April, 1922
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A New Method of Color Measurement for Oils’,z By Leon W. Parsonsa and Robert E. Wilson4 RESEARCH LABORATORY OF APPLIEDCHEMISTRY, MASSACHUSETTSINSTITUTE
This article discusses the need for and the development of a new and consistent method for the rapid and accurate measurement of the true color of practically all petroleum oils, from the lightest to the darkest. B y true color is meant a figure proportional to the amount of coloring matter present in the oil. The Lovibond method is shown to be very unsatisfactory, chiefly for the following reasons: ( a ) As a result of the wide separation of the light squares in the instrument and the “off” tint for certain colors it is impossible to obtain an accurate and reproducible measure of the color; (b) it necessitates the use of kerosene solutions in order to match any oil above 500 color. This process is time-consuming and necessitates an entirely different color scale bearing no simple relationship to the original Lovibond scale: (c) the most serious objection to the method is that the Lovibond scale is not “true,” i. e., its colors are not additive; hence the Lovibond color is not proportional to the actual amount of coloring matter in the oil, and hence the Looibond colors of mixed oils are not additive. The fundamental laws of color measurement are presented in detail, with data regarding their application to the problem in hand. The reasons for the deaelopment of each step in the new method are described in detail. The new method gives a rapid and accurate procedure for determining the true color of practically the whole range of petroleum oils from 1 to 5000 color, by making use of a single. standard. The Duboscq colorimeter is used and the depth of standard color solution necessary to match a given layer‘of the unknown oil is determined. The thickness of the unknown oil varies from a very thin, accurately
SCOPEOB INVESTIGATION HIS article describes the results of an extensive investigation of practical methods of color measurement for petroleum oils, which was carried out in preparation for a fundamental study of decolorization processes, The work was financed by the Vacuum Oil Company, and WRS carried out in cooperation with their Rochester laboratory. It is published with their kind permission. While practically all the fundamental experimental work has been carried out on petroleum oils, the same principles have been applied with quite satisfactory results in certain other fields of investigation in this Laboratory, notably, the colors of sugar solutions, of certain vegetable oils, of dyestuff solutions, and of black liquor from soda cooks. It should, therefore, be of considerable interest outside the field of petroleum oils. There are a t the present time three generally recognized methods of measuring the color of petroleum oils-the National Petroleum Association method, the Saybolt chromometer, and the Lovibond method. The National Petroleum Association method consists of viewing the oil in a 4-oz. bottle and comparing it with a set of six or seven arbitrarily chosen colors. With the Baybolt chromometer, a rough comparison is made through two vertical glass tubes, matching the color of an unknown oil with that of a colored disk,
T
1 Presented in abstract before the Section of Petroleum Chemistry a t the 61st Meeting of the American Chemical Society, Rochester, N. P., April 26 t o 29, 1921. 2 Published as Contribution No. 43 from the Research Laboratory of Applied Chemistry, Massachusetts Institute of Technology. 8 Assistant Director, Research Laboratory of Applied Chemistry. 4 Director, Research Laboratory of Applied Chemistry.
OF
TECHNOLOGY, C.4MBRlDGE, MASSACHESETTS
measured film for the darkest oils to a 10-em. layer for the lightest oils. The advantages of a true color scale are: (a)-It makes possible the determination of the actual amount of coloring matter removed by fuller’s earth under definite conditions, a quantity absolutely fundamental in an investigation of oil decolor izat ion. (b)-By a study of the efluent color curve (Plate X ) it is possible to determine the average color of a batch of filtered oil from the efluent color at any given time and the weight of oilfiltered up to that time. (c)-The true color of mixed diluted oils may be predicted accurately from the true color of the original oils used, information which is of fundamental importance in oil blending. A plot is presented (Plate V l I I ) showing the deviations of the Lovibond from the true color scale. B y means of this plot it is possible to convert Lovibond color to true color readings and thus secure the abooe-mentioned advantages of a true color scale, even if the Lovibond system should remain the basis for routine work. I n oiew of the many advantages of the new method, and its successf u l use for over a year in this laboratory, it is recommended for adoption not only for research where it is absolutely indispensable, but also for plant control. Specific directions are given for its use in the laboratory. B y a modification of the standard used, this method has been applied with quite satisfactory results in the investigation of other problems, for example, the colors of sugar solutions, of certain vegetable oils, of dyestuff solutions, and of black liquor from soda cooks.
the color standard being comparable to the color of potassium dichromate solutions. While both of these methods are adequate for many trade purposes, they are obviously of little value for accurate measurements or for research work on decolorization problems. The Lovibond method is a much more ambitious attempt to establish a color scale suitable for accurate measurements. It consists essentially in comparing the color of a cell (usually 0.25 in. thick) of the oil with successive members of a series of colored glasses which have been made to conform to an arbitrary color standard. In making these comparisons the general practice in the petroleum industry is to use a series of amber-colored glasses, although occasionally use is made of other members of the complete series of glasses, which includes the following colors-yellow, amber, red, and blue-matching the color as closely as possible by the most suitable combination of these four. The comparison with the oil in the cell is made in a rather crude manner by adding or taking away glasses until the two squares of color which are separated by a wide black partition appear to match. It was the intention of this Laboratory to adopt the Lovibond color scale in their investigations on the decolorization of oils, but a few weeks’ experience with the method indicated that it was very poorly adapted for the purpose in hand, especially for measuring the color of the unfiltered or partially decolorized cylinder stocks. The principal defects in the Lovibond method are as follows: 1-It is difficult, especially for an inexperienced operator, t o get accurate matches even in the best part of the scale because of the wide separation of the two squares of light. This difficulty could of course be obviated by a suitable optical device
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SO arranged as to give a split field with no marked line of division between the two color areas. 2-While the glasses match the color of the oils fairly well for colors between 3 or 4 and 200, they give a distinctly “off” tint for colors below or above this range, and especially for colors above 400”, where it is almost impossible to get a satisfactory match. Readings in this vicinity frequently vary by as much as 25’ for the same observer on the same oil. 3-For colors above 400’ Lovibond it is customary to use 5 per cent solutions of the oil in kerosene. This requires more time and necessitates an entirely different color scale which bears no simple relationship to the original Lovibond scale. Furthermore, it is applicable only to oils whose original color does not exceed 2000, and it cannot be employed successfully for the unfiltered cylinder stocks. If higher dilutions are used it is difficult and time-consuming to make them accurately. 4-The most serious objection to the Lovibond method is t h a t its scale is not “true,” i. e., its colors are not additive. For example, a 10 per cent solution of a 400’ oil in kerosene does not give a 40 color, as might be expected, but instead a color in the neighborhood of 78. Similarly a mixture of 50 parts of a 100” oil and 50 of a 400’ oil gives approximately a color of 283 instead of 250” as would be the case if a true color scale were used. It is obvious from this that the Lovibond color, as measured by the standard glasses, is not proportional to the amount of coloring matter in the oil, on account of a faulty scale used in making the standard glasses. This fact is an almost insuperable objection to the use of such a scale in any fundamental investigation of the laws governing decolorization.
Vol. 14,No. 4
unabsorbed after passing through thickness 1, being a constant depending upon the properties of the original material*
In view of the unsatisfactory nature of the existing technical methods of color measurement it soon appeared necessary to develop a new and consistent method of color measurement which would cover the whole range of lubricating oils from the original very dark oils down to the lightest filtrates, and, if possible, give a “true” or strictly additive color scale U / P .4/*over this range. It was furthermore essential that the method be both rapid and accurate, in view of the number It is not possible, of course! to predict how much of any of determinations which must be made, and the importance other color, such as yellow or red, mould be absorbed by one of the conclusions to be drawn. centimeter thickness, since this depends entirely upon its Several modified or new procedures were later tried out, individual characteristics. The important thing is, however, but eventually the one hereinafter described was found to that if the complete light absorption curve is known for be the most satisfactory for the whole range of darker oils any one thickness of the liquid, the curve for any other thickin which this Laboratory was especially interested. ness can be calculated readily and accurately. I n the ease of colored solutions, the effect of the concenFUNDAMENTAL PRINCIPLES OF COLORMEASUREMENT tration of the coloring matter on the absorption of light is Before proceeding to any detailed discussion of the best not known with any such mathematical certainty, Beer’s methods for determining the color of oils, it is necessary to Law states that the effect of doubling the concentration of discuss briefly the fundamental physical principles which coloring matter is the same as that of doubling the thickare applicable to the problem. To the human eye there ness. Most of the literature on the subject, however, is appears to be no evidence of any simple law indicating how devoted to showing that, as a matter of fact, Beer’s Law the color of a solution changes with increased thickness; does not hold at all accurately for most cases of colored even a casual observer realizes that certain light yellow solu- solutions. A perusal of this literature indicates, however, tions when viewed through a deep layer may give simply that the deviations from Beer’s Law are usually not large a darker but very pure yellow, whereas others quickly shift except in the case of aqueous solutions of ionized materials, to orange and red on the one hand, or green on the other, where factors such as the hydration of the ions and the degree merely by increasing the depth through which the white of ionization unquestionably have a serious disturbing inlight passes before reaching the eye. This apparently com- fluence. I n the case of organic coloring matter in organic plicated phenomenon is due merely t o the fact that the eye solvents, it is fairly well established that Beer’s Law holds is not able to distinguish separately light of different wave with reasonable degree of accuracy in most cases, though lengths, but integrates the effect of the spectrum as a whole this cannot be assumed safely in the absence of definite in such a way as to give a single sensation. data, however convenient it might be in working out a satisWhen, however, absorption of monochromatic light, or factory method of color measurement. It is obvious, therelight of a single wave length, is studied, a very simple law fore, that as a foundation for the work in hand it is necesis found to hold, namely, that the amount of light absorbed sary to determine the type of light absorption curve for in passing through a given thickness of a homogeneous ma- lubricating oils, and to find out whether or not Beer’s Law terial is a constant percentage of the light which enters that applies over the range of concentrations which would be section. Thus, if one centimeter of a given oil absorbs 20 encountered in practice. per cent of a certain wave length of blue light, the second OF FUNDAMENTAL PROPERTIES OF COLORcentimeter will absorb, not an equal amount, but 20 per cent DETERMINATION ING MATTER IN LUBRICATING OILS of the 80 per cent which enters it; in other words, 16 per cent of the original light. Similarly, the third centimeter In undertaking a preliminary investigation of the coloring will absorb 20 per cent of 64 per cent, or 12.8 per cent of this matter present in oils, the first and most important characterparticular color. The mathematical expression of this fact istic which was discovered was that practically all paraffin i s that log x=kl, where 2 is the fraction of light remaining base oils from very pale yellow spindle oils down to extremely m9Ycnr
0
L1
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dark-colored, unfiltered, cylinder stocks had essentially the to bring the heavier oils to this range of color where accurate same light absorption curve, providing the heavier oils were measurement is possible, either by dilution with a solvent diluted down or viewed through films which were sufficiently or by using varying thicknesses of layer down to a very thin thin. Even dark oils which differed markedly in their general but accurately measured film for the heavier oils. appearance and appeared a fluorescent purple, green, or It was also necessary to determine the validity of Beer’s red, when viewed by reflected light, gave almost identical Law for the coloring matter in oils. This was done by two colors when viewed through sufficiently thin films or in dilute different methods in two widely different ranges of color. In solutions, and it was possible to secure very satisfactory the first case a standard 10 per cent solution of dark-colored color matches by such means between widely varying original heavy oil in kerosene was made up. This solution was careoils. fully diluted further with kerosene to give concentrations Plate I, Curve ad, represents a rough approximation of of heavy oil varying from 1 to 4 per cent, and a constant the light absorption curve for a 1-mm. thickness of a fairly depth (15 mm.) of these solutions was compared with a single light-colored lubricating oil. It is realized that if extremely standard solution (containing 2 per cent oil) whose depth accurate absorption spectra were obtained-for example, was varied as necessary to get a precise color match. This with a spectrophotometer-these curves would show some depth was measured accurately in a Duboscq colorimeter, irregularities, but in general the curves which are plotted operated as described later. The results obtained on this represent the type of absorption spectrum obtained with a series of samples are shown graphically on Plate 11, where good spectroscope. It will be noted that the absorption in the red is extremely small, while the absorption in the blue and violet is very high. Using the above-mentioned law it is possible to calculate the light absorption curve for other thicknesses of the oils as indicated on Plate I. Curve eg, for example, indicates the light absorbed by a 30-mm. layer of the same oil, but since all oils give the same general type of light absorption curve, if used in fairly thin layers, it also represents approximately the light absorption curve for 1-mm. thickness of an oil which contains roughly (or exactly if Beer’s Law holds) thirty times as much coloring matter per unit volume. By inspection of this interesting series of curves, it is possible to obtain a clear picture of the series of changes which occur in the apparent color of the oil as seen by the eye. Taking an absorption curve for 1-mm. thickness, it will be noted that the light represented by the area below the horizontal line bd contains all the components of light in the correct proportion to give white light. But the part above the line and below the absorption curve ad contains a considerable deficiency in the blue end of the spectrum. The center of gravity of this area is, therefore, in the greenish yellow part of the curve, and the eye, which must average up the combined effect of 80 per cent white light plus 20 per cent colored, records the sensation of a pale, slightly greenish yellow color. As the thickness of the layer (or concentration of color) increases and we proceed to lower and lower absorption curves, it is obvious that less and less white light is coming through. This makes the colors appear much darker and furthermore the absorption curves are becoming steeper and the center of gravity is more and more displaced towards the orange-red end of the spectrum. The visual sensation given by a line such as eg would, therefore, be a very dark orange-red color. Intermediate absorption curves will, of course, give an intermediate sensation. it may be noted that, as Beer’s Law would require, the depth It will be seen thus that starting with a single light ab- of standard solution required to match the constant depth sorption curve it is possible t o account for all the observed of variable solutions was directly proportional (within the colors, ranging from the very pale lemon-yellow to a very limit of error) to the concentration of oil in the variable soludark orange-red or red-brown. The green or purple tinges tions. which show up by reflected light are not apparent when viewed When the film method of measuring heavy oils was finally by transmitted light. perfected, another verification of Beer’s Law was obtained It is furthermore obvious from an inspection of the curves by working with very dark-colored oils. The procedure that the best matches and the most accurate color measure- was as follows: Heavy oil, A, would be compared in a thin ments can be obtained in an intermediate range only, The film, whose thickness was accurately known, and matched colors of very light oils cannot be measured accurately be- against a certain depth of standard solution; Oil B, with cause there is so much white light coming through, while a widely differing color, would also be matched in the same the sensitivity of any match drops off very rapidly as the way. These two oils would then be mixed together in exactly absorption curves approach the bottom of Plate I, as may equal proportion. If Beer’s Law holds, the depth of standard be observed by noting the much closer spacing of lines for a required to match this mixture should be the arithmetic given increase in color. Small differences in the shape of mean of the two previous readings, and this was found to the curves also make trouble in thick layers. Any satis- be the case within the limit of experimental error. Repeated factory system of color measurement must, therefore, plan verification of Beer’s Law has been obtained in the laboratory
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by comparing colors of mixed oils on a small scale, and in the plant by blending larger quantities of oils, in some cases as high as 8000 gal.
Vol. 14, No. 4
metric work, differing only slightly in concentration from it. Measurements were made in the Duboscq instrument by placing the kerosene solution in one cup, and the solutions in the other. Various settings of the plunger were made and straight line relationships obtained between the readings of the standard plotted as abscissae and the readings of the other solutions plotted as ordinates. A summary of the readings of the various solutions matching a 10-mm. layer of the standard is given in Table 2. The deviation of these readings from 10 mm. is proportional to the variation in color due to the use of different solvents. TABLE2-COLORIMETER
AULld*d
x
I
READINGS F O R SOLUTIOXS OF MINERAL OIL I N VARIOUS SOI,VENTS Depth of Solution S Depth of Standard Used for Match Mni. Mm. Nature of Solvent in S 10 13.6 CHCls 10 cc14 12.4 10 11.2 CeHa 10 9.6 Gasoline
szdm5 = ~ ~ o m m .The depths of
Y&%= C7WOmm.
FA-%= adeomLa
PLATE 111-DETAIL
OF
FILMSPACERS
Table 1 indicates results obdained on comparing the color of mixed oils with the calculated value as obtained from the true color6 of the separate oils used and the weight of oil taken. It will be observed that there is a very close agreement between caIcuIat’ed and determined color. TABLEi-cALCULATeD AND DETERMINED COLOR O F MIXED OILS True Color and Per cent Calculated Measured True Color True Color by Weight of Oils Used of Mixture of Mixture 50% Qf oil (T. c. = 1790) 1595 1550 50% of oil (T. C. = 1400) 5 0 7 of oil (T C.=2420) 1910 1900 50$ of oil (T: C.=1400) 50Y0 of oil T.C -3200) 2300 2280 50% of oil IT. C : = 1400) 28 8 7 of oil (T. C.=490) 1390 1400 7 1 : 2 d of oil (T. C.=1760) 775 780 55% of oil (T. C. =240) 48% of oil (T. C. = 1430) 49.4% of oil (T. C. = 3200) 1700 1725 50.6% of oil (T. C . = 2403
The validity of the above relationship is, therefore, clearly established. , Beer’s Law has thus been shown to hold with surprising accuracy botJh for fairly light-colored kerosene solutions and very dark-colored heavy oils. It will be noted that in obtaining the foregoing results, the nature of the color solvent was not varied in the dilution experiments-in the first case the solution was more than 96 per cent kerosene in all cases, and in the other it was always practically 100 per cent heavy oil. Neither these results nor Beer’s Law will throw any light, however, upon the effect of diluting heavy oil with a widely different organic solvent such as benzene or chloroform, or possibly even kerosene. I n order to obtain some dat’a upon this point, a series of solutions of heavy oil was made up to a concentration of exactly 2 per cent in different solvents, and the color observed. I n this comparison, the standard of reference was a 2 per cent solution of heavy oil in kerosene.. The solution thus ohosen was identical in nature with the regular light standard, L. 8.(hereafter mentioned), used in all of our coloriThe derivation of the scale used is discussed later.
the colors in solvents widely differing in chemical nature from the oil are shown to be, in most cases, lower than those obtained in solvents similar to the heavy oil, such as kerosene and gasoline. Furthermore, the absorption curve in the chlorinated solvents was different from that of the original oil making it difficult to obtain accurate matches. It must, therefore, be concluded that, although the dilution law holds accurately as long as the solvent is not changed, it will not hold where the relative proportions of two dissimilar solvents are varied. One series of experiments were made by diluting heavy oil accurately with kerosene which indicated that, in this case, the dilution law held within 5 per cent, but by no means as accurately as Beer’s Law itself, where the solvent is kept substantially constant.
FACTORS II~VOLVED IN SELECTION OF METHOD
A discussion of the various phases in the development of a satisfactory method for measuring the color of oils is indicated below. COLOR STANDARD-The best color standard appeared to be a kerosene solution of heavy oil, since this gave exactly the same type of absorption spectrum as the oils themselves. This is a great advantage, for while it is possible to obtain fair matches with standards such as the Lovibond glasses, which have a distinctly different type of absorption curve, th’e depth of oil required to match a given glass is dependent to a very marked extent on the sensitivity of the eye of the observer to different colors and to the source of light. On the other hand, using a standard with the same type of absorption curve, it has been found experimentally that neither the observer (providing he has had a little experience) nor the source of light has any appreciable influence on the readings obtained. It has also been found that a single kerosene solution of dark oil matches all the lubricating oils from the very lightest to the dark cylinder stocks, providing a proper thickness of the oils is selected. The “grayness” which is, in the case of some oils, superimposed on their regular color, gives practically no trouble whatever when the color measurement is made in very thin films. I n some cases, however, it is more satisfactory to use a slightly different standard for oils which have a tendency toward cloudiness or grayness. This is referred to as the dark standard, D. S., and it is designed to have exactly the same color value as the ordinary or light standard, L. S. Details of operation are given later under “Laboratory Directions.” The standard solutions are diluted to approximately 35 Lovibond color and then standardized exactly by comparison in the colorimeter with a 50 color Lovibond glass, The latter is selected as the standard because up t o
April, 1922
THE JOURNAL OF INDUXTRIAL AND ENGINEERING CHEMISTRY
this point there is no deviation between the Lovibond and True Color curve. Mixtures of different oils were examined a t the Rochester laboratory in order to determine, first, whether they were more suitable for color standards, and second, their relative stability. In general it has been found that there is no advantage sf these over the regular L. S. standard either regarding accuracy of matching or stability of sample. Both L. S. and D. S. standard solutions have been found to be fairly stable over a period of several weeks. Any change which does take place, however, is usually a slight increase in color and occurs within the first 2 or 3 days after preparation of the solution; consequently, it is recommended that the solutions be allowed to stand from 1 to 3 days in order to come to equilibrium before the final color value is determined. They should be protected from exposure to light and should be meaaured at intervals against the 50 color Lovibond glass in order to check the color value. This single Lovibond glass (which may be separately standardized) therefore becomes the permanent reference standard, while the solution is used in all measurements on oils. MEASURING INSTRUMENT-The most suitable measuring instrument appeared to be the Duboscq type of colorimeter, since this brings the two fields in very close juxtaposition (two semicircles divided by a hair line), and permits the depth of the standard solution in one of the cups to be varied a t will and measured accurately. The match can be approached slowly from both sides without interruption of light or stepwise adjustments such as are necessitated by the use of Lovibond glasses. A distinct advantage of the Duboscq over the Lovibond is that the former is easily adaptable to measur-
2 73
ing colors of widely different types of colored solutions by merely changing the nature of a single standard. It has given extremely satisfactory results in several laboratories. DEVELOPNIENT OF THIN CELLS FOR DARK oIIe-The remaining question to be determined is the best method of bringing heavier oils into the proper color range for accurate matching. As pointed out previously, this can be done either by diluting the oil with a solvent such as kerosene or by viewing it through very thin films. The former method appeared more feasible at first sight and because it was not thought practicable to measure very thin films with sufficient accuracy, considerable work vias accordingly done along these lines. It was found, however, very difficult and time-consuming to measure the heavy oil accurately by diluting it with a known amount of kerosene. The method, furthermore, suffered from the uncertainty due-to the use of a solvent different fram the original oil. Attention was therefore turned to the possibility of securing very thin films of oil of accurately known thickness. After several attempts the demountable type of cell shown in Plate I11was developed, and with this it was found possible to reproduce thicknesses of 0.1 mm. within 1 per cent. The cell is made from two carefully prepared parallel glass plates of optical glass. On the upper surface of one plate are fastened, by means of Canada balsam, two small sections of microscope slides of a definite thickness, leaving a space in the middle of the plate to hold the film of oil. The cells may be made of any desired thickness. It is, therefore, possible to use a single standard solution and by merely varying the thickness of the layer of the unknown oil and measuring it accurately under all circumstances,
THE JOURNAL OF INDUSTRIAL A N D ENQINEERING CHE.MISTRY
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to determine the true color of any oil between No. 1 Lovibond and very dark unfiltered cylinder stocks which run above 4500 on the true color scale.
.
.~
,
.
.
~
__
.
~
. . . .
~
PROCEDURE FOR MEASURING COLOR-Fig. 1 shows the Duhoscq colorimeter with a cup in position ready to determine the color of an oil. B J ~moving the glass plunger the depth of liquid in the field of vision can be varied at will. A determination of the level is made by the aid of the verniers on the instrnment to 0.1 mm. Using always the same standard, L. S., t.he thickness of the unknown oil layer may be varied a t will in order to require a depth of standard sufficient to give accurate readings and yet not too deep to give a sharp match. A range of from 4 to 15 mm. of standard may be used in comparisons, hut the best accuracy is obtained by adjusting the thickness of the unknown oil layer so as to keep the L. S.reading between 4 and 12. In an actual measurement the plunger dipping into L. S. is brought very quickly to a point where the two fields observed in the eyepiece indicate an exact match between t,he layer of L. S. and the definite layer of unknown. This reading of the standard may be approached from both the light or dark side and a match in shade and intensity obtained very quickly and accurately. Two check readings, one from each side, are ordinarily sufficient, but for inexperienced observers five or six may well be taken and averaged, the total time necessary being very short. For oils of true color between 3 and 200 the osual 50-mm. cups may be used for the unknown, the plunger ordinarily being set a t 5, 10, 20, or 40 mm., depending on the depth of the color of the unknown oil. For oils of a true color less than 3, it is desirahle to unscrew the plunger entirely and put the oil in a special ccll 100 mm. deep, equipped with a glass cover which fits evenly on the top and squeezes out any excess oil. With this cup it is possible with the single standard to measure the color of oils down to 1” without any difficulty. Oils having a color range from 200 to 4500 obviouslymust be measured in one of a series of thin cells such as those d e scribed above. The following three thicknesses are suffieient to cover the entire range of true color from 200 to 4500, the latter figure representing the color of the darkest oil investiga.ted. Cell
1 2 3
No.
For Oils io True Color Range 1 2 0 t o 400 400 to IS00 IS00 t o 4500
Thickness of Cell 1.10 mm. 5% 0.380 mm. f 5 %
*
0.140mm.
i
5%
These limits and a conversion chart are shown graphically in Plate IY. This chart applies only to t.he exact dimen-
Yol. 14, No. 4
sions of the cells as indicated above and to a standard with a true color value of 35. Special charts may be very easily constructed by individual laboratories to correspond to the cells and standard chosen. These cells can be made readily by any expert optical worker. The exact thickness is determined by very accurate micrometric measurements on the two parallel plates and the assembled cells. By placing several drops of oil in the space between the film spacers, squeezing, out the excess by pressing down the top plate, and clamping the two plates together, a film of definite thickness is obtained. It is found that the presence of oil on the spacers has no measurable effect upon the readings obtained. I t was feared at the outset of the work that the use of very thin films could not he relied upon to give accurate results, hut this has been definitely dmproved, providing thc precautions mentioned later in the “Laboratory Directions” are observed. Even with the thinnest cells the readings are found to be reproducible within 1 or 2 per cent in the hands of an experienced operator. These films merely replace the 50-mm, cups in the Duboscq, the matches being obtained as before hy varying the dcpth of standard. If the reading does not come between 4 to 15 mm. the wrong cell has been selected for the oil. An experienced observer can very quickly tell in which cell a given unknown oil should he placed. CALCULATION OF RESULTS-From the foregoing discussion of the laws of light absorption, it is obvious that the true color (a figure proport,ional to the amount of coloring matter present) of an unknown oil can he calculated by the simple formula: Color of unknown - Depth of standard required ior match Depth of layer of the unknown oil Color of standard EXPERIMENTAL RESULTS From a large number of data on color measurement obtained a t the Rochester plant of the Vacuum Oil Company and in this Laboratory, the fundamental results have been collected and are illustrated graphically in the accompanying plates. Reference to these plates, rather-than to a detailed record of various colorimeter readings, shows more clearly the relation of the Lovibond system to the true color scale and also the scope and advantages of the new method of color measurement. VALIDITY AND SCOPE OF TRUE COLOR
SCALE-The readings of L. S. for several depths of oil solutions for several light oils are shown in Plate V. It is evident that a direct proportionalit,y exists between ihe depth of standard and the depth of a given oil necessary for a color match. S o . I-DUBOSCQ COLORIPHTBZ This is true both for light oils and for kerosene solutions of heavy oils, and follows necessarily from the above-mentioned law of light absorption as a function of thickness. Such a complete
April, 1922
THE JOURNAL OF INDUETRIAL AND ENGINEERING CHEMISTRY
series of points can, of course, only be conveniently obtained when the unknown oil is in the neighborhood of 10 to 60 color and hence is in one of the regular cups so that its thickness can be varied by small stages and matched by similar variations in the depth of the standard.
276
tory regarding the Lovibond color readings on a number of oils and their solutions in kerosene. On merely looking a t the Lovibond color values there was no consistent relation between the values obtained, but when the values were converted to the true color scale they approximated fairly closely the expected dilution laws. By means of Plate VIII, from any Lovibond color up to 400 there may be found the corresponding “true” color. Owing to the impossibility of obtaining satisfactory Lovibond readings above 400, the upper part of the curve cannot be determined with very great accuracy. The variation of the curve from the 45” dotted line indicates the variation of the Lovibond from a “true” color series. It is evident that for the light>eroils, up to about 50” Lovibond, the two scales are identical, but from this point on the deviation gradually increases. For oils above 500 color the Lovibond system required the use of a still different scale, based on the color of 5 per cent solutions which bore no simple relationship to the other scale. Using the new method the scale is a continuous straight line up to the highest colors measured. Table 3 indicates the relationship for light oils between our true color scale and other colors such as the National Petroleum Association number and the Lovibond (Series 500), the latter having been obtained from SL chart submitted to the laboratory. TABLE 3-RELATION
BETWEDN TRUECOLOR STANDARDS
AND
OTHER COLOR
s C D Prime white 0.5’ 0.5 Cream white 0.90 0.9 Extra pale 7.0; 7.0 Lemon pale 16.0 16.0 4 Orange pale 35.50 35.5 5 Light red 66;0° 75 6 Red 100 120 A = Association number. B = Association name. C = Lovibond color (Series 600) of oil matching corresponding Association number in Column A taken in the Association apparatus. D = T r u e color. A No. 1 1-1/2 2 3
From data such as are recorded in Plate I1 for various oils, it is also evident that a direct proportionality exists between the depth of standard and the concentration of color in a diluted sample of oil. This indicates definitely that a “true” color scale is obtained by this method, thanks to the fact that Beer’s Law holds for the case in question. In Plates VI and VI1 the readings for L. S. to match different heavy oils in two different film spacers show that the L. S. readings bear a constant ratio to one another regardless of the color of the oil tested. This ratio checked within 1 per cent with the relative thicknesses of the cells as determined with a micrometer, which again shows the value of using a single color standard and merely varying its depth. RELATION BETWEEN LOVIBOND COLOR AND ‘(TRUE” COLOR-
Plates VI11 and I X show the fundamental relationship between Lovibond color and “true” color. Plate IX is an enlarged section of Plate VI11 which shows the region covering the lighter oils of Lovibond color up to 150”. In establishing this relation a large volume of data had t o be accumulated. The Rochester laboratory furnished a large number of samples of oil on which the Lovibond color had been determined as accurately as possible. These were matched against the new color standard in the Duboscq colorimeter and their true color determined, thus giving the relationship between the two color scales, based, of course, upon the assumption of the Lovibond scale from 0 to 50 as the starting point of the true color scale. As is indicated on Plates VI11 and I X the points of all fall on a fairly smooth curve. The curve was further checked by comparison with data furnished by the Rochester labora-
276
T H E JOURNAL OF INDUSTRIAL AND ENGINEERING CHEiVIXTRY
PREDICTION OF THE COLOR O F MIXED OR DILUTED OILS-
Much difficulty has been experienced at the plant in attempting to blend various oils, particularly the heavy oils, and predict the final color by using the Lovibond scale. If the true color value be substituted for the Lovibond the accurate determination of the color of mixed oils becomes very simple since the colors are merely the weighted average of the colors of the original oils used. Even if the new colorimetric method should not be adopted, the relationship s h o m in Plates VI11 and IX between Lovibond and true color will be of great value in enabling one to predict the Lovibond color of a mixed or diluted oil, knowing the amounts and the Lovibond color of the original oils. This is done by finding the true color values corresponding t o each Lovibond value, calculating by simple proportionality the true color of the final mixture and then again referring to the chart to find the corresponding Lovibond color. Such results will not, of course, be as reliable as if the new method had been used throughout, because of the other inaccuracies of the Lovibond method, but the approximation will be reasonably good and should prove a great improvement over the present conditions where it is impossible to tell anything about these relationships The dotted lines on Plate VI11 show how to make the above-mentioned calculations for one case, using the Lovibond colors as a starting point. If it is desired to make such calculations on the basis of the 5 per cent solutions used in the Lovibond, it is merely necessary to keep in mind the fact that the true color of the original oil would be very nearly 20 times the true color of the 5 per cent solution, as indicated by the double scale of abscissae on Plate IX. RELtlTION BSTWEEN TOTAL COLOR O F BATCH AND EFFLUENT
coLoR-Plate
X shows a typical curve such as is obtained
Vol. 14,No. 4
during the filtration of oil through fuller’s earth. Here is plotted the true color against the weight of oil obtained from a filter, or, in other words, the color of successive samples of oil taken from the effluent stream from the filter as the filtration progresses. This curve shows graphically how much color is being left in, and removed from, the oil a t any given moment. From this amount it is possible to form conclusions with regard to the efficiency of filtration. Such a plot, using Lovibond color values, would not give information about the per cent of color removed, which factor is necessary for comparing accurately decolorizing efficiencies under widely different conditions. Furthermore, by using the new color scale, where the colors are additive, it is possible to calculate the color of the total batch up to any given time, or between any given times, merely by determining the effluent color and the weight of the batch a t various times during the run. The precise method of making the foregoing computation is illustrated in Plate X, as follows: The line AB represents the total color of the batch at t h e end of x: hours when the weight is ED grams. To determine this batch color in practice from the effluent color curve, the area EFCD is obtained with the aid of a planimeter and divided by the total weight a t the particular time. This gives directly the batch color. The effluent color curve and the weight of oil from the filter enable one to predict the total batch color a t any given time. This has been repeatedly verified both in the Laboratory and on a plant scale and is of great help in plant control. Such a calculation is not possible if the Lovibond scale is used. RESULTS BEARING ON NATURE OF COLORING MATTER-
One interesting point may be mentioned in passing, with regard to the bearing of the results obtained on the probable
THE JOURNAL OF INDUXTRIAL AND ENGINEERING CHEiWSTRY
April, 1922
nature of the coloring matter in the oils. I t has frequently been suggested that there might be several different coloring materials present in the oil, each of which was adsorbed to a different extent and gave a different kind of color to the oil. This might seem plausible in view of the very great difference in color between an original oil and a decolorized
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oil. In view of the fact, however, that a solution of the original oil can by mere dilution be made to match the same oil after it has been decolorized to any given extent, it is apparent that if there are two coloring matters in the oil they either have almost identically the same color absorption in different parts of the spectrum, which is improbable, or else they are adsorbed to the same relative extent by the fuller’s earth. In the latter case the coloring matter might as well for all practical purposes be considered as one single material. The only possible exception to this statement is in the case of the so-called “grayness” observed in the darker oils which is, however, a t best a small factor and is probably due to partly solidified paraffin or to fairly large colloidal particles of carbonaceous material. Certain oils from special districts have a purplish or other unusual tint when viewed in thick layers, but inspection of their dilute solutions or thin films shows that in even these the essential coloring matter is the same as in the other oils, but there is superimposed a small amount of material which has different color absorption and shows up quite differently in the thicker layers. Further work is now under way to determine any special characteristics which may be associated with the color of crude oils from different sources. LABORATORY DIRECTIOKS APPARATUS-The
necessary apparatus is as follows:
1-Duboscq colorimeter with two 50-mm. cups and one 100mm. cup (with glass cover). 2-Three film spacer3 as follows (see h’ote 1 for calibration). No 1 = 1.10 mm thick i 5 per cent No. 2 = O 380 mm thick 5 per cent Iio 3 = 0 140 mm. thick i 5 per cent
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3--A standard solution of oil €01 comparison, of about 36” Lovibond color (see Note 2 for calibration). METHOD OF PRocEDuRE-Let the standard be designated by L. S. and the unknown oil by X. The division of the oils into the following classes is made in order to require a depth of standard of 4 to 12 mm. Oils of True Color Range from 9 to 120-Obtain a uniform sample of X and pour it into the Duboscq cup, placing the cup on the right hand side of the instrument, L. 8. being in the left hand cup. Use the regular 50-mm. cup for oils of a true color between 3 and 120, and the 100-mm. cup for
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oils lighter than 3 (Note 3). Adjust the plunger quickly on the nearest mm. reading in X so as t o obtain a 5 to 20 mm. reading of L. S. Then move the L. S. plunger carefully until a color match is obtained, approaching a match from both the light and dark side (Note 4). One reading is sufficient, but three may be very quickly made and a check thereby obtained. Record as data the reading in mm., a, of L. S. necessary to match a definite layer, b mm., of oil X. Oils of True Color Rangefrom I20 to 4500-Stir X carefully to obtain uniformity and deliver a sample of several drops on the middle of the glass film plate which has the suitable spacers, as indicated in Plate IV. The film plates must be very clean and free from dust particles. Press down the top plate over the sample, squeezing out any excess of oil through the sides, and clamp the plates with suitable fasteners (Note 5). Clean the top and bottom surfaces and insert the film of oil beneath the plunger on the right hand side of the colorimeter, lowering the plunger until it rests lightly on the surface of the top plate, entirely covering the oil film. Move t8he L. S. plunger to obtain a match as before and record as data the reading in mm., a, of L. S. necessary to match the layer b (the thickness of film used) of X. CALCULATION OF TRUE COLOR VALUEL ~ ~ ~ ~ ~ = ~ ~ ~ At match. a mm.=reading of L.. S. for reading of b mm. of X oil For 1 t o 120 T . C. range, b=la&r of X oil in cup. For 120 to 4500 T. C. range, b = thickness of oil film in spacers. Then let C,=unknown
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color, and 35=color of I,. S.
Cx a and = or 35
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; cX -- b
(35).
If a is chosen arbitrarily a t 5 or 10 mm. then cx =
E or E’, respectively. b
b
When the film spacers are used the true color may be obtained easily from Plate IV, which gives true color readings for L. S. readings matching the films. If desired, simple charts or tables may be prepared to record the true color
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