August, 1923
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T h e Spect rop hot ornet ric I den ti ficati on of Dyes’*z . I-Acid
Dyes of the Patent Blue Type By Walter C. Holmes
COLORLABORATORY, BUREAUOF CHEMISTRY,‘WASHI~.GTON, D . C .
Y MEANS of the vidual dyes. Such conThe degree in which the intensity of the absorption of solutions s p e c tro-anal y t i c a l stants would afford a defiof dyes is modified by variations in concentration, solvent, alkalinity, methods and the nite mathematical expresacidity, and other conditions, is dependent upon the degree of alterspectroscopic data supplied sion of the essential propation of the dye in molecular form or aggregation, which, in turn, by F o r n i h e k and Granderties of the dyes. is conditioned upon constitutional differences. The quantitatioe rnouginlB it is possible to The Patent Blues, Xymeasurement of intensity of absorption under such variation in identify with comparative lene Blues, and Alphazuconditions, accordingly, affords spectrophotometric ratios, or conease and moderate certainty rines were selected as comstants, which have important diagnostic value for purposes of identhe majority of dyes which prising a group of dyes t ifieat ion. were marketed prior to offering such difficulty in A n analytical scheme for the identification of acid blues of the those y e a s . Their general identification as to afford a Patent Blue type is outlined, in which such spectrophotometric scheme of spectroscopic satisfactory test of the proratios are employed to supplement the spectroscopic data customarily identifietition has a wide posed scheme, while various obtained. A comprehensive table of constants for the identification range of effectiveness, afother acid blues of similar of the dyes in question is supplied. fording, in some instances, a general type were included satisfactory differentiation to renderthe data morecombetween dyes which may hardly be identified by other means. plete. The samples examined were those of the ordinary comAlthough the general value of their methods is beyond mercial product as supplied to the textile trades, with the exquestion, it is necessary to admit that they do not invariably ception of one sample of Alphazurine A, which was manufacsupply a satisfactory means of distinguishing between closely tured and purified with great care and was, accordingly, of a related dyes, and that the need for a more adequate basis higher grade of purity. The chemical structure of the pre-war of identification is becoming more evident each year with products may be ascertained by reference to the ‘(Farbstofftathe increasing multiplicity of such products upon the market. bellen” of Schultz. Of the American products Pontacyl Justification for the present investigation is found in a recog- Brilliant Blue A and Alphazurine FG are chemically identical, nition of that necessity and in a belief that the earlier respectively, with Xylene Blue AS and Erioglaucine A; investigators in the field have neglected to make full util- while Alphazurine A and Alphazurine 2 G are dyes, respecization of certain aspects of absorption, either through a tively, of the Blue A and Blue V types, in the manufailure to realize their value as an aid in identification or facture of which the distinctive intermediate is 2,5-disulfobecause suitable instruments for their quantitative study benzaldehyde. were not, available. A Hilger wave length spectrometer equipped with a In the analytical scheme of Formanek and Grandmougin Xutting photometer, a type of instrument combining good great emphasis is placed upon the diagnostic value of precise -accuracy with great convenience in operation, was employed. determinations of the spectral locations of the maxima of One-centimeter cells were used in all measurements except the absorption bands of solutions of the dyes in various sol- those carried out with comparatively concentrated aqueous vents, a particular wherein exact measurement is a matter solutions. Suitable thin cells for the latter solutions are of extreme difficulty. On the other hand, various phe- easily constructed by cementing to a microscope slide a thin nomena involving modification in the form or intensity of the sheet of mica cut in the form indicated in the accompanying absorption, which may be of equal or more important signifi- diagram. In operation a thin layer of petrolatum is applied cance, are utilized only in a qualitative and incomplete to the free surface of degree which greatly impairs the value which the methods the mica, a second slide would otherwise possess. The failure to supply a quanti- placed thereon, and tative record of such modifications in absorption is, further- the cell completed by more, a serious defect which operates to limit the utility of tightly winding the the methods in the hands of the inexperienced analyst. central and outer porThe present investigation was undertaken as a test of the tions with rubber bands. The subsequent introduction of effectiveness and limitations of an improved scheme of thesolution is easily effected. In the case of cells of the spectrophotometric identification, which, it was believed, thickness employed in the present investigation (0.0432 would be free from the more serious of these objections. cm.) it is possible, with a little practice in manipulation, It was proposed to investigate the spectroscopic behavior of to reduce the variation in thickness of the solution layer to dyes in a quantitative manner by means of the spectrophotom- within negligible limits. eter, with the expectation that it would be possible to Stock solutions of the dyes were prepared by dissolving obtain a series of spectroscopic and spectrophotometric 0.25 gram of dye in 250 cc. of distilled water, from which hich would be found susceptible of correlation all the solutions required were obtained by appropriate with structural differences and characteristics of the indi- dilution. In the study of the influence of alkalinity it was considered advisable to defer the addition of the alkali until 1 Reccived March 1, 1923. Presented before the Division of D y e Chemistry a t the 65th Meeting of the American Chemical Society, New the solution had been diluted to nearly its final volume, and Haven, Cunn , April 2 to 7, 1923. to carry out the measurements immediately after the comContribution No. 72 from the Color Laboratory, Bureau of Chempletion of the dilution. With the alcoholic solutions the istry, Washington, D. C. alcohol content was made approximately 91 per cent in all 3 “Untersuchung und Nachweis organischer Farbstoffe duf spektroinstances, although it was found that an appreciable variation skopischeni Wege,” published in final form in 1908, 1911, and 1913.
,
ol
2
INDUSTRIAL A N D ENGI NEERING CHEMISTRY
834
in that respect was required to influence the measurements in any particular. The general procedure followed consisted in the measurement of the absorption of the dyes throughout the visible spectrum, with such variation in concentration, solvent, alkalinity, or acidity as was necessary to define characteristic differences in behavior. It was found, however, that the
Vol. 15, No. 8
trated solutions in the region of 600pp, disappears in large measure. The maximum intensity of absorption-in proportion to dye present-is found in the alcoholic solution, the curve of which shows no indication of a hump and is narrower and steeper than the curves of aqueous solutions. It is believed by the writer that the symmetrical broadening of the absorption curves which occurs in passing from alcoholic to aqueous solution and becomes more pronounced with increasing concentration in the latter, may be accepted as evidence of an increasing degree of molecular association of the dye, while the local hump a t about 600pp is unquestionably an incipient modification of the general type which has been previously reported by the writer,6 and which is held by him to connote radical intramolecular rearrangement. The degree of decrease in relative magnitude of absorption in passage from alcoholic to dilute aqueous solution, abd from the latter to concentrated aqueous solution, constitutes a measure of the degree in which the phenomena referred to take place. The data which were obtained with change of concentration and of solvent, and which would appear to he of service for the purposes in view, are given in the following table of constants (p. 836). The values recorded in Columns 1 to 5 are the following: 1-The approximate spectral location (in millimicrons) of the absorption maximum in aqueous solution. 2-The same value for itlcoholic solutions of approximately 91 per cent alcoholic content. 3-The ratio of the magnitude of absorption a t the maximum in concentrated aqueous solution to that in dilute aqueous solution. 4-The ratio of the magnitude of absorption a t the maximum in dilute aqueous solution to that in dilute alcoholic solution. &The ratio of the magnitude of absorption a t the maximum in concentrated aqueous solution to that in dilute alcoholic solution.
540 560
580 660 620 640 WAVE LENGTH
660
683
FIQ. 1 Curve 1 = 30 parts Pontacyl Brilliant Blue A per million parts 92 per cent alcohol (1.002 cm layer) Curve 2 = 40 parts Pontacyl Brilliant Blue A per million parts water (1.002 cm layer) Curve 8 = 1000 parts Pontacyl Brilliant Blue A per million parts water (0.432cm. layer)
complete measurement of the absorption of the dyes investigated afforded relatively little of essential value which could not be obtained merely by means of the determination of the spectral location of the absorption maxima and by the measurement of the magnitude of the absorption a t that point.
INFLUENCE OF CONCENTRATION AND SOLVENT The general character and degree of modification of the absorption of the group of dyes under investigation with change in concentration of aqueous solution and with change from aqueous to organic solvent, is illustrated by the absorption curves for Pontacyl Brilliant Blue A recorded in Fig. 1. With change of concentration of aqueous solutions there occurs no decided alteration in the form of the curve. It is evident, however, that the dye does not conform to Beer’s law4 over the range in concentration employed, but that with increasing dilution there is an increase in the relative magnitude of the absorption, as measured by the intensity of absorption at the maximum. It is further evident that this phenomenon is accompanied by two types of minor modification in the form of the curve. With increasing dilution there occurs a symmetrical narrowing of the curve in proportion to its height, while an unsymmetrical development or “hump,” which may be observed in concen4
Pogg. A n n . , 86, 78 (1852).
The term “dilute solution” is applied to solutions of such strength RS will confine their absorption within the limits wherein accurate measurement is possible when examined with the spectrophotometer in 1-cm. layers. The concentrated aqueous solutions contained twenty-five times as much dye as the corresponding dilute solutions. (The employment of still more concentrated solutions was considered inadvisable because of the serious effect whlch even minute variations in the thickness of layer would have upon the accuracy of measurements in the extremely thin cells which would be required.) It was not attempted to define the location of the absorption maxima with great accuracy. It is necessary to recognize that commercial dyes are not pure producta and that minor variation in absorption may be expected between the output of different manufacturers. There is, furthermore, a definite limitation to the degree of precision with which it is possible to locate the maxima of absorption bands, and the results obtained by different investigators will be affected by the instrument employed and by variations in technic. In the case of the dyes employed in the present investigation, there will be found to be an average discrepancy of approximately two millimicrons between the locations of absorption maxima supplied by Formanek and Grandmougin3 and those supplied by Mulliken,e although the same methods of examination and type of instrument were utilized in their determination. The values recorded in the table for the locations of the absorption maxima are to be considered, accordingly, as approximations from which a minor degree of divergence is without significance. It will be seen that they have slight 6 “The Influence of Variation in Concentration on the Absorption Spectra of Solutions of Dyes.” Presented before the Division of Dye Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8,1922. 6 “Identification of Pure Organic Compounds,” Vol. 111, 1910.
INDUSTRIAL AND ENGINEERING CIfEMIXTRY
August, 1923
value in differentiating between the first six color types listed, which comprise the group of dyes under more immediate consideration. The resort to other solvents is without decided utility with these dyes, and in general merely results in a shift in absorption which is of about the same magnitude in all instances. Alphazurine FG and Cyanole Extra, among the dyes of secondary interest in the present investigation, constitute exceptions to this rule, having their maxima only slightly displaced by a change from aqueous to alcoholic solution. The various absorption ratios recorded are also of slight value in distinguishing between the individual members of the group studied, although there is reason to believe that similar ratios would prove of important service in that respect with certain other groups of dyes. However, they afford :t decisive differentiation between the two general types of amino substitution involved, clearly distinguishing the di-alkyl derivatives from the alkyl-benzyl derivatives. They will also prove of value in distinguishing the dyes under investigation as a class from other blues of different constitutional types.
INFLUENCE
OF
ALKALINITY
The dyes of the group investigated undergo two types of modification in absorption under the influence of alkalies. J17ith certain individual dyes an appreciable displacement of the absorption band occurs, while alterations in intensity of absorption take place, in general, which are dependent upon the nature and concentration of the alkali and may require a considerable period of time for completion. The imrnedkte effect of the presence of 0.1 per cent of sodium hydroxide may be that of increasing or of decreasing the magnitude of the absorption, depending upon the dye concerned, while that effect is followed by a gradual decrease in absorption, of which the rate and extent also vary with the individual dyes. Since the displacement of the absorption is always encountered with those dyes which contain a hydroxyl group and is not observed with other dyes, it may clearly be a'ttributed to a reaction of that group with the alkali. The nature of the gradual decrease in absorption which is observed in the alkaline solutions of all the dyes studied, appears to be complex. It seems probable that a progressive and extensive process of association of solute molecules accompanies the conversion of the dye into carbinol form and contributes to the loss in absorption observed. The data included in the table of constants were obtained with solutions containing 0.1 per cent of sodium hydroxide. The values recorded are as follows:
835
considerable measure of the individual dyes. The' differentiation between Alphazurine 2G and Xylene Blue &VS which they render possible is of particular utility in view of the close similarity of behavior of those dyes under the influence of other agents. st
INFLUENCE
OF
ACIDITY
The effect of the presence of acids upon the absorption of the dyes investigated is illustrated by the curves recorded for Pontacyl Brilliant Blue A in Fig. 2. To the eye, the color of solutions of the dyes passes with increasing acidity from. blue through green to yellow. The spectroscope reveals the fact that a yellow compound with absorption in the violet portion of the spectrum is progressively formed a t the expense of the original blue compound and renders it possible to follow the transition in a quantitative manner. The determination of the magnitude of residual absorption of the original blue dye affords a measure of the degree of transition under any definite condition of acidity. It is generally held that the alteration in color under discussion is consequent upon the direct addition of acid to the amino nitrogen atoms, the valency of which increases from three to five. The values listed in the table of constants are ratios of the residual absorption of the dyes under various degrees of acidity t o the absorption of neutral solutions of a corresponding dye content. The acid conditions for which the constants were obtained were as follows: 1C-Two per cent sulfuric acid (95 per cent). 11-One per cent sulfuric acid (95 per cent). 12-One-tenth per cent sulfuric acid (95 per cent). 13-Two per cent acetic acid (99.5 per cent).
*-The approximate spectral location of the absorption maximum. 7-The ratio of the magnitude of the absorption of the freshly prepared alkaline solution to that of a neutral solution of corresponding dye content. &The same ratio after the alkaline solution has stood 20 hours. *-The same ratio after the alkaline solution has stood 5 days.
The appreciable displacement of the absorption maxima of alkaline solutions of Patent Blue A, Patent Blue V, and Cyanole Extra has decisive value for the identification of those dyes. The immediate effect ,of alkalinity upon the,magnitude of the absorption is of no considerable practical importance. On the other hand, it will be seen that the stability of absorption under alkaline conditions is intimately related to constitutional differences in the dyes and influenced, to an appreciable extent, not only by the two general types of substitution within the amino group, but also by the three general types of substitution within the nonaminated nucleus. The constants obtained, accordingly, are characteristic in
WAVE LENGTH FIG.2 Curve 1 = 40 parts Pontacyl Brilliant Blue A per million parts Curve 2 = 100 parts Pontacyl Brillidnt Blue A per million 1 per cent H&O4 Curve 3 = 100 parts Pontacyl Brilliant Blue A per million 2 per cent H2S01 Curve 4 = 100 parts Pontacyl Brilliant Blue A per million 4 per cent H&&
water parts parts parts
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Vol. 15, No. 8
TABLE I SAMPLE Patent Blue A . . . . . . . . . . . . . . . Patent Blue A (Berlin).. . . . . . . Patent Blue A (H). ..... . . . . . . Patent Blue A. Averape., . . . . , Neptune Blue BG ( B ) . . . . . . . , , Patent Blue Superfine (H) . . . . . Patent Blue N (H). .. . . .. . . . . . Patent Blue V ( H ) . . . . , . . . . . . . Patent Blue V, Average.. , . . . . . Alphazurine A (Nat., Textile Dye , . . . . . .. . . . . . . . . . . . . . . . t AlphazurineA (Nat.)Special Dye Alphazurine A (Nat.), Average. Pontacyl Brilliant Blue .4 (duP) Xylene Blue AS (SI.. . . . . . . . , . Xylene Blue AS, Average,, . . , , Xylene Blue VS ( S ) . . . . . . . . . . . Alphazurine 2G (Nat.).. . . . . , , , Alphazurine FG (Nat.1,.. . . . . . Cyanole Extra ( C ) .. . . , . . . . . New Patent Blue B (By). . . . . . New Patent Blue 4 B (By) ... . . .
.
. .
(1) PP
(2)
(3)
(4)
(5)
(6) &P
(7)
(8)
637.5 637.5 637.5 637.5 637.5 637.5 637.5 637.5 637.5
63$?+) 631 630 630(C) 630 630 630 630 630
0.742 0.772 0.774 0.763 0.995 0.973 0.992 0.959 0.980
0.878 0.869 0.823 0.857 0.931 0.931 0.910 0.892 0.925
0.650 0.671 0.637 0.653 0.926 0.918 0.902 0.892 0.910
627.5 627.5 627.5 627.5 628.8 627.5 628.8 627.5 627.5(+)
1.120 1.106 1.175 1.137 1.048 1.028 1.033 1.170 1,093
1,003 1.003 1.060 1.022 1.043 0.997 0.999 1.014 1.013
637.5(-) 637.5 6 3 7 , 5 ( -1 637.5 637.5 637.5 640 640 630 612 5 625 615
630 630 630 630 630 630 632.5 631.3 628,8 611.3 617.5 608.8
0.763 0.781 0.772 0.787 0.789 0.788 0.973 0.958 0.807 0,951 0,909 0 969
0.911 0.880 0.896 0.839 0.832 0.836 0.918 0.939 0.984 0,834 0,932 0.841
0.697 0.690 0.694 0.664 0.657 0.660 0,900 0,892 0.801 0.792 0,846 0.816
637.5(-) 637.5 637.5(-) 637.5 637.5 637.5 640 638.8 630 606.3 625 615
0.964 0.999 0.982 0.960 0.937 0.949 0.993 0.988 0.970 0.980 0.976 0.963
0.508 0.616 0.512 0.516 0.558 0.537 0.819 0.858 0.587 0.766 0,640 0.626
The stability of the dyes to acid conditions is also closely related to structural differences. The influence of the type of substitution a t the amino nitrogen is extremely well defined. Although the effect of variation in the type of.nonaminated nuclear structure is relatively very much less clearly evident, the extremes of acidity employed develop sufficient differences to indicate a definite relationship. In the instances of Alphazurine A and Xylene Blue AS, the effect of 2 per cent of sulfuric acid is sufficiently distinctive to afford a means of decisive differentiation which is the more valuable because the dyes in question exhibit but slight dissimilarity in general behavior. In Table I all the constants obtained are recorded. All values are averages of two or more series of determinations in which a substantial agreement in results was obtained. This table constitutes a chart wherein various significant characteristics in the behavior of the individual dyes, which clearly originate in structural differences, have been recorded in definite spectrophotometric terms. Any of the constants utilized may be obtained with relative ease and substantial accuracy by means of q y modern type of spectrophotometer, while the fact that the majority of them are ratios serves to render the reliability with which they may be determined independent, in large measure, of differences in the type of instrument employed and in the technic and skill of the observer. The various constants recorded do not possess equal diagnostic value, and it will be unnecessary to carry out the entire series of incasurements in order to arrive a t a comparatively positive identification. The mere determination of the location of the absorption maximum in aqueous solution will be sufficient to distinguish the members of the series of the first six color types listed from the dyes of similar general type included in the tables. For the purpose of differentiating between the first six color types, it is recommended that the next step be that of thc determination of the relative magnitude of absorption in neutral solution and in the presence of 2 per cent of sulfuric acid. The value obtained will not only serve to indicate unmistakably the character of the substitution in the amino group, but will further suffice to distinguish between Alphazurine A and the remaining dyes of similar constitution as regards the nonaminated nuclei. The determination of the location of the absorption maximum in alkaline solution will then definitely establish the presence or absence of the Patent Blue type of nonaminated nucleus. It may then remain necessary to distinguish between Xylene Blue VS and Alphazurine 2G. The absorption of the dye in 2 per cent acetic acid will afford immediate evidence of value which should, however, be corroborated by the determination of the degree of stability to alkaline conditions. The determination of further constants may be considered unnecessary, but will be of advan-
~~
0.574 0.561 0.608 0.581 0.867 0.874 0.854 0.815 0.853 >0.01 >0.01 >0.01 >0.01
>0.01 >0.01 0.206 0.337 >0.01 0,340 24.1 11.1
0.293 0.470 0.298 0.474 0.313 0.449 0.301 0.464 Trace > 0 . 0 1 Trace > 0 . 0 1 Trace > 0 . 0 1 Trace > 0 . 0 1 Trace > 0 . 0 1
0.760 0.999 0.727 0.957 0.722 0.992 0.736 0.966 0.046 0.471 0.040 0,429 0,042 0.428 0,049 0.446 0.044 0.444
0.262 0.430 0.234 0.424 0.243 0.427 0.312 0.495 0.317 0,495 0.315 0,495 Trace >0.01 Trace >0.01 0.369 0,526 0.01 0.070 0.141 0.068 0.147 0,059
0.754 0.720 0.737 0.768 0.789 0,779 0.041 0.039 0.733 0.291 0,435 ,458
0.983 0,973 0.978 0.989 0,979 0.984 0.445 0.399 0,929 0.873 0.884 0,884
tage in confirming the preliminary verdict and eliminating the possibility of error. Although it is held by the writer that the scheme of identification outlined has been shown to be effective and may be accepted for application with the dyes investigated without reservation in any essential respect, it is considered advisable to qualify the recommendation of its adoption in certain minor respects. The variation in commercial dyes is such that the determination of exact and unvarying constants of any character for a given color type is not possible. It would be necessary for that reason alone to recommend that identification be based upon substantial agreement in respect to all significant factors, making allowance for a moderate degree of variation in individual values. It was further noted, in the course of the present investigation, that minor discrepancies occurred in duplicate absorption measurements in neutral solvents, which are apparently to be attributed to variations in hydrogen-ion concentration, while evidence was obtained that variation in temperature may have an appreciable influence upon the stability of absorption under alkaline conditions. It is recognized, accordingly, that it would be desirable to modify the procedure to the extent of making provision for a regulation of those factors, and that that course might render it necessary to revise certain of the constants which have been obtained under less definite conditions. Pending such revision of technic and of constants, it may be advisable for the analyst to verify the agreement of the sample under investigation with the color type to which he has assigned it by means of parallel determinations of values with a sample of the latter of known identity if it is available. It is believed, however, that the differences upon which identification has been based are so considerable, and that the influence of any probable variation in working conditions upon any of the constants which are essential to the scheme of identification will be so slight, that the methods outlined and the data supplied will serve to render possible the certain identification of any of the dyes investigated, even by an analyst who may not have access to samples of known identity for purposes of direct comparison. The general principles of identification employed in the present investigation may be expected to be found susceptible of extelisive application. Other types of dyes which offer difficulties in idenkification will be investigated along t similar lines as opportunity is afforded. ACKNOWLEDGMENT Acknowledgment is due to E. I. du Pont de Nemours & Company for samples of Pontacyl Brilliant Blue A and of various pre-war dyes, and to the National Aniline & Chemical Company for samples of the Alphazurineb.
