1671
V O L U M E 2 6 , N O . 10, O C T O B E R 1 9 5 4 Table I.
b
Measurements on Four Silver Chloride-Bromide Titrations
Bromide Titration Visual Template 15.8 15.9 4verageo 0.24 0.15 ub 25.5 25.6 -4verage 0.07 0.08 U 27.5 27.6 Average 0.13 0.10 U 19.6 19.6 Average 0 09 0.09 U Avernee milliliters of silver nitiatf.. Stnndard deviation in milliliter^..
Chloride Titration Visual Template 32.9 32.9 0.13 0.11 43.6 43.6 0.08 0.09 35.3 35.3 0.09 0.09 26.6 26.7 0.17 0.13
temperature to be controlled. The chief advantage arieing from this type of heater is that complete visual inspection is permitted during the whole heating operation. Besides being effective in heating the chromatograms, it has been useful in hastening d e p osition of the solutions of the carbohydrate products being studied. Efficient Laboratory Freeze-Drying Apparatus. &4. L. Tappel, Depart,ment of Food Technology, University of California, Davis, Calif. t'rom the frozen state is ofteii the only convenient biological materials without inducing undesirable chemical and physical changes. Freeze-drying has many useful applications in analytical procedures, including moisture determinations ( 4 ) , histological and histochemical analysis (1, d ) , and the preparation of large pieces of plant and animal tissue for radioactive tracer studies involving radioautography (6). The freeze-dryer described here has berri designed to satisfy theoretical requirements for efficiency, to be economical and easily constructed, and to be convenient in operation. iilthough most small freeze-dryers are designed specifically for use with Inaterials n-hich can be shell-frozen in the liquid state, this freezedryer wis designed to accommodate irregular solids. I t satisfies theorrtical requirements with a condenser temperature below -40" C., t: short and unrestricted vapor path between the sample and condenser, low total pressure, and rapid heat input into the sample to supply heat for vaporization of ice a t the maximum rate ( 3 ) . It is convenient in use bpcause the sample is visible, sample temperature and total pressures arr measured, and only one vacuum seal is broken in opening the freeze-dryer vacuum chamber. As it is made from laboratory equipment generally available, the construction is simple and inexpensive. The most costly piecrs of equipment are easily detached for use elsewhere. RYIS(;
D method from removing water from heat-labile
VOLUME
OF
TITRANT
Figure 1. Concentric Arcs Template Method of Determining Inflection Point
on that portion of the curve (Figure 1) where there is downward curvature. Fit the arc which coincides a t most points. At the common center (a small hole in the sheet) place a dot. Repeat the same procedure on that portion of the curve where the curvature is upward. Draw a line between the two dots. The inflection point is the intersection of this line and the curve. Figure 1 illustrates this procedure.
APPARATUS
The assembled freeze-drying apparatus is shown in Figure 1 as it appears during operation.
Although there is no obvious theoretical basis for this principle, in practice it has proved to give results which compare favorably in accuracy with other methods on the usual types of curves. Table I summarizes a set of measurements on four typical silver bromide-chloride titrations by 44 analysts. When the results are analyzed statistically, the visual method has significantly ( a t 9570 level) greater variability than the template mcthod.
Glass Plate Heater for Paper Chromatography. W.R. Fetzer and L. D. Ough, Clinton Foods Inc., Corn Processing Division, Clinton, Iowa. on developpd paper chromatograms depends L in a large"spots" number of cases upon subjecting the chromatoOCATING
grams to heat after treatment with detecting reagents. The usual procedure is to employ a thermostatically controlled oven which may or may not have windows permitting visual inspection. In this laboratory, satisfactory results have been obtained with a glass plate heater (Ra-Grid heater plate, Model K, obtainable from Blue Ridge Glass Corp., Kingsport, Tenn.), which has been used for over 2 years in lieu of an oven. Direct contact between heater and paper must be avoided to prevent scorching. Interposing a stainless steel screen of 0.25-inch mesh, together with imparting a rotary motion to the chromatogram, has been helpful. A variable transformer of the proper dimensions allows the
Figure 1. .4ssembled Apparatus
The stainless steel condenser, A , consists of a hollow annular chamber 18 inches in diameter and 2 inches high, with a 2-inch stainless steel pipe center well, B. The condenser is fabricated from stainless steel sheet and is of an all-welded construction except for the copper tubing heat transfer fluid inlet, C, outlet, D, and drain, E, which are silver-soldered in place. The top of the stainless steel condenser is machined flat, so that vacuum bell jars form a good vacuum seal. The stainless steel condenser is surrounded on the bottom and side by 2 inches of glass wool insulation, F , which is held in place by a sheet metal container. The insulation is sealed with an asphalt paint to prevent entrance
