ANALYTICAL EDITION
826
of Organic Precipitant Reagents SENSITIVITY usual Interface REAGENT method method Mg. per 5 CC. Dimethylglyoxime 0.02 0.005 Alizarin 0.6 0.01 p-Nitrobenzeneazoresorcinol 0.07 0.01 p-Dimethylaminobenzylidenerhodanine 0.001 0 00008
Table I-Sensitivity METAL Nickel Aluminum Magnesium Silver
Table 1shows the increase in sensitivity for a few organic precipitant reagents. The figures in the table are not to be
Vol. 3, No. 3
construed as accurate sensitivity figures, but merely the limits of an easily discernible reaction under similar conditions. Literature Cited (1) Harkins et al., J . A m . Chem. Soc., 39, 354 (1917); 39, 541 (1917); 43. 700 (1920): 43. 35 (1921). (2) Rolthoff, Ibid., 62, 2222 (1930). (3) Langmuir, I b i d , , 39, 1848 (191,). (4) Ruigh, z b i d . , si, 1466 (1929).
Studies on Turbidity in Sugar Products I-Relation between Intensity of Tyndall Beam and Depth and Concentration of Solution' F. W. Zerban and Louis Sattler NEW YORKSUGAR TRADELABORATORY, INC., 80 SOUTHST.,NEWYORK, N. Y.
T
Previous work on the optical measurement of turbe determined more easily by URBIDITY p l a y s an bidity in general, and with respect to sugar products a simple turbidity determinaimportant part in the in particular, is briefly reviewed. The Pulfrich photion than by filtration and manufacture of c a n e tometer, with which both the transmittancy and the weighing, as is the c u s t o m sugar. The juice expressed Tyndall-beam intensity of turbid solutions can be a t p r e s e n t . As a f u r t h e r from the cane contains susdetermined, is described. Several series of such example, t h e p r o b l e m of pended m a t t e r i n v a r i o u s measurements have been made on a raw sugar solution degrees of dispersion, from color d e t e r m i n a t i o n s in at varying depths and concentrations. It was found sugar products may be mencoarse, through fine particles, that with colored turbid solutions the Tyndall-beam to those of colloidal diment i on e d. Various investigaintensity is affected to such an extent by absorption t o r s h a v e f o u n d that i n sions. One of the objects of that the latter must be corrected for. The ratio beorder that the Lambert-Beer clarification is to remove the tween Tyndall-beam intensity and transmittancy is law may be applied to such suspended matter as far as within a certain range, a power function of the depth measurements, the turbidity possible. Even if this obor the concentration, according to the formulas R = must be reduced to a certain ject has been attained, the R1 X bn, and R = R, X cn, where b is the depth, c the m i n i m u m by careful filtrajuice becomes cloudy once concentration, R1the ratio for unit depth or concention. It is not known exmore upon c o n c e n t r a t i o n , tration, and R the ratio at any depth or concentration; actly w h a t t h a t minimum and the sirup m u s t a g a i n n is a constant which, at constant depth and varying be settled before crystallizashould be, except that there concentration, or vice versa, is independent of wave tion, because any suspended should be only a faint Tynlength. In the formula R = R1 X bn, the value of n dall beam. Opinions differ matter is partly adsorbed on varies approximately as the logarithm of the concenas to what is the best filterthe crystals, partly retained tration. The work is being continued to test the above ing medium to use. A careon them while the sugar is beformulas further and combine them into one equation. ful studv of turbiditv and ing centrifuged, and another transmittancy of s o l u t i o n s important part goes into the runoffs. When these are boiled to grain, the cycle is repeated, may be expected to throw more light on this disputed point. These few examples will suffice to show the importance of and when a low-grade sugar in which suspended matter has accumulated is used as seed, the resulting sugar gives a very turbidity measurements in the sugar industry. turbid solution. I n raw sugar manufacture, the juices, Available Radiometric Procedures sirups, and molasses are not filtered, but only settled, and The radiometric procedures available for this purpose fall the care with which this is done largely determines the filtering quality of the raw sugar. Turbidity in the liquors into three classes. In the first of these, the measurements are made by transof the refinery causes similar difficulties as in the raw sugar factory, and the suspended particles must be removed in order mitted light, using a photometric null device and comparing that a product of high quality may be obtained which must either with a standard dispersion in the same dispersion be free not only from color and dissolved impurities, but medium (turbidimeters of the Duboscq colorimeter type), or must also give a clear solution, If turbidity is t o be controlled comparing with the intensity of the light transmitted by the dispersion medium or a standard dispersion in a second abin the factory, we must be able to measure it. Turbidity measurements would also prove of value in sorption cell of the same characteristics (photometers, various analytical operations. The suspended matter in spectrophotometers). This group of methods is especially juices, sirups, and molasses affects the specific gravity of the adapted for measuring visible turbidity. The second group is based on measurements with a photolatter. To correct for this, it is customary to let the solution sfand until the suspended matter has settled out. The non- metric null device of the intensity of the Tyndall beam emitted settling particles should also be allowed for because they in- by the dispersed particles, in a manner similar to the one fluence the specific gravity. Their quantity can probably used in transmittancy measurements. If the comparison is made with a standard dispersion, the instruments are 1 Received April 14, 1931. Presented before the Division of Sugar termed nephelometers; if the comparison is made directly Chemistry at the 81st Meeting of the American Chemical Society, Indianwith the intensity of the incident light they are called tyndallapolis, Ind., March 30 to April 3, 1931.
July 15, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
meters (8). These two types of instruments are used particularly for determining invisible turbidities, but their range extends also into visible turbidities. The first two groups, which depend on the photometric principle, are free from personal errors so long as substantially monochromatic light is used, or the spectral curves of the two solutions to be compared are substantially the same. The third group of methods employs the criterion of complete extinction, and is thus subjective in character, The depth a t which an object, such as a wire or a small light, placed in or behind the column of liquid becomes invisible, serves as a measure of the turbidity. This method of procedure is to some extent independent of the color of the solution, but is subject to considerable error due to differences in the acuity of vision of different observers. Nearly all of the experimental work on the transmittancy of turbid liquids has been done on dispersions in colorless media, or by comparison with standards prepared with the same dispersion medium, so that the light absorption due to the latter was compensated for. Even under these conditions, the Lambert-Beer law, usually valid for molecularly dispersed systems, does not generally hold. The negative logarithm of the transmittancy does not increase at the same rate as the depth and concentration, but is a power function of these two variables. The Lambert-Beer formula, with the exponent equal to 1, is but a special case of the more general equation, If a certain instrument nevertheless shows a linear relationship between optical density and depth or concentration, this is generally due to the characteristics of its construction. The rate of change of the Tyndall intensity with depth and concentration, utilized in nephelometers and tyndallmfters, is much more complicated than that of the optical density of the turbid solutions. Wells (8) has developed a complete formula for the Tyndall intensity ratio, assuming perfectly diffused light. This formula contains eight constants and is not serviceable for practical purposes. For this reason it is often preferred to calibrate such instruments empirically for each type of solution to be investigated. For very low concentrations and depths, the calibration curve may be a straight line within the limits of error. This is shown, for instance, in the data of Tolman and co-workers (7) for a silica suspension up to a concentration of one gram per liter. The curve then bends asymptotically, and Wells (8) has shown that the exponential formula given above for transmittancy measurements fits the experimental results of several investigators quite closely. I n Tolman’s silica suspension, the curve reached a maximum a t around 2.5 grams per liter, and then the Tyndall intensity decreased with rising concentration because the light absorption overbalanced the Tyndall effect. The extinction method of measuring turbidity is based on the assumption that the simple dilution law holds. In reality, the conditions are much more complex, as has also been shown by Wells. I n practice, each instrument must be calibrated empirically for each type of dispersion to be measured. A choice between different instruments must be based primarily on the character of the turbid systems. For dispersions of very low concentrations, particularly those which appear clear by transmitted light, tyndallmeters or nephelometers must be used, as this is by far the most sensitive method. Even carefully purified water shows a distinct Tyndall cone when the light source has sufficient intensity. For medium concentrations, readings by transmitted light may be substituted. The extinction criterion permits of the greatest range in visible turbidity, but as has been stated above, it has other limitations because it is a subjective method greatly influenced by personal error. So far, only dispersions in which the light absorption due
327
to the dispersion medium is negligible, or corrected for by direct comparison with a standard dispersion in the same medium, have been considered. This condition can rarely be met in the sugar industry. I n technical sugar products, turbidity and coloring matter vary independently, and for this reason it is impossible to select any one standard turbid solution applicable to all products. Nevertheless, the need for turbidity measurements on sugar products has been so obvious that many attempts have been made to solve this problem. Several of the types of methods described above have been resorted to. Description of Various Existing Turbidimeters
A common procedure, largely employed in Hawaii and in the Philippines, for determining the clarity of juices and of raw sugar solutions, is based on the extinction criterion. The Kopke turbidimeter is an illustration of this principle. It is assumed that the simple dilution law holds, but it is evident from the discussion presented above that this apparatus can a t best give only approximately relative values which are not a linear function of the turbidity. Furthermore, it is very difficult for different observers, working independently, to check each other’s readings. The Jackson turbidimeter, recommended by Lindfors (6) for measurements of invisible turbidity, is also based on the extinction criterion. This instrument permits greater precision in the readings than the Kopke turbidimeter, but otherwise has the same limitations. Colorimeters of the Duboscq type and nephelometers of similar construction do not seem to have been employed for turbidity measurements in sugar products because of the complications due to the presence of coloring matter. The Tyndall phenomenon has been made use of in another way by several investigators. The turbidiscope of Horne and Rice (3) is based on this principle. It is admittedly meant only for rough comparisons, as no attempt was made in the original design to compensate for differences in the concentration of coloring matter. Lindfors (6) has recommended this apparatus and has improved the method of using it to obtain more nearly quantitative results. A series of turbidity standards is made up by dispersing bentonite in water, and sufficient caramel solution is added to each so that the color of the solution in all the tubes approaches that of the unknown. Lindfors further proposed to replace the slit of uniform width in the original instrument by slits of varying width, so that the results obtained by dilution may be checked by those based on varying depth. An ingenious apparatus for measuring turbidities of low concentration has been described by Ingersoll and Davis (4). The effect of color is overcome to a large extent by exciting a Tyndall beam of only 1 mm. thickness horizontally a t zero depth-that is, a t the surface of the liquid in an absorption cell open at the top. A measuring tube with concentrated potassium dichromate solution is placed above the Tyndall beam and the depth of dichromate solution determined a t which the cone becomes invisible. In other words, the extinction criterion is used to measure the Tyndall intensity. A red filter is placed in the primary beam to exclude fluorescence effects, and a Wratten filter No. 29F is interposed between the surface of the liquid of the absorption cell and the dichromate solution. When turbid liquors are diluted with practically cone-free water or white sugar sirup, the depth of the dichromate solution is a linear function of the turbidity, except a t the lowest concentrations where the curve ascends more steeply. This straight-line relationship is of advantage in practice. No experimental proof has been furnished that with this apparatus the turbidity in solutions of varying color is directly proportional to the readings. A simple and practical differential method of turbidity
328
ANALYTICAL EDITION
determination in sugar products has recently been developed by Balch ( I ) . The turbid sugar solution ie compared directly with a filtered portion of the same solution in a spectrophotometer, and the specific absorptive index is taken as a aeasure of the turbidity. Unlike the method of Ingersoll and Davis, this procedure is applicable only to turbidities of sufficient magnitude to affect the transmittancy appreciably. It is interesting to note that equal concentrations of suspended matter in a water-white solution and in a dark-colored caramel solution gave the same reading; this shows that the color of 'the solution is effectively compensated for. Balch proved experimentally that the Lambert-Beer law does not hold Btrictly for the suspended matter, but he states that the results give a good approximation. Honig (Z), who used essentially the same method, also calls attention to the fact 'that the results obtained are only relative. The data given by Balch in Table I of his paper show that for this particular suspension Beer's law holds quite closely. But there is a gradual increase in the values of (-log T ) / c from 0.074 for the lowest concentration, taken as the unit, to 0.080 for the highest concentration. If the T values actually read are substituted in the more general formula -log T = (-log TI)X c", the numerical value of n is found to be 1.032-that is, a trifle higher than the exponent 1 for Beer's law. It must be considered, however, that the transmittancy of turbid media is influenced by the characteristics of the primary beam and also by the shape and width of the absorption cell used. For this reason the readings obtained on a certain instrument with one type of cell do not necessarily agree h i t h readings for the same solution in another instrument 'with a different type of cell. Description of Improved Instrument
Vol. 3, No. 3
colorimeter and for reflectance measurements, or horizontally for determining transmittancies or Tyndall intensities, At the objective end it has two openings with centers 7 cm. apart, Each of the two apertures is formed by two V-shaped shutters moving symmetrically in opposite directions, with their planes in direct contact. The area of the aperture is varied, and a t the same time measured by means of drums mounted on the sides. The peripheries of the drums are calibrated in per cent transmission. The peculiar V-shaped construction of the shutters causes the percentage divisions to be much farther apart near the zero point than near the one hundred point. This makes it possible to read small transmissions with greater precision than when the percentage units are equidistant. Since both apertures may be varied independently, either one may be set at one hundred, and the samples may be reversed. The beams passing through the two apertures are brought into juxtaposition in the field of vision by a system of prisms and lenses. The color screens are placed in a revolving drum mounted in the ocular, and may be turned into the field in regular order. Eight color filters, covering the spectrum from the extreme blue to the extreme red end, are furnished for transmission measurements, and three less dense filters are supplied for the Tyndallintensity determinations. E'or transmission measurements up to a depth of 30 mm., the absorption cells are placed in holders fastened to the rear of the photometer apertures. If longer absorption tubes are required, they are placed on a special stand between the photometer and the light source. The lamp house is hemispherical. It has two circular openings in front, with centers 7 cm. apart to correspond with the photometer apertures. It furnishes two parallel beams of equal intensity. The light may be diffused by frosted glass disks. If the instrument is to be used for the measurement of As the problems of the sugar industry call for measure- Tyndall intensity, the lamp house and cell holders are removed .bents of color as well as of visible and invisible turbidity, it and an auxiliary apparatus placed against the rear of the would be desirable to have an instrument, preferably moderate photometer in its horizontal position. This attachment i n price, with which all these measurements could be made consists essentially of a cylindrical housing, and is constructed interchangeably. The usual spectrophotometers are suitable in such a way that no stray light can enter the photometer. for two of these types of measurement-that is, of color and On the side opposite the left photometer opening is a cylindri.of visible turbidity. The tyndallmeter of Mecklenburg and cal chamber. Behind this and somewhat to the right is the Valentiner is not equipped for transmission measurements lamp, placed so that the primary beam of light passes through and is very expensive. Recently a photometer originated by a square opening in the wall of the compartment and then Pulfrich (6) has been placed on the market by Carl Zeiss; through the sample a t an angle of 45 degrees from the direction this may be used as a colorimeter or turbidimeter like the of the observation. The angle of 45 degrees was chosen beDuboscq type of apparatus, or as a photometer for measure- cause this furnishes greater Tyndall intensity than the usual ments with transmitted or reflected light, and of Tyndall angle of 90 degrees. The cell is placed in a holder in such a intensity. Auxiliary equipment is furnished for determining position that the primary beam strikes the rear face normally, gloss and fluorescence, and for specifying colors by the mono- whereas the front face is sighted obliquely through the left chromatic or trichromatic system. The instrument has no photometer aperture. The actual measurements are made dispersing prism, but color filters with fairly narrow spectral by substitution in order to eliminate errors due to inequalities bands are used. This disadvantage is, however, largely offset in the intensity of the light beam passing through the sample by the features already mentioned, and also by the fact that and that of the comparison beam. The latter goes through the same glass cells are employed for both transmittancy any one of four milk glasses of different optical densities. measurements and determinations of Tyndall intensity. The sample is measured against the glass which furnishes the This is important because the edge effects caused by the smallest difference in intensity from that of the sample. shape and width of the cells are thus the same in the two Then the sample is removed, the standard block is substituted types of measurements. The reference standard for Tyndall for it, and this is now measured against the same milk glass intensity is a turbid glass block which is calibrated by the used in case of the sample. The ratio between the intensity found for the sample and manufacturers on the basis of the intensity of the light source under the same conditions under which the instrument that for the standard furnishes a relative measure of the is used. The values obtained can thus be expressed in Tyndall intensity of the sample. The readings are taken absolute intensity ratios if desired, but this will be necessary with a color filter, blue, green, or red, interposed in the ocular. only for intercomparisons of measurements made with Their effective wave lengths were found to be 449, 529, and 621 mp, respectively. These wave lengths were obtained different standard blocks. The construction of the photometer and some of its ac- by determining with the Keuffel and Esser color analyzer, cessories will be only briefly outlined because complete de- the spectral-transmittancy curve of a carefully filtered raw scriptions have been published by the manufacturers. The sugar solution. The transmittancy of the same sugar solution photometer can be placed either vertically to be used as a was also measured with the Pulfrich photometer for the three
July 16, 1931
IhTDUSTRIAL AND ENGINEERlNG CHEMISTRY
color filters, and the effective wave lengths of the screens found by locating the transmittancies observed on the spectral-transmittancy curve of the solution. Comparison between Tyndall Intensity and Transmittancy with Pulfrich Photometer
The primary object of the present investigation is to ascertain the properties of turbid and colored sugar solutions with respect to both transmitted and scattered light, with the use of the spectrophotometer as well as of the Pulfrich photometer. The first problem selected is a comparative study of the Tyndall intensity and transmittancy of such solutions with the Pulfrich photometer. Balch's spectrophotometric method for measuring turbidity by transmitted light has already been mentioned. The Pulfrich instrument, equipped with a monochromatic light source, can be used for this purpose, but it remains to be seen with what precision this can be done when light filters are used instead of spectrally purified light. Experimental data on the relation between Tyndall intensity of highly colored turbid media and the depth and concentration of such dispersions have not been found in the literature and must be supplied before the problem of turbidity determinations, based on Tyndall intensity in media of varying color, can be attacked. The technic invqlved demands the greatest scrupulousness and care. Absolute cleanliness is essential, even more so than in transmittancy measurements, since the Tyndall phenomenon is a much more sensitive criterion. Much preliminary work had to be done. The cell holder furnished by the manufacturers was equipped only for the 2.5-mm. cell, and a special holder had to be constructed which accommodates cells of smaller and greater depths up to about 25 mm. A dark-colored raw sugar was used to establish the relation between Tyndall-beam intensity and depth of solution. The large number of factors which can disturb both the color and turbidity values of a given sample make it imperative that a uniform system be adopted for the preparation of the sample for examination. The sugar is weighed out in a wide-mouth Erlenmeyer flask, and enough boiling hot distilled water is added on the balance to bring the solutions slightly above 60" Brix. The Erlenmeyer flask is placed in a beaker of water a t 80" C., and the contents stirred gently. It was found that a total heating time of 15 minutes was adequate for the size of samples employed in this laboratory. These averaged around 40 grams of sugar. After the sugar is in solution it is cooled quickly to room temperature, and coarse, suspended particles, such as bag fibers, scale, etc., are removed by whirling for 15 minutes in a small hand centrifuge at 1100 r.p.m. Finally, the density is adjusted to 60" Brix by refractometer. Other solutions were made by mixing the original solution in various proportions with a white sugar sirup of 60" Brix, decolorized with Suchar, and carefully filtered with barium sulfate of x-ray grade and with specially purified asbestos. As it is well known that such solutions containing complex colloidal systems and subject to attack by enzymes do not keep for any length of time, it was necessary in each series of measurements a t varying depth to prepare a fresh solution from a different portion of the same raw sugar. The readings for each series are therefore strictly comparable only as to varying depth, but not so far as varying concentration is concerned. The Tyndall-intensity readings on solutions containing 6, 15, 24, 54, and 77.1 grams of colored dry substance are shown in Table I. With solution 1, red screen, the Tyndall intensity increases steadily with depth. In most of the other series it first increases, reaches a maximum, and then drops again. In solu-
329
tions 4 and 5 , blue screen, the maximum has already been reached at a depth less than 1.06 mm., and the figures descend from the very beginning. Although theoretically the Tyndall intensity itself would steadily increase with depth if there were no absorption, this increase is counteracted by the growth in absorption as the depth becomes greater. To ascertain the magnitude of this effect, it was necessary to determine also the transmittancies of the same solutions in the same cells. This gave the results assembled in Table 11. Table I-'Iyndall DEPTH 1
Intensity Readings a t Varying Depth 2
3
4
5
BLUE SCREEN
Mm. 1.06 2.53 5.09 10.01 20.06
35.2 68.1 93.4 115.3 76.4
60.4 124.0 152.8 113.3 34.6
132.2 201.7 158.5 81.1 12.1
161.2 148.6 72.6 9.32
..
184.2 130.8 31.8 1.43
..
GREEN SCREEN
1.06 2.53 5.09 10.01
20.06
129.6 201.7 288.6 002.3 445.7
168.5 421.9 615.4 943.8 324.2
429.1 926.8 1306 1100 292.3
794.4 1210 1131 3E7.6 128.6
931.0 1042
2014 4884
3342 3753 6426 4373 737.4
S18,O 141.9 3.69
R E D SCREEN
1.06 2.53 5.09 10.01
20.06
393.8 665.2 1050 1824 1861
640.1 1430 2659 3816 3286
1435 3259 5455 6323 3779
6783 5363 1133
Table 11-Transmittancies of Same Solutions a t Varying Depths DEPTII 1 2 3 4 5 BLUE SCREEN
Jim. I.?6 2.03 5.09 10.01 20.06
90.8 79.2 47.7 20.9
81.8 60.5 37.4 15.9 2.63
95.3 89.0 79.0 64.3 41.6
89.8 75.4 55.9 34.6 12.1
66.2
74.5 46.3 16.0 4.8 0.36
48.5
17.6 3.2 0.13
35.0 8.44 0.78
...
...
...
67.1 37.1 14.1 2.20
57.0 23.9 5 80 0.42
79.8 55.5 32.5 11.7 1.42
71.4 43.8 20.0 4.64 0.24
GREEN SCREEN
1.06 2.53 5.09 10.01 20.06
82.3 64.5 35,5 19 1 3.61
...
...
RED SCREEN
1.06 2.53 5.09 10.01 20.06
96.5 92.1 88.2 76.8 56.0
94.1 84.7 71.7 52.4 28.8
89.9 76.7 54.4
38.9 14.2
To save space, the corresponding values of (-log T ) / b are not given here. They have been calculated, however, and the results show that the precision of the instrument is not so high as might be desired, especially in the readings near the hundred point. This is no doubt largely owing to the fact that spectral bands are used instead of spectrally purified light. The relation between -log Tand depth is nearly linear, proving a close approximation to Beer's law. If the calculations are made on the basis of the formula -log T = (-log TI) x b", the average value of n is found to be 0.981-that is, slightly less than unity. The next step was to ascertain in what way the Tyndall intensity must be corrected for absorption, and it was found that when the former is divided by the corresponding transmittancy, the resulting figures, instead of rising and falling as the Tyndall intensities do, increase steadily with the depth. A further examination showed that the ratio between Tyndall intensity and transmittancy is a power function of the depth. When the logs of the ratios were plotted against the logs of the depths, straight lines were obtained within a certain range. From the straight-line relationship between the logs of the ratio as defined above and the log of the depth, it was concluded that the general formula, R = R1 X b", is applicable, where R is the ratio for any depth, and R1 that for unit depth. The millimeter was chosen as the unit. It was found that with each separate solution the value of n was the same for all
ANALYTICAL EDITION
330
three wave lengths, within the experimental error. This would indicate that the differences in the transmittancies a t different wave lengths observed by Balch, are compensated for by similar differences in the Tyndall ratios at different wave lengths. With the experimental data as a basis, the numerical value of n was found to be 0.726 for solution 1; 1.080 for solution 2; 1.179 for solution 3; 1.268 for solution 4; and 1.312 for solution 5. Although it has been emphasized that the five solutions of differing concentration cannot be compared directly, it is nevertheless interesting to note that the values of n vary approximately as the logs of the concentration of the colored dry substance. The calculated values for R1 are given in Table 111. These figures and those for n were substituted in the formula shown above, and the results for R at different depths, calculated in this manner, are also shown in the same table, together with the ratios actually observed. Table 111-Percentage Ratio between Tyndall Intensity a n d Transm i t t a n c y for Varying Depth (Found, and calculated by formula R i= RI X bn) a 4 DEPTH 1 2 56 BLUE SCREEN
Mm. a
Calcd.
0.435
0.686
1.52
2.83
4.89
1.06
Found Calcd.
0.388 0.454
8.738 0.730
1.77 1.63
3.33 3-05
5.50 5.28
2.53
Found Calcd.
0.860 0.853
2.05 1.87
4.36 4.55
8.45 9.19
5.09
Found Calcd.
1.41 1.42
4.09 3.98
9.91 10.4
22.7 22.3
Found Calcd.
2.42 2.32
7.13 8.25
16.9 23.0
74.0 52.6
10.01
error in the same direction. Fourthly, it has been observed that when the thick cells, of 10- and 20-mm. depth, are used for Tyndall readings, there are reflection effects from the side walls of the cells which, as has been explained above, are set a t an angle of 45 degrees. It follows that with this particular instrument measurements of Tyndall intensity should be confined to cells not over 5 mm. thick. The power law which has been found to govern the relation between the ratio R and the thickness also holds with respect to concentration and under the same limitations. Four different concentrations were used-namely, a 60 ' Brix solution of raw sugar, and portions of this same solution diluted in three proportions with a 60" Brix white sugar sirup which had been' purified as stated previously. The colored dry-substance concentrations were 9, 18, 36, and 77.1 grams in 100 ml., respectively. The thickness of the cell was 5.09 mm. Table IV shows the found and calculated values for R, on the basis of n = 1.470, R1 (concentration = 1 gram of colored dry substance in 100 ml.) for the blue screen = 0.0582, for the green screen = 0.1950, and for the red screen = 0.4626. A correction was applied for the slight Tyndall turbidity of the white sirup. Table IV- Percentage Ratio between Tyndall Intensity a n d Transmittancy for Varying Concentrations (Found, and calculated by formula R RI X cn) ' CONCN. IN 100 ML. FOUND CALCULATED
-
Grams
15.2 16.5
a
Calcd.
1.25
2.00
5.03
10.4
18.2
1.06
Found Calcd.
1.36 1.31
1.88 2.13
5.21 5.39
11.8 11.2
19.3 19.6
2.53
Found Calcd.
2.27 2.46
5.60 5.45
14.4 15.0
32.6 33.8
58.3 61.5
5.09
Found Calcd.
3.65 4.08
11.0 11.6
36.8 34.3
80.2 82.0
10.01
Found Calcd,
7.81 6.67
27.3 24.1
57.6 76.1
20.06
Found Calcd.
26.8 60.6
81.0 173
a
Calcd.
3.91
6.26
14.6
25.4
45.3
1.06
Found Calcd.
4.07 4.08
6.80 6.66
16.0 15.7
25.2 27.4
51.5 48.9
2.53
Found Calcd.
7.22 7.67
16 9 17.1
42 5 43.7
88.0 82.5
5.09
Found Calcd.
11.9 12.8
37.1 36.3
100 99.6
209 200
359 383
10.01
Found Calcd.
23.8 20.8
72.8 75.3
163 221
458 472
1086 930
266 Found 33.2 114 502 Calcd. 34.5 190 RI (Rfor 1 mm.). b Averages of two series of measurements.
798 1140
4058 2316
34.0 52.3
GREEN SCREEN
163 193
1.77 4.41 11.6 34.6
GREEN SCREEN
3.66 3.84
164 154 402 373
RED SCREEN
20.06
2.01 4.75 10.6 42.7
1s
36 77.1 9 18 36 77.1
Foiind Calcd.
10.7 11.1
BLUE SCREEN
9
42.9 41.3
20.06
13.2. 20.8
VOl. 3, No. 3
155 153
In the case of the most dilute solution, number 1, there is good agreement between found and calculated values for all thicknesses. As the concentration increases, the deviations become greater, first for the 20-mm. cell, and then also for the 10-mm. cell. Up to a thickness of 5 mm. there are good checks a t all concentrations. The deviations are readily explained. I n the first place, the simple power formula is only a first approximation, covering a limited range. Secondly, photometric readings near the one hundred point are uncertain, and near the zero point the possible percentage error is large. Thirdly, errors in both the Tyndall intensity and in the transmittancy measurements are reflected in the ratio between the two, and they may give a combined
5.77 15.8 38.6 154
5.93 14.8 38.9 116
R E D SCREEN
Good agreement is shown between found and calculated values for the first three concentrations, but the calculated figures for the highest concentration are much lower than those observed. Some other series which were run with cells of smaller and of greater depth gave erratic results which were so evident that no calculations have been attempted. It@wasfound subsequently that it is difficult to obtain a perfectly homogeneous mixture of the two sirups. If the sample has been shaken violently, as for example, where the solution has been diluted with white sugar sirup, it is necessary to allow the colloids to reach equilibrium. The following figures show the magnitude of change immediately on shaking a sample violently: TURBIDITY Before shaking After shaking
% Blue Green Red
18.7 506.3 610.5
PERCENTOB ORIGINAL
% 28.0 744.5 992.1
149.8 147.2 162.8
I n the continuation of this work, colored turbid solutions of smaller specific gravity will be used, as has been done by Lindfors and by Balch. This will make it easier to vary color and turbidity independently. Literature Cited (1) Balch, IND.END.CHEM.,Anal. Ed., 3, 124 (1931). (2) Honig, Arch. Sutkerind., 37,111,231 (1929). (3) Horne and Rice, IND.ENG.CHEM.,16, 626 (1924). (4) Ingersoll and Davis, Ibid., Anal. Ed., 2, 248 (1930). (5) Lindfors, IND. END.CHEM.,17, 1155 (1925). (6) Pulfrich, Z.Instrumentenk., 45, 34,61,109 (1925). (7) Tolman, Reyerson, Vliet, Gerke, and Brooks, J . A m , Chem. Soc., 41, 300 (1919). (8) Wells, Chem. Rew., 3, 331 (1927).
