DECEMBER, 1952
0
615
A TECHNIQUE TOR HANDLING SMALL PRECIPITATES SEYMOUR
2.
LEWIN
New York University, New York, New Yolk
ITIS occasionally necessary to obtain in a form suitable for a melting-point determination amounts of precipitate too small to be collected on a filter paper. A common but tedious procedure involves centrifugation, removal of supernatant liquid with a micropipet, one or more washing operations, and drying of the residue in the centrifuge tube. A more convenient and rapid technique requiring no special apparatus or experience is the following: The precipitate is allowed to settle in a 3-inch test tube, and all but a small amount (about one drop) of the supernatant liquid is removed by means of a dropper. The remaining drop of liquid is agitated t o suspend the solid in it, the test tube is held a t a small angle from the horizontal, and one end of a piece of capillary tubing 1.5-2.0 mm. in diameter and 100 mm. long is dipped into the suspension. Capillary action causes the suspension to be transferred entirely to the tubing of small diameter. The position of the column of liquid in the capillary tuhe can be readily controlled by very slight tilting of the tube from the horizontal. A strip of filter paper about 3 mm. wide and 50 mm. long is rolled between the fingers along its length, and is inserted into one end of the capillary for a distance of
about 15 mm.; the column of liquid being kept away from this end of the tuhe (the figure). The column of
rilte. paper&ing
capinary ~~b~
T W . S into ~ ~ ~
liquid is now slowly allowed to approach the edge of the filter paper while rotating the capillary tube. When the suspension touches the paper, the liquid gradually becomes absorbed, the solid collecting at the point of juncture. Washing of the precipitate is accomplished by dipping up some wash liquid, suspending the solid in it by rotating the capillary tube, and removing the liquid as above. The capillary and its contents may be allowed to dry in a desiccator, and then sealed a t one end. By this technique precipitates of the order of 0.1 mg. have been successfully transferred in easily visible form to melting point tubes.
