G. W. A. Fowles
The University Southampton, England
Combining Preparative and Quantitative Procedures in Qualitative Analysis
In most British universities, qualitative analysis takes up quite a large proportion of the time that is available for the teaching of practical inorganic chemistry in degree courses, but over the past decade a number of departments have attempted to change the subject matter of the practical courses so as to make them more interesting and instructive. The purpose of this article is to review briefly some of the advantages and disadvantages of teaching qnalitative analysis, and to outline an alternative approach that has been developed a t Southampton University in recent years.' To set the scene, it is worth noting that a student starting on an Honours Degree course in an English university will normally have his General Certificate of Education (G.C.E.) with a t least three good advanced level passes, one being in Chemistry. The student, while studying at school for his G.C.E., will have re-
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EDITOR'S NOTE:We call redem' attention to other articles similar in purpose to this one. For example, alternatives to the typical qualitative anelysis by use of a, separations scheme have been suggested in Tms JOURNAL, by Richard E. Frank, 34, 383 (1957) and by Carl Mahr 38, 407 (1961), and for high school 38,146 (1961). chemistry by William Vasilakes, TEIS JOURNAL,
ceived instruction in practical chemistry for several hours each week for two or three years, and for mnch of this time will have been concerned with inorganic chemistry. Because of the nature of the present practical examinations, most attention will have been concentrated on exercises in volumetric and qnalitative analysis. Many teachers are profoundly dissatisfied with this practical examination, but nevertheless feel that qnalitative analysis may still he worth teaching a t this level since it trains the student in new manipulative skills and introduces him to elementary ideas of equilibria, etc. It is mnch less certain, however, that universities are justified in spending so much time teaching qnalitative analysis a t a more advanced level. Thus although the student may acquire considerable manipulative skill, and learn neat and tidy habits if semimicro methods are adopted, the advantages are less distinct when we come to consider the linking of practical and theoretical studies. I t is not, for instance, of any significant help in the understanding of the more advanced treatments of equilibria necessary at the degree level. The analytical schemes do normally embrace many of the so-called
Volume
39, Number 8, August 1962
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401
evaporate; add o m drop quinine iodide reagent which less common metals, and the enthusiasts for qualitative analysis at the degree level claim that the practical work introduces the student to the properties of compounds of these elements, and helps to correlate many of the less interesting facts of inorganic chemistry. However, there can be little doubt but that the main advantage lies in the ease with which this type of course can be administered and examinations set. It requires no imagination either to set up a series of artificial fouror six-radical mixtures, or to devise and mark a practical examination. It demands little from the average student, who tends to feel that the analysis of such artificial mixtures is of little practical value. Surely, however, we teachers of inorganic chemistry have not so lost our interest in our vocation that we design our practical course in order to make life easy for ourselves (and dull for our students). Examinations must not determine our teaching syllabus, whether it be for theoretical or practical work. In the university world, a t least, where we set our own examinations, we must first decide what should be taught, and then set our examinations accordingly. In considering alternative practical courses to qualitative analysis, we must try to retain as many as possible of the good features of this work. I n our second year work a t Southampton, we have completely ahandoned the formal course in semimicro qualitative analysis, and in its place we have devised a series of experiments that make use of the same experimental techniques but avoid the cookery-book analytical schemes. Our experiments are designed to illustrate the characteristic properties of the elements and their compounds on the basis of the periodic table. The qualitative tests are supplemented by suitable preparative and quantitative work. Up till now we have only produced schemes for the various transition metal groups and for the silicon, nitrogen, and oxygen groups, hut in the coming year we shall extend it to the remaining groups of elements. One feature of our experimental sheets is the inclusion of suitable questions that ensure that the student really understands the significance of the experiments. As an illustration, procedures are given for vanadium and for the copper, silver, and gold subgroup. In devising some of the preparative experiments we are much indebted to various volumes of inorganic syntheses and to textbooks of experimental inorganic chemistry. Thanks are due to Mr. E. Cartmell for valuable discussions. Detailed procedures for the other groups may be obtained by writing to the author. Group VA-Vanadium
Prepare a solution (A) by dissolving about 2 g of ammonium metavanadate in 50 ml 1.0 M NaOH (approx.). Agitate, add 50 ml2 M HBO,, and dilute to 250 ml. @ustion: What reactions occur in the preparation of this solution? Qualitative ezpwiments: (1) Treat am11 volumes of solution A separately with (a) H a , (h) SO*, and (c) Zn. Note and recmd color changes. Questions: What oxidation states are reached in these reactions? What species are present in the final solutions? (2) Dilute a small volume of A several times with distilled water. Add several drops HsOX. Record colm. Test stability of coloration by adding (to separate solutions) small amounts of (a) sodium phosphate, and (b) sodium fluoride.
402 / Journal of Chemical Educofion
Questions: What species produces the color when &OX is added? How does the stabilitv of the vanadium color compare with that of titanium? siggest reasons for your ohservabinnn. Quantitativesurvey of valacy states: (1) To 25 ml of solution A add 2 M H.50. and a. few crystals of sodium sulphite. Bail to remove excess SOa (10 min), then titrate against approx. 0.02M KMnO, at 80°C. (2) To 25 ml of solution A add 20 m l 2 M &SO, and shake vigorously with 2% ~ i n oamalgam in a stoppered bottle until there is no further color change. Decant solution quickly into 50 ml of original solution A, then wash out the bottle with two 100-ml portions of 2 M H$O+ shaking well before decanting each time. Titrate the final solution against the approx. 0.02 M KMnO. a t mDc -. Qaestion: What do you conclude from a comparison of the titres in experiments (1) and (2)? ~~~~~
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~~~~
-"
Group IB--Coppar and Silver
+
1.
4 d 4M NaOH. Heat to boiling.
2.
+ excess 4M NH40H
Questions: W h t oxidation states are present in the above experiments?
Comment on the stability of copper and silver complexes with NH. and CN-. Quantitativeinvestigations: P ~ e p a r a t z aof tn's-(thiourea)coppe7(I)chloride. Dissolve 5 g thiourea in 25 ml hot water. Add 1 g copper powder and 5 ml eanc. HC1. Heat on boiling water bath until copper has dissolved, then filter solution hot. Allow filtrate t o cool slowly, then recrystsllise product from 57" aqueous thiourea. Filter off crystals and wash with acetone. Analysis for eoppw. Weigh out accurately duplicate 0.5 g samples and dissolve in 50 ml warm water containing 2 3 drope conc. HCI. Add 10 ml conc. NH,OH and heat untilprecipitats settles well, then filter through Gooch, washing with cold airfree water. Dissolve in warm conc. HNOs (about 10 ml), add 2 ml conc. HaS04,and evaporate t o fumes of SOs (fume chamber). Cool, dilute to 300 ml, and add 2 gsodium hydrogen sulfite. Heat almost t o boiling and add 20 ml of 2% ammonium thiocyanate by drops with constant stirring. Allow t o stand overnight, then filter on tight Gooch (or No. 4 sinter), washing with cold 0.170 ammonium thiocyanate containing a little SO,; finally, wash with 20% ethanol until filtrateis free from SCN-. From the weights of CuSCN and the weights of the samples of the complex, calculate the percentage of copper in tris(thiaurea)copper(I) chloride. Questias: Suggest a structure for the complex.. I n what way might you be able t o prove this? Suggest an analytical method for chlorinein this compound.
