THE DETERMINATION OF THE EQUIVALENT WEIGHT OF MAGNESIUM
Some of the common 6eginning experimats on finding equieralat weights of metals are criticized. A method is suggested fur imprwng the accuracy and shortening the time of experiments i n determining the epuivalat weight of metals by displacement of hydrogen from hydrochloric acid.
. . . . . .
Most laboratory manuals give experiments for determining the equivalent weights of various metals. Most depend on the direct union of a metal with oxygen or the reaction of a metal with water or acid with the liberation of hydrogen for data to calculate the equivalent weight of the metal. Most of the methods I have seen are either very time-consuming or rather inaccurate. Burning magnesium ribbon and weighing the ash usually gives values for the combining weight of magnesium that are too high, due to incomplete oxidation. The treatment of a weighed amount of zinc with acid and the collection and measurement of the hydrogen released furnishes a method for determining the equivalent weight of zinc. This method in my estimation takes too much time, as the zinc has to be weighed accurately for each student. Since such a small amount of zinc is required, each sample of zinc has to be weighed on an analytical balance and it is doubtful whether very many freshman chemistry laboratories possess sufficient analytical balances to prevent undue congestion of the students around them. This same difliculty is experienced in determining the equivalent weight of magnesium by finding the amount of oxygen it combines with on burning. The weighings of the students are more or less inaccurate, since these experiments usually are made near the beginning of the first-year chemistry course and the students' laboratory technic is far from satisfadory at that time. The results obtained often T i e r so much from the theoretical that the student frequently is led to question the value of the experiment. The procedure of issuing the student gelatin capsules containing weighed quantities of the metal was tried but was found to be too time-consuming. Where the classes are large the assistant has to spend a considerable amount of time weighing the quantities of metal for the capsules and the student loses time in waiting for the capsule to dissolve. It was thought that the use of magnesium ribbon offered a means of eliminating numerous weighings. A long, clean piece of magnesium ribbon was weighed and measured and the student given a definite length of the ribbon. The piece of ribbon was inserted beneath a gas-measuring tube filled with dilute acid and inverted in a beaker of water. The bubbles of hydrogen, however, attached themselves to the piece of magnesium and caused it to float to the top of the tube. The concentration of the acid at 524
VOL. 9, No. 3
EQUIVALENT WEIGHT OF MAGNESIUM
525
the top of the gas-measuring tube was rapidly reduced and the action from then on was very slow. The amount of add used was also comparatively large. The following method was finally decided upon and is very economical from the standpoint of time and materials. The whole experiment including the time used in distributing the materials takes only about fifteen or twenty minutes. The apparatus required is shown in the diagram. The following are required: a 50-cc. gas-measuring tube, a 150-cc. beaker, a piece of glass tubing inch outside diameter, about 3 or 4 em. in length, and nearly closed at one end, a piece of glass tubing about 4 or 5 inches in length with one end drawn out and bent into a slight book, a short-stemmed funnel, a short piece of rubber tub'mg, and a buret clamp. Assemble the apparatus as shown in the diagram. Clean and weigh a strip of magnesium ribbon a meter or more in length. This is sufficiently long for the average class, as a strip three centimeters in length is enough for each student. Fill the gas-measuring tube with water. Place t h e index finger over the open end of the tube, invert it and remove the finger after placing the open end of the tube below the surface of the water contained in a 150-cc. beaker. This operation should be accomplished in such a manner that no air gets into the gas-measuring tube. The beaker may be set on a ring stand and the tube loosely clamped in a vertical position. Bend about '/z cm. of each end of the ribbon up at an angle of about 45' and slip the ribbon in the short piece of glass tubing that has been nearly sealed at one end. The small hole in the tube should be between one and two millimeters in diameter. Bending the ends of the ribbon up will cause it to exert a spring action and keep it from slipping out of the tube. Take forceps and insert the piece of glass tubing containing the metal, small end up, in the open end of the gas-measuring tube without allowing any air t o enter. Add concentrated hydrochloric acid to the gas-measuring tube by
526
JOURNAL OF CHEMICAL EDUCATION
MARCH, 1932
inserting the end of the bent tube connected to the funnel bearing the acid beneath the open end of the gas-measuring tube and allowing 5 cc. of concentrated hydrochloric acid to run into the gas-measuring tube. The tube connected to the funnel should be filled with acid by opening the buret clamp sdiciently to fill the tube, before inserting it beneath the gasmeasuring tube; othemise air bubbles will be driven into the gas-measuring tube. Remove the funnel and acid-bearing tube. The reaction proceeds quite rapidly. If the action should stop, shake the gas-measuring tube to bring fresh acid in contact with the metal. Wben all of the metal has been dissolved, place the finger over the open end of the tube and carry the tube to a tall cylinder or tank filled with water. Place the open end of the tube below the surface of the water, remove the finger, adjust the height of the tube until the level of the water on the inside is the same as the level on the outside, and read the volume of the gas. Read the barometric pressure and room temperature. Calculate the volume of the gas under standard conditions and, from this volume and the density of hydrogen (0.0000898 g./cc.), its weight. Calculate the equivalent weight of magnesium from the proportion: Weight of metal : Weight of hydrogen : : Equivalent weight of the metal : 1.008.
This method was found to give very satisfactory results with aluminum wire as the metal. The action is fairly slow however unless the aluminum wire is twisted around a piece of copper or nichrome wire before being inserted in the small piece of tubing. If a gas-measuring tube is not available a buret may be used. The method will obviously prove quite satisfactory for determining the equivalent weight of any reactive metal obtainable in wire or ribbon form.
