letters Audio Tapes To the Editor: Many of the young faculty in our department are always anxious to read the J o u m l because of its excellent articles. For example, the article "Old Technology Revisited, The Use of Audio Tapes" by David Goodrick [J. Chem. Educ. 1993,70,511 was very interesting. About 10 years ago, a student with dyslexia helped me to develop another use for audio tapes. He could not make satisf&ory exam grades when hchad to read the trsts. So I had a student assistant rerord the ttst qurstlons onto an audio tape. The student then listened to the tape and read along. With this reading assistance, he was able to understand the questions and, thus, answer effectively. His test performance after the use of the tapes improved to the top 15 ner cent of the class. Afterwards. the .aeatest difficultv . in helpingstudenrs with dyslexia w:ts to h w r thmi d o r m me of their need for this helpful uwchlng assistance. F. L. Himes Department of Agronomy Ohio State Univemity Columbus, OH 43210
Environmentally Acceptable Determination of Iron To the Editor: In a recent article [J. Chem. Educ. 1992, 69, 9351 the authors describe "AMore EnvironmentallyAcceptable Determination of Iron in an Ore". In this procedure the ore is dissolved in concentrated HCI, the iron reduced with Zn, and then titratedwith a standard solution of KMn04.This experiment did not go far enough. The authors missed a golden opportunity to elevate the procedure to modem day techniques. As I nointed out in this Journal some vears aeo IJ. Chem. ~ d u l980,57,7031 : an appropriate ~ ~ $ ~ ( ~ e t Estandodof ard Additions) would increase the accuracv and recision of this experiment while reducing the investment in time, chemicals. and ~rocedure.It is Dossible to do this entire analysis i i one ~ r l e n m e ~flask,-eliminate er all liquid transfers.. perform the Drocedure exactlv as described except usiw . simple volum~triiequipment such as graduated cylinders fbi all work including the vrcpara~ionof'the titrant KMnO,, use only one calibraGd b&e< eliminate the standardization of the titrant completely, and still allow for an increase in accuracy and precision. All one has to do is to weigh out a length of pure iron wire with each ore sample (weighed separately), prepare the titrant with the approximate concentration required (graduated cylinders will be fine), then follow the exact procedure for the iron ore as before. The arithmetic wiU look as follows.Assume S1to be the weight of pure iron in the first sample, Wl to he the weight of ore, and P to be the percentage of iron in the ore. Let Vl be the volume of titrant r e quired. Then: Vl == K (Sl+P Wl) Assuming three determinations there will be two other equations like the one above. If the weights are given in milligrams, then the constant Krepresents the number of milliliters of titrant per milligram of iron. If the entire equation is divided by S1then the following is obtained: Vl -='Wl --K+ -
Sl Sl The equation above is that of a straight line with slope equal to K P and intercept equal to K. With three determi-
860
Journal of Chemical Education
nations there will be three such eauations and a simple linear least squares can obtain the siope and intercept A d the standard deviations of both. Dividina the slope bv the intercept gives the percentage of Fe invthe ore.-one can equate the standard deviation of the intercept with the constant K, and that of the slope with P, thus the errors can also be determined. Lest one be concerned that the concentration of the titrant has been forgotten, it is included in the value of K. The calculation of the number of milliliters of titrant for one milligram of Fe is straightforward a s shown below where M i s the molarity ofthe titrant. K= mL titrant = 1mg Fe x -x
lgFe 1000 mg
1mol Fe 55.85 g
1molKMn04 5 mol Fe
M
The only caveat here is that sample calculations should be done in advance to insure that 40-45 mL of titrant is used. For 0.05 M titrant the value of K is approximately 0.07 mWmg. The weights of pure iron and ore should differ for each run to minimize the errors in the arithmetic itself, hut their sum should be less than one a a m to keep the buret on scale. Note also that aging of the titrant should have a minimal effect on this procedure.
Morris Bader Moravian College Bethlehem. PA 18018 A Marrakchi Experience: In the Teaching of Crystal Structures To the Editor: The note by Martin [J. Chem. Educ. 1992, 69, 4951 was valuable in pointing out one of the problems encountered in the teaching of crystal structures and sharing the experience he developed to overcome it. As a lecturer at the Faculty of Sciences Semlalia of the University Cadi Ayyad of Marrakech (Morocco). I was facina manv facets of the problem such as the lack of three-dimensionaivision by students and the understandina of the constitution of crystals. However, at the beginning if the academic year 1991-1992, I started a new experience with my colleagues. Before giving any lecture or tutorial on the subject, we started by inviting our students to take a laboratory course during which they were given simple explanations on the three-dimensionalityof crystallime materials. In addition, hall bearings were used by students, under intensive supervision fmm lecturers, to form the close-packed structures, and a cage-like cube was used to introduce the basic cubic structures (simde. centered. and face-centered) and to explain the unit cell hotiou, how atoms are shared be: tween unit cells, their number in a unit cell, and the coordination number. This laboratory class was successful and enabled us to overcome, at least partially, the difficulties of explaining crystal structures to our undergraduate secondyear students durina lectures and tutorials. Back to Martin's note. Incidentally,the reference given by him contains a typo; it should be as follows: Ho, S:M, Douglas, B. E. J. Chem. Educ. 1969,46,207. Ahmed Learouri Vlsltlng Professor on a FL brlgh;~ward Depanment of Chem ca Englneer ng Iowa State University Ames. IA50011
