Color Index Light-Colored Petroleum Products I. >I. DILLER, 218 Linden Blvd., Brooklyn, N. Y.
J. C. DEAN, R. J. DEGRAY, AND J. W. WILSON, JR. Socoiiy-Vacuum Oil Co., Inc., General Laboratories, Brooklyn, N. Y.
I. C. I. data may be calculated from the color index if desired, although this is usually unnecessar? in petroleum technology. The Saybolt chromometer has been shown to be unreliable, and to be importantly affected by the surface tension, refractive index, and specific dispersion of the oil being tested. This color index may be converted to Saybolt color by the establishment of suitable curves for the types of product in question. The conversion is prepared statistically to allow for the unreliability of the Saybolt method. The required colorimeter can be adjusted during manufacture to the specified standard, so that the readings obtained are independent of one’s particular instrument.
The applicability of the system tentatively known as “photoelectric color” to the evaluation of lightcolored petroleum products has been demonstrated, thus making possible a continuous color scale for all petroleum products. This color index consists of two parameters which are read directly on a special photoelectric instrument. The two parameters completely designate the color of materials such as oils in terms which are of direct technological significance and which express directly the appearance values. Light-colored oils are read with a violet filter instead of North Sky and red and converted to this color index by a suitable equation, so that no change in sample depth is required. For light oils, the second parameter is zero.
T
HE color of petroleum products is of importance in
the colors of dark- and light-colored products. Several attempts of this nature have been made prior to and since the development of the Saybolt instrument, leading to the development of the Stammer colorimeter (17, 26), methods of using the Lovibond glasses (6, 16, 18, 20, PI), and color systems known as “true color” (23, 24, 26, 28), and “optical density” (12). All these methods failed in their objective for one reason or another. None of them is directly related to accepted systems of color definition (14, 19, 22), which, with several others, have been described by Gardner (13). All are not only subject to peculiarities of the individual instrument, but lean heavily upon the personal equation. Furthermore, for light-colored oik, these systems require cell depths appreciably greater than used for darker oils, which prevents use of a continuous color scale. The Stammer and the Duboscq type colorin’eters ( I 5 ) , in which the depth Of oil is varied until a s t a d a r d is matched, suffer from the same faults, including the variations in hue and, in addition, frequent failure of Bouguer’s law to apply to petroleum has certain advantages, but is products* “True frequently invalidated by comparison with dilutions Of oils of different hue, or because Beer’s and Bouguer’s laws do not hold for such dilutions. (According to Bouguer’s law the logarithm Of the transmittance is proportiona1 to depth: = kd.) Story and Kalichevsky (27) proposed the use of a photoelectric colorimeter, primarily as a substitute for the eye. This eliminated the subjective nature of color measurement of petroleum products. A new color index for the designation and determination of color, previously called “photoelectric color” has been proposed (11), and the designation of the colors of lubricating oils and darker colored petroleum products by means of this index has been described. “Photoelectric color” was originally chosen for want of a better name to distinguish this system of color designation and measurement from Lovibond,
their processing and marketing. Hitherto, for its measurement, the petroleum industry has employed several systems, all of which are visual, and depend upon matching the color of the sample with that of a standard glass. Committee D-2 of the A. S. T. M. on Petroleum Products and Lubricants specifies two such methods, the Union colorimeter (4) for lubricating oils and the Saybolt chromometer (6) for light-colored products. The color of oils customarily measured with the Union colorimeter has been the subject of a previous paper (11), which described a system for color designation and measurement. It is the purpose of this paper to discuss in a similar manner the colors of oils usually determined with the Saybolt chromometer, and to propose means for handling them by the method already described (11).
The Saybolt chromometer consists essentially of two glass tubes 50 cm. (20 inches) in length, which are illuminated from the bottom by light reflected from a mirror. A prismatic optical head provides a circular field of vision, one half of which is illuminated by the light from the sam le, and the other by light pmsing through the color standard. $he level of the oil in the sample tube is adjwted 80 that its color matches that of the standard disk in the other tube. The height of oil is read and is converted to Saybolt chromometer color by an arbitrary table. The instrument, invented by George Saybolt, has been brought to its present stage of standardieation largely as the result of A. S. T. M. activity (I,,%’, S,7). Both the Union colorimeter and the Saybolt chromometer are arbitrarily standardized and are not related to one another by any fundamental means. Furthermore, the specifications of each of these instruments are so drawn that sufficient latitude is allowed in the selection of color disks to permit an appreciable variation of results from instrument to instrument. For these reasons, among many others, the industry has needed one system of color measurement which will be applicable to all types of petroleum products, and which will provide a continuous relationship between 367
368
INDUSTRIAL AND ENGINEERING CHEMISTRY
Saybolt, or Union, but it is probable that the industry will find one which is more appropriate as this color index becomes accepted. This color index conveys full color information concerning the oil. I n addition to presenting this information in terms which are of immediate and direct significance in petroleum technology, advantage is taken of the substantially invariant color properties of these series of products-that is, darkcolored petroleum products, light-colored petroleum products, fatty oils (Q), etc. B y so doing, all the desired information can be given despite the use of less than three parameters, in the present instance, only one parameter. The color index consists of two terms, the second being zero when the color is ‘‘normal” or when it is that of lightcolored petroleum products. The first term is the North Sky reading (or its computed equivalent in the case of light products as shown below), and the second is the deviation of the red reading from the normal at that S o r t h Sky reading. This normal is obtained from Figure 3 of another paper of this series ( 2 1 ) . By reporting deviation from the normal red reading rather than the red reading itself, one can tell a t a glance whether the oil is greenish or reddish and to what extent. For ixample, a color index of 90 3 would represent a North Sky reading of 90 and a red reading of 100 and the oil is on the reddish side of normal. An index of 90 - 3 represents a North Sky reading of 90 and a red reading of 94 and the oil is greenish to the extent of 3 units. When the second teim is zero, it will often be omitted, as in Figure 6, a chart for the conversion of this color index to Saybolt color. This system expresses and measures directly what the eye sees. It does so by measuring the equivalent of the energy received by the average eye looking through the sample toward C illuminant, and it also measures variations in hue. Xhile it serves for darker oils, modification was necessary in the case of light-colored products, to magnify the readings without changing (increasing) sample depth.
+
30 20
--
1
I
D
I
1
I
1 1
’
1 I
400
450
500
1
COLOR I N D E X
.30
~
550
A 0
1000
c
984 97 2
D
94 8
MM
650
20
700
W A V E LENGTH.m!J
SPECTROPHOTO~fETRICCURVES FOR SOLVESTS FIGURE 1. TYPICAL
One way to magnify the reading would be to increase the sample depth appreciably above the present 1.8 em. However, this increase would be objectionable because (1) errors due to bubbles and turbidity would be equally magnified; (2) the energy loss due to fluoroscence would be increased; (3) Bouguer’s lam would not hold rigidly, owing to the effect of fluorescence and other factors; and (4)it would entail redesign of a successfully simplified instrument TI ith
Vol. 15, No. 6
X - T R I S T I M U L U S VALUE X 100
CHART,VIOLETREADINGS TO X FIGURE 2 . COSVERSIOX
consequent increases in complexity, cost, and likelihood of error. The use of two instruments, as is present practice, u ould be even more objectionable. I n the case of light-colored petroleum products, the eye “sees” and is influenced almost exclusively by light transmission of the oil in the violet range. Accordingly, magnification of the reading, without change in sample depth, can be accomplished by the use of the violet filter. Readings of light-colored petroleum oils with this filter have a direct relationship with those made with the North Sky filter. Thus, the disturbing effects of turbidity, bubbles, and fluorescence are not equally magnified, and no reliance need be placed upon the applicability of Bouguer’s law. One instrument, therefore, provides a continuous color scale for all petroleum products. A further advantage lies in the fact that the color of these products can be obtained with less than 10 ml. of sample, instead of the 50 ml. or more required for other methods. I n the range of darker oils, no monochromatic filter could be found which was the equivalent of the North Sky filter. However, with lighter colored oils, the changes in the spectral curves are confined to the portion below 550 mp with virtu illy no absorption above this wave length. Curves for these oils are smooth in shape and show greater transmittance as the wave length increases. The ultimate is a curve for a water-rhite oil which consists substantially of a straight line at 90 per cent transmission. As these water-\vhite oils oxidize and darken in color, their spectral curws deviate from this line, but only in the region below 550 mp. This is illustrated by Figure 1, showing the progressive darkening of a water-white oil. Thus, the average eye, in viewing C illuminant through these light-colored oils, is influenc.4 only by changes in the blue and violet region, so that a measure of the absorption below 550 mp is a11 that is required. The instrument with the violet filter in position has an excellent response in this region, and thereby performs the same function for light-colored oils that the K’orth Sky filter does with darker ones. Indeed, the authors’ experimental work has confirmed that the relationship between violet and S o r t h Sky filter readings in this range is linear, A further peculiarity of light-colored petroleum products, as distinguished from darker ones, is that because of the high degree of refining gken them there is a negligible light
ANALYTICAL EDITION
June 15, 1943
absorption at wave lengths greater than 550 mp, Also, for oils of the same color index, there is no appreciable difference in the steepness of their spectral curves. Hence, only one parameter suffices for a full description of the color and the red reading for determination of variations of hue is not required.
369
same filters were used. However, since such oils have practically identical spectral curves above 550 m,u, and there are but slight differences in their hues, the response of the red filter was substantially the same in all cases; so that this filter reading is not required. The same reference standard used previously (11)namely, distilled water-was employed throughout, and the colorimeter was set by this standard to a scale reading of 100 prior to all determinations.
Definitions The terminology used by the petroleum industry in dealing with light-colored products is not rigidly standardized. Therefore, for the purpose of this discussion, the following definitions have been established : REFINEDOIL. Any oil which has been rendered light in color (straw color t o colorless). KEROSEKE.-4ny refined oil suitable for use as an illurninant in a wick lamp. SOLVENT.Any refined oil of narrow boiling range suitable for solvent purposes. USEDOIL. Any oil suitable as a lubricant which has been subjected to service conditions. Description of the RIethod As has been described ( I I ) , the color index of dark oils is taken n-ith the Korth Sky filter, and optionally with the red filter. For the reasons already given, refined oils must be meawred with the violet filter, which gives readings related to those found with the Korth Sky filter, and thus provides a continuation of the scale. This filter v a s so chosen that, n-ith paraffinic kerosene and paraffinic solvents, it gives exactly ten times the deflection obtained with the Yorth Sky filter. Thus, the equivalent North Sky reading of these products is computed, using the equation:
reading s.s. = 90 + violet filter 10
40 50
TABLEI. COLORDATAos REFISEDOILS 1
2 3
2
k
9
10 11
Iiature Paraffinic Aromatic Aromatic Paraffinic Aromatic Paraffinic Paraffinic Paraffinic Aromatic Aromatic Paraffinic
Spectrophotometric Y Z
x
0.880 0.881 0.880 0.873 0.859 0.856 0.832 0.820 0.814 0.783 0.778
0.899 0.903 0.903 0.899 0.895 0.892 0.898 0.890 0.881 0.879 0.876
1.056 1.051 1.026 1.022 0.941 0.938 0.801 0.752 0.709 0.527 0.526
70
eo
90
I
100
2 - T R I S T I MULUS VALUE X 100
For values up to 96.0 the Korth Sky filter is used, but above 96.0 the violet filter and the above equation are employed. Peadings are taken to the nearest half-scale division, and the first term of the color index is reported to the nearest 0.0.5 unit. Thus, a n oil with a .i-iolet filter reading of 7 8 3 n-ill have a color index of 97.85.
Oil
~ 60
Color Index
s.s 100.0 102.0 100.5 100.0 100.0 99.0 98.0 07.5 98.0 96.0 94.0
v 100.0
101.0 96.0 95.0 88.0 84.0 75.0 72.0 72.0 56 0 18.0
Since the colors of certain types of refined oils are highly unstable, and are markedly affected by exposure to light, it is essential that measurements be taken as quickly as accuracy permits. Because of the bleaching action of the light beam, the color of the sample is lightened appreciably in the vicinity of the focal point of this beam. The oil in the sample tube should, therefore, be agitated between check readings to disperse the bleached material. The instrument enables measurements to be made a t the rate of six samples per minute.
Experimental COLORIMETER . 4 S D FILTERS.For this work the colorimeter :heady described (11) and based on Diller's (8, IO) was employed. 111 order to extend the proposed system to light-colored oils, the
FIGURE 3. COSVERSION CHART,VIOLETREADINGS TO Z
As has been previously emphasized, the instrument as a \\-hole, and not merely the filters alone, must be designed and manufactured to yield the standard readings. The temperature of the light source, characteristics of the photocell, optical system, etc., as well as the filter determine the response. Of particular importance to this nork is the fact that the instrliment is so designed as to minimize the effects of variations in refractive index and fluorescence of the sample. CORRELATIOX WITH I. C. I. VALUES. As in the previous investigation, the readings of the colorimeter can be correlated direcJy with I. C. I. data. This mas accomplished by having eleven representative oils evaluated by the Electrical Testing Laboratories with a Hardy spectrophotometer, and obtaining data simultaneously with the photoelectric colorimeter. These samples represented two series of refined oils having widely differing physical characteristics, one bcing of paraffinic and the other of aromatic nature. The spectrophotometric data were converted to tristimulus values by the 30 selected ordinates method described by Hardy ( I C ) . These values and readings taken v, ith the tv-o filters are shon n in Table I. The conversion of S o r t h Sky filter readings to Y tristimulus values has already been established (II), and the conversion factor of 0.90 as used here also. This factor results from the use of distilled water, which has a transmission of 90 per cent a s the standard for the color index. I n order to obtain conversions to X and 2, the values obtained spectrally iyere plotted against violet filter readings as shown in Figures 2 and 3. Figure 2 is the more sensitive curve mentioned in the previous paper (11) for obtaining
J
370
Voi. 15, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
the X tristimulus value for light-colored oils. The most representative curves for these points were straight lines. It is logical to expect that such correlations exist, since, as already shown, the only variations in the spectrophotometric curves occur a t wave lengths below about 550 mp. I n this range, readings with the violet filter, as well as the 2 function and a portion of the X function, are affected. This relationship is further confirmed by Figure 4 wherein for these eleven representative samples X is plotted against 2, which results in a straight line. Using Figures 2 and 3, X and 2 tristimulus values were obtained from the violet filter readings, These and the Y tristimulus values obtained as above are given in Table 11. The mean deviation of the authors’ values obtained spectrophotometrically is shown to be * 0.012. The trichromatic coefficients were calculated from the tristimulus values, and are shown in Table 111. A mean deviation of ~0.004 is indicated. Finally, from Y , x, and y the luminous transmission, dominant wave length, and purity were obtained. These data are shown in Table IV, wherein mean deviations from the true values of ~ 0 . 9per cent in transmission, *3 mp in dominant wave length, and *1.5 per cent in purity are found.
xx
TABLE 11. TRISTIMULUS VALUES 100 Y x 100 zx Color index b
Color index b
100 Color Spectrala index’, 105.6 106.5 105.1 108.0 102.3 102.6 102.2 101.4 94.1 93.5 89.0 93.8 78.8 80.1 76.6 75.2 75.6 70.9 58.0 52.7 49.0 52.6
Oil Spectraln Spectrala 1 88.0 88.4 2 88.0 3 87.3 4 85.9 5 85.6 6 83.2 7 82.0 8 81.4 9 78.3 10 77.8 11 Mean deviation from spectra *1.2 a Computed from Hardy spectrophotometric ourves. b Converted from readings of photoelectrio colorimeter.
TABLE 111. TRICHROMATIC COEFFICIENTS Y
X
Oil
Spectral
index Color
1 0.310 2 0.311 0.313 3 0.312 4 5 0.319 6 0.319 7 0.329 0.333 8 9 0.338 0.358 10 0.357 11 Mean deviation from spectral
Spectral
2
index Color
Spectral
0.317 0.318 0.322 0.322 0.332 0.332 0.355 0.362 0.367 0.401 0.402
index Color 0.374 0.375 0.365 0.364 0.347 0.338 0.315 0.308 0.307 0.260 0.233
*0.004
Saybolt Chromometer It was advisable to establish a correlation between this new system and the one now in general use. However, early in the work with tjhe Saybolt instrument, it was found that that instrument did not yield comparable results for the different classes of oils and no one correlation curve could be drawn. Dependent upon other physical characteristics, each class of oils required an individual curve. In Table V, data obtained on two series of solvents are given. These include the samples tested spectrophotometrically together with others evaluated photoelectrically. It is evident from this table that the Saybolt chromometer is primarily measuring purity, and only secondarily luminous transmission. As is indicated, the dominant wave length remains constant throughout both series. Saybolt colors and purity are plotted graphically in Figure 5. Two curves result which, even though slightly irregular in shape, illustrate that basically, purity is being measured.
TABLE IV. LUMINOUS TRANSMISSION, DOMINANT WAVELENGTH,AND PURITY
The above may be summarized as indicating that these color indices for refined oils may be converted to I. C. I. values with reasonable accuracy. This correlation is only for petroleum oils, and is possible because their spectral curves have the same generic shape.
Luminous Transmission, % Color Oil Spectral index 90.0 1 91.8 2 90.5 3 90.0 4 90.0 5 89.1 6 88.2 7 87.8 8 88.2 9 86.4 10 84.6 11 hlean deviation *0.9%
Dominant Wave Length, m# Color Spectral index 569 569 569 568 569 569 569 569 569 569.5 570 ~3 mp
Purity, % Color Spectral index 0.2 0.1 0.8 0.6 2.2 2.2 2.0 2.6 6.5 7.1 6.5 9.6 15.2 15.2 17.5 18.1 18.0 22.3 30.5 ’35.6 37.5 35.5 j=1.57*
Purity of a color may be considered as that percentage of a pure spectral color which when mixed with C illuminant of the I. C. I. system will match the color (14). Since Saybolt chromometer colors are related to purity, the instrument is substantially measuring the concentration of colored bodies of the same dominant wave length. This is proved by correlating violet filter readings with “modified Saybolt” colors, which are calculated by the following equation: (20)(number of disks) Modified Saybolt color = depth of oil in inches
0
50
70
0
I 80
‘
i
I
l
82 84 86 X - T R I S T I MULUS VALUE X 100
FIGURE 4. REL.4TIONSHIP
OF
x AND
88
This equation is based upon the validity of Bouguer’s and Beer’s laws in this range. If modified Saybolt colors of any one family of oils are plotted graphically against the logarithms of the violet filter transmissions, a linear relationship is found. Each family of oils is represented by a separate curve, but all are straight lines over the range in which these laws hold.
June 15, 1943
ANALYTICAL EDITION
Obviously, therefore, some other factor is involved which affects the optical properties of the oils. I n Table VI, other data obtained with the series of oils mentioned above are shown, which indicate that despite identical color indices, the Saybolt colors vary with surface tension, refractive index, and specific dispersion. I n this particular instance, the variations are due entirely to differences in chemical composition. The oils studied here are all darker than those most frequently evaluated by the Saybolt chromometer, but were chosen primarily because they illustrate the discrepancies which may be obtained with this instrument. Similar discrepancies exist throughout the Saybolt range, as indicated by Figure 6, although they are not so pronounced.
40
35
30 ** 25 >
t
5
371
20 I5 IO 5
0
-20
-15
-10
-5
0
+5 t10 +IS SAYBOLT COLOS
+20
t25
+30
+30+
Ts4BLE
OF PURITY AND SAYBOLT COLOR FIGURE5. RELATIONSHIP
vi1
1'1.
SAYBOLT COLOR, REFRACTIVE INDEX, DISPERSION, AND SURFACE TENSION
(All samples gave violet filter readings of 7 0 . 0 ) Saybolt Refractive Specific Color Index Dispersion
TABLEV. CORRELATION OF SAYBOLT COLORS WITH I. C. I. DATA 00
(Computed from Diller colorimeter readings and Hardy spectrophotometric curves) Dominant Luminous Saybolt Wave Length Transmission Purity Oil Color mp % % A.
Paraffinic 1' 2 3 4a 5 6 7 8=
9 10 11a 12 13 14= 15 16 17"
B.
+30+ ++24 30
+21 +20 1-15 +IO +8
+:
-3 -3 -6 -7 10 19 Too dark
-
89.9 90.0 90.0 89.9 89.6 89.1 89.1 89.2 88.7 88.2 89.8 87.8 87.8 89.0 87.3 85.5 87.6
569 579 569 568 569 572 570 569 571 572 569 572 57 1 569 571 571 570
0.2 0.2 1.5 2.0 3.1 6.8 8.7 6.5 10.0 12.7 15.2 15.7
90.3 90.0 90.3 90.0 90.0 89.5 89.1 88.2 88.4 87.8 87.3 86.9 87.9 86.0 85.5
569 566 569 568 568 569 570 57 1 569 57 1 57 1 571 569.5 571 57 1
0.8 1.0 2.2 3.8 6.5 6.5 10.8 16.0 22.3 20.0 24.5 27.8 35.8 32.2 38.0
16.8
18.1 20.0 36.0 35.5
Aromatic la
2 35 4 5 6'3 7 8 9'L 10 11 12 136 14 15 a
+SO+ 29 +27 23 19 18 +15 +9 +5
+ ++ +
+: -4 -7 -8 - 12
102
SPECIFIC
Surface Tension Dynes/cm. 20.5
If the optical system of the Saybolt chromometer is analyzed, it becomes apparent that the physical characteristics listed in Table VI affect the results obtained. A constant source of light exists, and the oil level in one tube is so adjusted that the short-wave (violet) portion of the light energy reaching the eye is substantially equal to that passing through the standard disk and the empty tube. The energy loss in the sample tube is substantially equal to thdt lost because of refraction, reflection from the side walls, and absorption by the oil layer. Surface tension, in a sense, determines the contour of the meniscus which is formed at the liquid-air interface. Specific dispersion and refractive index are closely related to the energy loss a t the interface, and also to the angle a t which light leaves the oil surface. Thus, all three determine the average angle a t which light from the interface is directed toward the side walls of the tube. The more nearly the light strikes the side walls a t right angles to them, the greater will be the energy loss. Higher surface tensions, refractive indices, and specific
I. C . I. data obtained from Hardy spectrophotometer curves.
However, despite the apparent relationship between Saybolt color and purity, several anomalies have been encountered. In one experiment, a solution of off-colored kerosene in carbon tetrachloride was prepared and found to match a standard disk at a height of 23.75 cm. (9.5 inches). Various amounts of water were introduced above this solution in the tube, and the color was measured again. Matches were obtained when the total heights were 9.5 inches in each case. Apparently then, even though the instrument measures color concentration, it indicated the same concentration when the colored bodies were in the ratio
9.5:8.5:7.5. A second experiment was conducted, wherein mixtures of refined oils were prepared so that all had the same violet reading,
and were closely matched visually. However, when evaluated by the Saybolt chromometer, their colors ranged from - 4 to - 17. Thus, even though all had the same purity, or color concentration, based upon 1. C. I. data, their Saybolt colors indicated concentrations over a range directly proportional t o the heights in the tubes of from 5 to 9.13 cm. (2 to 3.625 inches).
-20
FI(;lrRE
6.
-15
-10
-5
0
t5 +IO +I5 SAYBOLT COLOR
+PO
+25
t30
t3DC
CONVERSION CHART, COLOR I i i D E X TO S.4YBOLT
COLOR
INDUSTRIAL AND ENGINEERING CHEMISTRY
372
dispersions individually result in greater energy losses. I n order to compensate for this added loss above the oil layer, its depth must be reduced to decrease the loss due to absorption. Hence, a low Saybolt color reading results. To illustrate the effect of surface tension, a potassium dichromate solution having a Saybolt color of +10 was prepared and measured in a sample tube of the chromometer coated with a thin film of highly viscous light-colored oil. A value of +15 was obtained, but when a few drops of wetting agent solution were added, the color was found to be +6, despite the fact that the color index was unchanged. The personal equation involved in color measurements with the Saybolt instrument also cannot be disregarded. It has been established that there are certain individuals incapable of reproducible results. These operators frequently state that because of a different “shade” of color they are unahle to match it with the standard. This is not the case with color-blind observers, or those less sensitive to minor variations in hue, who are able to obtain accurate readings even with abnormal oils. A. S. T. M. Method D156-38 specifies the standard disks as follows: Whole Disk
Y
= 0.860 t o 0.865 x = 0 . 3 4 2 to 0.350 y = 0.367 to 0.378
Half Disk 0.888 to 0,891 z = 0 , 3 2 7 to 0 . 3 3 1 21 = 0.344 to 0.350
Y =
Using the graphs prepared by Hardy ( I d ) , bhese limits are equivalent to the following: Whole Disk Luminous transmission, yo 86.0 to 86.5 Dominant wave length, mp 569 to 574 Purity, % 2 2 . 2 to 2 7 . 2
Half Disk
Luminous transmission, 9% Dominant wave length, mrr Purity, %
88.8 to 8 9 . 1 569 to 574 1 2 . 0 t o 14.5
It has been shown that paraffinic and aromatic refined oils have the same dominant wave length of about 570 mp, which is within the limits of the disks. Since the dominant wave lengths of the oil colors do not match that of the disk, they must have been varied before the light reached the eye, and that variation may be caused by differences in surface tension and refractive index. Since the energy loss is disproportionate for light of certain wave lengths, and because of the shape of the spectral curves, this results in a slight variation in dominant wave length that is easily detected by operators with keen color perception. The effects of refract’ive index and surface tension are thus shown to invalidate certain Saybolt color measurements. These factors are also significant in photoelectric measurements. However, in the photoelectric colorimeter used in this work, measurements are not made through a liquid-air interface, so that surface tension has no effect. Refractive index also applies to the photoelectric measurement, hut the use of a round sample tube in conjunction with the converging-diverging light beam of Diller’s instrument (8, 10) minimizes the refractive error, because the light enters the tube a t substantially zero incidence. Provision is also made for reducing as far as possible the effect of fluorescence.
Vol. 15, No. 6
I t will be noted that all thess curves and also those of Figure 5 exhibit points of inflection. These deviations from smooth curves are the results of the arbitrary table used in converting depths of oil to Saybolt colors (6),and the peculiar relationship betrreen Saybolt colors and modified Saybolt colors. If these are plotted graphically, one against the other, an irregular line with the same inflection points will result. These inflections will appear in any curve obtained by plotting Saybolt values against any fundamental value such as color index or purity. Figure 6 was prepared by evaluating over a thousand separate samples. However, similar curves can be obtained by a simpler procedure involving the linear relationship of modified Saybolt colors and the logarithms of the violet filter readings-for example, a more limited number of sRmples could be tested with both instruments. By computing modified Saybolt colors from the heights in the sample tube, and recording the violet transmission on the logarithmic (concentration) scale, values mill be obtained through which straight lines may be drawn. These lines may then he converted to Saybolt colors and violet filter readings, yielding curves such as those shown in Figure 6. (For any given sample, instrument, and operator, deviations from these curves should be no greater than one Saybolt unit.)
Standardization and Reproducibility Xeans for checking the setting of the instrument for the production of standard readings have been described (11). The same standard aqueous solutions mere used in this study. Since the colors of some off-test solvents and kerosenes are highly unstable, it was obviously impossible to check the reproducibility of the instrument on the samples after a period of storage. However, the colors of the series of oils listed in Table I. were determined simultaneously with two instruments to demonstrate the concordance between them (Table VII). One instrument was used a t the Electrical Testing Laboratories, and the second at the authors’ laboratory. By means of telephone communication, it was possible to have both readings taken nithin 5 minutes of each other. The differences between the results are no greater than the limits of error previously established (11). TABLEvu. REPRODUCIBILITY OF RESULTS Oil 1 2 3 4
5 6 7 S
9 10 11
North Sky Filter A B 99.5 102.0 100.5 100.0 100.0 99.0 98.0 97.5 98.0 96.0 94.0
100.5 102.0 100.5 100.0
100.0 99.0 97.5 97.5 98.0 96.0 94.0
A Violet Filter 100.0 101 0
96. ’ ) 94.5 88.0
84.0 74.5 72.0 72.0 56.0 48.0
B
100.0 101.0 96.0 95.5 88.0 84.0 75.5 72.0 72.0 56.0 48.0
Average deviation t 0 . 2 4 scale division.
Correlation of Color Index with Saybolt Values
Conclusion
Since, as stated above, other physical charact,eristics markedly affect Saybolt values but not color index, no rigid relationship between them can be established, and each family of oils must be evaluated to fix its own correlation curve. Figure 6 shows typical correlation curves of four types of oil, prepared by evaluating paraffinic solvents, aromatic solvents, solutions of used oils in paraffinic solvents, and highly refined paraffinic spindle oils. The divergence of these curves is in accordance with the explanation already advanced.
The photoelectric colorimeter may be used for measuring colors of refined oils, thus providing a continuous scale for all petroleum products. This has been made possible by the use of a violet filter, which is directly related to the North Sky filter, and which has its response in the blue-violet region in which all variations in spectral absorption occur. Readings obtained with these two filters may be converted to I. C. I. data if desired. The Saybolt chromometer, customarily used for measuring the colors of theso oils, has been s h d i e d and fonnd to be
June 15, 1943
ANALYTICAL EDITION
unreliable in that erroneous and anomalous results may be obtained. Variations in physical characteristics of the oils, such as surface tension and refractive index, are probably chiefly responsible for these errors. The color index described abore can be correlated with Saybolt values by the use of suitable conversion curves.
Acknowledgment Acknowledgment is made to D. B. Judd of the Sational Bureau of Standards, who has carefully reviewed this paper and offered suggestions. Frequent references to a private report by T’. A. Kalichevsky and B. W. Story were of great assistance in this Ivork. The instrument used in this investigation is known as the Hellige-Diller photoelectric colorimeter, Model 405-A.
Literature Cited .im. Soc. Testing Materials, Proc. Am. SOC. Testing Materials, 23, 352 (1923). Ibid., 24, 524 (1924). Ihid.. 34, 895 (1934). Am. Soc. Testing Materials, Standards on Petroleum Products and Lubricants, A . S. T. 11.Designation D155-39T. I6id.. A. S. T. >I. Designation Dl56-38. Campbell, A , , “Petroleum Refining”, 2nd ed., pp. 74-85, New York, Petroleum Age, 1922. Delhridge, T . G., Proc. Am. Soc. Testing 3lateriaZ.s. 22 I, 425-9 (1922). Diller, I. M.,J . Biol. Chem., 115, 315-22 (1936). Diller, I. AI., paper presented before t h e American Oil Chemists’ Society, fall meeting, 1941. Diller, I. XI,, U.S.P a t e n t 2,232,169 (Feb. 18, 1941).
(11) Diller, I.
373
M.,DeGray, R. J., and
Wilson, J. W., J r . , ISD. ENG. ED.,14, 607-14 (1942). (12) Ferris, F. W., and McIlvain, J. M., Ibid., 6, 23-9 (1934). CHEM., h A L .
(13) Gardner, H . A , , “Physical and Chemical Examination of Paints, Varnishes, Lacquers and Color”, 9th ed., Washington, D . C., Institute of P a i n t and Varnish Research, 1939. (14) H a r d y , il.C., “Handbook of Colorimetry”, Cambridge, Mass., Technology Press, 1936. (15) Hellige, F., Petroleum Z., 10, 725 (1914). (16) Herbrich, J., J . Inst. Petroleum Tech., 18, 140 (1932); Ann. chim. anal. chim. a p p l . , 14, 193-201 (1932). (17) Holde, D . , “Examination of Hydrocarbon Oils and of Saponifiable F a t s and Waxes”, 2nd ed. rev., New York, John Wiley & Sons, 1923. (18) Inst. Petroleum Technologists, Standard Methods K. Z . , P. S. 2, P. S.2a. (19) J u d d . D. B., J . Optical Soc. Am., 23, 359-74 (1933). (20) Lovihond, F. E., Proc. Opticil Convention, 1926, I, 211-14. (21) Lovibond, J. W., “Light and Color Theories and Their Relation t o Light and Color Standardization”, London, E. and F. N. Spon, 1916. (22) Munsell, A . H., “A Color Kotation” and “A Color .4tlas”, Baltimore, h l d . , Munsell Color Co., 1933. (23) Kelson, IT. L., Oil Gas J . , 37, KO. 3, 74 (June 2 , 1938). and Wilson, R . E., J. IND.ESG. CHEM.,14, 269(24) Parsons, L . W,, 78 (1922). (25) Redwood, B., “Treatise on Petroleum”, 3rd ed., P a r t 11, pp. 214-15, London, C . Griffin and Co., 1913. (26) Rogers, T . H . . Grimm, F. T., and Lemmon, ?;. E., ISD.ENG. CHEU.,18, 164-9 (1926). (27) Story, B. TT,, and Kalichevsky, I-.A , , ISD.ENG.C H E Y . , AXAL. E D . , 5, 214-17 (1933). (28) Vinock, H . , Re.finer S a t u r a l Gasoline M f r . , 16, 601 (1937). PRESENT before E ~ the Division of Petroleum Chemistry a t t h e 104th Meeting of the ERICIN IN CHEMICAL S O C I E T Y , Buffalo,N. Y.
Determination of Iodine in Tetraiodo-
phenolphthalein SAMUEL WEIIVER, BYRON E. LEACH’, AND MARY JANE BRATZ Paul-Lewis Laboratories, Inc., Milw-aukee, Wis.
B
UTLER and Burdette ( 1 ) in 1939 proposed a method for
determining iodine in tetraiodophenolphthalein that n as much more rapid and accurate than the procedure of the L-,S. Pharmacopoeia of that time. It consisted of three steps: (1) digestion lvith alkaline permanganate solution to decompose the material and release the iodine as iodate, (2) acidification and treatment with sodium bisulfite to reduce the manganese and iodine to their loner oxidation states, and (3) pal tial neutralization 1% ith ammonium carbonate and titration of the iodide Ti-ith sib-er nitrate, using eosin or diiodofluorescein as an adsorption indicator.
Adsorption Indicator Methods The clear admiitages of the Butler-Burdette method led to its adoption by tlie U.S.Pharmacopoeia XI1 (6)with certain inot1ific:itions. The U. d. P. XI1 method is almost identical Tvith one that has been successfully used in this laboratory since October, 1941, but in which starch-iodine is used as the titlsorption indicator. If eosin or tliiodofluorescein is used as adsorption indicator in the titration of the iodide, tlie amounts of acid and alkali added after tlie digestion n-it11 permanganate must be ex:lctly sucli that the pH at the time of titration is between 4.5 1
Present address, University of Illinois, Uibana, Ill.
and 7 . 5 , preferably 6.0. On the other hand, Kolthoff’s (4) sbarch-iodine complex will give sharp end points at pH’s as low as 0.5. Place 0.15 to 0.20 gram, accurately weighed, of the tetraiodophenolphthalein or its sodium salt in a 500-ml. Erlenmeyer flask, add 10 ml. of 5 per cent sodium hydroxide solution, and when the sample has dissolved on the water bath or steam bath add 25 ml. of saturated potassium permanganate solution. Digest 45 minutes on the water bath, cool, add 75 ml. of water and 10 ml. of 1 to 1 sulfuric acid, and then add 3 11.1sodium bisulfite from a buret with constant swirling of the flask until the solution is colorless. Add dilute potassium permanganate solution drop by drop till a faint permanent yellow appears. Add 10 ml. of A‘ ammonium carbonate solution and 4 drops of dilute starch solution and titrate with 0.1 11-silver nitrate bolution. KOTES. The end point \ d l be the disappearance of the last trace of blue, green, or gray, leaying the solution and the precipitate n-ith a clear canary yelloJv color. If the silver iodide coagulates, the end point is best observed on the precipitate; if, as sometinies happens, it fails t o coagulate, the end point is observed in the body of the solution. The end point is usually very sharp. From 10 to 18 mi. of sodium biaulfite L3-ill be needed to reduce the iodate and permanganate. After one has observed the amount needed in a few titrations, it is preferable to add a slight excess at once from a graduate qlinder, so as t o avoid the loss b y volatilization of the iodine during the reduction from iodate to iodide: 15 nil. of 3 .I1 sodium bisulfite would be a convenient amount. o coagulate and the On rare occasions, the starch cc,niplrx gives n vei
