ANALYTICAL CHEMISTRY
168 Table 11. Spectral Characteristics of Rhodium-Hypochlorite Solutions at Various pH Values PH 2 6
8 10
Visual Color Lavender Blue Green Yellow-orange
Transmittancy Minimum, mp
Transmittancy Maximum, mp
565 665 665 435
460 480 480 385,640
maximum blue color, within the appropriate p H range, required an extremely high ratio of hypochlorite t o rhodium. For a study of the system to deduce the composition of the complex by the method of continuous variation or by the method of mole ratios, the concentration of rhodium was increased and the concentration of hypochlorite was decreased to bring the ratio of reactants into a range suitable to these methods. It was then observed that for a given ratio of hypochlorite to rhodium, various colors appeared as the pH of the solution was varied. Starting with the acidic solution of rhodium and hypochlorite (in amounts to give a final solution 0.001 M in each reactant), slow addition of sodium hydroxide to increase the pH caused the appearance of different colors, as listed in Table 11. The color changes were completely reversible upon decreasing the pH by slow addition of dilute hydrochloric acid. The production of a green color at a pH intermediate between that at which the blue and the yellow-orange color appeared suggested that the visual green color might be a composite of a blue- and a yellow-colored species. The spectral curves for the different solutions had transmittancy minima and maxima shown
in Table 11. The absorption of the yellow-orange solution a t 435 mp is sharp and intense, but the spectral curve for the green solution shows no indication of increased absorption at this wave length. The yellow-orange variety appears to be a colloidal material. All the color systems are being investigated further. ACKNOWLEDGMENT
This investigation was supportea by the United States Atomic Energy Commission, under the terms of a research c0ntrac.t with The University of Texas. LITERATURE CITED
(1) Alvarez, E.P., Chem. News, 91, 216 (1905). (2) Ayres, G.H., ANAL.CHEM.,21, 652 (1949). (3) Bouvet, Pierre, Ann. pharm. franp., 5,293 (1947). (4) Currah, J. E.,McBryde, W. -4.E., Cruikshank, A. J., and Beamish, F. E., IND. ENG.CHEM.,ANAL.ED.,18, 120 (1946). (5) Demarpay, Compt rend., 101, 951 (1885). (6) Hillebrand, W.F.,and Lundell, G. E. F., “Applied Inorganio Analysis,” New York, John Wiley & Sons, 1929. (7) Ivanov, V. N., J.Russ.Phys. Chem. Soc., 50,460 (1913). (8) “Scott’s Standard Methods of Analysis,” Furman, N. H., ed., 5th ed., Vol. 1, pp. 746-7,New York, D.Van Nostrand Co., 1939. (9) Thompson, S. 0.. Beamish, F. E., and Scott, M., IXD.ENO. CHEM.,ANAL.ED.,9, 420 (1937). (10) Whitmore, W. F., and Schneider, H., Mikrochemie, 17, 279 (1935). (11) Wolbling, H., Ber., 67, 773 (1934). RECEIVEDM a y
26, 1951. Presented before the Division of Analytical Chemistry a t the 119th Meeting of the AMERIClN CHEMICALSocmrr. Boston, Mass.
Spectrophotometric Studies on Raw Cane Sugars in Solution F. W. ZERBAN, JAMES MARTIN, AND CARL ERB, New York Sugar Trade Laboratory, .Yew York, N. Y . LOUIS SATTLER, Brooklyn College, Brooklyn, N . Y . In a previous study of the color of refined sugars in solution it was found that the absorbancy index determined at 560 mp, but not at other wave lengths, is equivalent to the color expressed in terms of brightness, purity, and dominant wave length. This work has been extended to 102 raw cane sugars from eleven countries. The transmittancy of 60 Brix solutions, filtered through Celite analytical filter aid, was measured throughout the visible spectrum. The plotted transmittancy curves were smooth, and similar to those of the refined sugars. The brightness, purity, and dominant wave length were com-
A
PREVIOUS study (6) on the color of 76 refined sugars in solution proved the correctness of the findings of Peters and Phelps (4)that the absorbancy index a t wave length 560 m p is equivalent, colorimetrically, to the integral absorption over the visible spectrum. In the present work the color of the sugar solution, as perceived by the eye, was computed in terms of the monochromatic analysis-i.e., brightness (luminance), excitation purity, and dominant wave length-by the selected ordinate method of Hardy ( 2 )for illuminant C of the International Commission on Illumination, and the color thus expressed was compared statistically with the absorbancy index, a, a t wave length 560 mp, and with the transmittancy for the same wave length a t 60 Brix and a thirkness of 5 em. It was shown that the absorb-
puted from the transmittancies of the solutions, and a statistical analysis showed a correlation coefficient of 0.9992 between those three and the transmittancy determined under the same conditions at 560 mp. Consequently, the corresponding absorbancy index at that wave length gives a measure of the color of the sugar within the limit of error of transmittancy measurements. Absorbancy indexes at any other wave length give erroneous color values. It is concluded that the color of raw cane sugars can be determined in the same manner as refined sugars, confirming the statement of Peters and Phelps.
ancy index of refined sugars a t wave length 560 mp is equivalent to the color within the limit of error of transmittancy measurements. This study has been extended to 102 raw cane sugars, including 40 from Cuba, 10 each from Puerto Rico, Hawaii, the Philippines, and British West lndies, 8 from Louisiana, 5 from Peru, 4 from Australia, 2 each from-the Dominican Republic and Florida, and 1from Mauritius. Solutions of approximately 60 Brix were prepared and filtered with Celite analytical filter aid, instead of with asbestos, for reasons explained in the previous paper. The transmittanciee of the filtered solutions were measured with a Coleman Universal spectrophotometer a t 13 points from 405 to 700 mp, in cells of 2.46-
V O L U M E 24, N O . 1, J A N U A R Y 1 9 5 2 Table I.
NO.
18 22 43 41 23 21 25 64 26 27 42 14 70 6 24 73 33 72 17 12 51 28 62 93 88 40 50 44 16 82 86 60 94 78 63 29 98 59 4 52 61 53 5 1,5 83 32
g;
34 76 74 11 35
30 7 101 97
19 55 10 46 81 31 69 8 36 92
I
48 6.i 71
1 on 66 56
91 8i 34 X1 37
13 20 45 85 96 54
Filtered Solutions in Order of Increasing Absorbancy Index T 560 m p &, Domin.
Brightness 94.37 93,lO 93.08 93.20 92.34 92.06 89.81 89.55 89.47 89.65 89.44 88.57 89.06 88.87 88.53 88.47 88,57 88.21 87,90 87 68 87.19 87.44 87.09 87.24 86.99 86.BO 86,83 86.37 86.06 85 29 85.31 8.5 23 85.02 84.90 84.72 84.74 83.91 84.47 84.43 84.42 84.18 84.32 83.67 84.16 83.60 84.11 83.07 83 42 83.38 82.77 83.24 83.07 83 45 83,21 83.03 82.14 82.21 82.53 82.45 82 .51 82,36 81 77 82.47 81.74 81.36 81.90 80.57 80 72
80.67 50.02 ,9.97 80.17 7 9 . 43 29.95 19 71
79.41 29.64 18.51
79.22 78 4 2 78 66 78.94 77 42 77 R3
9
78.00 77,i8
9n
76.81
58
26.68
$? ,
.1
lo? 99 38 9,; 2 3
7; 89 08 49 80 79
169
~6.87
-7.5 - ?8 04 "ii~-n 25 7 5 78 74.14 74 84 74.23 z3.75 ,3.62 72.45 72.63 70 52 65.94
Purity 4.91 6.90 6.45 6.68 7.74 8.24 11.22 11.84 12.10 9.80 10.15 14.98 14.87 12.% 13.50 13.34 10.31 14,87 12,75 14.68 15.41 12.56 15.81 14.63 13,13 13.88 11.22 11.76 15.51 18.60 16 56 17.23 18.e8 17.56 18 60 18.14 21.20 17.72 15.84 16.88 19.03 16.99 17.26 16.99 19.06 14.68 22.20 18.76 15.30 22.95 18.01 17.36 14 58 16.16 18.82 23 73 22.47 17.85 18.68 16.99 18.44 22.65 15.68 21.73 20.21 18.14 23 57 21.23 19,16 26.d5 27.75 23.89 24..i6 22.60 24.24 21.91 14.94 26,92 20 77 27.41 24.72 21 01 25.95 29.26 24.48 23.68 30.87 26.38 21.53 30.78 31.82 32.45 20.32 33.63 27 00 28.10 33.47 32.23 34.09 32.42 38.43 42.45
Wave Length 575I4 573.7 573.7 573.3 573.6 574.3 573.9 573.7 577.2 573.8 574.3 574.0 573.7 574.9 574.3 573.9 574.0 573.8 574.6 574.4 573.9 574.4 574 7 574.0 574.1 574.8 574.3 573.8 575 1 574 9 574 6 575.0 574.9 574 7 574 6 575.1 573.4 575 3 575.2 575.0
574.9 575.0 57s.7 1 57.5. 574.7 574.4 573.2 575.0 575.3 575.3 574.6
575.7 574.3 574.9 575.4 575.1 57.5.6 574.8 575,2 574.9 574 9 574 9 574.3 575.3 575.6 574..j 573,5 57.5 8 575.1 575.4
574.9 ;75.4 07.5 . 4
575.5 ,575.4
573.4 575.3 57,s. 5 57.5,'7 575 1
530
MU
MU
0.232 0.899 0.278 1.284 0.291 1.244 0.292 1.261 0.311 1.385 0.329 1.481 0.413 2,076 0,430 2.161 0.434 2.076 0.434 1,844 0.443 1.891 0.452 2.549 0.458 2.733 0.459 2.161 0.472 2.336 0.477 2.495 0 480 2.007 0.487 2.676 0.506 2.270 0.516 2 534 0.531 2.848 0,536 2.336 0 536 2.823 0 543 2.708 0.545 2.457 0 550 2.464 0.574 2.175 0.580 2.292 0 600 2,774 0 619 3.279 0 62,5 3.028 0 630 2 993 0 630 3.279 0 637 3.279 0 641 3.288 0 646 3 063 0 654 3.645 0.639 3 170 0 670 2 857 0,673 3.036 0.675 3 420 0.677 3 010 0.682 3 072 0.691 3 010 0 701 3.468 0.705 2 840 0 711 3 799 0 720 3.233 0,723 2.865 0.725 3.925 0.726 3 335 0 731 3.036 0.731 2.790 0 734 2.857 0.737 3.316 0 748 4.089 0.757 3.820 0 762 3.242 0.762 3.372 0,768 3.134 0,780 3.391 0.782 3,947 0.783 3 080 0.791 3.851 0.808 3.585 0 809 3.372 0 837 4.134 0.851 3.768 0 862 3.633 0 862 4.437 0 867 4 584 0 867 4 089 0.868 4.145 0 888 3 936 0.893 4 271 0 90: 4.318 0 912 3.655 0 921 4 584 0.431 3 820 0 942 4 , 5 4 7 0.946 4 318 0 917 3.913
57.5 4 573.2 57.5.8 0.975 ,573 7 0.979 575 .5 0 986 576 1 1.013 1 018 1 .031 073.4 1.068 ,576 . 9 1.078 575.8 1 . 0 9 2 575,9 1,107 1 135 57.5 2 576.4 1.141 576.1 l.lf3 576.1 1,192 576.0 1,192 576.3 1.194 576.5 1.269 576.0 1.280 576.4 1.352 577.0 1.637
;g
'20
4 486 4 933
4 283
+o
101 086
4 7.50 4 559
5.157 5 376
5.346 4 056
6 467 4 841 4 949 5 638 5.498 5 719 5.498 6 383 7.328
60 Biix, 2.46 Mm.
Found 90.4 88.5 88.1 88.0 87.3 86.6 83.5 82.9 82.7 82.7 82.4 82.1 81.9 81.8 81.4 81.2 81.1 80.8 80.2 79.8 79.3 79.1 79.1 78.9 78.8 78.6 77.8 77.6 76.9 76.3 976.1 75.9 75.9 75.7 75.6 75.4 75.1 75.0 74.6 74.5 74.4 74.4 74 2 73.9 73.6 73,s 73.3 73.0 72.9 72.8 72.8 72.6 72.6 72.6 72.5 72.1 71.8 71.7 71.7 71.5 71.1 71.1 71.0 70.8 70.2 70.2 69.4 68.9
68.6 68.6 68.4 68.4 68.4 67 8 67.7 67.3 67.1 66.6 66 6 66 2 66.1 66.1 65.3 65.2 65.0 64.2 64.1 63.2 62.7 62.4 62 0 61.6 60.9 60.7 60.1 59.4 59.4 59.3 57.4 57.1 55.4 48.9
Calcd. 89.8 87.9 87.8 88.0 86.7 86.4 83.0 82.6 82.8 82.5 82.2 81.5 82.3 81.7 81 2 81 1 80 7 80 9 80.0 80.0 79 2 79 2 79.2 79.2 78.5 78.5 77.9 77.2 77.4 76.6 76.2 76.2 76.1 75.7 75.6 75.6 74.7 75.0 74.7 74.8 74.7 74.6 73.6 74.4 73.7 73.9 73.4 73.4 72.8 73.0 72.9 72.6 72.7 72.6 72.8 72.0 72.0 71.7 71.7 71.5 71.5 71.2 71.2 71.0 70.2 70.7 69.4 69.2 68.8 68.9 69.0
68.7 68.4 68.1 68.0 67.6 67.1 66.4 66.6 66.3 66.3 66.1 65.2 65.1
65.1 64.6 64.2 63.2 62.3 62.6 62.2 61.8 60 6 60.1 60.2 59.3 59.4 59.0 57.3 57.3 54.7 47.6
420me 560 ma 3.88 4.62 4.27 4.32 4.45 4.50 5.03 5.03 4.78 4.28 4.27 5.64 5.97 4 71 4.95 5.23 4.18 5.50 4.49 4.91' 5.36 4.36 5.27 4.99 4.51 4.48 3.79 3.95 4.62 5.30 4.85 4.75 5,21 5.15 5.13 4.74 5.57 4.81 4.26 4.51 5.07 4.45 4.50 4.36 4.95 4.03 5.34 4.49 3.96 5.42 4.59 4.15 3.82 3.89 4.50 5,47 5.05 4.25 4.43 4.08 4.35 5.03 3.93 4.87 4.44 4.17 4.94 4.43 4.22 5.15 5.29 4.72 4.78 4.43 4.78 4.77 4.01 4.93 4.10 4.83 4.56 4.13 4.60 5.01 4.34 4.05 5.00 4.52 4.27 4.78 4.92 4.83 3.57 4.79 4.16 4.16 4.73 4.60 4.51 4.30 4.72 4.48
mm. thickness. The transmittancies, on the basis of a concentration of 1 gram of dry substance in 1 ml. of solution, and of I-mm. thickness, were plotted against wave lengths. Smooth curves were obtained, three of which are shown in Figure 1, for samples 38, 66, and 70 (Table I). The brightness, purity, and dominant wave length were calculated, as described in detail in (6) and the results of this monochromatic analysis are shown in the second, third, and fourth columns of Table I. The absorbancy indexes for wave lengths 560 and 420 mp are given in the next two columns. A comparison between columns 2 and 5 shows that, as with the refined sugars, the brightness decreases in nearly the same order as the absorbancy index a t 560 mp increases. There is no such relationship between the brightness and the absorbancy index a t 420 mp; although the a values increase in a general way as the brightness decreases, there are large fluctuations up or down. The significance of these differences is discussed below, Lorge and associates (3) fitted a multiple regression equation to the data, and obtained the following equation for the relationship between the absorbancy index a t 560 mp and the color computed by monochromatic analysis: a, 560 mp = -0.05373 brightness
-0.00402 purity -0.00352 dominant wave length 7.30264 (1)
+
The correlation coefficient for this equation is 0.9981, much better than for the corresponding equation for refined sugars ( 5 ) . The equation also shows that the relative contribution of the purity and the dominant wave length is much smaller than that of the brightness. As the absorbancy indexes are logarithms, a still better correlation was to be expected, as before, between the transmittancy a t 560 mp and the color expressed by monochromatic analysis. The transmittancies observed had to be corrected to exactly 60.00 refractometric Brix (0.7719 gram of dry substance in 1 ml. of solution) because the filtered solutions had a slightly higher Brix owing to evaporation during filtration. The corrected observed transmittancies for a thickness of 2.46 mm. are shown in column 7 of Table I. Lorge computed the following equation for the relation between the observed transmittancy and the monochromatic analysis:
T, 560 mp, 60 Brix, 2.46 mm.
+
=
+
1.7119 brightness 0.1675 purity 0.0673 dominant wave length - 111.2555 (2) The correlation coefficient for this equation is 0.9992, much higher than for Equation 1 and for the corresponding equation for refined sugars (6). The transmittancies calculated by this equation are shown in column 8 of Table I. The differences betxeen the found and calculated values average 0.22% transmittancy for the 102 samples. They exceed 0.5% in only nine cases, and are over 1% in only one case. With this last exception, they are well within the limit of error of transmittancy measurements in the range of 65 to 90% transmittancy.
ANALYTICAL CHEMISTRY
170 Table 11. Correlation between Absorbancy Index or Transmittancy at 560 M p and Brightness, Purity, and Dominant Wave Length
a 560my T , 560 my, 60 Brix, 2 . 4 6 mm.
Brightness
Purity
Dominant Wave Length
0.9975 0.9981
0.9391 0.9362
0.8062 0.8067
1001
to the actual color, as perceived by the eye. They vary from 3.57 t o 5.97, differing as much as 167%. To cite an example of what this means, the absorbancy index of sugar 66 (Table I and Figure 1) a t wave length 420 mp is 2.2% higher than that of No. 38, while that at wave length 560 mp, equivalent to the actual color, is 30.8% lower. The same is true for refined sugars, as noted from Table I1 of ( 5 ) . The absorbancy index at 420 mp of sugar 59 is only 0.5% higher than that of No. 84, but a t wave length 560 mp it is 43% higher, and the "color" determined by the method of Gillett, Meads, and Holven ( 1 ) is 123% higher. Similar examples can be found in the same tahles.
e REFINED SUGARS RAW SUGARS
A 3000-
0
u
A
2500-
-
LT w
z>-2000--
5
I 7 O l
- i'r
I5 00-
Q
-
RATIOS 420mu 560mp
IOOO-
5.97 "70 "66 -____ 3.57 * 3 6 -4.78
II
' 4 k
Figure 1.
r 5.00 70
rlf 30
-
a
1
I
'
505
'
'
' 605 ' '
l I 705
,
,
,
l , SO5
,
,
'
t
Figure 2.
l
"
'
l
'
"
"
74
"
"
78
'
t
'
~
'
t
'
'
L
62 86 90 AVERAGE BRIGHTNESS
94
96
Purity v s . Brightness of Raw Sugar Solutions
9
WAVE LENGTH
Transmittancy Curves of Three Raw Sugars
I t is interesting to note the correlation coefficients between the absorbancy index and the transmittancy a t 560 mp on the one hand, and the brightness, purity, and dominant Tvave length on the other, as recorded in Table 11. In spite of the fact that the dominant wave length varies only between 573.3 and 577.0, there is nevertheless a fairly high correlation between it and the absorbancy index or the corresponding transmittancy a t 560 mp. The higher correlation coefficients of Equations 1 and 2 for raw sugars, as compared to those previously found for refined sugars, are accounted for by the fact that the transmittancies of the refined sugar solutions were measured a t the greatest available thickness, 5 cm. They were, therefore, in the range of 90 to 100% a t 560 mp, except for the six darkest sugars, and Fere affected by relatively greater observational errors. This strengthens the conclusion reached by the writers and other investigators that cells of a t least 20-cm. thickness should be used for refined sugars, t o have the readings fall within the most precise range b e h e e n 30 and 60 or 70% transmittancy. The sum total of the different coloring matters in refined sugars generally has higher &-ratios 420 mp/560 mp (5) than in raw sugars (Table I, column 9), denoting a higher proportion of individual yellowish coloring matters. This is confirmed by the higher purity a t equal brightness, as shown in Figure 2, in which the circles denote the average purity for seven successive groups of 10 samples each of the i o refined sugars, and triangles denote the average purity for six successive groups of 15 samples each, followed by the group of the last 12 samples of the 102 raw sugars. The refined sugars have throughout a higher purity a t equal brightness than the raw sugars. The &-ratios 420 mp/560 mp, given in Table I, column 9, also explain why the absorbancy index a t 420 mp bears little relation
This proves beyond doubt that the color, as perceived by the eye, and its chemical corollary, color concentration, cannot be determined by transmittancy measurements at the blue end of the spectrum or a t any wave length substantially different from 560 mp, but can be found accurately only at this particular wave length. ACKNOU LEDGBl ENT
Thanks are due to Irving Lorge, Teachers College, Columbia University, for the statistical analysis of the experimental data. LITERATURE CITED
(1) Gillett, T. R., Meads, F. P., and Holven, A. L., ~ ~ N A CHEM., L. 21, 1128 (1949). (2) Hardy, A. C., "Handbook of Colorimetry," Cambridge, Technology Press, 1936. (3) Lorge, Irving, Sattler, Louis, and Zerban, F. W., IND.EXQ. CHEM..ANAL.ED..4. 435 11932). (4) Peters, H. H., and Phelps, E'. P.,'Bur. Standards Technol. Paper 338 (1927).
(5) Zerban, F. W., Sattler, Louis, and Afartin, James, ANAL.CHEX., 23, 308 (1951). RECEIVED April 19, 1951. Presented before the Division of Sugar Chemistry, Symposium on Measurement of Color of Sugar Products, at t h e 119th CHEVICAL SOCIETY, Boston, Mass. Meeting of the AMERICAN
