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DETERMINATION OF THE EQUIVALENT WEIGHT OF METALS' A Freshman Research Project2 ENNO WOLTHUIS, DALE DEVRIES, and MARVIN POUTSMA Calvin College, Grand Rapids, Michigan
ON
THE conviction that a proper understanding of the fundamentals of chemistry requires an appreciation of the quantitative nature of the subject, we are convinced that the laboratory work in a general chemistry course should include a number of quantitative experiments, which, incidentally, also greatly increase student interest. One such experiment deals with the determination of the equivalent weight of a metal. This is the subject of the project here reported. Nearly every laboratory manual for general college chemistry contains one or more experiments on this subject. Many of these, either because of the way the directions are written or because of difficulties in manipulation, give results which can hardly be called quantitative. Therefore we set out to investigate the possibility of devising a simple but accurate method for determining the equivalent weight of a metal, suitable for use in college chemistry, and preferably a method applicable to more than one metal so that unknowns could be distributed to the students. The periodical literature reveals only a few studies on the determination of equivalent weight as a lahoratory experiment. Most of them employed the measurement of the volume of hydrogen evolved by reaction of a eiven weieht of metal with excess acid. Reef (1) applied this method to magnesium but gave no data on the arcuracy to be expected. Shah (2) compared the results obtained by measuring the volume of water displaced by hydrogen with those obtained by measurine the volume of hvdroeen collected over water. Lincoln and Klug (i) improved the eudiometer method to results zinc and magnesium within 0.5% error but used equipment not generally available to students in general chemistry. Neser (4) also measured the volume of hydrogen evolved but gave no ex~erimentaldata. Ellis and Einner (5) report a gravimetric method in which a gram of zinc or-cadmium reacted with acid, the solution was evaporated to dryness. after They presented no data to indicate the accuracy obtainable. Murphy (6) reduced copper oxide with ammonia and obtained the equivalent weight of copper with an average error of 3%. The and Linner was for
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' Adapted from apaper presented before the Division of Chemicnl Education a t the 130th Meeting of the American Chemical Society, Atlantic City, September, 1956. 2This paper reports the research done by Messrs. DeVries and Poutsma as part of their freshman year's work a t Calvin College. VOLUME 34, NO. 3, MARCH, 1957
Fisure 1.
Apparatus for Dryin. Metal Chlo.ids.
further study and improvement. Zinc was used in working out the details of the method, after ~vhich were tried by the approved procedure. The following procedure was developed and tested in the general chemistry laboratory course. LABORATORY PROCEDURE Heat gently a clean, 125-ml. Erlenmeyer flask in a. Bunsen flame to remove traces of moisture, set i t upon a. paper towel on the desk, stopper it lightly with a clean cork, and allow it to cool to room temperature. Weigh the corked flask accurately on a balance to the nearest tenth of a milligram. Add to the flask 0.3 ta 0.5 g. of metal (whose equivalent weight is t o be determined) and again weigh accurately. Add about 3 ml. of 6 N HCI. and cover the flask at once with a small watch elass. Suspend the flask, as in Figure 1, from s. ring stand bymeans of a elamp without rubber tips. If the metal is not completely dissolved in 10 minutes, warm the flask slightly. If some metal still remains, carefully add about 0.5 ml. (10 drops) of concentrated HNOI. After the metal has completely reacted, rinse the condensed vapors on the underside of the watch glass into the flask with a few drops of distilled water from a medicine dropper.
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Now heat gently with a small flame under the gauze to evaporate all liquid. During this time the liquid should not boil but
should merely be heated enough to allow a, steady stream of vapors to leave the flask. When the contents of the flask are dry, and not before, suspend a llO'C. thermometer in the flask as shorn in Figure 1, such that the thermometer bulb is midway between top and bottom of the flask. Continue heating with a smell flame so that the thermometer records a temperature of 100"-llO°C. for onehalf hour. Stop heating, stopper the flask with the cork, allow it to cool to room temperature, remove the stopper momentarily and replace it, and weigh again accurately. Assemble the data obtained in tabular form, and calculate the equivalent weight of the metal as that weight which combines with 35.46 g. ohlorine.
VARIABLES STUDIED
A brief discussion follows of the laboratory work performed in developing the foregoing procedure. Metal kind and quality. Complete solution of the metal in the acid is an obvious requirement. Even some samples of C.P. zinc failed to meet this test. Only the best grade of metal available should be used. Of the various metals tried only zinc, cadmium, and manganese gave good results. The others, magnesium, tin, iron, and aluminum, gave residues not completely soluble in water but soluble in acid. These residues are probably oxychlorides and/or oxides, formed during the heating period. Acid quality and quantity. The acid used mas technical grade and was tested to make sure that it contained no non-volatile material. Except for some of the larger samples of aluminum and magnesium, 3 ml. of 6 N HC1 was adequate for complete reaction. Where necessary, more acid could he used. Only in the case of cadmium was it necessary to use a mixture of HCl and HNOa to dissolve the metal. Mechanical losses. The method of Ellis and Linner obviously suffered from losses of salt by spattering during evaporation. Therefore, from the outset it was decided to use a small flask with a narrow mouth. It was also found necessary to cover this flask with a watch glass during solution of the metal. Failure to observe this precaution led to errors of 2% and over. The small droplets which collect on the underside of the watch glass were found by actual test to contain metal chloride. Drying time and temperature. The minimum drying temperature was first determined by using a ther-
mostatically controlled electric oven. It was found that st least 150°C. was required to give good results in a reasonable time, say one hour. Then, to eliminate the oven, it was found that equally good results were obtained by heating with a flame for one-half hour a t 150°C. as registered on a thermometer suspended to within inch of the bottom of the flask, or a t llO°C. if the thermometer is in the center of the flask as in Figure 1. These experiments then reduced the required conditions to those which can be obtained in any laboratory. Weighing the chloride. I n the early experiments the chloride was cooled in a desiccator, but later it was found that, if the flask is stoppered, it may be cooled on the desk as well. Accurate weighing of the chloride also requires the flask to be stoppered, especially on humid days. Nature of the residue. As pointed out above, the chloride should be completely water soluble. This was found to be true only when zinc, cadmium, or manganese were used. To check the composition of the residue, and for the sake of experience, some of the residues were analyzed for chlorine content by the Volhard titration. These titration results, together with the insolubility of some metal residues, indicated that the chlorides of the metals, iron, magnesium, tin, and aluminum, tend to lose chlorine du~ingthe heating to form oxychlorides and oxides. EXPERIMENTAL RESULTS
Using zinc in a series of five experiments by the method outlined above, the students who worked on this project obtained equivalent weight values ranging from 32.77 to 32.96, averaging 32.83, the average error being 0.43%. The procedure was then carried out by 108 students in college chemistry classes. Zinc was used, but its identity was withheld from the students. The result was an average equivalent weight of 32.77, or an average error of 0.25%. The distribution of these results is shown graphically in Figure 2. With cadmium, a series of five experiments resulted in equivalent weight values ranging from 55.73 to 56.17, averaging 55.95, the average error being 0.44%. Using manganese in a similar series of five experiments, the range was 27.33 to 27.57, the average 27.47, or 0% error. SUMMARY
An accurate but simple laboratory experiment for general college chemistry on the determination of the equivalent weight of a metal has been developed by students in their freshman year a t college. The method gives very good results for the metals, zinc, cadmium, and manganese, and should constitute a valuable addition to the college laboratory program. LITERATURE CITED
(1) REEF,V. E., J. CREM.EDUC., 9, 524-6 (1932). (2) SHAH, N. M., J. CREM.Eonc., 12,492-3(1935). (3) LINCOLN, A. T., AND H. P. KLUO, J. CHEM. EDUC., 12, 589 Equivalent Welght: 32.17 32.43 32.69 32.95 33.21 % Error: -1.6 -0.8 0 +0.8 +1.6 Figure 2.
Determination of Equiral.nt Weight of zinc. Disttibution of Student Results
(\----,. lQ25l~
NESER, G. O., J. CHEM.EDUC., 13,510 (1936). (5) ELLIS, R. H., A N D E. R. LINNER, J. CBEM.EDUC., 26, 528-9
(4)
(1949).
(6)
MURPHY, D . B., J. CHEM.EDUC., 27, 463 (1950). JOURNAL OF CHEMICAL EDUCATION
