The ratio arm at the left is a Leeds & Northrup four-dial decade resistance box, range 1 to 10,000 ohms, and is used for the measurement. The null instrument is a type R Leeds & Northrup lamp and scale galvanometer, No. 2500, with b suspension, rated sensitivity, 0.0005 μа/mm.

To perform a measurement of transmittancy, the arc is struck and allowed to burn during 10 minutes to reach full intensity. At the same time the current is turned into the phototubes, which are allowed also to "warm up." A pair of matched absorption cells, of proper thickness, with plane parallel end plates, and thoroughly cleaned, are chosen. One is filled with the solution to be tested and the other with the solvent. With the pair of spectral filters to give the desired wave length in each beam, the solvent cell on the carriage is centered in the beam at the right, and with the dials of the resistance box set to read 10,000 ohms, the galvanometer scale reading is brought

accessories.

to zero by turning the rheostat knobs. By means of the 1,000-ohm rheostat this may be done exactly. The galvanometer switch is thrown off and the solution cell is centered in the beam. The galvanometer is switched on and the scale reading is brought to zero by means of the resistance box. The dial reading is then the transmittancy, T, of the solution. With the galvanometer well-protected against vibration, these measurements may be exactly duplicated. The galvanometer is sensitive to dial settings in the fourth figure under favorable conditions of illumination, but accurate readings in the third place are sufficient.

For routine quantitative measurements of sugar color, a 200-watt Mazda projection bulb is used along with the special filters for 560 mμ described in section 4 (c), p. 314. These filters may be set in the blocks to replace a pair of the mercury-arc filters.

(d) KEANE AND BRICE SUGAR PHOTOMETER

This apparatus was designed primarily for the industrial grading of white sugars on the basis of (1) the appearance of a sugar in terms of

its reflectance for white light, (2) the apparent color of an unfiltered solution in terms of its transmittancy for red and green light, and (3) the turbidity of the solution in terms of its transmittancy for red light. Since the color filters used transmit very wide spectral bands, the results obtained are not related to those obtainable with a spectrophotometer, but are peculiar to the photocell-filter combination. The following description of the instrument is condensed from the article of the authors [28]. The optical arrangement of the photometer is shown schematically in figure 74 and the wiring diagram is figure 75. A beam of light from a 50-watt projection lamp, J, passing through a color filter, F, is made parallel by lens, L, and is divided by a clearglass plate, A, roughly 10 percent of the light being reflected onto the 'compensating" photocell, P2, while the main beam passes through a compartment containing an absorption cell, C. After reflection by a mirror, M, the collimated beam strikes the standard white reflecting surface, S, at an angle of 45°, and a cone of diffusely reflected light is

66

FIGURE 74.—Photoelectric photometer of Keane and Brice.

received by the "measuring" photocell, P1, placed directly above and with its sensitive surface parallel to the standard reflecting surface. A sliding shutter operated by screw, B, provides a means of making small adjustments in the light intensity received by P2. The parts, F, C, and S, are mounted on slides and can be withdrawn from the light beam or replaced as required.

The diagram in figure 75 illustrates the compensating photoelectric circuit of the apparatus. P and P2 are photronic cells in a parallel connection with a 22-ohm mirror galvanometer, G, of sensitivity 0.36 μa/mm and a 35-ohm precision-wound, 12.5 cm diameter Leeds & Northrup potentiometer rheostat, R. The uniform scale of the latter is calibrated from 0 to 100. The conditions under which the scale indicates light transmission or reflectance include (1) suitable photocells, (2) a low resistance value for the potentiometer rheostat, and (3) moderate illumination of the photocells.

For reflectance measurements, the granulated sugar is poured into a metal dish 63 mm in diameter and 16 mm deep and the top surface is carefully smoothed until it is flush with the edge of the dish. Slight overfilling or underfilling will cause appreciable error, since the distance from the reflecting surface to the photocell is only 55 mm. The

reflectance standard is an opaque white glass plate with a finely ground surface having a reflectance of 0.856 relative to that of a freshly prepared magnesium oxide surface measured in place in the instrument. With the standard white plate in position in the light beam, the galvanometer is adjusted to read zero, the rheostat scale set to 85.6, the lamp turned on, and the shutter, B, moved until the galvanometer again reads zero. The standard plate is then replaced by the sugar sample by moving the slide. The galvanometer deflects and the circuit balance is restored by adjusting the rheostat. The scale reading then indicates the reflectance of the sample relative to magnesium oxide. A comparison of readings made by the authors for individual samples indicated the necessity of screening the sugars to uniform grain size.

For transmittancy measurements, a 150-g sample of sugar is dissolved in distilled water at room temperature and, without filtering is made up to 250 ml. The solution, free from air bubbles, is transferred to a 150-mm absorption cell with plane parallel end plates.

A reference cell, similar to, but only half as long as, the solution cell, is also filled with distilled water. The reasons for using the 75

Figure 75.—Compensating photoelectric circuit of Keane and Brice.

P1, Measuring photocell; P2, compensating photocell; G, galvanometer: P, potentiometer rheostat.

mm solvent cell are: (1) The sugar solution used contains only 51.2 percent of water; (2) water of this depth appreciably absorbs red light beyond about 690 mμ with a maximum absorption at 760 mμ; and (3) the red filter used, freely transmits light absorbed by the water, and the photronic cell shows appreciable response to radiation in this part of the spectrum. One starts with the 75-mm cell of water in the light beam and, with the standard white plate in position, the scale is set to read 100. After the circuit is balanced by turning screw B, the water cell is replaced by the 150-mm solution cell and when the circuit is balanced by means of the rheostat, the scale indicates the transmittancy of the solution. The transmittancies are measured first with a blue-green filter (Corning light shade blue-green, No. 428, 3.4 mm thick) and then with a red filter (Corning traffic red No. 245). The apparent color index is then computed from the ratio of the transmittancies by the formula

I.=100(1 T:

and the index of turbidity by the formula

I, 100(1-T).

Turbidity measurement is further considered in chapter XX, *p. 341.

* Nees, [32] using a Lange photoelectric colorimeter [35], determines color and turbidity in the unfiltered solution with blue and yellow filters. The apparatus is first calibrated by reading the relative percentage of absorption of blue and yellow light by a given unit of color and turbidity. From this relationship both color and turbidity are calculated and expressed as the percentage of absorption of blue light.

(e) MERCURY-ARC SPECTRAL FILTERS

The mercury arc is useful, not only for the wave-length calibration of spectrometers, but because the intense light of three lines of the mercury spectrum may be isolated in sufficient purity by means of properly selected filters. It constitutes an admirable light source for abridged spectrophotometry of sugar solutions, as indicated above. Spectral filters for this purpose may be composed either of colored glasses, single and in various combinations, or of dyed gelatine film. properly mounted. The composition of various glass spectral filters is to the found in the paper by Gibson, Tyndall, and McNicholas [8], and in the catalog of Corning Glass Works entitled "Glass Color Filters." The filters given in table 39 have been used both in visual photometry and with the photoelectric apparatus described under (c), p. 319. It is to be emphasized that when photoelectric cells are used, high spectral purity is essential, and since all of the filters transmit more or less red and infrared, this radiation must be removed from the radiation reaching the photocell. This may be done by interposing a water solution of copper sulfate of such thickness and concentration that the number of grams of CuSO4.5H2O per liter is equal to 178 divided by the length of the cell in centimeters. For visual photometry the red may be removed by means of Corning Dark Shade Blue-Green No. 430, 4 mm thick. In table 39 are given the mercury wave length isolated, the designation of the glass components, the thickness, and the transmission of each of the filters of this particular set at the wave length isolated. There is also included a list of dyed gelatine filters under the manufacturer's designation. These are described in the catalog of the Eastman Kodak Co., Rochester, N. Y., under the title Wratten Light Filters. These filters also transmit some visible red.

Several subvarieties of both serpentine and amphibole are referred to in the literature as being suitable for laboratory filter aids. A large proportion of long fibers is desirable for the filtration of sugar sirups and the variety chosen should be resistant to the drastic chemical treatment described below, the purpose of which is to remove estremely fine material that may cause cloudy filtrates, and other substances that might affect the color of the solutions. Hard columnar fiber

bundles (sometimes 20 cm or more in length) should be broken to a length of 3 or 4 cm and loosened sufficiently to allow easy permeation of liquid. The asbestos fiber, in a suitable iron container such as a sand bath, is treated with a solution of sodium hydroxide, specific gravity 1.43 (40-percent NaOH), 250 ml to each 25 g. The vessel is covered and the mixture is boiled for 30 minutes with occasional stirring, no attempt being made to maintain the above concentration. The mixture is filtered by suction in a Büchner funnel without paper and washed with relays of clear tap water until substantially all alkali is removed from the filter pad. The washed asbestos, after pressing in the Büchner funnel, is transferred to a glass flask and treated with a mixture of 250 ml of hydrochloric acid, specific gravity 1.20, and 25 ml of nitric acid, specific gravity 1.42, for each 25 g of asbestos originally taken. The fiber pad is disintegrated and mixed with the acid by shaking and the mixture is heated for 30 minutes on the steam bath. The contents of the flask are then diluted with distilled water, filtered, and washed with hot distilled water until the washings give no reaction for acid or for chlorine ions. The fiber is dried at 100° to 110° C and stored in a clean container.

Asbestos for general analytical purposes has been prepared in the same manner. For the clarification of sugar solutions, three grades of long-fibered asbestos, designated consecutively according to fiber length as XXX, XX, and A 18 have been found satisfactory. Whether the asbestos is crude fiber or acid-washed, it should be subjected to the treatment described above for the removal of fines.

(b) ASBESTOS FILTERS

Several forms of filters are suitable for sirup filtration with asbestos. A 25-ml or larger Gooch crucible fitted with a disk of 200-mesh bolting silk to retain the asbestos is convenient and low-priced [40]. A good grade of filter paper (not hardened) may be substituted for the silk. Small Büchner funnels with filter paper also may be used. The size of the filter should be chosen with regard to the amount of solution to be filtered which, in turn depends upon the depth of color of the solution.

Jena glass filters [41] in the form of cylindrical funnels, 60- or 120ml capacity with 4-cm filter element, have proved very satisfactory. These filters are designated as 11-G-1 and 11-G-2 for the 60-ml capacity, and 11-a-G-1 and 11-a-G-2 for the 120-ml capacity. The final figure designates the pore size of the filter element, No. 1 being the larger. The pore size, No. 1, is used for preliminary filtration, and the No. 2 for the final filtration.

To form the filtering pad, the asbestos in water suspension is poured into the filter, sucked down with the aspirator, and packed tightly by pressing and tapping with a flattened glass rod. The flat pad, which should be about 5 mm thick when tightly packed, is then washed a few times with water and drained by suction.

Where many samples are to be run daily, a battery of filters may be arranged as shown in figure 76. Each filter or Gooch adapter is fitted to an 8-ounce wide-mouthed bottle through a two-hole rubber stopper, the other hole being fitted with a glass tee with a two-way (Geisler) stopcock, which leads to a vacuum header, and which in

18 Powhatan Mining Co., Woodlawn, Baltimore, Md.