. Another and better explanation would be to say that the dimethyl nitrogen acted as a base in the sulfuric acid system, and the consequent acquirement of an H + would prevent resonance. Such a change would be pictured in equation (9).
With the free electron pair of the dimethyl nitrogen held by the hydrogen it would not be possible to form the quinoid structure necessary for resonance.
Both the am and the dimethyl nitrogens would he positive. It is hoped that these suggestions will aid teachers in their interpretation of the action of the common indicat0r-s. LITERATURE CITED
(1) WILLARDAND FWAN. "Elementary quantitative analysis," D. Van Nostrand Co., Inc., New York City, 1937, p. i5. AND SANDELL, "Textbook of quantitative analy(2) K~LTHOFF sis," The Macmillan Co., New York City, 1936,. -D. 428. (3) WHELAND AND PAULING,"A quantum mechanical discussion of orientation of substituents in aromatic molecules," J . Am. Chem. Sac., 57,2086-95 (1935). (4) BURY, "Auxochromes and resonance," ibid., 57, 2 1 1 5 7 (1935). (5) WILLST~TTBR AND PICCARD, " a e r die Farbsalze von Wurster." Ber.. 41, 1458-75 (1908).
(6) ACREE."On the theory of indicators and the reactions of phthaleins and their salts," Am. Chem. J., 39, 531 (1908). (7) Kossn AND MARSHALL, "Phenolphthalein and its colorless salts, 5. Am. Chem. Soc.. 34, 1429 (1912). (8) GREENAND PERKIN, "The constitution of phenolphthalein," J . Chem. SOL..85, 399 (1904). (9) KOBERAND M ~ ~ ~ u ~ ~ ~ , " ~ h e n o l p hand t h aits l ecolorin less salts (potassium salt of phenolphthalic acid)," J. Am. C h . Soc., 33, 68 (1911). (10) Cf., Reference 9, p. 62. (11) ACREEAND SLAGLE,"On the theory of indicators and the reactions of phthaleins and the& salts, 111," Am. Chem. J.. 42, 118 (1909). NIETZK~ AND BURCKHARDT, "Uher chinaide derivate des phenolphthaleins," Bn., 30, 175-80 (1897). GREEN AND KING, "Zur konstitution der Phenol- und Hydrochinonphthaleinsalze, . 11," Ber., 40,3724-34 (1907). Cf. Reference, 9, p. 69. LUND,"The fading of phenolphthalein in basic solutions." Beret. 18th Skau. Naturforskemode, Copenhagen, 1929, 412-13; through Chem. Abstr.. 24, 2659 (1930). LUND. "The constitution of phenolphthalein, 11. The fading of phenolphthalein in alkaline solutions," I. C h a . Soc.. 1930, 1844-52.
