SOLUTIOYS FOR COLORIMETRIC STASDARDS. IV. Some Factors affecting the Color of Indicator Solutions* BY M. G . MELLON A N D G . W . F E R X E R
In some earlier work it was observed that certain indicators intended for use in the colorimetric determination of hydrogen ion concentration seemed to give erroneous results for the pH range through which they were supposed to undergo their transformation of color. The variation in hue for the same indicator supplied by different manufacturers was particularly dist'urbing. This experience raised the question as to whose indicators one might use with most assurance, and whose method of use should be followed. If either or both of these points are of significance, any information regarding them should be of interest to individuals in various kinds of experimental work involving the use of such compounds. It is of special importance to those attempting to prepare permanent standards designed to match the color of indicator solutions. The present work was undertaken with the object of securing information regarding the effect on certain indicator solutions of the following factors: ( I ) the source (or degree of purity) of the indicator itself; ( 2 ) the method of preparation of the stock solution of any given indicator; and (3) aging, particularly the effect of light.
Previous Work Of the earlier relevant work mention may be made of some of the significant contributions. Several authors have noted variations in the hue of indicators from different sources. Hunter,' on testing samples of congo red from four manufacturers, found a divergence of 40 to j 5 percent from the hue values for a purified product. Methyl orange showed a similar variation. After testing six different samples Collins2 stated that methyl orange for indicator work should be so specified when ordering. Schlegel and Steuber3 found that brom thymol blue from different sources, or successive lots from the same source, may give widely divergent results. Similar observations have been made upon the method of preparing the stock solutions of indicators, and the effect of different substances, such as ethanol, upon the change of hue of indicators. Thiel and Springemann,?Thiel, Wiilfken and Dassler,j and Michaelis and Mizutanis investigated the effect * Presented hefore the Division of Phvsiral and Inorganic Chemistry at the meeting of the .4merican Chemical doriety at Cinciinati, Ohio, Septemher 8-12, 1930. Biochem. J., 19, 42 (r92j). J. Ind. Eng. Chem., 12, 800 (1920). Ind. Eng. Chem., 19, 631 (1927). Z. anorg. allgem. Chem., 176, 64 (1928). Z. anorg. allgem. Chem. 136, 406 (1904). Biochem. Z., 147, 7 ( 1 9 2 4 ) .
1026
M. G. MELLON AND G. W. F E R N E R
of ethanol. Kolthoff’ states that little is known about the influence of different solvents on the sensitivity of indicators. Although he has called attention to a significant “alcohol error” for several indicators in solutions containing from I O to 7 0 percent alcohol, the concentration is very much less than the smaller of these values when one is using one ml. of a 95 percent alcoholic solution in 50 ml. of a buffer solution. Clarks states that brom cresol purple dissolved directly in alcohol has a different hue than when prepared according to his method. I n connection with the stability of indicator solutions Thiel and Springemann9 showed that various indicators fade when exposed to light in the presence of organic solvents. Some faded samples regained their color on standing. Brightman and others1° made a spectrophotometric study of the end point and fading of phenolsulfonphthalein indicators. Marsh” found that indicator solutions react with the bottles in which they may be stored, while Schlegel and SteuberP recommended keeping the solutions in Pyrex flasks, or coating ordinary glass with paraffin or wax.
Experimental Data Preparation of Materials. In purifying the materials used and in preparing the solutions the usual precautions for careful work were observed, such as using “conductivity water,” recrystallizing salts three times, preserving solutions in Pyrex containers, and adhering to Clark’s directions’* in preparing buffer solutions. The latter were made from the stock solutions just before use. I n addition to regular commercial material, intended for use as indicators, samples of methyl orangel and methyl redla were purified according to methods given in the respective references. Otherwise the other indicators were used as purchased, since most individuals probably use them in this form, both because various authors so recommend and because of a natural hesitancy to purify materials retailing at their present price. I n this case also it was of particular interest to ascertain whether different samples were equally good. Most of the indicators were purchased from such reputable firms as the National Aniline Company, Coleman and Bell, Hynson, Wescott and Dunning, the LaMotte Chemical Company, and the J. T. Baker Chemical Company, each of whom stated that they prepared their own products. In this data reference to the source of each indicator used was purposely omitted since it is not the object of the paper to advertise any concern’s product. In case of any marked variations among the results for several different products one should use this particular indicator with suspicion. Most of the indicators ’“Indicators,” 92, 183,184 (1926). “Determination of Hydrogen Ions”, 95 (1927). 9 2 . anorg. allgem. Chem., 176, I 12 (1928). 10 J. Am. Chem. SOC.,40, 1940 (1918). 11 Science, 59, 216 (1924). l2 “Determination of Hydrogen Ions”, 192 (1927). 13 Desvergnes; Ann. ehim. anal. chim. appl., 2, 209, (1920). 8
SOLUTIONS FOR COLORIMETRIC STANDARDS
1027
were purchased shortly before use, but a few had been in stock at least five years before the solutions were prepared. In some cases the solid indicators from the several manufacturers appeared much alike; in others there was a marked difference in the colors, little of which WLLSobservable, however, when the solutions were prepared. Stock solutions of the indicators were prepared by several different methods. One tenth percent aqueous solutions of methyl orange were used throughout, as recommended by K01thoff.l~ Two hundredths percent solutions of methyl red in 60 percent ethanol were prepared according to Coleman and Bell’s dire~ti0ns.l~ Tropaolin 00 was prepared in 0.1 percent aqueous solutions,16 and in 0.I percent solutions in 50 percent ethanol.16 The recommendation of Coleman and BelP was followed in preparing a 0 . 0 2 percent solution of cresolphthalein in 95 percent ethanol. Three types of solutions were used for the indicators of Clark and Lubs : (I) aqueous solutions prepared according to Clark’s directions16 by adding definite amounts of sodium hydroxide to 0.1 g. of the indicator and diluting to 2 5 0 ml.; ( 2 ) alcoholic solutions made by dissolving 0.1 g. of the indicator in 5 2 ml. of neutral 95 percent ethanol, adding the amount of sodium hydroxide specified by Clark, and diluting with water according to Taub’s procedure17; and (3) according to Kolthoff14by dissolving 0.1g. of the solid in 50 ml. of 95 percent ethanol and diluting with water to 2 5 0 ml. With the exceptions noted, Clark’s directions were followed in preparing solutions for the comparison of the different products and for the determination of the effect of ultraviolet radiation: likewise, a single product was used in testing different methods of preparation and in preparing the solution for exposure to ultraviolet radiation. With the exception of tropaolin 00,a sample of each indicator was exposed for two hours to a quartz mercury lamp (Cooper-Hewitt type, designed for operation at four amperes and 2 2 0 volts) by placing a thin layer of the solid on a glass about 30 cm. from the lamp. Solutions were then prepared similar to those for the unexposed samples.
Detemination of Color. Observations of the various lots of the different indicators, in solutions of different kinds, prepared before and after exposure to ultraviolet radiation, were made by means of a Keuffel and Esser color analyzer (Model C). The procedure followed differed from that reported in an earlier paper1* only in the use of two 400 w.Mazda lamps for illumination of the samples in the five cm. tubes. Solutions for the determination of the spectral transmission curves were prepared by diluting one ml. of the stock solutions to 50 ml. with appropriate buffer solutions. Most of the buffers selected were 0.2 pH unit beyond the limits of the useful range of the indicators in order to insure a practically com“Indicators”, 63,64, 72 (1926). “Catalog and Price List”, 68 (1928). 16 “Determination of Hydrogen I o d ’ , 94 (1927). J. Am. Pharm.Assocn., 16, 118 (1927). I* Mellon and Martin: J. Phys. Chem., 31, 161 (1927). I4 l5
S . G . MELLON A S D G. W , FERTER
1028
plete conversion to either the basic or acidic color. One sample of each solution so prepared, except tropaolin 00, was exposed in a quartz container to the same conditions of ultraviolet radiation as were the solids. The data obtained are plotted on a semilogarithmic basis using the percent transmittancy as ordinates and the wave length in millimicrons as abscissas. All of the curves for a given indicator, except thymol blue, are on the same graph. The point used in locating the position of the curves at each interval of ten millimicrons represents the average of five readings on the color analyzer. Of the fourteen indicators studied the curves for cresolphthalein
420
460-
5UO sm s o 620 Nuve-iengfh,in M i ! / i m j c r o n s
660
700
FIG. I
METHYL ORANGE. Curves showing the effect of different samples and different conditions on the spectral transmittancy values for a 5 em. tube containing a solution made hy diluting one ml. of a 0 . 1 percent solution to j o ml. with a buffer solution.
SOLUTIONS FOR COLORIMETRIC STAXDARDS
1029
alone have been omitted since they checked each other so closely. On all the graphs a given number of curve, as N o . I , refers to the product of the same manufacturer. I t may be mentioned that similar spectral transmission curves have already been p u b l i ~ h e dfor~ individual ~ ~ ~ ~ ~ samples ~~ of practically all of the indicators studied, but in these cases the object was not to determine spectrophoto. metrically differences of color.
FIG.2 TROPAOLIS 00.Curves showing the effect of different samples and different conditions on the spectral transrnittancy values for a j ern. tube containing a solution made by diluting one ml. of a 0.1percent solution to 50 ml. with a buffer solution. I s Brode: J. Am. Chern. SOC.,46, 581 (192%); M,ellon and Martin: Ref. 18;Baker and Davidson: Phot. J., 62,375 (1922);Gibbsand haplro: J. Am. Chern. Sac., 50,2798 (1928).
1030
M. G. MELLON AND G. W. F E R N E R
FIG.3 BROM PHENOL BLUE. Curves showing the effect of different samples and m e r e n t conditions on the spectral transmittancy values for a 5 cm. tube containing a solution made by diluting one ml. of a 0.04 per cent solution to 50 ml. with a buffer solution.
SOLCTIONS FOR COLORIMETRIC STANDARDS
FIQ.4 BROM CRESOL PURPLE. Curves showing the effect of different samples and different conditions on the spectral transmittancy values for a 5 em. tube containing a solution made by diluting one ml. of a 0.04 percent solution to 50 ml. with a buffer solution.
103 I
If.G. MELLOX AND G . R'. F E R N E R
1032
/I 420
%0
i
t I m 330 w Move -fergth, in Mi/, nicron 6
160
700
FIG.5 BROM THYMOL BLUE. Curves showing the effect of different samples and different conditions on the spectral transrnittancy values for a 5 cm. tube containing a solution made by diluting one ml. of a 0.04 percent solution to jo ml. with a buffer solution.
SOLUTIOXS FOR COLORIMETRIC STASDARDS
B
i
8;
No. 41
10.2 A/c. ( 7 ) No.2 R/c. (Kl N0.2 /rrudf Job. N0.2 /rradf Joo//Y
FIG.6 PHENOL RED. Curves showing the effect of different samples and different conditions on the spectral transmittancy values for a 5 cm. tube containing a solution made b y diluting one ml. of a 0.04 percent solution to 50 ml. with a buffer solution.
M. G . MELLON AND G . W. FERNER
I
FIG 7 BROM CRESOL GREEN. C w e s showing the effect of different eamples and merent oonditiona on the spectral tranamittancy values for a 5 cm. tube containing a solution made by diluting one ml. of a 0.04 percent solution to 50 ml. with a buffer solution.
SOLUTIONS FOR COLORIMETRIC STANDARDS
---t-t--pN
------pH
-
0
No./
0
No.2
*/ 6.2
c No.3
No.+ No.5 o No. fi-ffeery&i No.Sa /rmdt Soh. A 10. Si /modi? &/id P
4
I
m
%O
so
J*o
a
420
Move-Lengfh, in Mi//iimicrons
660
;oo
FIQ.8
METHYL RED. Curves showing the effect of Merent samples and dif-
ferent condtions on the spectral transmittancy values for a 5 cm. tube containing a solution made by diluting one ml. of a 0.02 percent solution to 50 ml. with a b d e r solution.
1036
31. ti. M E L L O S .4SD G . TV. FERSER
FIG.9 CRESOL RED. Curves showing the effect of different samples and different conditions on the spectral transmittancy values for a 5 cm. tube containing a solution made hy diluting one mi. of a 0.04 percent solution to 50 ml. with a buffer solution.
S O L C T I O N S FOR COLORIMETRIC STASDARDS
126
140
;.lo
Muye-Length, / n M i / / i m i c r o n s FIG.I O THYMOL BLUE. Curves showing the effect of different samples and different conditions on the spectral transmittancy values for a 5 em. tube containing a solution made by diluting one ml. of a 0.04 percent solution to j o ml. with a buffer solution.
1038
M. G. MELLON AND G. W. F E R N E R
60
.WO
S?V
.BO
120
Wove-Lenqfh, in Mi/limicrom
460
7 0
FIG.1 1 THYMOL BLUE. Curvea showing the effect of different samples and different conditions on the spectral transmittancy values for a 5 cm. tube containing a solution made by diluting one ml. of a 0.04 per-cent solution to 50 ml. with a buffer solution.
SOLUTIONS FOR COLORIMETRIC STANDARDS
CHLOR PHENOL RED. Curves showing the effect of Merent samples and Merent conditions on the spectral transmittancy values for a 5 cm. tube containing a solution made by diluting one ml. of a 0.04 percent solution to 50 ml. with a butrer solution.
I040
If. G . J I E L L O S A S D G . \V. F E R S E H
BROM PHENOL RED. Curves showing the effect of different samples and different conditions on the spectral transmittancp values for a 5 cm. tube containing a solution made hy diluting one ml. of a 0.04 percent solution t o 50 ml. with a huffer solution.
SOLCTIOSS FOR COLORIMETRIC STAKDARDS
1041
Discussion For the purpose of the present investigation it seemed that curves coordinating transniittancy and wave length would be of most value as a basis for formulating conclusions. If two solutions, for example, yield curves practically superimposable upon each other, within the limits obtainable with the spectrophotometer used, one may conclude that the samples exhibit the same color. But if there is a marked divergence between the curves for two different solutions, this fact in itself is definite evidence of divergence in the colorimetric characteristics of the two systems. Obviously, the spectrophotometric curves show merely whether there is a difference between samples: where a difference is found little direct evidence is provided to indicate the cause. Having in mind that our present interest is in the divergence of the curves for any given indicator, we may consider the data from the view point of several different factors thought to be of possible significance in connection with the use of indicator solutions. These factors are discussed separately below. Effect of Cltraviolet Radiation. The chief visible effect of ultraviolet radiation on the solid indicators was to cause considerable darkening during exposure of brom phenol blue, brom cresol green, brom cresol purple, and brom thymol blue. Solutions of the exposed solids gave curves which were practically identical with those for the solutions of the unexposed solids. Any differences of appearance of the irradiated solids did not extend to the hue of the solutions. I n view of the effect on the solutions, however, perhaps a longer exposure would have produced more significant results. Fading, varying from an almost negligible amount to a complete disappearance of hue, occurred in all of the solutions which were exposed to ultraviolet radiation. Absence of color would indicate the destruction in the solution of whatever had been functioning in the system as a selective absorber of visual radiation. Such effects raise the question of the possible action of sunlight on any solutions found to be sensitive to ultraviolet radiation. Effect of the iTlethod of preparinq the Solutzons. From a study of the curves it is evident that differences in the method of preparing the indicator solutions had no appreciable effect in the case of tropaolin 00, brom phenol blue, brom cresol green, brom cresol purple, brom thymol blue, phenol red and cresol red. Only one kind of solution was used for methyl orange, methyl red and cresolphthalein. For the remaining indicators the curves obtained for the different solutions varied more or less. I n most cases the solutions prepared according to Taub’s method compare quite favorably with those prepared following Clark’s directions. Those prepared according to Kolthoff’s suggestion varied to a greater or less extent from those prepared by the other two methods. From the curve it would seem that the small amount of alcohol used in preparing the solutions made no appreciable difference in the color. It is a question T\ hether the variations in the solutions prepared by Kolthoff’s method were
1042
hl. G . MELLOS AND G.
W.
FERNER
due to the absence of sodium hydroxide in the stock solutions or to the presence of alcohol. Since the solutions prepared by Taub's method contained alcohol and agreed fairly well with those prepared by Clark's method, it is reasonable to assume that the differences are connected with the neutralization of the indicator in preparing the solutions, rather than with the presence of the alcohol. E f e c t of the Source of the Indicators. The appearance of the solid indicators from the different sources varied considerably. While these differences were quite marked, they did not extend to the solutions; in fact, some of the solids appearing most nearly alike showed the greatest variation in their spectral transmission curves. The curves for the different samples of methyl orange, tropaolin 00, brom phenol blue, brom cresol green, brom cresol purple, phenol red, and cresolphthalein, compare quite favorably with each other. The small variations in these indicators would probably be of no significance in the colorimetric determination of pH values. The remaining indicators, presenting variations more or less marked, are considered individually. For methyl red the agreement was fairly good with the exception of one old sample (more than five years) and of the region of shorter wave lengths a t pH 6 . 2 . For thymol blue, a t both sets of pH values, two samples stand out as being different in color. One of these (KO.4) was an old sample. The curves for chlor phenol red show the widest variation, especially at the higher pH value and for one old sample ( S o . I ) . Qualitative tests on this sample showed its pH range to be approximately 6.4 to 7.6 inst,ead of the normal range of 4.8 to 6.4. Thus, the hue of the solution was still yellow at a pH value where it should have been fully red*. The two available samples of brom phenol red, one of which was old (No. I ) , gave curves quite different from each other. In connection with this indicator, as also with chlor red phenol, it may be mentioned that the manufacturer of the old sample stated it is their belief that results, such as t'hose encountered with chlor phenol red, may be due to a decomposition of the indicator or to an incomplete halogenation a t the time of preparing the solid material. I n the present instance the old sample of chlor phenol red was not showing its normal transformation range at the time of purchase. For brom thymol blue the chief variation occurred, curiously, at the minimum point for the higher pH value. For cresol red one sample stands out as having a high transmittancy. In all such cases it may be kept in mind that a high transmittancy means that the solution is absorbing less visible radiation and hence may be referred to as being paler or having a higher relative brilliance. Presumably such a solution contains a smaller concentration of the selective absorber due to the lower degree of purity of the solid indicator. * Harden has just published [J. .4m. Chem. Soc., 52, 4611 (rg30)] a note regarding variations in different samples of chlor phenol red.
SOLUTIONS FOR COLORIMETRIC STANDARDS
I043
Summary and Conclusions
As a result of the foregoing spectrophotometric study of certain indicators, it may be concluded, on the basis of the curves obtained, that the source of the compounds, the method of preparing their solutions, and the action of ultraviolet radiation upon the latter are all of more or !ess significance in affecting the color of the solutions. These effects vary with the different compounds, but in general the actions may be summarized as follows: I. The decolorizing effect of ultraviolet radiation on buffer solutions of indicators shows their photochemical sensitivity. I n view of this action, and of the change in hue of some of the solids on irradiation, such systems probably should be protected from strong sources of photochemically active radiant energy. 2. Differences in the method of preparing the indicator solutions may have an observable effect upon the hue obtained. I t would seem advisable, therefore, to specify a uniform method of preparing such solutions whenever any variation would be liable to produce significant variations in the results obtained with the use of the indicator. 3. Indicators from different sources may show considerable variation in the color of their solutions. Whatever may be its cause, the fact of this variation’s occurrence makes necessary a careful inspection of any sample intended for use wherever the several attributes of color, hue, relative brilliance, and colorimetric purity, must have definite values, as in the possible preparation of permanent colorimetric standards to match indicator solutions. For any work of this kind indicators should meet definite specifications. Purdue Universaty, Lafayette, Indzana.
