and not to the analytical method. To decide whether the problem existed in preparing polymer samples or in the analytical procedure, a commercial polymer containing a phenolic primary antioxidant and a thiodipropionate secondary antioxidant was analyzed several times and an estimate made of the 95% confidence limit from the range data. Five determinations were made, each on a different day, on the polymer which had 0.204% distearyl thiodipropionate in it. The relative 2 u value calculated from the relative range of 3 . 6 8 z was =!=l1.97Z:. This was only slightly greater than the value obtained on standard materials. A number of polyethylene samples containing varying amounts of 4,4’-thiobis(6-terr-butyl-rn-cresol)were analyzed using the new method. Results were very nearly identical to the expected values, and precision of determination was satisfactory. Almost any typical synergist could probably be accurately determined in polyethylene. Two other secondary antioxidants mentioned in the
literature are thiophosphates and secondary phosphites. A thiophosphate was run and, even though the compound was only of technical grade, the data obtained showed that the sulfur was converted t o the sulfone as with other sulfur-containing antioxidants. A discussion of secondary phosphites has already been given. The procedure described in this paper has allowed quantitative determination of secondary antioxidants in polypropylene and polyethylene with great precision. It was shown that diorgano sulfides and tertiary phosphites could be quantitatively oxidized to the corresponding sulfones and phosphates with no interference from other stabilizers or additives. The proposed method has been applied to numerous commercially available polymers with good success, and probably can be used on other polyolefins with equally good results. RECEIVEDfor review February 1, 1971. Accepted July 19,1971.
Evaluation of Color Changes of Indicators Specific Color Discrimination of Phthalein and Sulfonephthalein Indicators V. M. Bhuchar, V. P. Kukreja, and S. R. Das National Physical Laboratory, New Delhi-I2, India The sensitivity, pH’s of maximum color change, and rapidity of calor change of phthalein, sulfonephthalein, and Congo red indicators have been evaluated in terms of Specific Color Discrimination steps, pH,,,, and the half bandwidth of change of SCD in pH units. For rapid calculations of these values, the 1963 Commission Internationale de L’ Eclairage recommendation for perceptually-more-uniform color spacing has been given preference to the MacAdam Ellipses and the RUCS system. The chromatic separations calculated from the coordinates obtained by these systems have been compared and correlated in terms of the standard deviation of color matching. The study has been reported with the hope that the above values may form the reference values of indicators.
resentation, the MacAdam Ellipses obviously would transform t o circles and equidistant color points represent an equal color discrimination. The color discriminations obtained from the U-V methods were correlated with those obtained by MacAdam Ellipses, with a view t o check their use in subsequent work. EXPERIMENTAL
IN A PREVIOUS COMMUNICATION ( I ) on the evaluation of color changes of two screened indicators based on phenol red, we have introduced the idea of specific color discrimination (SCD) as a measure of sensitivity of color change of an indicator. In the present work, we have evaluated the SCD for some phthalein and sulfonephthalein indicators that are usually used in acidimetric titrations, namely, phenolphthalein (mol wt 31 8), o-cresolphthalein (mol wt 348), thymolphthalein (mol wt 430); phenol red (mol wt 354), cresol red (mol wt 384), thymol blue (mol wt 466). Congo red (mol wt 696) (an azo indicator) was included as an extension of the study and also with a view t o cover a wider range of color transition. The Commission Internationale de L‘ Eclairage (CIE) coordinates have been calculated also in terms of the RUCS coordinates U , V [Rectangular Uniform Chromaticity Scale ( 2 ) of representing the colors]. In this rep-
Hilger-Watts Uvispec-spectrophotometer No. 700.308 and Beckman’s Expandomatic pH meter, Model No. 76005 were used. Indicator Solution. Normally, B.D.H. indicators were used. The o-cresophthalein was prepared by a method similar t o the one described by Singhal and Tandon ( 3 ) and Patrovsky ( 4 ) and crystallized from alcohol; mp 221 “C. A 2 x lO-*M solution of each of the indicators in acetone was used in case of phthalein indicators. In case of sulfonephthaleins, 2 X 10-4M aqueous solutions were used. Buffer Solutions. These were prepared according to the Indian Standards Specification (5). Procedure. Five milliliters of 2 x lO-4M indicator solution was taken and diluted with about 10 ml of water. The solution was buffered appropriately to obtain the desired pH; This was checked on the expanded scale of the p H meter and the volume was finally made to 25 ml with water. A final concentration of 4 x lO-5M of the indicator was thus obtained. In case of the phthaleins, the medium obtained in this manner was therefore 20% acetone-containing-water. The values of pH of the solutions are shown in Figure 1. Absorption spectra of each of the indicator solutions of the indicators mentioned above were taken at different pH’s,
(1) V. M. Bhuchar and s. R. Das, J . Opt. Soc. Arne?., 54, 817 (1964). (2) F. C . Breckenridge, and W . R. Schaub, ibid., 27, 226 (1937): 29, 370 (1939).
(3) G. K. Singhal and K. N. Tandon, Talanta, 14, 1127 (1967). (4) V. Patrovsky, ibid., 10, 175 (1963) ( 5 ) Indian Standards Institute, Delhi, India, I.S. Specification No. 3225- 1965.
ANALYTICAL CHEMISTRY, VOL. 43, NO, 13, NOVEMBER 1 9 7 1
1847
0 4oc
o 300
t
1
0 200
i
I
I
12.5
d
4
I
10.01
4
Figure 1. The X , y plot of the change of color with pH of various indicators (pH’s are indicated on each curve) Phenolphthalein Cresol phthalein Thymol phthalein Phenol red Cresol red Thymol blue Congo red
generally in 0.2 pH-unit steps, at 10-nm intervals in a 10-mm cuvette. The chromaticity coordinates were calculated using C.I.E. distribution function for a standard illuminant C (6). The color points a t different pH values are shown in Figure 1. From the color coordinates, the chromaticity difference was calculated as the number of standard deviations of color matching on the basis of MacAdam Ellipses (7). The standard deviation, ds, was obtained from the equation dS2 = gii dx2
+ 2g12 dxdy +
gp2
Ac/APH
(’i,j)As/ApH
dy2
Here dx and dy are the changes in x and y coordinates, respectively, and gll, 2g1z, and g2~are the values of constants of ellipses corresponding t o the color points. As a unit-color-step corresponds to 3 standard deviations of color matching, the Ac corresponding t o the change of p H under consideration, could be calculated and compared. In order t o evaluate quantitatively the sensitivity of the various indicators at different pH’s, the number of color discrimination steps for a small constant pH difference, say 0.1 pH (6) H. D. Murray, “Color in Theory and Practice,” Chapman and Hall, Ltd., London, 1952, p 330. ( 7 ) D. L. MacAdam, J . Opt. SOC.Amer., 32, 267 (1942). 1848
unit, could be considered. But as this is difficult t o achieve experimentally, we followed a more convenient procedure of calculating the average number of color discrimination steps for one pH unit, a t a rate of color change as obtained experimentally for the indicator between two close pH values. We have called this average number of color discrimination steps for one pH unit the Specific Color Discrimination. The specific color discrimination was obtained as
The specific color discrimination (SCD) of each of the indicators with the change of pH is plotted in Figure 2. The corresponding coordinates U , V on the RUCS system were calculated from the following relations ( 2 ) :
u = 0 075 v=
+
0 823 ( X y - 1) 1 x - 7.05336 L‘ - 1.64023
3.69700 x - 5.07713 ._ - 1.36896 .-. - o,50000 -1 x - 7.05336 I’ - 1.64023 --
These are plotted in Figure 3. In this paper U , V should be considered as chromaticity coordinates on the basis of RUCS system of Breckenridge and Schaub ( 2 ) . The chromatic separations, Au, between respective points
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
40
-
30
-
n,
l
.
1
~
~
~
0
?"
-
"
8
"
9
" 10
"
12
I1
Figure 2B. Specific Color Discrimination (SCD) of sulfonephthalein indicators with p H (MacAdam's Ellipses method). Phenol red Cresol red Thymol blue Congo red
-X-X-A-A-@--@-
-m--n-
from the RUCS coordinates U, V were obtained from the relation
bn
-
Figure 2A. Specific Color Descrimination (SCD) of phthalein indicators with p H (MacAdam's Ellipses method).
- -X- - - - X- - -A- - - -A- - -0- - -0-
r
Phenolphthalein Cresolphthalein Thymolphthalein
In terms of standard deviation As of color matching, which defines an area of shades of an individual hue indistinguishable from each other, the Au appeared to be a large unit. To evaluate Au in terms of As, the As/ApH values (MacAdam Ellipses method) were plotted against 1000 Au/ApH for a few indicators as shown in Figure 4. These curves show nearly the same slope of tan 1: 1. The values of Ac/ApH for different indicators obtained by the RUCS system according to the following equation are shown in Figure 5. Ac/ApH
=
(I/%)
X (1000) Au/APH
GREEN
BLUE
t
-8.0
Figure 3. The U-V-coordinates of the changes of color with p H of various indicators in RUCS System (Index as in Figure 1)
. I 140
pH's are indicated on each curve
L
I
1
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1849
/ /
/
t
/ /
Figure 4. Correlation of the As/ApH values determination from MacAdam’s Ellipses and 1000A(r/ApHby RUCS method (Index as in Figure 1)
60-
50-
Figure 5. Specific color discrimination (SCD) of phthalein and sulfonephthalein indicators (RUCS-system) (Index as in Figure 1) 1850
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
97
0 20 0.15
,
I
0 I?
I
1
0 I9
I
I
I
0 21
I
0.2)
1
, 0 25
1
I
0 27
1
I
I
0.29
U--+
Figure 6. The u-u coordinates of the change of color with p H of indicators according to CIE’s recommendation (8) of the preceptually more uniform chromatic spacing The curves show the same pattern and nearly the same values as given in Figure 2 . A simplified transformation t o the perceptually more uniform chromatic spacing has been recommended (8) by the C.I.E. in 1959 and 1963 in which u =
L’
=
4x
X + 15Y + 3 2 6Y ~~
X + 15Y = 3 2
Calculations according t o this recommendation have been done for the progress of color change in the case of the indicators phenolphthalein, o-cresolphthalein, and the thymol blue. The values of u, L‘ coordinates so obtained are shown in Figure 6. The shape of the curves obtained from the Breckenridge and Schaub’s calculations ( 2 ) and the CIE‘s 1963 recommendation (8) are similar in plot, but the numerical values are very different in the latter case. The values of sA/AH for the three indicators obtained by either method are close enough and are given in Table I for comparison. DISCUSSION
In the above investigation we have preferred using a 20% acetone-containing aqueous solution of the phthalein indicators, because these, especially the thymolphthalein, tends t o precipitate out at lower pH values of the solution, if acetone concentration becomes less than 20%. Even though the maximum color change comes off a t a slightly higher p H in presence of a 2 0 x acetone containing aqueous (8) C.I.E., Compte Rendu, Brussels, June 1959, Vol. A , p 3 7 ; i b i d , Vienne, June 1963, Vol. A, p 34.
Table I. Comparison of As/ApH Values by Breckenridge and Schaub’s Method and by CIE 1963 Recommendations Breckenridge and Schaub’s CIE 1963 method recommenpH interval (2) dation (8) Indicator Phenolphthalein 9.40- 9.60 19 18 10.20-10.30 126 118 11.00-12.20 12 11 o-Cresolphthalein 8.88- 9.08 188 181 9.47- 9.81 39 36 10.01-10.31 25 25 Thymol blue 1.30- 1.50 58 54 1.50- 1.60 71 85 2.12- 2.25 32 34 7.70- 7.90 15 15 8.58- 8 . 8 0 63 54
solution than in aqueous solution, the above medium was adopted for comparison sake. The CIE coordinates x, y of the color points of the different indicators have been calculated as also their RUCS coordinates U , V as a part of the investigation. The range of color of these indicators is spread over all the four quadrants of color scale, each quadrant giving a different principal color-namely, red, green, blue, and purple. For reason that by the method of representation by U , V coordinates, a n equal amount of color extension indicates nearly the same chromatic discrimination, a relation between the specific color discrimination as determined by MacAdam Ellipses and U , V coordinates of the RUCS system, was calculated. The two modes
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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Table 11. QSCDValues at Maximum Color Change of Phthalein and Sulfonephthalein Indicators Indicator Phenolphthalein U-Cresolphthalein Thymolphthaline Phenol red Cresol red
Range of color change Alkaline Alkaline Alkaline Alkaline Acidic Alkaline
Thymol blue Congo red a Specific color discrimination. pH of maximum color change.
Acidic Alkaline
gave nearly similar values over a large range of color extension. F r o m comparison of the changes of color of phthalein indicators with those of the corresponding sulfonephthalein indicators, the following observations were made; some well known characteristics were confirmed: Figure 2, A and B, represent the progress of specific color discrimination (SCD) values of the different indicators with change of pH. These values a t maximum color change are given i n Table 11. Table I1 brings out the following: o-Cresolphthalein is the most sensitive of the indicators. At the p H where the color changes maximally, a just discriminable color change would require a change of 0.13 p H unit only. But phthalein indicators suffer from the defect that they tend t o precipitate out in aqueous solutions when these are acidic. The phthalein indicators are much more sensitive than the corresponding sulfonephthalein indicators. The rapidity of change of color in the case of the phthalein indicators, as shown by half bandwidth of color change, is 2 times more than in the respective sulfphonephthalein indicator a t corresponding pH’s. There is a single range of p H (alkaline range) for color change of phthaleins while there are two ranges of pH of color changes in the case of sulfonephthaleins, one a t the lower p H range and the other at the higher p H range. The color changes of the phthaleins a t very high acidity (9) of nearly 9 N HC1 which correspond t o those of the sulfonephthaleins in acidic pH’s have not been dealt with because of their little consequence. The specific color discrimination curve of a particular sample of o-cresolphthalein (Russian origin) gave erratic results. The indicator was given up as an impure product. The results reported in the present study (Figures 1 and 2 ) are those of a n indicator prepared as described (3) and show a single peak in SCD/pH plot as expected. At the higher pH’s, the color changes of phthaleins run parallel t o the color changes as occur in sulfonephthaleins. These have been ascribed t o corresponding lactonic/sulfonic quinone phenolic changes in the molecular structure of the indicator. Parallel running of the color extension indicates different degrees of color saturation in corresponding phthaleins and sulfonephthaleins. (9) I. M. Kolthoff, “Acid Base Indicators,” The Macmillan Company, New York, N.Y., 1937. 1852
b~Hmca 10.26 9.00 10.50 10.90 7.68 8.32 0.55
1.75 7.90 8.70 1.55 8.90 5.05
%CD at max color change 45 71 35
Half band width of change of SCD in pH units 0.40 0.40
0.50
30
0.40
35 16 14 11 23 19 20 27 17
0.85 0.85
0.70 1.oo 0.90 0.80 1.15 0.90 0.72
In each p H range of color change-Le., acidic or alkaline region-cresol red shows two close positions of maximum color change. This was suspected to be due to the indicator being not pure. The cresol red was developed by thin layer chromatography on silica gel (10). Methyl ethyl ketone was shaken with water (50:lO); the methyl ethyl ketone layer was used for developing. Three distinct spots were observed. The main spot constituting the indicator was the one having R, value of 0.3. The two humps in the SCD/pH curve may be ascribed t o the presence of impurities. The change of color, viz., yellow t o violet of phenol red, even though it is a sulphonephthalein has been described as possessing a single p H range of color change, viz., 7.6-9.2. The plot of specific color discrimination As/ApH of this indicator (Figure 2) made it evident that this indicator also shows two positions of pH,,, even in this range. As in the case of cresol red, the TLC resolved this indicator into three spots indicating it also was an impure product. However, it was expected that this indicator, in common with other sulfonephthalein indicators of the series, should possess two ranges of color changes. Qualitatively, it was noticed that the indicator transforms from yellow (pH 6 ) t o red in a 2N hydrochloric acid solution. The transformation in this range is very gradual. This is of little consequence in the acid-base titration. It was therefore not pursued. King (ZZ) represented the changes of color of various indicators with change of p H in a color diagram. He has indicated the rate of color change on a scale of least perceptible difference based on the Lovibond-Schofield system. But no values of merit of the rate of color change were adopted by him as have been done in the present paper (Table 11). Comparison of sensitivity and rapidity of color changes could not be made in his case except only partly from the diagram for each indicator. In the present study, this comparison would become apparent from Table I1 and Figure 5. Reilley et a/. (12) have discussed the color changes a t considerable length. They have considered this in more details than the chromaticity steps alone, with which we have concerned ourselves. They included in their discussion the relative grayness of the indicator and the effect of memory. (10) E. Stahl, “Thin Layer Chromatography,” Springer Verlag, Berlin, 1965, p 346. (1 1) J. King, Atlalyst, 77, 742 (1952). (12) C. N. Reilley, H. A. Flaschka, S.I.F. Laurent, and Bertel Laurent, ANAL.CHEW., 32, 1218 (1960).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
For the latter they have assigned a n arbitrary value of 10 memory steps for each distinguishable color .field and the number of memory steps per angle of each of the color fields differs. This evaluation becomes, consequently, fairly complicated. They have, however, arrived at their conclusions with the following limitations. They presumed that one is concerned simply with two color points of the indicator, i.e., one before and the other after the end point; and that the indicator at these points is present in two limiting forms, viz., 100% free or 100% converted form. This is hardly true; no doubt one is also concerned with the progress of the color changes during the course of the reaction, especially in the case of acid-base indicators whose color transition occurs over a wide range of about 2 pH units. The end point in these indicators depends upon the nature of titrants and the medium used. The optical concentration of the indicator depends upon the pH of the solution at a particular moment. In the present paper, we have attempted t o show the color changes of the indicators step by step in terms of SCD values as a measure of the sensitivity of the indicator.
The rapidity of color changes has been shown by the halfbandwidth (Table 11). The effect of memory is a n important consideration, but has not been taken into consideration in the present paper. The authors feel that the CIE recommendation of 1963 on the perceptually more-nearly-uniform-color spacing [three dimensional (S)] would take care of the effect of luminosity (grayness) at least partly. The u-v coordinates of the perceptual1 y-more-nearly-uniform color spacing recommended by the C.I.E. have been checked for their application t o color change of indicators (Table I) (Figure 6). They are far easier t o calculate and have the advantage that the coordinates have positive values and are valid for luminance values 1 t o 100. Accordingly, acceptable values of SCD are obtained with less effort and this mode of calculation is recommended for other indicators. RECEIVED for review November 23, 1970. Accepted May 28, 1971. Taken in part from the Ph.D. thesis of V.P.K., Delhi University, 1971.
Rotated Platinum Cell for Controlled-Potential Coulometry Ray G . Clem Nuclear Chernistry Diuision and Lawrence Radiation Laboratory, University of California, Berkeley, Calif. 94720 A rotated platinum cell which is a conceptual departure from the stationary platinum gauze-stirred solutiontype cells has been constructed. It has fast sparging characteristics, low sample-volume requirements, and permits the attainment of the highest electrolysis rate constants ever achieved with a platinum electrode. Constants of 0.13, 0.082, and 0.064 sec-1 are found for Au(lll), Fe(lll), and nitrite, respectively. The platinum cell i s a gauze-lined cylinder, partially open at the top, and closed at the bottom. Centrifugal force holds the sample solution against the cylinder wall in a thin film. The film i s contacted and efficiently stirred with a stationary, coaxial, counter electrode-reference combination probe. This cell is freely interchangeable with the rotated mercury cell in an apparatus previously described ( I ) . THEPRESENTED, NOVEL,rotated platinum cell is a conceptual departure from the widely used stationary, platinum gauze, stirred solution designs and is the result of an extension of previous work which led to the development of the rotated mercury cell ( I ) . The cell, which is made entirely of platinum, is a gauzelined cylinder, closed at the bottom, and partially open at the top. It is held inside a plastic cylinder, with a partially open, plastic, screw-cap top. The plastic cylinder is attached t o a copper turntable, and the cell-turntable combination is mounted in the previously described 1800 rpm motor driven apparatus ( I ) . The sample solution, upon rotating the cell, is held against the cell wall with centrifugal force and is contacted and efficiently stirred with the coaxial, combination, reference-auxiliary electrode probe. This efficient stirring combined with a favorable working electrode surface-area to solution-volume ratio results in the attainment of what quite (1) R. G. Clem, F. Jakob, D. H. Anderberg, and L. D. Ornelas, ANAL.CHEM., 43, 1398 (1971).
possibly are the highest electrolysis rate constants ever achieved with a platinum electrode. This claim is qualified below. As expected, many of the desirable qualities of the rotated mercury cell are found in the presented rotated platinum cell since both are based on the same principles. Hence, it also features very rapid sparging characteristics. Usually, 20 seconds of sparging is sufficient t o reduce the oxygen current to a value below the level of the background current. Also, it presently requires only 2 milliliters of sample solution although there appears to be no reason why a cell with a much smaller capacity could not be constructed. Additionally, surface phenomena are less troublesome since the platinum surface employed in this work is smaller than that of the three, large surface, high-speed cells to which the rotated cell is compared below. EXPERIMENTAL
Instrumentation and Reagents. The digital instrumentation used was described in a previous paper (2). A Beckman No. 39270 saturated calomel electrode was used as reference. Stock 0.5M “21-0.025M sulfamic acid, 0.5M HCI, and 1 M acetate buffer, pH 4.7 supporting electrolyte solutions were prepared by dilution of reagent grade chemicals. All water used was distilled. Stock solutions of the electroactive substances were prepared in the following manner. A weighed amount of gold metal was dissolved in aqua regia and the excess aqua regia displaced with hydrochloric acid. Sufficient sulfamic acid was added to give a concentration of 0.025M o n final dilution of the stock Au(II1) solution. A weighed amount of iron was dissolved in a mixture of hydrochloric acid and hydrogen peroxide. This solution was evaporated to a small volume to remove excess HCI, and to destroy the excess peroxide, then made to volume with water. (2) R. G. Clem and W. W. Goldsworthy, ibid., p 918.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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