Apparatus for Photoelectric Titrations HUBERT N. ALYEA Princeton University, Princeton, New Jersey
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HE simplicity, sturdiness, low cost, and ease of cleanmg and replacement of parts in the apparatus in the accompanying sketch are self-evident. I t differs in principle-fro& the usual photoelectric arrangement in that light is passed vertically, rather than horizontally, through the solution whose depth of color is to be measured. It may be used for titration to some given colored endpoint, or without the buret for comparing different intensities of the same color in different solutions. For example, for the particular use for which it was designed, it was fifty times more sensitive than the eye in detecting a color change in a pink phenolphthalein solution. The specifications which follow serve to emphasize the merits of the apparatus. The galvanometer is a Leeds and Northrup portable reflecting galvanometer, D.c., No. 2420A, sensitivity of thirty millimeters deflection per microampere. Uniform illurgination is achieved with an ordinary drug-store bed-lamp of the spotlight bull's-eye variety. A 50-ohm slide-wire*resistance in series with a 60-watt bulb in the bed-lamp controls intensity of illumination; and the 3.5-volt lamp of the galvanometer is also lighted on the same current by tapping off an appropriate length of the resistance. A B-battery should be used if the 110-A.C. is not steady. The photosensitive cell is a Weston Photronic Cell, Model 594. Increased sensitivity is achieved by covering.the cell with a filter which excludes lig.bt of the same &lor as the solution to be measured. For red solutions a 3" X 3" square of Corning Glass Light Filter G 984 B, 4 mm. thick, was used. The cell in which the colored solution is placed is an ordinary 50-ml. pyrex beaker. This size exactly fits the well of the Photronic Cell; and different pyrex beakers are sufficiently identical in their transmissions in the visible spectrum so that they may be used interchangeably, a factor of considerable advantage. When a filter is employed, automobile cork gaskets are glued onto
both sides of the filter, as illustrated in the figure. To clean the photocell and filter, they are simply lifted from each other, and from the base board.
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Reproducibility of the galvanometer is assured during titrations by checking frequently against two standard deflections namely, (1) with the light passing through the G 984 B filter and (2) with the light passing through both a square of Coming Glass "heat resisting yellow" Noviol 0 filter and the G 984 B filter.
