AN EXPERIMENT ON SAMPLING JOHN A. BISHOP Newark College of Engineering, Newark, New Jersey
IR MOST of the textbooks on quantitative analysis, fhe initial experiment suggested is the determination of the water of hydration of BaC12.2H20. This is done both as an introduction to the use of the analytical balance and also as a means of teaching the students the importance of cooling samples in a dessicator. By comparison with the true percentage of HzO, t,he student can also gain confidence in his ability. Some years ago the author decided to vary this experiment by using samples which were poorly mixed, in order to show the students the importance of having a properly taken, uniform sample. As done a t Newark College of Engineering the student received a sample prepared from BaClz.2H10 and a non-hydrated salt, such as NaCI. This sample is not well mixed, but the components have been accurately weighed so that the theoretical percentage of water can be calculated. The student has to grind and mix the complete sample before weighing out one-gram samples for analysis. He discovers that he must have a mortar and pestle combination which is clean, dry, and cool. He also learns about mixing on paper by rolling. Poor precision in his results indicates incomplete mixing, a t which point he finds that the sample is spoiled and he must get a fresh sample. The student's reaction to this is to worry about the uniformity of every sample he gets. In most cases good agreement with the theoretical result is obtained, as is shown by the figure, where the
VOLUME 35, NO. 1, JANUARY, 1958
absolute deviation is plotted against the percentage of determinations having that deviation. This graph is the summation of several hundred determinations, the percentage in perfect agreement (about 4 7 3 being about the same as the percentage of students who requested new samples because of poor precision. As new classes add their results, the graph approaches closer to the ideal probability curve, as might be expected. The fact that the peak of the curve comes toward the nerzative side mav indicat.e that students actually do not get perfect drying
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Distribution of Error in Determination of water
The results to date indicate that the addition to the traditional initial experiment is justified, and that it is one which may he of interest to other teachers of analytical chemistry.
