V O L U M E 2 4 , NO. 5, M A Y 1952 829

of Chemical Analysis,” pp. 94-5, Sew York, John FYiley tk Sons, 1938.

(8) Maxwell, J. A., and Graham, R. P., Chem. Revs., 46, 471 (1950).

(9) Rodden, C. J., “A4nalytical Chemistry of the Manhattan Project,” National Xuclear Energy Series, Division VIII, Vol. 1, p. 515, Xew York, McGraw-Hill Book Co., 1950.

(10) Russell, A. S., Evans, D. C., and Rowell, S. Ti’., J . Chem. SOC., 1926, 1872.

(11) Sinclair, E. E., and Casto, C. C., U. S. Atomic Energy Com- mission, R e p t . C-4.360.6 (March 1946).

(12) Willard, H. H., and Greathouse, L. H., J . Am. Chem. Soc., 39, 2366 (1917).

RECEIVED for review August 18, 1951. From B thesis presented by Leven S. Haalegrore to the Graduate School of Emory University, December 1949, in partial fulfillment of t h e requirements for the degree of master of science.

-4ccepted February 18. 1952.

recovering manganese from the cathode, the mercury being dis- tilled off in a stream of nitrogen.

LITERATURE CITED

(1) Furman, N. H., Bricker, C. E., and McDuffie, B., J . Wash. Acad. Sci., 38, 159 (1948).

(2) Grimes, VV. R., and Casto, C. C., U. S. Atomic Energy Com- mission, Collected Paper 79 (March 1947) : Kational Nuclear Energy Series, Division VIII, Vol. 2, iYew York, McGraw- Hill Book Co.. 1951.

(3) Holler, A. C., and Yeager, J. P., ISD. ENG. CHEM., ASAL. ED.,

(4) Khlopin, N. Y., 2. anal. Chem., 107, 104 (1936). (5) Latimer, W. hf., “Oxidation Potentials,” S e w York, Prentice-

14, 719 (1942).

Hall, 1938. (6) Lingane, J. J.. and Neites, L., ANAL. C m x . , 19, 159 11947). ( 7 ) Lundell, G. E. F., and Hoffman, J. I., “Outlines of Methods

Measurement of Reflectance o f White Sugar T. R. GILLETT AND P. F. MEADS

California and Hawaiian Sugar Rejining Corp., Ltd., Crockett, Calif.

Color is an extremely important factor in determining the qualit). of w-hite granulated sugars. Up to the present time no suitable devices have been de- veloped for use in plant control of the color of w-hite sugars. An investigation designed to develop a suitable control procedure led to the development of a photoelectric reflectance method for determining the color of granulated sugars controlled w-ithin reasonably narrow grain size distributions. This method has been used for a number of years for plant control purposes w-ith considerable satisfaction. Production personnel know at all times the color of the refined sugar and are able to correct production methods promptly in order to maintain uniform quality in the product. This is very desirable in meeting product specifications and customer requirements.

ECAUSE color is an extremely important factor in the B evaluation of the quality of white sugars, a great deal of study and investigation has been carried on in the industry t o develop suitable means for measuring the color of such products. This has involved measurement of t he color of dry white sugar as well as of solutions of white sugar. Both types of measurement are significant for quality evaluation and for control purposes. Solution colors are of particular importance where the white sugar is dissolved prior to use in industrial food processes. On the other hand, the white appearance of granulated sugar is of significance where i t is used or compared in the dry form.

The primary concern is the visual appearance of the dry sugar or the sugar solution. Accordingly, all color measurements in this laboratory have been placed on a visual basis-Le., re- ferred to suitable visual procedures. In this limited sense, the color methods may be considered someJThat empirical. How- ever, the photoelectric transmittance and reflectance measure- ments utilized are basic and their fundamental nature is re- stricted only by equipment limitations, which have had no effect on the usefulness of the methods for control and comparison.

Both dry and solution methods of color measurement have been employed in this laboratory. Solution colors are used as a basis for controlling as well as evaluating color. This type of measurement is generally more precise because the color is accentuated in solution, and transmittance measurements are more readily made than reflectance measurements. The method developed in this laboratory for photoelectrically measuring the solution color of white sugars has been described (5’). This instrument and procedure have proved very satisfactory for routine color work in the laboratory, but they have not been particularly adaptable to color control in the refinery.

Considerable study was given to the possibility of developing

an instrument that would give a close approximation of the color of a dry granulated sugar, and would be sufficiently simple and rapid for use by operating personnel. This investigation ulti- mately led t o the design and construction of a photoelectric reflectometer which has proved very satisfactory in service for the past 10 years. The instrument essentially replaced the previous method of visual comparison against a series of dry standard sugars. With this change, inaccuracies resulting from visual observation and personal judgment have largely been eliminated.

The problems involved in developing an instrument for meas- uring the color of dry white sugar are obvious. In the first place, the amount of color involved is extremely small-for example, high quality white sugars, of normal granulated grain size will reflect 94% or more of the incident light. Lower quality sugars, which occasionally may he enrountered in the trade, might reflect as little as 90% of the light. Obviously, measure- ment of the small differences in light reflected by various sugars is very difficult, and differences in grain size of white sugars seriously affect the nature of the reflection. 1Iore finely divided particles appear whiter than coarpe particles both to the eye and to photoelectric devices Furthermore, differences in luster influence results, and differences in apparent grayness sometimes confuse the detection of yellowness in the sugar. These and other factors make the measurement of color of dry sugar an exceedingly complicated problem.

Keane and Brice ( 5 ) found that, in general! the coarse-grained sugars had lower reflectances than the fine-grained sugars and that variations in coloring matter appeared to have a more pronounced effect on the reflectance of the coarse sugars than of fine sugars. They concluded that a plot of reflectance against grain size should yield

Grain size variations are particularly important.

830 A N A L Y T I C A L C H E M I S T R Y

a family of curves, one curve for eaeh value of apparent color. The present investigation has substantiated this conclusion. Browne and Zerhan ( 1 ) have a180 commented on the effect of grain size on reflectance.values, referring pmtieularly t o the work a t the Java Sugar Experiment Station,

In the initial stages of this investigation a number of different methods were tried. Among these were experimental arrange- ment8 involving measurement of multiplereflectance, fluorescence, transmittance through a thin layer of dry sugar, and transmit- tance through a thin layer of moist sugar. None of these methods appeared particularly promising and the investi- gation was therefore directed t o development of a method for measuring diffuse reflectance from the surface of the sugar. This method wa8 adopted as a hasis for the instrument that was ultimately developed.

DESCRIPTION OF INSTRUMENT

The general arrangement of the photoelectric reflectometer is shown in Figure 1.

Figure 1. Optical Arrangement of Photoelectric Reflectometer

In principle, light is directed upward through a color filter a t

the photoele&ic celk. This'is the same ([eneral arrangement

~ ~~~~ ~~ ~~ ~~ ~ . . watt, llO-&lt, projection la& ~ The color filter is a Corning glass Daylight type No. 590. This li$ht blue filter modifies the spectral character of the light to simulate conditions under which white sugars are generally examined visually under day- light from the north,

mol0 J CELL

-PYoTo CELL

Figure 2. Photoelectric Reflectometer Circuit

A number of variations utilizing two photoelectric cells in balanced circuits were tried. The hasic photoelectric circuit which WSB finally adopted (Figure 2) is a modification of similar circuits in this lahoboratory (2.4).

The slide-wire units have been expanded by the u8e of a series resistance, in a manner similar t o that used in the instrument developed for solution color measurements (S). This multiply- ing effect results in t h e t o t d 10% absorption range of white sugmrs being spread over the greater part of the full slidewire scale, thus giving much greater sensitivity in the instrument. As a matter of fact, the scale spread is increased about seven times. T o Secure optimum sensitivity, a multiple reflecting lamp and scale galvanometer are employed.

Figure 3. Photoe1eotrie:Retleetometer

~ ~~~~ ~~ --., ~ ~ - - absolute refleetances of 'ahout ~ 87%"as related t o magnesium carbonate are used. For initial balancing of the circuit, the whit.e

glai8 plate, replacing one of the tiles. It was determided that the clear photo glass plate had no significant effect on the meas- urements.

A photograph of the completed instrument is shown in Figure 3.

OPERATION OF INSTRUMENT

hitlance point is in the direction of great.er reflectance, ai granu- lated sugars generally have higher reflectancos (whiter) than the reference tile.

The lefehand tile of the instrument is then raised, the steel ring placed on the glass plate, and a sample of sugar poured into the metal ring and then covered with a suitable opaque cover.

The slide-wire is rotated until the galvanometer again indi- cates .a balance.

The color reading is taken from the slidewire scale. The sugar is then swept off the glass plate by a movement of

the steel ring t o the left, and drops through the hopper to a re- ceiving pan. Last trace8 of the sugar are removed with a soft cloth.

CALIBRATION

As i t wa6 intended that solution colors would remain the stand- ard for color determination, the reflectance instrument is cali- brated in terms of solution colors. For this purpose the color8 of a number of special samples of sugars of three different grain sizes were carefully determined on both the laboratory solution color instrument and the new reflectometer, The reflectance measurements have been converted from slidewire scale readings to -log R values and are compared in Figure 4 with the solution color values. Each of the three types of sugar gave a linear relationship in terms of -log R and -log T. Also, as would heexpected, thehesugarsshowa higherreflectance(1ower -1ogR value) for a particular solution color than do the coarser grain eugars. Table I shows the average screen analysis and particle size index of these sugars.

V O L U M E 2 4 , NO. 5, M A Y 1952

k Y

0.45

831

The presence of colloidal matter has a marked effect on the relationship between dry and solution colors. Certain types and quantities of colloidal matter impart a gray cast to the dry sugar and in- fluence the amount of light absorbed and hence affect the color reading indicated by the reflectom- eter. On the other hand, the solution instru- ment corrects for the presence of colloidal mat- ter and thus accentuates the differences between the two color values.

The luster of the cryatals may also have some bearing on the reflected color, although this hap not been conclusively demonstrated. I t seem? reasonable, however, that a lusterless sugar with chalky crystal surfaces nil1 show different reflecst-

. L O G T.<* 0.20 0. 25 0 30 0.3 0.40

Table 1. Average Screen inalyses of Sugar Samples (Per cent)

Calculated On On On On On On On Particle 20 28 35 48 80 100 150 Balance Size Index

Coarse granulated T 3 . 7 7 6 . 1 18.5 1 . 3 . , . , , 0 . 1 33 Regular granulated . T 9 . 4 4 0 . 3 4 3 . 2 7 . 1 48 Fine granulated , . . 0 1 2 . 1 1 4 . 3 3 4 : 8 3 4 ' 8 13 6 98

T = Trace. Particle size index is average particle size, expressed a6 Tyler mesh value, calculated froiri

screen analysis, assuming sugar retained on a particular screen has average size midway between tha t screen and the next coarser screen.

In view of this linear relationship it is possible to calibrate the instrument scale in terms of -log R, -log T , or some other proportional unit. In this particular case the instrument is also calibrated in terms of C and H color units ( 6 ) , since the solution color instrument is calibrated in these same units. Rhich are used for routine control in the refinery.

DISCUSSION OF RESU-LTS

Initial calibration involving the samples mentioned above has been checked by the subsequent determination of colors on the solution colorimeter compared with those on the reflectometer on consecutive-shift composite production samples of granulated sugar. In this particular list, the standard deviation is about 15 C and H color units, the maximum amounting to 30 C and H units. For the most part, this instrument will give results within 25 units of the colors determined on the solution colorim- eter.

can U N I T S SOLUTION COLOR

Figure 4. Relation between Reflected and Solution Color of White Sugars

Although the differences obtained between reflected and solu- tion colors are generally less than 25 color units, there are oc- casions when the differences exceed this value. These larger differences appear to be due to the following causes:

Some of these larger differences are undoubtedly due to un- usual variations in the particle size distribution of the produc- tion sugars. While sugars arc normally screened to rather close tolerances in this refinery, on infrequent occasions the normal small variations may be exceeded. The effect of rather large differences in grain size is shown rather clearly in Figure 3. Smaller variations in particle size will produce proportionately less, but still significant, changes in reflected color.

The manner in which color is distributed in the sugar crystal probably also affects the relationship between dry and Lolution colors to R considerable extent. For those sugars in which the color is largely confined to the film on the crystal surface, the dry color will appear darker than would be expected from the solu- tion color. On the other hand, where the color is more or lese uniformly distributed throughout the crystal, the two colors will appear more nearly alike. The distribution of this color is de- pendent on many factors, such as the character and quality of raw sugar, method of refining, crystallization, etc.

ance values than clear, dust-free sugar crystals.

Even though the instrument has limitations associated with grain size variations and distribution of color within the crystal lattice, it has been of very considerable assistance to operating personnel in the maintenance of production color within proper quality specifications. Its use has been restricted to sugars with the normal granulated sugar particle size, although, as is evident from the curves shown in Figure 4, appropriate scales could be developed for products of other grain sizes. However, it is very essential that the particle size distribution be maintained uni- fornily within narrow limits for each puticular grade. The use of this instrument has reduced considerably the amount of sugar returned for reprocessing because of off-color charac- teristics. Obvioudy, this has provided considerable monetary benefit s.

SUMMARY

An investigation of methods of measuring t,he apparent color of dry white sugars led to the development of a photoelectric reflectometer which utilizes a balanced photoelectric circuit and the principle of diffuse reflectance. In its development numeroue

Table 11. Typical Coniparative Results on Dry Sugar Reflectometer (C and H units)

Sample Solution Color Reflected Color Differencr 1 480 505 + 2.5 2 435 460 +25 3 460 460 0 4 430 430 0 5 430 430 0

440 440 470 490 460 460

1 1 460 445 -15 5 12 430 425

13 420 400 - 20 1 4 420 410 - 10 13 410 395 -15

16 420 140 f20 17 420 410 - 10 18 370 360 - 10 19 410 395 - 15 20 400 400 0

2b 3 80 390 +I0

28 475 470 - :, 29 450 44.5 30 470 490 f 2 0

27 390 400 +IO -

31 4 80 475 - 5 32 500 490 - 10 33 476 500 +25 34 4 50 140 - 10 35 470 490 + 20

36 470 470 0

38 430 425 - 5

40 495 490 - 0

37 460 448 -15

39 470 475 + p Standard deviation 1 1 5 unit>

832 A N A L Y T I C A L C H E M I S T R Y

Methods of Sugar Analysis,” 3rd ed., p. 620, Kew York, John U’iley & Sons, 1941.

(2) Gillett, T. R., and Holven, -4. L., Ind. Eng. Chem., 35,210 (1943). (3) Gillett, T. R., Meads, P. F., and Holven, A. L., ANAL. CHEM.,

(4) Holven, A. L., and Gillett, T. R., Facts About Sugar, 30, 169

(5) Keane, J. C., and Brice, B. A, , IXD. EXG. CHEM., ANAL. ED., 9,

(6) Spencer, G. L., and hleade, G. P., “Cane Sugar Handbook,”

21,1228 (1949).

(1935).

258 (1937)..

8th ed., p. 552, New York, John Wiley & Sons, 1945.

factors were considered relating t o the effect of grain size, luster, distribution of color in the crystal, etc., on the reflectance value of different types of white sugars. Good correlation has been shown between reflected color and solution color of a white sugar, pro- viding the grain size and other characteristics are uniform. The instrument is calibrated in terms of solution color and has been used for over 10 years by operating personnel for determining the reflected color of production sugars. Results are obtained quickly and with sufficient reliability to avoid packing of off- colored sugars.

LITERATURE CITED

(1) Browne, C. A., and Zerban, F. W., “Physical and Chemical

RECEIVED for review June 15, 1951. Accepted Januaay 9, 1952. Pre- sented before the Division of Sugar Chemistry, Symposium on Measurement of Color of Sugar Products, at the 119th Meeting of the AMERICAN CHEMI- c . 4 ~ SOCIETY, Boston, Mass.

Color Standards for Sugar Cane Sirup and Edible Sugar Cane Molasses

C. B. BROEG AND-C. F. WALTON, JR. Sugar Branch, Production and Marketing Administration, U. S. Department of Agriculture, Washington, D. C.

Although color has been very important in the com- mercial marketing of sugar cane sirup and edible sugar cane molasses, no w-idely accepted method for measuring color has been in use. Some distributors have their own color standards, but these are not in general use. Others depend entirely on visual in- spection. Comparison of color measurements by means of Lovibond glasses, liquid color standards, other glasses, and colorimeters indicated that a liquid color standard which is flexible, easily used, and inexpensive would be the most useful until fur- ther work developed standards of a permanent na- ture. Liquid color standards have been recom- mended for use in grading these sugar cane products in connection with the C . S. Department of Agricul- ture’s permissive grades for sirup and molasses. This fills a gap in the marketing of these products, and the results indicate that the color standards are adaptable to a number of liquid sugar products.

N CONNECTIOS with the development of specifications for I grading sugar cane sirup and edible sugar cane molasses, the need for color standards became a major factor. Preliminary specifications contained only general and somewhat vague de- scriptions of color, and producers and distributors of these prod- ucts pointed out that such color specifications were confusing, inadequate, and a source of dispute in transactions based on such general terms. Consequently, it was decided tha t definite color standards should be developed for these products. Be- cause sugar cane sirup and edible sugar cane molasses are pro- duced generally by individual farmers or small factories and rel- atively small units of production are involved, the color standards should be as simple as possible.

Color standards have long been used for grading maple sirup. Liquid standards of caramel solutions were devised by Balch (8) and for a number of years were distributed by the Department of Agriculture. Recently, Brice and his associates ( 5 ) have de- veloped permanent glass standards which have largely replaced the liquid standards. Some of the large sirup distributors have developed color Standards for their own use (Penick & Ford, Ltd., Inc., and .Imerican Molasses Co.). Sugar cane sirup buyers in the southeastern Gulf States use color as one of the criteria for purchasing sirup, but the practice has been to rely on visual in- spection alone and not t o use standards. This condition has led

to uncertainty and confusion in the grading of sirups. A number of methods for measuring or comparing colors were investigated, in the hope of finding color standards that would be widely ap- plicable and simple to use.

Research on the color of sugar products and on methods for measuring color has generally been along two lines: measure- ment of color by comparison with a standard color, and measure- ment of absolute color. Among the many investigators of color are Ventzke (II), Stammer ( I O ) , Bates and associates (S ) , Peters and Phelps (9) , Brewster ( 4 ) , Keane and Brice (a), Zerban (12), Zerban et al. (13), and Gillett, Meads, and Holven (7).

Because in grading it was not considered important to measure the absolute color of sirups and molasses as such, but rather to compare the amount of color in a sample with that of a known color standard, this work was largely directed toward the investi- gation of color comparison and development of a suitable color standard for making the comparison. Attention was given to availability and reproducibility of the color standards, simplicity of use, cost, flexibility, and applicability to a variety of sugar products. It seemed desirable to have standards which would be immediately available and easily reprodGcible, could be used a t any place without special laboratory facilities, would be reason- able enough in cost to permit even the smallest producer to use them, and would be flexible enough to permit changes to be made in a short time and a t little expense.

Brief studies were made of several glasses and li uid color standards and several colorimeters. The standard 8asses in- vestigated were: Lovibond glasses series 52 (yellow) and brew- ers’ red glasses and combination of these two series, Hellige amber C and D series glasses, and glasses developed by Penick & Ford, which consisted of two yellow and t v o red glasses of different optical densities for use singly or in combinations. The glass standards for maple sirup developed by Brice and his associates were not investigated. A standardized caramel solu- tion of various concentrations and standards prepared from a Rolution of mineral salts were the only liquid colors studied. An Am5nco neutral wedge colorimeter and a Duboscq color com- parator were used for comparative purposes, and a Coleman photoelectric colorimeter was used for measuring the transmit- tance of samples.

Liquid colors, instruments, and numerous samples of sugar cane sirup and edible sugar cane molasses were collected and their “color” was determined by transmittance and by comparison with various standard colors. 4 clear cell providing a 0.125-inch depth of sample was prepared for general use except in the rase of the Hellige standards, where a 1-mm. depth was used. The