Optical Density Color Measurement for Petroleum Oils S. W. FERRIS AND J. M. MCILVAIN, The Atlantic Refining Company, Philadelphia, Pa. ETHODS used in this country for assigning numerical color values to petroleum lubricating oils may be grouped as follows: CLASS1. A given thickness of oil is matched against one of a set of glass or liquid standards, primarily on the basis of hue. In this class fall the Lovibond (7) and the National Petroleum Association (1) scales. CLASS2. A variable thickness of oil is matched against a standard color disk, as in the method initially described by Delbridge (6) and later applied in the TagRobinson1 colorimeter ( I S ) ; or against a fixed depth of liquid standard, as in the methods of Rogers, Grimm, and Lemmon (10)a n d of W e i r , Houghton, and Majewski (12). In. at least one of these (12) t h e m a t c h i s made on the basis of e q u a l i t y of brightness. A variant of methods of this class consists in matching y 110 a fixed thickness of F 100 0 10 W 30 40 50 60 the oil sample against a variable thickness DEPTH OF SAMPLE 1 (NN.) of oil standard, as in m e t h o d of FIGURE 1. REVERSAL IN APPARENT t h e TRUECOLOR WITH DIFFERENT DEPTHS Parsons and Wilson OF OILS (9). The color values d e r i v e d from the methods of class 2, with the exception of the Tag-Robinson colorimeter, have been called “true colors.” CLASS3. The fraction of light absorbed by a certain thickness of oil is determined photometrically. Examples of this class are the instrument pro osed by Cox (3) which uses a sectored flicker disk, and that of Fftory and Kalichevsky (11) in which the distance between sample and illuminant is varied and the inversesquare law employed. Apparatus in which the relative transmission of unknown and standard oils are indicated by the res onse of photoelectric cells have been developed by several of tEe instrument companies. In all the methods cited except one (3)white light has been used as illuminant.
The methods of class 1 fail to meet specification 2. (For example, &’ mixture of equal parts of two finished oils from the same stock, which have respective N. P. A. colors of 4 and 8, may have an N. P. A. color of 6.5, whereas Equation 1 would require an N. P. A. color of 6 for the mixture. Story and Kalichevsky (11) show that the Lovibond scale leads to oil colors which are far from additive in the sense of Equation 1.) The methods of class 2 when restricted to matching a variable thickness of oil against a fixed thickness of one standard, as prescribed in the most recently published example (12) of the class, yield color values which satisfy specifications 1, 2, and 6, but do not meet specifications 3 and 4, and could be better with regard to specification 5. The methods of class 3 fail to meet either specification 1 (3) or specification 2 (11). (The method of Cox is open to objection because of his choice of blue light as illuminant, rather than because of his photometric arrangement.) When photocells are used with white light as illuminant it is well to remember that the response curve of the cell may vary markedly from that of the normal eye (the so-called visibility curve). O’Brien (8) has shown that filters can be devised for a t least one type of photocell, such that the response curve of the combination is in satisfactory agreement with that of the eye.
While several of these methods have been useful in the specification of color for sales work, from the viewpoint of the refiner each has more or less serious shortcomings. In order to serve him best, a color scale must meet six specifications: 1. Samples should be accorded values in agreement with ordinary visual inspection. 2. Values should be additive in the sense that the color C, of a mixture of two oils having colors C, and Cb, will be given by the equation
FIGURE 2. VARIATION OF APPARENT TRUE COLOROF DARKOILS*MATCHED AGAINST VARIOUS DEPTHS OF STANDARD 0.1 t o 2.0 per cent solution in ethylene dichloride.
This paper presents experimental data to show how a color scale for petroleum lubricating oils was developed, which meets all six of the above specifications. A complete description of the final method is also given, with examples of determinations on a large number of oils.
where V , and Vb are the respective percentages (by volume) of the oils whose colors are C, and Cb. 3. Apparatus, standards, and color values should be reproducible in any laboratory. 4. Color values should be based on fundamental rather than arbitrary units. 5. The method should be usable in routine work. 6. Consistent color values should be obtainable on oils ranging from light finished products t o the darkest tars and bottoms.
EXPERIMENTAL One of the most serious objections to the true color method of determining oil colors is the change in apparent true color with the nature of the standard oil (IO) or with the depth of standard against which the match is made. Figure 1 shows the results of an experiment in which two oils of nearly the same N. P. A. colors were compared in the Duboscq colorimeter, using white light. (These and subsequent
1 The Saybolt chromometer for measuring the color of gaolines and naphthas fal!s in thia clasa.
23
ANALYTICAL EDITION
24
samples are described in Table I). Sample 1 is taken as standard and is therefore assigned the same true color at all depths. It is obvious that sample 2 will be assigned a true color less than, equal to, or greater than sample 1, if the depths matched are varied from lesser to greater. Furthermore these effects become greater when the difference of type between sample and standard increase. This is exemplified in Figure 2, which shows the results of matching ethylene dichloride solutions of various dark oils against a 2.5 N. P. A. oil standard. Another difficulty with the true color method lies in the choice of a proper diluent for dark oils. Figure 3 illustrates the spread in true color values which may be introduced by the use of a poor solvent such as kerosene. It is evident that for this particular oil benzene is a superior diluent
Vol. 6, No. 1 KEROSENE SOLUTIONS
22.
-
-
18
60nl.1.
+
0
.
BENZENE SOLUTIONS
16
TABLEI. DESCRIPTION OF SAMPLES
I4
OILS UEEDIN TESTS
I2
VISCOEITY E. 0. SECONDS
SAMPLE 1 2 35 4" 5a 6a
7a 8a 9= loa
11" 12n 13" 14a 15"
16" 17" 18 19" 20a 21" 22a 23" 24 25 26 27 28
A. P. I.
TYPE
7 N. P. A. oil standard 7.25.N. P. A. clayed distillate Distillate Crude residuum Crude residuum Crude residuum Cracked tar Crude residuum Crude residuum Crude residuum Crude residuum Crude residuum Finished oil Finished oil Finished oil Distillate Crude residuum Finished oil Finished oil Finished oil Finished oil Finished oil Distillate Distillate Finished oil Finished oil Finished oil Finished oil -SOLUTIONS
..
20:4 18.0 17.1 14.8 10.8 20.6 17.2 13.3 14.2 22.8 21.5 21.5 19.4 20.9 8.2 26.0 22.0 25.0 24.6 28.4 20.6 24.2 24.6 20.4 23.8 22.0
looo F. At 210° F.
... ...
... ... ... ...
196
..... . ... .. .. .. .. .. .. ... ... ...
145 505 294 278 503 612 305 906 590 360 505
--
.. .. ..
160 270 231 401
... 3 10
252 2141 632 279 170 170 159 123 1816 e . .
... ... ... ... ... ...
.. 79 .. ..
...
I N TESTSb VOLUMB OIL 100 V O L U M l SOLUTION DILUENT
SAMPLE OIL 29 Sample 15 30 Sample 16 31 Sample 17 32 S a m d e 19 33 Sample 4 34 Sample 9 35 Sample 10 36 Sample 20 37 Sample 21 38 Sample 22 39 Sample 23 40 Sample 15 41 Sample 28 42 Sample 10 a Used only in solution. b Excluding thoee made in true color in calculation of true color.
USED
At
2.50 1.56 0.10 6.25
