1 ,5-di(2'-sulfo-4'-methylanilino)anthraquinone (Anthraquinone Violet

[COMMUNICATION NO. 841 FROM T H E

KODAKRESEARCH LABORATORIES]

SOME OBSERVATIONS UPON THE RELATION BETWEEN ABSORPTION SPECTRA AND CONSTITUTION OF CERTAIN ACID ANTHRAQUINONE DYES C. F. H. ALLEN, C. V. WILSOX,

AND

G. F. FRAME

Received December 16, 1942

A sufficient amount of information is now available from the studies on isomers and homologs of the acid anthraquinone dye, Toluidine Blue (1, 2, 3), so that one may attempt to relate spectral absorption characteristics and molecular structura This relation to be discussed in this paper is limited to the effects of groups in the blue and green dyes in the 1,4- and 1 ,5-series; it is only a part of the general topic of color and constitution of anthraquinone dyes, which is summarized in Houben (10). The blue and green series are of the general types I and 11, respectively. The isomers and homologs are derived by varying the

I S

I1 S = NaS03

= NaSOs

nature of the groups in the rings A, B, and C. The older dyes considered (the formulas of which are not given in this paper) are the sodium salts of 1,5-di(2’-sulfo-4’-methylanilino)anthraquinone (Anthraquinone Violet) (4), 1,4-di-(2’-sulfo-4’-methylanilino)anthraquinone(Alizarin Cyanine Green) (5), 1,4-di-(3’-sulfo-4’-methylanilino)anthraquinone(Alizarin Direct Green) ( 6 ) , 7,s-dihydroxy-l , 4-di-(2’-sulfo-4’-methylanilino)anthraquinone(Alizarin Viridin) (7), and 1-amino-2-bromo-4-(2’-sulfo-4’-methylanilino)anthraquinone (Alizarin Pure Blue B) (8). Toluidine Blue is the sodium salt of 1,5-di-(2’-sulfo4’-methylani1ino)-4,8-dihydroxyanthraquinoneland Toluidine Green is the 1,4-isomer (1). I n Fig. 1are shown the characteristic curves for four dyes, two in the 1,4- and two in the 1,5-series. The peaks of each are listed in Table I; it will be noted that there are two maxima in the longer wave lengths of all except the violet dye.l An inspection of these curves reveals the great differencesb e h e e n the 1,4-and 1,5-series; thus, the differences between the second and third peaks in the green 1 This dye may not be strictly comparable, for it appears to have four bands in the ultraviolet with a suggestion of a fifth a t about 384 mp. The band head a t 330 has been assumed to be comparable t o the third band in the other dyes, and the values in Table I1 are calculated from it.

169

170

ALLEN, WILSON, AND FRAME

series in the ultraviolet is more than double that in the blue series (125 vs. 35-55), the third peak in the former appearing in the visible. The effect of the two hydroxyl groups in the 4,8- and 5,8-positions is also evident in Fig. 1. The difference between the first and second peaks (Table 11) is approximately twice as great in the dyes containing these groups so arranged. Further, there is a much greater spread (50% more) between the third and fourth peaks in the blue series. That is, in this series, these hydroxyl groups have pushed the red absorption towards the longer wave lengths without greatly TABLE I

I I1 I11 IV

Anthraquinone Violet (4) Alizarin Cyanine Green (5)* Toluidine Blue Toluidine Green

400

1

246 253 244 243

272 285 300 290

1

1

308 410 335 415

500

330 608 654 644

560 646 696 692

700

WAVE LENGTH IN MILLIMICRONS FIG. 1. ISOMERIC D Y E S I N 1,4- AND 1 , 5 - S E R I E S

-, Anthraquinone Violet (1,5); ---------, Alizarin Cyanine Green (1,4); Toluidine Blue (1,5,4,8);--,Toluidine Green (1,4,5,8).

---e-,

affecting the portion of the band in the ultraviolet. Further, the two hydroxyl groups in these positions have moved the red absorption towards the longer wave lengths; in the green series this amounts to 36 and 46 mp (Table I), while in the blue it is of the order of 100 mp. This effect seems to be confined to the 4,8- and 5,8-positions, for the 6,7- and 7,8-isomers resemble in the visible the unsubstituted Alizarin Cyanine Green (Fig. 2); even so, the effect is beginning to appear (12 and 8 mp) in the 7,8-isomer (Alizarin Viridin), which has only one OH in an alpha position (Table 111). Finally, the introduction of two OH groups into the 6,7-position of Alizarin Cyanine Green has moved the red

171

SPECTRA O F ANTHRAQUINONE DYES

absorption in the opposite direction, slightly to the left (6 and 8 mk) (Fig. 2) (Table 111). If this latter can be considered to be the normal effect of introducing hydroxyl groups, then the large shift in the opposite direction observed in the Toluidine Green must be connected with a peculiarity of the alpha position. This feature is the possibility of chelation of the hydroxyl hydrogen with the TABLE I1 DIFFEREKCES BETWEEN ABSORPTION MAXIMAOF TABLE I Anthraquinone Violet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alizarin Cyanine Green.. . . . . . . . . . . . . . . . . . . . . . . . . . Toluidine Blue.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toluidine Green... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I

.I 1

I

26 32 56 47

58 125 35 125

j

i

230 198 319 229

1

I

I, I

38 42 48

TABLE I11 COMPARISON OF HYDROXYLATED DYES(mp)

IV I I1 I11

I

DYE

NO.

1

Alizarin Cyanine Green Toluidine Green Alizarin Viridin 6,7-Dihydroxy-l,4-ditoluidinoanthraquinone

j

I

460

I

OH

608 644 620

7, 8 6,7

602

646 692 654 638

I

5oQ 600 WAVE LENGTH IN MILLIMICRON5

Pi

FIG.2. ALIZARINCYANINE GREENAND THREE DIHYDROXY DERIVATIVES

---, Alizarin Cyanine Green; -, Toluidine Green (5,s);---------, Alizarin Viridin (7,8); -*-., 6,7-Dihydroxy isomer of Toluidine Green. carbonyl oxygen. Further confirmation is thus afforded of the generally accepted idea of hydrogen bonding of dpha-hydroxylated anthraquinones (13). I n general, the introduction of halogen atoms into Alizarin Cyanine Green has a bathochromic effect, but their position in the molecule has a very great influence. I n the case of the 5 ,&dichloro derivative (a-series), in the ultraviolet, the distance between the peaks is 48 mp, as compared to 32 mp in the unsubstituted dye (Table IV). The 5,8-dichloro derivative thus resembles

172


Toluidine Green, where the difference is 47; that is, two chlorine atoms have an effect similar to two hydroxyl groups, when both are in the 5,8-positions (Fig. 3). From Table IV it will be noticed that it is a different head of each that is about the same as one of the parent dye, the hydroxyl group having moved the first peak towards the shorter wave lengths, whereas the chlorine atom has shifted the second in the opposite direction. The main band in the red, however, is greatly broadened by the chlorine atoms; the right head is moved 14 mp to the longer TABLE I V ABSORPTIONMAXIMAOF HALOGENATED DYES (mp) Alizarin Cyanine Green., . . . . . . . . . . . . . . . . . . . . . . . . . ' 5,8-Dichloro derivative.. . . . . . . . . . . . . . . . . . . . . . . . . . 5,6,7,8-tetrachloro derivative., . . . . . . . . . . . . . . . . . . . 6,7-Dichloro derivative.. . . . . . . . . . . . . . . . . . . . . . . . . . . Toluidine Green., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ' 2,a-Dichloro derivative of Toluidine Green . . . . . . . . .

1

~

~

1

1 i

253 285 252 1 300 270 ' 266 (326?)1 243 290

I I

410 425 425 428 415 420

'

608 568 570 628 1 644 600 I ~

1

I

646 660 640 662 692 643

1.2 1.0

'

QB

E

0.6

2

04

0

0.2

c_ cn z

E

400

500

600

700

WAVE LENGTH IN MILLIMICRONS

FIG.3. ALIZARINCYANINEGREENAND Two DICHLORODERIVATIVES Alizarin Cyanine Green; - - - -, 6,7-Dichloro derivative; -.-.-,

,

5,8-Dichloro derivative.

wave lengths (as compared to 46 for the 5 ,8-hydroxyl groups) but the left one is shifted 40 in the opposite direction, resulting in a wide band with a difference of 92 between the two maxima. I n the case of the 6,7-dichloro derivative @-series), the introduction of the halogen has shifted the entire red band towards the longer wave length about 20 mp, but otherwise has had no appreciable effect, So far, owing to side reactions leading to dyes of uncertain structure, a dye having chlorine atoms in the 2,3-positions has not been obtained, However, a 2,3-dichloro derivative of Toluidine Green has been prepared; the location of the maxima (Table IV) is about the same as in the unsubstituted Alizarin Cyanine Green, the halogen

173

SPECTRA OF ANTHRAQUINONE DYE8

having a very slight bathochromic effect. While this appears to be opposite to the 6,7-dichloro dye, a strict comparison cannot be made, since one has two hydroxyl groups in the alpha position. From these few examples, it may be said that when halogen atoms are present in the beta positions, they have much less influence upon the absorption than when they are in the alpha position. Chlorine atoms in the benzene residue also have a bathochromic effect; comparisons with this type are made in the blue series (Table V). The ultraviolet is essentially the same in the 4’-chloro and 4’-methyl (Toluidine Blue) dyes, but the band in the visible, though of the same size, is not as far to the right in the case of the halogenated dye. TABLE V ABSORPTION MAXIMAOF DYESRELATEDTO ANTHRAQUINONE VIOLET(IN mp) 1

Anthraquinone Violet.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 272 4’-Chloro-4,8-dihydroxy derivative.. . . . . . . . . . . . . . . 242 ’ 296 4’-Methyl-4,8-dihydroxy derivative (Toluidine Blue). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 300

FIG.

1 1 NO.

I

I1 I

7 7

,

’

I1 I I1

I

~

~

330 332

622

335

654

560 j

662 696

TABLE VI COMPARISON OF BASESAND DYES* DIFF. DIPF. LAST Two BASE AND

SUBSTANCE

DYE

Alizarin Cyanine Green base Alizarin Cyanine Green dye Toluidine Green base Toluidine Green dye Toluidine Blue base Toluidine Blue dye Alizarin Pure Blue base Alizarin Pure Blue dye

400 412 414 416

600 608 642 644 630 654

642 646 676 692 676 696

42 38 34 48 46 42 34 30

8, 4

2, 16 24,20 14,10

I1 1 I * The absorption of the bases was observed in dioxane solution, and of the dyes in water.

All the foregoing comparisons have been made on the dyes, which are salts of sulfonic acids. Since the unsulfonated quinones and dyes are not always dissolved by the same solvents, their curves may not be strictly comparable. Bearing this in mind, it can be said that the introduction of sulfonic acid groups into the toluidine residues has a relatively small effect (Figs. 4-7). This is towards the right (2-20 mp) (Table VI) and is somewhat more pronounced in the blue series, being about double when the hydroxyl groups are present. Sulfonation of the dye base always results in the introduction of the sulfonic acid group in the 2’-position of the toluidine residue (1, 5). The isomeric 3’-sulfonic acids can be secured by the use of sodium 4-aminotoluene-2-sulfonate under pressure (2, 3, 9). It is interesting to note that the isomeric 4-aminotoluene-3-sulfonate fails to react under the same conditions; this is undoubtedly

174


on account of its chelated structure. Comparison of the absorption curves of this variety of isomers (Figs. 8-10) shows small differences (Table VII). Of the two common dyes, Alizarin Cyanine Green and Alizarin Direct Green, there is a very small shift in the violet end, but the width of the main absorption band in the red end is quite different, the first having more blue absorption. More significant is the nature of the main absorption band in the red-it has but one


FIG.4. EFFECTOF SULFONATION OF ALIZARIN CYANINEGREEN. COMPARISON OF 1,4-AND ~,5-DITOLUIDINOANTHRAQUINONES

, Alizarin

Cyanine Green, base in dioxane; Anthraquinone Violet, base in dioxane.

5

>

aY

---------, Same, sulfonated; -.-.-,

1.o

0.0 0.6

04 0

0.2 500

600

WAVE LENGTH

700

800

IN MILLIMICRONS

FIG. 5. EFFECT OF SULFONATION OF TOLUIDINE BLUE -, Base in dioxane; - - - -, Same, sulfonated

peak, whereas with the 2’-isomer there are two maxima. Since the second peak is present in the unsulfonated 1,4-ditoluidinoanthraquinone,its presence in one sulfonated dye and its absence in the isomer must be related to the location of the sulfonic acid group. This same behavior is exhibited by the isomeric sulfonic acids of other dyes, and is discussed below. Finally, 1 ,4-di-~-hydroxyethylamino-5,8-dihydroxyanthraquinone and its

175

SPECTRA OF ANTHRAQUINONE DYES

c


FIG.6. EFFECT OF SULFONATION OF TOLUIDINE GREEN , Base in dioxane; - - - -, Same, sulfonated

c

1.0

FIG.

DYE

soa

8

Alizarin Cyanine Green Alizarin Direct Green Toluidine Green Toluidine Green Isomer Toluidine Blue Toluidine Blue Isomer

2' 3' 2' 3' 2' 3'

8

10 10 9 9

1

x.4Xm.4

~

~

243 242 244 240

I ~

23; 290 290 300 294

1

415 422 325 338

644

I

646 692

650 654

I

696

660

sulfato ester may be considered (Fig. 11 and Table VIII). Here it will be noted that there is no peak in the 325-425 range, from which it follows that the presence

176


of this probably depends upon the aromatic toluidine residues. There is just the suggestion of a head at about 566 (576), perhaps related to the aliphatic residues, as it does not appear in any of the other dyes. It will also be noticed that the introduction of the sulfate group has had two effects: it has considerably depressed the peak farthest to the right, and it has

r

WAVE LENGTH IN MILLIMICRONS FIG.8. EFFECT OF POSITION OF SULFONIC ACID GROUP Alizarin Cyanine Green (2'-SOaNa); - - - -, Alizarin Direct Green

-, (3'-SOsNa).

2.6 2.4

\

- \d,,\

> 2.2 -

5 20-

\ \ \

Z 1.8-I

4 E o

\

1.6 1.4' 1.2 1.00.8 -

ll

0.Gc4 0.2

200

-,

I

\

/c-

-

-.

\

,

I

, '\

300

' 0

400

500

600

7 3

WAVE LENGTH IN MILLIMICRONS FIG.9. EFFECT OF POSITION OF SULFONIC ACID GROUP Toluidine Blue (2'-SOaNa), - - - -, Isomer (3'-SOaNa)

moved the entire red absorption about ten mp to the left. The latter effect is ust opposite to what was observed in the aromatic-substituted anthraquinone. The depressing of the right-hand peak is insufficient to obliterate it, as seems to have happened with the 3'-sulfonic acids of the other series. Even more instructive is the comparison between the 1,4-di-P-sulfatoethyI-

2.4

0402 -

2002

4

6

0 3 c M 2 4 6 64002 4 6 0 5 0 0 2 4 6 0 6 0 0 2 4 6 87002 4 6 8 % I WAVE LENGTH IN MILLIMICRONS

FIG.10. TOLUIDINE GREEN AND ITS ISOMERICSULFONIC ACID , Toluidine Green (2’-SO8Na) ; - - - -, Isomer (3’-S03Na) TABLE VI11

COMPARISON OF 8-HYDROXYETHYLATED DYES

11

11

1

DYE

PIG.

Sulfated 1,4-Di-D-hydroxyethylaminoanthraquinone 5,8-Dihydroxy-l, 4-di-8-hydroxy-

DIFFEXENCE IN 1 AND 2,4AND 5

I ~

~

578? 624 ~

ethylaminoanthraquinone,

11

unsulfated 5,8-Dihydroxy-l, 4-di-&hydroxy-

664

I

36

696

1

56

ethylaminoanthraquinone,

1 1

sulfated Toluidine Green Toluidine Blue

200

300

400

500

40

6 00

700

1

50

42

800


FIG. 11. DERIVATIVES O F 1,4-DI-@-HYDROXYETHYLAMINOANTHRAQUINONE 1,4-Di-8-hydroxyethylamino-5,8-dihydroxyanthraquinone; ---------, 1,4-Di-8-sulfatoethylamino-5,s-dihydroxyanthraquinone; -.-.- , 1,4-Di-@-sulfatoethylaminoanthraquinone. 177

-,

178


anthraquinones with (Fig. 11, Curve 11) and without (Fig. 11, Curve 111) hydroxyl groups in the 5,8-positions. In the latter there is but one peak in the ultraviolet, and the absorption in the red is considerably to the left; furthermore, the two peaks in the red are closer together, and of about equal magnitude. This might make it appear that the second peak in the ultraviolet was dependent upon the presence of the hydroxyl groups, but this cannot be true since both Anthraquinone Violet and Alizarin Cyanine Green, in which there are no hydroxyl groups, have double absorption in this region. The effect of the 5,8-hydroxyl groups is just the same here as in the Toluidino series-the red peaks are moved to the right 26 and 36 mp. There is no legitimate comparison in the ultraviolet, since it is unknown with which of the two bands of I1 the single peak of I11 corresponds. The basal structure of acid anthraquinone dyes has long been accepted as that represented in general formula I; when aromatic groups occur in the amino side chain, it is known that they are preferentially attacked on sulfonation. I n the first paper of this series (l),the structure of Toluidine Blue was written as having hydrogen bonds, with the sulfonic acid group represented by the letter S. From the examination of a number of dyes it has become evident that a sulfonic acid group in the 2’-position has a different effect from one in the 3‘- or 4’-positions, as evidenced by the change in shape of the absorption curves; that is, the grouping - SOsM is not linked alike in the two instances. S o w from an inspection of the formula, it is seen that while a -SOa- group in the 3’- or 4’-positions could have no effect on the bonding, when the group is in the 2‘-position, it can partake in ring formation with the imino hydrogen, making a different hydrogen bond system, as shown in 111. 02

I11 All dyes sulfonated in the 2‘-position are now assumed to have the possibility of such a bond arrangement, and the formulas IVZand V must be considered as contributing to the complete structures of Toluidine Blue and Toluidine Green,

IV

v


179

whereas in the isomeric sulfonic acids the imino hydrogen can only be bonded to the carbonyl oxygen as shown in the partial formula VI, VIa.

H

\

0’’

KR

C

C

I

II

\ / C VI

VIa

The absorption curves of these dyes, as well as others not shown in the figures, are of two general types, differing sharply in the nature of the main band in the visible. One type has a smooth curve while the other is characterized by a double head. The variations were first noticed among the isomeric sulfonic acids, and attributed to the location of the sulfo group, but later it was found that the unsulfonated “dye bases” themselves exhibited similar phenomena. Thus, 1,5-di-p-toluidinoanthraquinonehas the smooth curve, whereas the 1,4-isomer shows two maxima on the main band (Fig. 4). Based upon an inspection of a number of dyes, it has now been concluded that when there are in the 1- and 4-positions two groups which are able to furnish electrons by a mesomeric shift (VII), the main band of the absorption curve will have a double head. If but one group of this type is present (VIII), only a single head will be observed. The twofold shift of electrons, originating, for instance, with unshared pairs of electrons of nitrogen or oxygen atoms, appears to be responsible for the differentiation of the one head into two.

1-11

VI11

This interpretation accounts in part for the nature of the curves of the 1 5- and 1 4-ditoluidinoanthraquinones. Kow if the unshared pair of electrons in question on the nitrogen atom is restrained from a shift towards the anthraquinone nucleus, it is to be expected that the type of curve with a single head will result. Such a restraint can be effected by a sulfonic acid group in the benzene ring attached to the nitrogen atom. It has been found that sulfonation in the 3’- (meta) or 4’- (para) position has just this effect; the restraint is attributed to the strong inductive effect of the positive sulfur atom, which is transmitted through the aromatic ring system to the nitrogen atom and its unshared pair of electrons. However, when the sulfonic acid group is in the 2’-position (ortho to the imino nitrogen), a hydrogen bond will be formed by the sharing of an electron pair of an oxygen atom with the hydrogen

180


of the imino group (111). The hydrogen, in turn, will to some extent release its electrons shared with the nitrogen atom, so that the effect of the sulfonic acid group on the rest of the molecule will be diminished,-a sort of internal “short circuit”. This interpretation is supported by the absorption curves of the dyes having an aliphatic side chain, a t the end of which is the sulfate group (Fig. 11). Since the effect cannot be transmitted (as readily) along an aliphatic chain, these dyes should have two maxima, and they do. The conclusions discussed above were reached after a study of a large number of dyes, all of which were essentially of two general types; a t the same time these were largely isomers (1,4- and 1,5-series). As a sort of test, it is now of interest to venture predictions as to dyes of other, less closely-related structures. First, consider Alizarin Saphirol (IX), (11). The main band should have two heads, for the following reasons: (a) It was shown above that the differentiation of the absorption curve into a band with two maxima may be attributed to the twofold mesomeric shift of electrons into the anthraquinone system, these electrons originating with the substituents in positions 1 and 4. In IX, as in

IX Toluidine Blue (IV), the NH2 and OH are a suitable source of electrons. (b) A sulfonic acid group in position 2 or 3 of the anthraquinone nucleus, by its inductive effect, would not be expected to restrain, but rather, to enhance the shift of electrons towards the ring from positions 1 and 4,and the short circuit described above should work in the same direction. Hence, the introduction of n sulfonic acid group into position 2 or 3 of the anthraquinone system should result in dyes which do not exhibit the single but the more complicated doubleheaded absorption curve. This is, indeed, the case (Fig. 12). Second, consider

X Alizarin Rubinol G (X), (12). In this dye, the necessary groups are present in the 1- and 4-positions, but the effect of the carbonyl oxygen is to counteract the

181


mesomeric shift of the electrons into the ring at position 1. Consequently, the absorption band would be expected to have a single head, as is actually the case (Fig. 12). In each figure, the abscissa is in millimicrons, while the ordinate is optical density; the latter is defined as loglo 9, where T = transmission. The solution was examined in a cell 1 cm. in thickness. We are indebted to Mr. E. E. Richardson and his staff for the absorption curves. The dyes used were mostly those the preparation of which has been given in the earlier papers (1, 2, 3). Anthraquinone Violet (4) and Alizarin Pure Blue (7) were secured following directions from the patent literature, but using intermediates free from isomers. We are indebted to Dr. J. G. Baxter for a very pure specimen of 1,4-di-p-toluidinoanthraquinone,and to Dr. J. B. Dickey for the sodium 1 ,4-di-~-sulfatoethylaminoanthraquinone.

500

400

WAVE

600

700

LENGTH IN MILLlMiCROMS

FIG. 12. PARTIAL ABSORPTION CURVESO F Two DYES

, Alizarin Rubinol G (1:4OOO); ---------, Alizarin Saphirol B (1:3OOO) SUMMARY

The absorption spectra of a number of acid anthraquinone dyes have been recorded, and attention called to the effect of certain atoms or groups of atoms in various positions in the molecule. An explanation has been suggested to account for the marked difference in the nature of the main bands in the visible. The influence of the position of the sulfonic acid group has been related to the possibilities of hydrogen bonding. The correctness of the assumptions has been verified in two instances. ROCHESTER, N. Y.

REFERENCES (1) ALLEN,FRAME,AND WILSON,J. Org. Chem., 6, 732 (1941). (2) ALLEN,WILSON,AND FRAME, J . Org. Chem., 7, 68 (1942). (3) ALLEN,FRAME, AND WILSON,J. Org. Chem., 7, 63 (1942).

182


(4) SCHULTZ, “Farbstofftabellen,” 7. Auflage. Akademische Verlagsgesellschaft m. b. H., Leipeig, 1931, No. 1208, page 536. German patent 108,274; [Frdl., 6, 311 (18971900)1. (5) SCHULTZ, No. 1201, p. 532. German patent 91,149; [Frdl., 4, 315 (1894-7)] (6) SCHULTZ, No. 1202, p. 533. German patent 181,879; [Frdl., 8, 318 (1906-7)]. (7) SCHULTZ, No. 1193, p. 528. (8) SCHULTZ, No. 1199, p. 531. German patent 126,392; [Frdl., 6, 360 (1900-2)]. (9) German patent 206,645; [Frdl., 9, 720 (1908-IO)]. (10) HOUBEN,“Das Anthracene und die Anthrachinone,” Verlag Georg Thieme, Leipeig, 1929, p. 15. (11) SCHULTZ, No. 1187, p. 525. (12) SCHULTZ, No. 1210, p. 537. (13) PAULING, “The Nature of the Chemical Bond,” Cornell University Press, 1940, pp. 318, 329.

1 ,5-di(2'-sulfo-4'-methylanilino)anthraquinone (Anthraquinone Violet

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