Martin 1. Thompson and John W. Bixlerl
Lake Forest College Lake Forest, Illinois 60045
Abridged Qualitative Analysis with Limited Reagents
W e wish to describe a short experiment done in our introductory chemistry course for science majors. Although qualitative analysis as a separate course was dropped from our curriculum four years ago, we felt the need to retain the following positive characteristics of it: a continually enthusiastic response from the students, and a vehicle for demonstrating some fundamental aspects of solution equilibria, descriptive inorganic chemistry, and simple separation techniques. I n contrast to the more traditional approach we desired to avoid the consumption of large amounts of laboratory time and to introduce a more creative experience for the student. The experiment with limited reagents involves the detection and identification of four cations using only the following reagents: 6 M HC1, 3 M H,SOn, 6 M NaOH, 6 A t NH3, and deionized water. The students may mix or dilute these reagents as desired, but no additional reagents or flame tests are permitted. The students work in partnerships and each pair of students is given a list of four uniquely assigned cations. With the aid of reference books, they prepare a preliminary outline of their plan of attack. After discussing their preliminary scheme with the instructor in a brief interview, they work out the experimental de'Now at State University College at Brockport, New Yolk 14420.
tails using known ions, both singly and in mixtures. When work on the known is complete, the instructor provides each of the partners with a different unknown solution, containing from two to four of the cations in the known. Each member of the partnership analyzes his own unknown, without assistance from the other, by employing the scheme developed by both. General instruction sheets are given to the students a t least one week before beginning the experiment. Since they often begin this experiment before all pertinent aspects of solution equilibria have been covered in lecture, a very general discussion of the various types of possible precipitates and complexes is given on the instruction sheets. Competing reactions, the moderate solubility of certain sulfates, complex anion formation with excess hydroxide or chloride, ammine complexes, and the behavior of the NHa-NH4+ buffer system are some of the items mentioned. No specific information concerning any of the cations to be identified is given. Each partnership's set of four cations is assigned a t this time. The cations are chosen by the instructor from the following list: Ca2+, S++, Baa+, Fe3+, Co2+,NiZ+,Mn2+,A13+,Cra+,Zn2+, Cu2+,Cd2+, Hg2+,Sn4+, Sb3+,Ag+, HgZ2+and Pb2+. Fifteen different combinations of four cations have been used over the past four years. The sets have varying degrees of difficulty, and are assigned according to the student's ability. Two or three of the assigned
Volume 48, Number 2, Februory 1971
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Jovan Pupezin,I Gabor J ~ n c s o , ~ and W. Alexander Van Hooka University of Tennessee Knoxville, Tennessee 37916
The Vapor Pressure of Water: A Good Reference System?
The general features of the phase diagram of water are well known to chemistry undergraduates. I n fact this particular system is almost the required example for introduction to the one-component phase diagram. Also, since the temperature scales are defined in terms of certain fixed properties of water the student gets exposed to its phase diagram again, this time from a slightly diierent point of view. The effect of this continual emphasis on the idea of water as a reference substance is that with one more small mental step the measured (as opposed to the defined) properties become imbued with an aura of exactness which they do not in fact possess. This is especially true of those properties, such as vapor pressure, which are tabulated in the standard compilations (1). We recently became interested in this problem be-
1 Permanent address: Boris Kidrich Institute of Nuclear Sciences, Belgrade, Yugoslavia. 3 Permanent Address: Central Research Institute for Physics, Hungarian Aoademy of Sciences, Budapest. To whom correspondence should be addressed.
1 1 4 / Journal o f Chemicol Education
cause of some work on aqueous (Z-4) and aqueous solution (6) vapor pressure isotope effects which we are engaged in. We wanted precise data on the vapor pressure of ordinary water and ice a t pressures below 10 mm Hg and were amazed to discover, at least a t low temperatures, that the available compilations (1) in some details contradicted one another and sometimes basic theory as well. (For example refs. (la)-(lc), all based on the same calculation (6), equate the vapor pressure of water and ice a t the ice point, not, as would be correct, at the triple point.) A little more work in the library soon showed that for this supposedly well known reference substance there is actually very little data in the important region around the triple point. Furthermore, those data which exist there are old, and the precision is not up to the capabilities of modern methods. We therefore concluded that a discussion of the actual limitations on the experimental data and the implications of these limitations would be of interest to the student. We will be especially interested in the region between the triple point and the steam point but will also pay some attention to other parts of the phase diagram.
