made use of the Clark and Lubs buffer mixtures, as given in table 35, which were checked electrometrically. The data were then smoothed by use of the equation pH=k log (alkaline form)/(acid form), where pH is the hydrogen-ion exponent, k is the apparent or total dissociation constant of the dye, and "alkaline form" and "acid form" designate, respectively, the concentrations of the indicator completely transformed into the alkaline or acid form by excess of base or acid. In table 37 the drop ratios are given in the first column, the first figure of the ratio being the alkaline form and the second figure the acid form of the dye. The pH values corresponding to these ratios are found in the succeeding columns headed by the indicator used. TABLE 37.-pH values corresponding to various drop ratios 1 The following is the Gillespie procedure for the preparation and use of the color standards: Test tubes, preferably without flanged tops, 15-mm bore and 150-mm length, selected for uniformity of bore as already described under (a), are cleaned, rinsed, and drained. The tubes may be held in pairs by means of a rubber band wound around them in the form of a figure 8. It is convenient to use test-tube racks, one for each indicator, each holding two rows of tubes, accommodating one tube of each pair in front and one in back. For any desired indicator a set of color standards is prepared by placing from 1 to 9 drops in the back row of tubes, and 9 to 1 drops in the front row. A drop of 0.05 N sodium hydroxide is then added to each of the tubes in the back row (2 drops in the case of thymol blue) to develop the alkaline color. A 0.2-percent solution of stick soda is sufficiently accurate for this solution. To each of the tubes in the front row is added the kind and quantity of acid indicated at the bottom of each column in table 37. According to Clark, the 0.05 N hydrochloric acid is prepared with sufficient accuracy by diluting 1 ml of concentrated hydrochloric acid (sp gr 1.19) to 290 ml. The volume is at once made up in all the tubes to a constant height, corresponding to 5.5 ml with distilled water. Each pair of tubes thus constitutes a colorimetric standard and is to be labeled with the corresponding pH value given in table 37. These dilute standards are not considered stable and daily renewal is recommended. A third A block comparator for matching solutions and standards, as shown in Gillespie's article, is illustrated in figure 60. This is a modification of the comparator described in section (a) in that there are three holes in series instead of two to accommodate the extra tube. series of vertical holes with a third observation hole would permit comparison with two adjacent standards in the indicator range. To perform a test, 10 drops of the desired indicator solution is added to a clean test tube, and the test solution is added in amount to match the height of the standard solutions in their tubes. The sample tube with the indicator is placed in the comparator and is backed by two tubes containing distilled water. Pairs of the indicator tubes are placed in two of the holes of the other series and are backed by a tube precautions pointed out in the previous section are to be observed in regard to volume and concentration relations. 5. REFERENCES [1] F. J. Bates and Associates, Intern. Sugar J. 22, 654 (1922) (Abstract). [2] J. F. Brewster and W. G. Raines, Louisiana Planter 69, 167 (1923); Intern. Sugar J. 25, 88 (1923). [3] W. M. Clark, The Determination of Hydrogen Ions, 3d Ed. (The Williams & Wilkins Co., Baltimore, Md., 1928). [4] J. Reilly and W. N. Rae, Physico-Chemical Methods, 2d Ed. (D. Van Nostrand Co., Inc., New York, N. Y., 1932). [5] E. Biilmann, Bul. soc. chim. 41, 276 (1927). [6] F. Haber and Z. Klemenziewicz, Z. physik. Chem. 67, 385 (1909). [7] W. S. Hughes, J. Am. Chem. Soc. 44, 2860 (1922). [8] P. M. T. Kerridge, Biochem. J. 19, 611 (1925). [9] D. A. MacInnes and M. Dole, Ind. Eng. Chem., Anal. Ed. 1, 57 (1929). [10] D. A. MacInnes and M. Dole, J. Am. Chem. Soc. 52, 29 (1930). [11] G. R. Robertson, Ind. Eng. Chem., Anal. Ed. 3, 5 (1931). [12] M. R. Thompson, J. Research NBS 9, 833 (1933) RP511. [13] G. A. Perley, Trans. Am. Inst. Chem. Eng. 29, 257 (1933). [14] D. A. MacInnes and L. G. Longsworth, Trans. Electrochem. Soc. 71, 73 (1937). [15] K. Horowitz, Z. physik. Chem. 115, 424 (1925). [16] L. Michaelis, Naturwissenschaften 14, 33 (1926). [17] M. Dole, J. Am. Chem. Soc. 53, 4260 (1931); 54, 2120, 3095 (1932). [19] K. H. Goode, J. Am. Chem. Soc. 44, 26 (1922). [20] L. W. Elder and W. H. Wright, Proc. Nat. Acad. Sci. 14, 936 (1928). [21] H. M. Partridge, J. Am. Chem. Soc. 51, 2 (1929). [22] R. Müller, Z. physik. Chem. 155, 451 (1931). [23] A. DuBridge, Phys. Rev. 37, 392 (1931). [24] C. Morton, J. Sci. Inst. 9, 289 (1932). [25] E. C. Gilbert and A. Cobb, Ind. Eng. Chem., Anal. Ed. 5, 69 (1933). [26] L. G. Longsworth and D. A. MacInnes, J. Bact. 29, 595 (1935); 31, 287 (1936); 32, 567 (1936). [27] N. Crites, Rep. Assn. Hawaiian Sugar Tech. Chem. Eng. Sect. 14, 41 (1935). [28] R. T. Balch and H. S. Paine, Planter Sugar Mfr. 75, 347 (1925). [29] G. A. Perley, Ind. Eng. Chem., Anal. Ed. 11, 316 (1939). [30] G. A. Perley, Ind. Eng. Chem., Anal. Ed. 11, 319 (1939). [31] W. M. Clark and H. A. Lubs, J. Biol. Chem. 25, 497 (1916). [32] W. M. Clark and H. A. Lubs, J. Bact. 2, 1, 109, 191 (1917). [33] B. Cohen, U. S. Pub. Health Service Rep. 41, 3051 (1927). [34] G. L. Spencer and G. P. Meade, Handbook for Cane Sugar Manufacturers and Their Chemists, 7th Ed. (John Wiley & Sons, Inc., New York, N. Y., 1929). [35] G. P. Meade and R. Baus, Planter Sugar Mfr. 74, 509 (1925). [36] L. J. Gillespie, J. Am. Chem. Soc. 42, 742 (1920); Soil Sci. 9, 115 (1920). [37] A. S. Brown, J. Am. Chem. Soc. 56, 646 (1934). [38] H. S. Harned, J. Am. Chem. Soc. 51, 416 (1929). [39] C. K. Rule and V. K. La Mer, J. Am. Chem. Soc. 58, 2339 (1938). [40] E. R. Smith and J. K. Taylor, J. Research NBS 20, 837 (1938) RP1108. [41] H. S. Harned and R. W. Ehlers, J. Am. Chem. Soc. 55, 652 (1933). [42] L. R. Bacon, J. W. Hensley and T. H. Vaughn, Ind. Eng. Chem., 33, 723 (1941). XIX COLORIMETRY 1. INTRODUCTION The maintenance and improvement of the quality and appearance of sugar products leads naturally to the recognition of the importance of colorimetry and of the need for adequate apparatus and methods for the measurement of sugar color. The early literature on this subject dealt with methods used in connection with the refining of sugar with bone char; thus Ventzke [1] in 1860-61 published the description of a "decolorimeter," by means of which colors of solutions were compared before and after char treatment. In these articles Ventzke referred to the work done by Payen about 25 years previously. In 1861 Stammer [2] published a description of the instrument and method for sugar colorimetry that bears his name, and which, little changed, is still in use. In 1873 von Vierordt studied the measurement of color in diluted molasses with a spectroscope, the entrance slit of which was divided into upper and lower halves by independent jaws actuated by micrometer screws. To each screw was attached a drum which bore a graduated scale reading from 0 to 100. The opening and closing of the slits thus served to vary the light intensity in a measurable manner in either half of the field of view. The description of this apparatus is contained in a reprint in the Bureau's possession. This was reprinted in 1873 by Schmidt & Haensch, Berlin, from an article by Vierordt, but the journal source is not cited. The table in the reprint gives log transmittancy corresponding to scale readings, and layer thickness and dilution were taken into consideration in the text, the latter on a volumetric basis. This approach to a spectrophotometric method apparently received little attention from sugar technologists until many years later. In 1920 Bates and Associates [3] published a description of an abridged spectrophotometer (to be described later) with which three pure spectral lines of the mercury arc were used. This was the beginning of a systematic investigation of color in sugar products at the National Bureau of Standards. Spectral-transmission and absorption curves of a large number of varied sugar products were obtained over a greater part of the visible spectrum [4] and these afforded a qualitative and quantitative measure of sugar color and its characterization, and a way to the scientific classification of sugar products according to color. These results served as a basis for further investigation and demonstrated the possibility of using abridged spectrophotometric methods and simplified apparatus whereby sugar colorimetry, on a spectrophotometric basis, was made quite as simple as by the Stammer method and without the objectionable features of the latter. In 1936 the International Commission for Uniform Methods of Sugar Analysis [5] at its London meeting unanimously adopted the resolution that "*** spectrophotometry is to be considered the basis of all colorimetric measurements in the sugar industry. It is therefore recommended that absolute measurement be introduced as far as possible into factory and commercial control, the measurements to be performed with the monochromatic light of the mercury arc, namely at the wave lengths 4358, 5461, and 5789 A, and that measurements of log t at 5600 A be calculated from the measurements at 5461 and 5780 A. It is to be understood that any country or group of workers may use other wave lengths as desired." The adoption of the above resolution was prompted by practical, as well as theoretical considerations. Not only is the measurement of absorption more exact with monochromatic illumination than with white light as used in former days, but it is much easier for the observer, and there seems to be no other means of interrelating such measurements over the whole gamut of sugar color. Spectrophotometric equipment has been in use for several years in many laboratories in the sugar industry for purposes of research and control and this use is increasing, particularly in the field of simplified or abridged spectrophotometry adaptable to routine observations. 2. COLOR NOMENCLATURE IN THE SUGAR INDUSTRY [4] In the work at the National Bureau of Standards on color phenomena in sugars, the recommendations contained in the various reports of the Committee on Standards and Nomenclature and of the Progress Committees of the Optical Society of America have been adopted. Special care has been taken to conform to the recommendations of the Committee on Spectrophotometry [6, 7] and of the Committee on Colorimetry (Preliminary Draft) 1919, and subsequent reports of this latter committee published from time to time, insofar as those recommendations adequately cover the ground. In certain cases, however, it has been found necessary to coin new terms and symbols. Where this has been necessary, the new symbols and terms have been selected and defined in such a manner as to cover the desired ground and at the same time be an extension of, and not 903232 0- 50-21 in conflict with, the recommendations contained in the committees' reports. To one not thoroughly familiar with the subject, the elaborate and extensive system, with its fine distinctions of meaning to be hereinafter set forth, may appear somewhat pedantic and academic. Not so, however, to him whose daily work is in this field, for he is continually inconvenienced and annoyed by the circumlocution and misunderstandings occasioned by lack of suitable terms and symbols to express his ideas and findings cogently and without ambiguity. It will be noted in the list of terms below that there are many that are not explicitly used in the color work on sugar products. The terms that are most used in experimental sugar work are relatively few in number (T, t, log t, log t, c, b, λ, Q, n, as defined below). The others are necessarily given for the purpose of precisely defining these terms and to bring out the small but important differences between them and certain other similar terms. In the case of homogeneous light passing through homogeneous substances, such as a plane parallel polished plate of glass, we have the following terms [6], all being functions of wave length: E T=E、 E radiant energy (of light source). = = E radiant energy incident upon the first surface. = transmission. It is the fraction of the incident light which is transmitted, and not lost either by reflection or absorption. E2 E2 TE-EE -transmittance; that is, the transmission after cor recting for losses by reflection [8, 9]. The transmittance per unit of thickness, which is called transmissivity, t, may be calculated from the transmittance, T, for any thickness, b, by means of the relation t='√T=transmissivity, which is known as the Lambert law. No exceptions to this law have ever been noted. For transparent solutions: Toln. transmission of a given cell containing the solution. Tsolv. transmission of the same (or a duplicate) cell containing pure T = solvent. TroinTsoln. =transmittancy; that is, the transmission after cor rection for reflection at the surfaces and for absorption, if any, by the pure solvent. Since the absorption of pure water is negligible for our purposes, T has practically the same significance as T above. The symbols T, t, A, referring to solutions, are distinguished from T, t, A, referring to solids (or homogeneous substances), in handwriting and typewriting by underscoring the former, and in printing by the use of |