W. L. Masterton Universitv of Connecticut
Storrs
I
1
Analysis of an Aluminum-Zinc Alloy A general chemistry laboratory experiment
One of the most commonly performed quantit,ative experiments in general chemistry is the determinatiou of t,he gram equivalent weight of a metal by measuring the volume of hydrogen evolved in the reaction of a veighed metal sample with excess acid. The experiment as it is ordinarily carried out involves the use of pure samples of aluminum, zinc, and magnesium as "unknowns." Since these metals are rather easily identified by visual observation or simple chemical tests, it was felt desirable to substitute for them a series of alloys of varying composition. Aluminum and zinc were chosen as alloy constituents because of the wide difference in their gram equivalent weights. The analysis of such alloys, in addition to offering a greater challenge to students, illustrates a practical application of the concept of gram equivalent weight and of the use of the gas laws. The alloys were prepared by melting together pieces of block aluminum and zinc pellets in a clay crucible. The molten mixture was stirred to ensure homogeneity and cooled to just below the melting point, where the two metals form a solid solution. The metal ingot formed was quenched in water, the oxide coating adhering to the surface removed, and the alloy converted to turnings on a lathe. A series of five alloy samples each weighing about 1 kg and varying in composition from 27 to 57% aluminum was prepared. Sufficient alloy r a s issued to each student to enable him to weigh out duplicate samples varying in mass
558
/
Journal of Chemical Education
from 0.120 to 0.180 g. The procedure followed in the experiment was similar to that described in Selwood.' The volume of hydrogen generated mas found by determining the weight of water required to completely fill the gas bottle in which the hydrogen was collected. Each student was instructed to report the per cent of aluminum in his sample, making use of the relations Volume
HASTP) = Wt Zn
X
Weight sample
=
- -+ 11.2 1 32.1 g
Wt Zn
\Vt A1 X
11.2 1 8.99 g
+ Wt A1
A data sheet was submitted with each report so that errors in calculations, of which there were many, could be detected. The precision of the experiment, determined from results submitted for about 1000 samples, is indicated by data for the alloy containing 57% '0; 34% of the results were within *1% of the median, 59y0 within *3% and 74% within *5y0. The principal errors arise from inaccuracies in weighing the sample and in measuring the volume of hydrogen evolved. It can be calculated that with the apparatus used, an error of 0.001 g i n the weight of alloy sample produces an error in the reported yo of aluminum of about 0.5%. An error of 0.5 ml in the volume of the hydrogen collected gives rise to a deviation of the same magnitude. Smwooo,
3rd ed
P. W., "Experiments in General Chemistrr,"
, Henry Holt and Company, Inc., New York, 1959, p. 25.
