Construction of a Self-Filling Pycnometer in Five Minutes
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One of the first experiments in the beginning chemistry laboratory is often the determination of the density of a liquid, either known or unknown. As large elementary laboratory classes are not usually equipped with any special devices for obtaining densities, the likely alternative is to use C the somewhat unwieldy 10-ml graduated cylinder and a suitable balance. If less than 10 ml of liquid is available the best volume measurement will likely contain just two significant figures; yet the mass of the liquid can easily he read to three or four figures on a top loading balance. To overcome this disparity, a pycnometer can he used whose limitations in measuring depend primarily an the balance itself. A self-filling pycnometer with a small capacity can be readily constructed from a 10-em length of 5- or 6-mm a d . soft glass tuhing, a hot Tirrill burner flame, and a fragment of unglazed ceramic (even sewer pipe works fine). The glass tubing is held just below the tip of the inner cone of the flame and rotated gently until the glass softens in two places and begins to sag. All drafts and breezes must be rigorously excluded so that the flame will be precisely symmetrical. The glass is removed from the flame and pulled firmly hut smoothly with both hands to yield two segments of capillary separated by an unchanged "bulb". If the flame was not symmetrical the capillary sections are likely to differ appreciably in their diameters, but even so a workable pycnometer is still possible. The capillary section with the smaller diameter is scratched gently with the ceramic chip about 3 em from the bulb and broken off. A similar operation is performed an the large^. capillary about 8 cm from the bulb. This longer tip is heated very cautiously in a low yellow flame to produce the two curvatures shown in the figure. In order for the pycnometer to self-fill completely, loop B should lie between C and D; the distance between A and B should be about 15-20 mm. Several differeht types of containers can he used tci hold the liquid and the pycnometer: a l-ml beaker, a microcmcible, or a homemade micro-test tube constructed from 6-mm soft glass tubing. The container is clamped firmly and end A is dipped into the liquid so that B hangs on the rim of the container. Filling will proceed automatically by capillary action and the syphon effect, but the time required for completion will vary greatly depending on the diameter of the curved capillary and the viscosity of the liquid. When a pycnometer having a capacity of 0.25 ml and a capillary diameter of 0.6 mm was used, acetone required 31 sec; water, 61 see: and t-butyl alcohol, aver 8 mi". By use of a side arm test tube and a one hole stopper to fit the bulh at C, the filling time for any liquid ordinarily encountered can be reduced to a matter of seconds by applying gentle suction with a dropper bulh having a small hale in the tip, a hole that can be closed or opened at will with a finger in order to apply or release the suction. This same apparatus can he used after the final weighing to recover the liquid present in the pycnometer. A slight loss from evaporation through the open capillaries will occur with a low-boiling liquid, hut usually this loss is negligible. A pycnameter filled with acetone lost only 2 mg ofliquid in 11min. Although this operation will not give density directly, hut rather specific gravity hased on a standard liquid, usually water, it does give the young student a meaningful contact with the proper use of significant figures and the notation used to express specific gravities. The preparation of the pycnometer itself affords uncontestehle evidence that the outer portion of a burner flame is much hotter than the inner cone.
Thomas McCullough, CSC St. Edward's University Austin. Texas 78704
546
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Journal of Chemical Education
