sugar factory laboratories. The light source may be either a mercury arc with the appropriate filters or an incandescent tungsten lamp with which a special 560-mμ filter is used. Glass plates calibrated for transmission at the various wave lengths are used as standards. The apparatus, figure 67, consists of a Bausch & Lomb Duboscq colori

FIGURE 66.-Stammer colorimeter modified for abridged spectrophotometry.

meter with 10-cm cups, to the vertical column of which a shelf with a heavy bracket is fastened with screws to leave a clearance of 1 cm when the cups are in their lowest position. A carrier for the standard plates with a 25-mm center orifice, and with the ends cut as shown, slides between guides on the shelf and has a slot that engages in a

metal stop at the center of the shelf to center properly the orifice under either cup. A 60° prism enclosed in the triangular housing of the eyepiece telescope permits observation with the head in a natural, unstrained position. The adjustable mirror at the bottom of the colorimeter is silvered glass with the outer surface ground to diffuse the reflected light.

Spectral filters for obtaining light from ordinary tungsten illumination at wave length 560 mμ or close thereto have been devised by Brewster [18] and by Gibson [19]. These permit very satisfactory measurement of transmittancy at this wave length. The composition of these filters is given in table 38.

The components are sealed together with Canada balsam and may be bound with black tape or mounted in metal. The filters may be cut to such diameter as to be mounted in the telescope tube of the instrument or may be used over the eyepiece. In the Brewster filter the gelatine sheets are sealed between the glass components. In the Gibson filter the Jena glasses are sealed between the Corning components.

In the Brewster filter the spectral centroid of the transmitted light for incandescent tungsten illumination at 2,848° K was computed to be 558.8 mμ. For the original Gibson filter and certain replicas made at this Bureau, the optical centroid, under the same conditions as above, was found to be located at 560.0 mμ.'

16

17

A second filter consisting of a blue glass, Jena BG-12, thickness 3.42 m, is used by Brewster for the special case of very pale sugar solutions, such as those of high-grade refinery products. The optical centroid of this filter was computed for 2,848°K as being at 459.9 mu." The photometric standards are plane parallel plates of polished glass, the color of which is known as "carbon amber." This glass is obtainable in shades varying from brown to palest straw. These are to be calibrated in terms of transmission, T, at wave length 560 mμ. This calibration is best obtained by sending the plates to the National Bureau of Standards with a request for calibration, as above specified. The transmission value may be controlled by the shade and thickness of the glass, and it is advisable to have plates of three values,

16 At the time this was written there was difficulty in obtaining exact duplication in the two Jena glasses used in the Gibson filter. However, it is doubtful that a slight shift of the optical centroid on either side of 1560 by 1 or 2 m, would greatly affect the results in sugar colorimetry. Attempts to duplicate this filter by substituting glasses with transmission properties too greatly different from those specified by Gibson would likely result in an inferior filter.

17 In order to measure transmittancy in extremely pale sugar solutions at 560 mp, excessive layer thickness is required beyond the 10 cm available in the standard type of Duboscq colorimeter. Employment of the 460-mu filter is therefore a compromise between having an instrument constructed with much longer cells or choosing a wave length at which the light absorption is greater.

one each with T-560 m about 50, 70, and 80, respectively. This makes for flexibility of the method, permitting measurements over a sufficient range of shades. The glass is customarily supplied in the form of 2-inch polished squares, but other shapes and sizes may be specified to conform to the construction of the instrument. For measurements with pale solutions and the 460-mu filter, a plate consisting of colorless optical glass calibrated in terms of T at 460 mμ is used. To measure the transmittancy of a solution a portion is added to each colorimeter cup, a standard is placed in the carrier and centered under one of the cups, which is adjusted in height to a scale reading of 1 cm. With the spectral filter

in place, the observer adjusts the height of the opposite cup to a photometric match and records the scale reading. Three to five such settings are made, and the standard is shifted by sliding the carrier and centering the standard under the opposite cup, which is then adjusted to the 1-cm scale reading as above, and a second series of the same number of readings is recorded. This substitution method of comparison is the usual one employed with direct-reading spectrophotometers and serves to compensate for any error of zero setting.

The readings in centimeters are added, and the sum is divided by the total number of readings to give the mean. The blank setting (1 cm) is deducted, and the remainder represents that thickness, b, of the solution whose transmittancy, T, equals the calibrated transmission of the standard at the specified wave length. Assuming that the dry substance concentration, c, in grams per milliliter, as given in table 114, (corresponding to the refractometric Brix of the

solution) is already known, the final value, log t, is found by substituting the values of b, c, and -log T (table 129) in the LambertBeer equation, log t=1/cb (-log T).

(d) ZEISS-PULFRICH PHOTOMETER

This instrument, with accessories and methods of manipulation for various photometric purposes, was first described by Pulfrich [20]. The diagram in figure 68 illustrates the photometer (accessories not shown). The optics will be understood from a glance at the drawing.

It consists in principle of two parallel telescopes having a common eyepiece, and is enclosed in a cast housing. The device for measurably varying the intensity of the light is shown in the drawing.

The two diaphragm leaves, with 90° openings, are movable toward or away from each other by equal amounts by means of the micrometer screws. These screws are turned by means of the drum upon which scales are engraved. When the drum is set at the scale-reading 100, the diaphragm is full open; and at 0 (a complete drum revolution) it is completely closed. One scale reads directly in values of transmission and the other in extinction (-log T). The graduations are read from a common index line engraved on a small glass plate. Since both telescopes are provided with photometric devices, the substitu

P1

Ok

P2

tion method may be used. The instrument may be mounted either in a vertical or a horizontal position. Supports, absorption cells, and illuminating lamps with appropriate housing, including a mercuryvapor lamp, are available. The Zeiss pamphlet [21] describes these in detail.

Four different kinds of color filters designated S, K, L, and Hg, respectively, are supplied for use with the photometer, the Hg series being used with the mercury arc, and the others with incandescent tungsten or other white-light source.

There are nine filters in the S series characterized by high optical density and narrowest transmitted spectral band of the three series. The seven filters of the K series transmit relatively wider bands and more light than the S series, while the three L filters have high transmission and are used for faint luminosities. Landt [22] states that the Pulfrich photometer with a mercury arc as light source, and with the appropriate spectral filters, may be recommended for purposes of color measurement in the sugar industry. doubt one of the 560-mμ filters described under (c) p. 314 could be used to advantage with this instrument.

FIGURE 68.-Zeiss - Pulfrich photometer (schematic section). 01, 02, Object lenses; M1, M2, photom

eter drums with scales; L1, L2, len

ses; P, P1, prisms; Pa, biprism; Ok,

ocular lens.

5. PHOTOELECTRIC PHOTOMETERS

No

Photoelectric cells of various kinds have become available at moderate cost within the past few years, and their use in the construction of colorimetric apparatus has become widespread. This is true particularly in the field of analytical chemistry, a review of which, with an extensive bibliography, has been published by Müller [23]. The advantages and limitations of photoelectric cells in colorimetry have been discussed by Gibson [24].

Among the advantages of photoelectric over visual methods may be named: Greater objectivity of measurement; relief from eyestrain and fatigue; and, under good conditions, greater speed. The precision and

reliability of results depend upon the design and construction of the apparatus. Specific literature references to the measurement of sugar color with certain photoelectric instruments are given in references [25 to 33] at the end of the present chapter. Some of the instruments there cited had been designed primarily for sugar colorimetry, while others evidently were intended for purposes of analytical chemistry. For the measurement of transmittancy of sugar solutions over a great range, as, for example, in comparing the decolorizing efficiency of carbons, a flexibility is demanded that is not usually present in analytical apparatus. This may be attained by providing for (1) the accommodation of a variety of thicknesses of absorbing layer up to 20 cm and (2) for the employment of monochromatic light. The second of these considerations is more important in photoelectric than in visual photom

Discussion of the numerous optical and electrical arrangements that have been employed in the construction of photoelectric photometers

cannot be attempted FIGURE 69.-Objective sugar colorimeter of Sandera. here. This is to be

found in the references cited. The instruments described in the following pages have been used particularly for the measurement of sugar color and will serve to illustrate some principles of design and construction.

(a) SANDERA OBJECTIVE SUGAR COLORIMETER [25]

The apparatus devised by Sandera is illustrated in figure 69. Light from the incandescent filament inclosed at O passes through the diaphragm and lens at e to form a practically parallel beam, which is transmitted by the solvent in cell, a, or solution in cell, b. These cells rest upon a movable carrier, whereby the cells may be interchanged as desired. The transmitted beam enters the window of the phototube, A (a potassium cell) and strikes the sensitive surface, K. The voltage is supplied from a battery. The photoelectric current is amplified by means of the vacuum tube, while the millivoltmeter in the plate circuit is the indicating instrument. By means of the variable resistance, R3, the sensitivity of the millivoltmeter may be varied so that dark- and light-colored solutions may be measured equally well.