1780
ANALYTICAL CHEMISTRY
aid of the calibration curve, determine the weight of magnesium in the sample. EXPERIMENTAL
In order to test the accuracy of the method, synthetic samples of known magnesium content were prepared by dissolving 0.1gram samples of very pure nickel in the recommended manner, and then adding aliquots of standard magnesium solution plus aliquots of solutions of various other metal impurities. The aliquots of most of the metal impurities contained 100 y of the metal in question; in the case of cobalt, 500-7 portions were added. The results obtained by the recommended procedure are recorded in Table I. 'The values shown in the last column have been corrected for a small amount of magnesium (approximately 2 y ) found in the reagents plus nickel. I n order to test the reproducibility of the new method, magnesium was determined in several types of cathode nickel samples (Table 11). Sample Cathaloy A-31 contained about 4% of tungsten and the average value obtained several years ago
on sample 220 (melt H-1400) by eight different laboratories, using photometric, spectrochemical, and gravimetric methods, was 0.040% magnesium.
Table 11. 220,
Determination of Magnesium in Electronic Nickel Samples 220 Melt H-1400,
%
%
0.038 0.038
0.041 0.041 0.041
0.040 0.039
225,
%
0.066 0.066 0.064
Cathaloy A-30,
Cathaloy A-3 1,
0.028 0.025 0.027 0.028
0.038
999 None detected
%
%
0.038 0.038
0.039
LITERATURE CITED
(1) Luke, C. L., and Campbell, 11.E., ANAL.CHEW,25, 1592 (1953). RECEIVED for review March 27, 1964. Accepted July 29, 1954,
Recent Developments in White Sugar Colorimetry T. R. GILLETT and W. D. HEATH California and Hawaiian Sugar Refining Corp., Ltd., Crockett, Calif,
Color is especially important in the sugar industry, where color removal is one of the primary objectives of the refining process. Color measurement is required in the research laboratory, and in the plant for control of operations and for maintenance of product quality. Many procedures for sugar color measurement have been developed, but no one method has yet received general acceptance in the industry. Today, most laboratories use methods and instruments which they have devised or adapted. This lack of uniformity has led to considerable confusion in units and terminology, which has made i t nearl? impossible to formulate uniform quality standards in the sugar and sugar-consuming industries. These difficulties have been recognized for some time and considerable work has been done in an effort to correct the situation. Some recent developments in this field are outlined in this paper, and a proposed method and instrument for measuring the color of white sugar solutions are discussed.
S
UGAR colors, in general, range from the brown of raw sugars
to the slight yellowish or white color of refined products. While it would be extremely desirable to utilize a single method and instrument for both light and dark sugars, it is not yet possible to do this in a simple manner. Therefore, separate methods are generally utilized for light and dark colored products. In early years, color determinations were generally made by visual methods which involved comparisons of sugar solutions with various color standards of glass or solutions of mineral salts. Some of these methods are still in use ( 1 , 11). Since the photocell has been developed, photoelectric procedures have gained general acceptance. These procedures are now widely used because of their better reproducibility as compared to visual observations. Most photoelectric methods are based on a measurement of the amount of light which can be transmitted through the sugar solution as compared to that which can be transmitted through a reference standard, such as distilled water. Colorimeters, which employ glass color filters, are usually used for this measurement. In recent years, spectrophotometers have also received considerable application due to their greater accurkcy and ability to iso-
late relatively narrow wave bands of light. However, because of their more complicated nature and higher cost, spectrophotometers are primarily employed for research work, and are not too generally used for routine measurements. Although photoelectric color instruments and methods offer numerous advantages over visual methods, various problems are involved in obtaining accurate and uniform procedures. Small amounts of light-scattering particles are sometimes present in sugar solutions, and this turbid material has a pronounced effect on the transmittancy reading. For dark sugars, filtration can usually be used for its removal. In white sugars, however, filtration is difficult ( S ) , and can effect some color removal. Filtration procedures previously recommended for white sugars (10) are too slow and cumbersome for routine use. To overcome these difficulties, turbidity compensation has been advocated by several investigators (3, 6, 8, 9) as a substitite for filtration. Turbidity compensation usually requires measurements a t two points in the spectrum and the application of an empirically determined corrective factor, based on the relationship between the two readings. This correction is valid only where the character and size distribution of the turbid particles remain relatively constant. Fortunately, the turbidity of most adequately refined white sugars is uniform enough to permit this simplification, and turbidity compensation has been used for control analyses for these products by many laboratories in the sugar and allied industries. Various other problems exist in the preparation of solutions prior to color determination, the establishment of suitable and uniform terminology, and the development of a simple but arcurate photoelectric instrument. RECENT DEVELOPMENTS
In the past 2 to 3 years, many investigators have been working to solve some of these problems in sugar color measurement (4). Considerable attention has also been devoted to developing standardized methods and terminology which, if generally adopted, would eliminate some of the present confusion in this field. Zerban (IS) recently proposed a standard color method for raw sugars as a result of his work with the Association of Official Agricultural Chemists. Broeg and Walton (1)have developed visual color standards for cane sirups and edible molasses.
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V O L U M E 2 6 , N O . 11, N O V E M B E R 1 9 5 4 Deitz and assoriates have s t u d i d some principles of color measurement extensivelv and have suggested a new terminology ( 2 ) . This is a revision of the terminology originally proposed by Peters and Phelps ( I O ) , which has been generally accepted. Several terms defined by Peters and Phelps are as follows:
T, =
Table I. Common Features of 36 IIethods for Determining Color of White Sugar Nethods Employing Designated Feature Feature Visual comparison Photoelectric comparison .%pproximate wave length 420 mp 450 m p 560 mp Other Solution density 50% solids Above 60% solids Other Filtration of solution Compensation for turbidity Solution DH adjustment Instrument . Colorimeter ~DectroDhotometer
Tao~n.
= transmittancy of the solution Tm~v. -log T , -log t = -= specific absorptive index hc where Tao~n. = t,ransmittance of a given cell containing the solution Ts,,~,.= transmittance of the same or a duplicate cell containing pure solvent,, or t'he same mixture in same relative proportions minus the constituent of interest b = length of absorbing path of liquid, cm. c = concentration of solution, grams per ml.
Deitz has now proposed that this nomenclature be modified slight.ly i n order to recognize the light-scat,t,eringeffects sometimes present in sugar solutions. The term absorbancy index is suggested for use instead of specific ahrorptive index, in cases where no light scattering exists and t.he solutions are optically clear. as=
-log T , = nhsorhancy index bc
~~
\There light scattering exists, h v rccommrnds the use of thc term attenuation index. -log 2', a: = ~- - attenuation index of solution bc
( I n the alisence of light scattering, tmheattenuation index, a
