a method for determining the hydrogen equivalent of sodium

SHORTRIDGE HIGH SCHOOL, INDIANAPOLIS,. INDIANA. In the past the writer has experienced considerable difficulty in weigh- ing metallic sodium with any ...
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In the past the writer has experienced considerable difficulty in weighing metallic sodium with any degree of accuracy for the purpose of determining its hydrogen equivalent. The determination of the hydrogen equivalent of sodium, magnesium, and aluminum is made as an introduction to the study of valence and subatomic structure. Inasmuch as a poor determination may do more harm than good it is important that the quantity of sodium should be weighed rather accurately. Recently, Fernelius and Schurman (1) described a method for filling tubes with the alkali metals. They also suggested the use of such tubes as a means of securing accurate quantities of sodium. By the measurement of the bore of a tube and the calculation of the area of the cross-section; the length of the tube necessary to supply a given weight of sodium is found by dividing the amount of sodium by the product of area of the cross-section of the tnbe and the density of sodium.* The method of filling the tubes as described by Fernelins and Schurman was modified by the writer as follows. A rather long narrow pyrex test tube was filled to within 15 mm. of the top with meltedparaffin. Metallicsodium, reasonably free from oxide was dropped into this tube of melted paraffin and melted in the bottom of the tnbe under the paraffin. Glass tubing of 3 mm. diameter and approximately 75 mm. longer than the test tube was closed tightly with the forefinger and introduced through the paraffin into the melted sodium (see accompanying Twris figure). The forefinger was removed and the tube filled FILLING METALLIC with sodium due to hydrostatic pressure. The forefinger WITH soDIuaa was again placed over the end of the tube and the latter removed to the air where it was held until the sodium had solidified. The lower end of the tube was of course sealed from the air by the melted paraffin solidifying. The tube was then cut off at the upper surface of the sodium and sealed with paraffin. Paraffin is kept out of the tube as it is introduced, due to the air expanding. Several tubes can be filled in a few minutes by this method.

* The method of estimating the amount of sodium used depends upon the accuracy of measuring the diameter of the tube. If the diameter of the tube can he measured accurately to within 0.1 mm. the volume of gas evolved will check the data presented. Using the usual millimeter rule the diameter of the tuhe cannot be estimated much closer than 0.2 mm., hence about twice as much deviation in the volume of gas evolved from the mean is to he expected. This error is not sufficiently great to outweigh the convenience of the method. 2165




It does not require a vacuum, an inert gas, or glass-blowing operations. Lengths of these tubes were cut such as would give one-tenth gram of sodium, and the hydrogen equivalent was determined with this quantity. To determine the hydrogen equivalent, a water-filled gas-measuring tube was inverted in a large crystallizing dish of water and the tube clamped into position on a ring stand. A three-inch length of rubber tubing was plugged a t one end and the cut length of sodium-filled tube slipped in at the other. This was then manipulated under the gas-measuring tube and the hydrogen collected. A short piece of copper wire bent at right angles served to dislodge any bubbles that get caught in the tube and shut off the water. The following table gives the results obtained by four different teachers of our department, using the method as a lecture demonstration.

This method has the nierit of being rapid, easy in manipulation, and gives more accurate results than the old method of weighing the sodium directly. The tubes are easily filled and may be conveniently stored. Literature Cited S SCHURMAN, "A Method for Showing the Metallic Luster of (1) F B R N E L ~and the Alkali Metals," J. CHBM.EDUC., 6, 1765-6 (Oct., 1929).