ANALYTICAL CHEMISTRY
512
Grinding with a finer grade of Carborundum might result in still better dispersions. In use the pestle is rapidly rotated in the vertical chuck of a variable.speed stirring motor, the is moved Over its ground surface from below. Unless very small quantities me used, the material being ground is rapidly forced out, and a t interVal8 Of a few seconds must be back Onto the grinding surfaces. This can be done quickly, without Stopping the motor, by collecting the expressed suspension with a spatula.
similar apparatus hut replaced the carbon dioxide bubbler with a conventional stirrer. A magnetic stirrer employed in a Beckman ModelB spectrophotometer has heen described by Hume (2). Because of the greabr versatility of the Beckman Model DU spectrophotometer, an interchangeable cell compartment wa8 constructed to adapt this instrument for photometric titrations. Alterations of the instrument proper required far this modification.
The spherical surfaces of the mortar and pestle provide a perfect fit over the entire grinding area. The spherical contact also permits continuous movement of the mortar over the surface of the rotating pestle and thereby avoids an axial dead spot. Grinding is extremely rapid. Good mineral oil mulls of some materials have heen prepared by this means in less than 15 seconds of grinding. The degree of subdivision of the particles is greater than the writer has obtained with any other apparatus. The mull is more readily removed from the small mortar and pestle than from larger devices. Borosilicate glass has been found very satisfactory for the apparatus and there has been no indication of contmnination of the mulls by the glass. Homever, if it were desired to grind hard materials, the apparatus could he constructed of hardened steel. It should he useful for grinding small quantities of materials other than mineral oil mulls.
A Beckman test tube adaptor assembly for the Model DU s p c trophotometer was modified for construction of the titration assembly. A '/4-ineh hole was drilled in the center of the top of the compartment cover and fitted with a rubber grommet of suitable size to permit the light-tight insertion of a buret tip. The test tube holder d a t f o m WZLSremoved and the two guide rods
Photometric Titration Assembly for Beckman Model DU Spectrophotometer
firmlv on the h n w d id1 eomDartment
Thus the buret is ko&ed
with~ihisattachment. The &e ~~
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removed and the motor
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inches, which-alioiid it to be inserted und; the celr cornpa& ment. The armature bearing support bolts were changed from
This arrangement made the controls readily accessible it all times.
Donn W. Klingmon, Donold 7. Hooker, ond Charles . . V. Bonk, Institute for Atomic Research and Depqirtment of Chemistry, Iowa Slate College, Ames, low. :+r^+:n"o "CT.."rl" WEEtechniaue of Dhotometric t.,,,,,,., ul.vlyo ,," "imnla Olulw.r solu1 tion t o many cokplex analytical problems. Various apparatus for photometric titrations have been described by previous workers. Bricker and Sweetser ( 1 ) replaced the cell compartment cover with a felt pad through which a buret tip was inserted and carbon dioxide was bubbled through the titration cell to ensure mixing of the reagents. Reilley and Schweitzer ( 3 )employed a
BURET SUPPORT
BURET
Figure 2. Photographof m o u n t e d assembly
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was a 180-ml. electrolysis beaker. The bottom, inch of one side of the beaker was flattened slightly to permlt it to fit firmly between the euide bars. A similar cell for use in the ultravlolet region~ofthe'spectrum was constructed from a fused silica tube. The titration cell, containing the sample and magnetic stirring bar, wzu placed in the cell compartment and the stirring motor started. The stirring rate was adjusted such that the vortex, of the solution did not interfere with the light beam. The additton of each increment of titrant was terminated by rapidly rotatlng the stopcock from off to on to off. This technique left a small and reproducible droplet on the buret tip. This photometric titration attachment has been employed in this laboratory in studying chelate kinetics, molar absorptivities, and routine analytical determinations. These particular applications will be described in!orthcoming publications. L I T E R ~ ~ U RCITED E SUPPORTS
Figure 1.
Schematic diagram of titration assembly
