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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
THE HESS-IVES TINT-PHOTOMETER AND ITS USE WITH RAW SUGARS’ By George P. Meade and Joseph B. Harris CENTRAL CONTROL LABORATORY, CUBAN-AMERICAN SUGARCo., CARDENAS, CUBA
The Hess:Ives tint-photometer is described by the makers as “an instrument for comparing different shades and hues of light colored materials or liquids with each other and with fixed standards, in order t o differentiate and classify them and give them numerical expression.” Light from a n artificial daylight lamp placed a t the back of the instrument shines on two blocks of magnesia from which i t is reflected by means of a mirror through two adjustable light openings or slits into the instrument itself. The light from each slit is distributed over the corresponding half of a circular field by means of a patent device called an optical mixing wheel, which is rotated rapidly b y a small motor. The left-hand slit may be adjusted from full open t o tight closed by means of a shutter operated by a lever. The lever moves over a scale which reads zero when the slit is closed and I O O when i t is open. The right-hand slit is adjustable through narrower limits, the adjustment being used t o bring the two halves of the field t o equal intensity when the lever actuating the left-hand slit is brought t o I O O on the scale. When a sample is placed on the magnesia block before the right-hand opening the light entering this aperture is reduced, and the corresponding half of the field appears darker. To measure the amount of the reduction the lever is moved down the scale until the field is matched again, and the amount the left-hand aperture has been closed is read on the scale. If the sample is of a gray shade without any specific color the field can be matched without the use of a color screen, but for colored materials readings are made by matching the field through each of the three color screens, red, green and blue-violet, provided with the instrument. When working with liquids the solution t o be examined is placed in a small glass cell with clear glass bottom on a shelf interposed between the right-hand magnesia block and the right-hand light opening, so t h a t the light passes through the column of liquid. Provision is made so t h a t no light enters the liquid except through the bottom. It is essential t o place a cell containing distilled water t o the same depth as the solution under examination on the shelf before the left-hand light opening, t o compensate for the light cut off by the glass and t h e water of the solution. If this is done the same reading will be obtained irrespective of the depth of the column of liquid, provided the same amount of color is maintained. Our work has been done entirely with liquids, and no study has been made by us of the use of the instrument with solid materials. The pamphlet accompanying the tintometer is vague, pafticularly as regards the arrangement of the attachment for liquids, and considerable study is 1 Presented at the 59th Meeting of the American Chemical Society, St. Louis, Mo., April 15, 1920.
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necessary t o set the instrument up properly. The position of the scale makes i t awkward t o read, but otherwise the tintometer as i t comes from the manufacturers is well made and mechanically easy t o operate. SCALE READINGS
The problem which presents itself when the instrument is put into practical use is the meaning of the scale readings. The readings are in “per cent luminosity” or, subtractions of t h e readings from “one hundred per cent darkness.’’ But how can these percentages be interpreted in terms of color concentration? Journal articles dealing with the tint-photometer which have come t o our attention since this study was taken u p also recognize this difficulty. Kress and McNaughton’ add the three readings through the three color screens together and subtract from 300, calling the results “parts black.” As will be seen later this method will serve only for readings in one part of the scale, and will then be only approximate. Zerbanz says, ‘(since t h e readings obtained in degrees of the instrument bear no direct relation t o t h e color concentration, i t was first necessary t o standardize the instrument for the purpose of translating the readings into concentrations.” This he did b y making an arbitrary standard solution of dark molasses, 3 0 g. per liter, and making u p a series of dilutions containing I per cent, 2 per cent, 3 per cent, etc., up t o 2 0 per cent, then by 2 per cent intervals up t o 30 per cent, and by j per cent intervals t o I O O per cent. Each of these solutions was then read in the tintometer through the three color screens and the readings recorded. He called the color of his standard molasses solution 100, and b y comparing readings of other solutions with those obtained on the various concentrations of his standard he was able t o express any scale reading in terms of the standard. For reading solutions other t h a n sugar solutions ( e . g., colorimetric determinations of iron and of the polyphenols) he made similar series of readings on standard solutions of t h e substance t o be examined, but here he encountered the difficulty t h a t “the color readings of t h e solution t o be analyzed did not always agree in each color region with the readings obtained on those of known concentration.” INTERPRETATION O F SCALE READINGS
The tint-photometer was purchased by this laboratory for determining the color of raw sugars, and practically all of our study has been done on solutions of raw sugars. A weighed amount of sugar was dissolved in a given quantity of water, a liberal amount of kieselguhr added, filtered through paper, pouring back until t h e filtrate was brilliant. A series of solutions of a certain raw sugar were made up containing, respectively, I g., 2 g., 3 g., in 2 5 cc. and readings were made on each of t h e solutions through each of the color screens. The tabulation shows t h e readings, which are presented graphically in Fig. I . 1 “A Numerical Expression for Color as Given by the Ives TintPhotometer,” THIS JOURNAL, 8 (1916), 7 1 1 . 2 “The Color of Sugar Cane Products,” Louisiana BuEZetin 166, March 1919.
July,
1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
The instrument is accurate t o one degree of t h e scale, the blue field being somewhat more difficult t o match than t h e others. TABLE I-READINGS ON
V A R I O U S S T R E N G T H S O L U T I O N S O F T H E S A M E RAW S U G A R , S H O W I N G T H E V A R I A T I O N OF T H E S C A L E READINGS FOR CONCENTRATION
Sugar in 2 5 cc. G. 1 2
3 4
5 6
7 8
9 10
Red Screen
Green Screen
87 75 65
74 55
40 zn 22 16 12 8 6 4
__
-5 .7
50 43 37 32 27 24
Blue Screen 47 24
12 7
Too dark to read accurately
I t is seen from these figures t h a t neither t h e scale readings themselves nor the scale readings subtracted from I O O are direct expressions of the relative amounts of color, but t h a t as the amount of sugar is increased
68 7
ing must be reduced until i t is 7 4 per cent of its size, or, in other words, t h e lever must be moved down t h e scale t o 74. The conditions are now t h e same as when t h e instrument was in adjustment a t 100;t h e same amount of light is entering both apertures and t h e field is evenly matched. The first solution is now replaced by one containing 2 g. of the sugar. This second gram of sugar will cut off the same percentage of light t h a t the first gram did, t h a t is, i t will permit only 74 per cent of the light now entering t o pass, and the left-hand opening must be closed t o 74 per cent of its present width in order t o match t h e field again. (.74 X .74 = .548 or j j in practice.) A third gram of sugar will again reduce t h e light 74 per cent of the amount entering after the field is matched with the z g., and the scale must be moved t o 74 per cent of 4 4 8 , or ,406, a reading of 4 0 in practice. This relationship between the scale readings and the amount of material taken can be expressed algebraically by the equation y = K Z , where y: = any scale reading KI= the scale reading for one unit of the material x, = the number of units of material which will give the scale reading y.
With this equation, given the scale reading for a unit amount of material, i t is possible t o calculate what reading would be obtained with any other amount of the same material; or, given the scale readings of unit quantities of two materials, the amount of color in the one as compared t o t h e other may be computed. For example: To find what 5 g. of sugar would read through the red screen, given that one gram reads 87, y = . S i s = 498 or 49.8 against jo in practice (Table I)
Clem+
o f Suqqr
FIG.1
the scale readings decrease by a constantly decreasing amount. A study of these curves and figures and of other similar series brought out t h e fact t h a t t h e readings in any series run in powers of t h e first reading of the series, i. e., the reading for one unit of material, no matter which color screen or what class of material was used. (For mathematical purposes, the scale readings must be considered as decimal fractions.) For example in the first series given in Table I : .87 X .87 = .757 X .87 = .658 X .87 = . j 7 4 , etc The scale does not admit of t h e reading of fractions. The reason why the readings follow this rule lies in the make-up of the instrument itself. Consider t h e two halves of the field evenly matched with the scale set a t 100,i. e., with the same amount of light entering both apertures. Now if a solution containing one gram of t h e sugar used in Table I is placed before t h e right-hand opening, i t will permit only 74 per cent of the light t o pass when viewed through the green screen. T o match t h e field again, the left-hand open-
Five grams of Raw Sugar A read 58 through the green screen; the same quantity of Raw Sugar B reads 41. Problem: To find how much more color B has than A or how much of A would be required t o give the reading of 5 g. of B. Solving the equation for x :
- .3873
y = log .41 = x = log = 1,636 log K log 3 8 .2366
T h a t is, through the green screen, B has 1.636 times as much color as A, or it would require 1.636 times as much of A to give the reading of any given quantity of B. To prove this in practice, a solution Containing 8.2 g. of A (1.64 X 5 g . ) was read through the green screen. One observer read 41.the other 42, as against a reading of 41 for 5 g . of B. Through the red screen, j g. of A read 78, and 8.2 g. read 67. What should the 8.2 g. have read according to the equation? y TABLE
FOR
=
.7S1e4
,665 or 66.5 on the scale
TRANSLATING ‘SCALE
READINGS
TO
UNITS
O F COLOR
I t is also possible by means of t h e equation t o compare all scale readings with some fixed number as a standard. This has t h e advantage t h a t K becomes a constant in t h e logarithmic calculation. T o avoid repeating t h e logarithmic calculation, we adopted K = gg as a standard, and calculated a table by means of t h e equation for each scale reading from I O O t o I (Table 11). By this table all solutions are
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
688
compared t o a hypothetical solution which would give a reading of 99 for all three color screens. The number 99 was selected because i t represents the smallest amount of color which can be read in the instrument, and therefore the color corresponding thereto can be t a k e n as unity. The table gives opposite each scale reading (y) the number of units ( x ) of the hypothetical solution ( K = 99) which it would take t o give t h e scale .reading; or, taking t h e color of the hypothetical solution as unity, the number of color units corresponding t o t h a t scale reading. TABLE II--FOR TRANSLATING HEW-IVESTINT-PHOTOMETER S C A L E READINGS TO UNITS OF COLOR y = Kx,where y = scale reading; x = units of color: K = 99 (constant) Units Scale Scale Units Scale Units Scale Units Scale Units Readof of Readof ReadRead- of Readof ing Color inn Color ing Color ing Color ing Color Y
X
100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84
0.0 1.0 2.0 3.0 4.0 5.1 6.1 7.2 8.3 9.4 10.5 11.6 12.7 13.8 15.0 16.2 17.4 18.6 19.8 21.0
80
82 81
X
X
Y
20 19 18 17 16 15 14 13 12 11
IO 9 8 7 6 5 4 3 2 1
X
160.0 165.0 170.5 176.0 182.0 189.0 196.0 203.0 211.0 219.0 229.0 239.5 251.0 264.0 280.0 298.0 320.0 349.0 389.0 458.0
Any of the illustrations used t o show the workings of the equation can now be worked out much more simply by the table. For example: To find what 5 g. of sugar would read through the red screen, given that I g. reads 87. From the table, 87 = 13.8 color units 13.8 X 5 = 69.0 From the tahle, 69 corresponds to a scale reading of 49.8 ( 5 0 ) Sugar A reads 58 through the green screen Sugar B reads 41 through the green screen From the table, 58 = 54.2 color units; 41 = 88.7 color units 88.7 Then B has = 1.64 times as much color as A 54.2
-
By 'means of t h e table, then, any scale reading for a n y color screen can be immediately translated into units okcolor, irrespective of the class of material under examii." ion. L
r
DETERMINATION O F C O L O R O F RAW SUGARS
The following method for determining the color in raw sugars has been adopted: Dissolve 20 g. of sugar in distilled water and make up t o I O O CC. Filter through paper in which are placed two or three teaspoonsful of kieselguhr, poyring back until t h e filtrate i s brilliant. Transfer z j cc. of this solution t o the observation cell on the right side of the instrument, and place a cell containing 2 5 cc. of distilled water on the left-hand side. Read the solution through each of the three screens, record the readings, and note the color units for these readings as taken from the table. Add the color units thus obtained and divide by three, recording this as the color of the sugar. Whole numbers only a r e used.
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I n case the sugar is SO dark t h a t any of the readings fall below 2 0 it is best t o take a smaller amount of the solution (diluting i t t o 2 5 cc. if desired). The result can be calculated t o 5 g. The instrument is rather hard t o read accurately in the lower part of t h e scale, and one degree error with the lower numbers of the scale represents a much greater amount of color than with t h e higher scale readings. The use of 5 g. and the division of the sum of the color units by three were adopted as they were found t o give convenient sized numbers for our work. The time required t o make a color determination in this way is about the same as t h a t required for a direct polarization. TABLE 111-TYPICAL RESULTSOF COLOR DETERMINATIONS ON RAWSUGARS Class SCALE READINGS UNITS F R O M TABLE Red Green Rliie of Red Green Blue 3 Sugar Screen Screen Screen Screen Screen Screen or Color Cuban Raws 1920 Crop. 64 40 12 44.4 91.1 211.0 116 65 41 16 42.8 88.7 182.0 105 66 47 25 41.3 75.1 137.8 95 70 50 21 35.5 68.9 155.0 87 72 52 25 32.6 6 5 . 0 137.8 78 71 53 31 34.0 6 3 . 1 116.5 71 35.5 59.4 98.9 64 70 55 37 80 62 35 22.2 47.6 104.4 58 Cuban Raws 1919 Crop ...... 59 29 IO 5 2 . 5 123.0 229.0 135 60 42 16 37.0 86.0 182.0 102 72 53 41 32.6 63.1 88.7 ' 62 Washed Raw 1919 Crop 84 76 53 17.4 27.3 63.1 36 1920 C r o p . . . .. 99 97 85 1.0 3.0 16.2 7
...
......
Refinery High Remelt..
. .
95
89
73
Refinery Molasses Sugar One Gram. 73
51
23
...
5.1
11.6
31.2
16
31.2 63.1 150.6 82 (Calculated to 5 9.) 82 X 5 = 410
SUMMARY
The scale readings of the Hess-Ives tint-photometer are meaningless in themselves as they do not express directly the relative amounts of color.
It was found t h a t the scale readings for solutions containing I , 2 , 3 , 4, etc., units of material run in powers of t h e reading for one unit, considering t h e scale readings as decimal fractions. This is due t o the mechanical make-up of t h e instrument, and is true no matter which color screen or what class of material is used. Expressed algebraically, this relationship between t h e scale readings and t h e amounts of color becomes y = Kx, where y is any scale reading, K is the reading for one unit of material, and x is t h e number of units of material required t o give the scale reading y. By means of this equation solved for x the color of two materials may be compared, given scale readings for equal quantities; or all scale readings may be compared t o a standard. T o avoid the repeated calculation, a table has been calculated which gives the units of color corresponding t o each scale reading from 100 t o I . A convenient method for determining the color of raw sugars, using t h e table, is given, together with results for various sugars.
