M. V. Olson and J. M. Crawford Dartmouth College Honover, New Hampshire 03755
Semi-Micro Ian-Exchange in the Freshman Laboratory
The determination via ion-exchange of the total ion equivalents in a salt solution is a widely used analytical experiment. The method not only yields good quantitative results hut has the added virtue of exposing students to a chromatographic medium of widespread practical importance. Indeed, the utility of ion-exchange resins, in applications as diverse as the scrubbing of radioactive ions from nuclear reactor effluents to the sequencing of proteins, is sufficient by itself to warrant the inclusion of ionexchange experiments in the undergraduate laboratory. Unfortunately, the procedure that is generally used ( I ) suffers from some practical drawbacks that limit its attractiveness in introductory labs. Two trivial, but chronically annoying problems are associated with the large mesh size resins t h a t are normally employed (usually 2&50 or 50-100 mesh). First, columns prepared from these resins usually have natural flow rates that are much too fast for efficient exchange; good results, therefore, require careful flow rate control at a stppcock. Given the innate impatience of students in elementary laboratories, inadequate flow regulation is frequently a source of systematic error. Secondly, massive disruption of the column by air bubbles results if the liquid level ever drops below the top of the resin bed; once again, the margin for error is smaller than would he ideal in a general chemistry experiment. A more substantive drawhack is that coarse resins require relatively large bed volumes (typically, 2&30 ml), producing a column that is poorly adapted to small-scale analyses. We have developed a semi-micro technique, based on a commercial, research-quality resin, that obviates all these difficulties: the natural, unimpeded flow rates of the columns are slow enough to give complete exchange, the columns can he allowed to "run dry" without disruption of the resin bed, and bed volumes are on the order of 1 ml. This technique fonns the basis for an integrated unit of work, extending over several laboratory periods, which has been used with good success at Dartmouth during the past three years. Students start with qualitative and standard quantitative experiments and then progress to more sophisticated biological and environmental applications. Some of the latter are well-suited for unstructured projects in which students have wide latitude in planning their own work. Qualitative Experiments During the first period each student prepares two columns which are used in all subsequent operations. One of them is immediately employed in a series of qualitative experiments which are designed to reinforce the student's conceptual understanding of ion-exchange phenomena. Using solutions of copper sulfate, hydrochloric acid, and sodium hydroxide the sequence of transformations shown in eqns (1)-(5) is carried out; a t every step the effluent's acidity and sulfate content are monitored, and the location of the cupric ion is noted visually. The reactants present in stoichiometric excess are underlined. 'These results, as well as all subsequent data, were obtained by an experienced worker. 546 /
Journal of Chemical Education
In our experience, this sequence of steps, which requires only about an hour to carry out on a semi-micro column, holds a t least one muprise for most students. Since we start the unit a t the very beginning of general chemistry, students are encouraged to focus largely on the stoichiometry of the transformations; the reversibility of absorption, as in steps (1) and (2), is rationalized qualitatively using LeChatelier's principle. Standard Quantitative Experiments Students develop quantitative proficiency by analyzing known samples of common salts. The samples are individually weighed, dissolved in water, and then charged onto a hydrogen-form column. The displaced hydrogen ions are assayed by a standard titration with hase. This experiment demands facility with the two common methods of determining the number of moles in a sample: measuring the weight of a pure solid of known molecular weight and measuring the volume of a solution of known concentration. BY using salts of varied charze tvoes.. e m.~ h a s i sis also placed on the sroirhiometrv ol'the displacement reaction. The technique, despite its semi-micro scale, 1s caoable of excellent results. Data are presented here for a representative experiment: multiple samples of sodium chloride, barium nitrate, and ferrous ammonium sulfate were analyzed for the ratio of the number of moles of H + displaced to the number of moles of salt charged. The results are indicative of the inherent precision of the method (values given are the means and their standard deviations; the integers in parentheses indicate the number of replications): NaC1, 1.003 0.003 (8); Ba(NO&, 2.007 + 0.007 (6): F e ( N H ~ ) 2 ( S 0 4 h 6 H . 0.06 (4).' - 2.04.02 . . After a few trials, inexperienced students are generally able to obtain agreement of -*I% between their oredicted and observed values. Obviously, if the exercise is carried out in this way, some other basis must he used for grading than the quality of a student's internal agreement. We are encouraged to continue our use of known samples, however, by the observation that many students are strongly motivated to refine their technique until they obtain good results.
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Sea Water Once students acquire proficiency at analyzing standard salts, they apply the cation-exchange method to samples of greater intrinsic interest. Two obvious candidates are sea water and hlwd plasma. The salt concentrations of these two readily available solutions are high enough to make 1-2-ml aliquots ample for a semi-micro total ion determination. Analytical data for representative sea water and blood plasma samples are given in Table 1from liter-
Table 1. Ionic Comoosition of Sea Water and Blood plasma (equivalents/liter).
-=
,,msA
(3.5%salinity)b ~a * K+
0.010 0.109 0.021
Mgg* Cali total cations
HCOz> .". ,. HPOP
o.oo.6 0.005 0.155
0.103 0.027
0.002 nc-
a
".""o
-.
".-&
... ... ..
proteins organic acids total anions
I asma ~ ma'
0.003
0.619 0.559
Cl-
B
0.142
0.479
0.002
0.016 0.006 0.155
0.619
'Includes all components normally present in concentrations greater than lU-"eqiv/l. b Ref. (2). These figures apply to sea water of average salinity, which contains 3.570 by weight of tots1 ionic solutes. r
Ref. (3).
Table 2. Analyses of Sea Water Samplesfor Chloride and Total Equivalent Ion Concentrationsa
Proveneetown, Maas.
0.592f0.002
0.538f0.001
(Cl-)/ (ionhot 0.909+0.004
Salmon Creek,
0.567
+ 0.004
0.520 f 0.001
0.918 -f 0.007
(ion)ct
Source
".I:
