Elementary QuantitativeInorganic Analysis A Strategy and Some Experiments H. E. Blayden, P. J. Jones, and M. Webster The University, Southampton SO9 5NH, England T h e requirements of a n introductory undergraduate course in practical inorganic chemistry are varied and depend on the aims of the whole course with its mixof lectures, tutorials, and laboratory work. The practical work must be intellectuallv challengine and provide a sound training in the use of laGoratory app&atusfor quantitative and qualitative manipulations for students with disparate backgrounds. he skill'include for our purposes: accurate weighing, quantitative transference of solids and liquids, and the use of volumetric glassware. Project work is commonly used in advanced courses, hut all too often an introductory course consists of a series of independent experiments. Particularly where the students circulate among a limited number of experiments there is the risk of the experiment3 being regarded as very mechanical and simply repeating what a group of students had carried out the previous week. We also have the constraint that the background to many of the physical techniques that would routinely be used in an advanced course has not been presented and the pedagogic value of experiments using these must he limited. There has been over the years considerable discussion over the nature and value of laboratory work ( 1 3 ) ;many individual experiments have appeared in this Journal and a smaller number of descriptions of more radical courses presented (4). We have been offering for a number of years a practical course to our first-year undergraduate students that has eenerallv been well received hv them, and, most importantiy, whileindividual experimenis have changed, it has offered a course that has smod the test o f t ime. A hrief description of the course is given below with the hope that it might be of interest to other organizers, and we also give the syntheses and outline analyses of three of the compounds we are currently using. course Descrlptlon The duration of the course is about five weeks with six hours per week spent in the laboratory (2 X 3 h) with each student workine on his or her own. Students have to plan the work to fit in w s h thelaboratory timetable,and theyworkat their own soeed. The practical work consists of the ytraightforward synthesis of a n (inorganic) compound the composition of which the student is not told, followed by the chemical an&sis of three elements or groups using (1) gravimetric analysis, (2) volumetric analysis, and (3) an instrumental method of analysis. The instrumental methods available include: flame photometer, visibleRiV spectrophotometer (fixed wavelength), and various potentiometric titrations. From the three analyses of the compound, the empirical formula can be estahlished (or a t least the relative proportions of the three species if a total analysis has not been carried out). Normallv the experimental methods being used are first tested by analyzing a suhstance of known composition. At this point the student should have agood idea of the compound that was prepared and can write plausible formulations including a discussion of possible isomers, etc. After completing theanalysis, each experiment is followed by a chemically related extension-up to three short exercises. ~~~
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These may include use of IR spectroscopy on the compound synthesized (if this is informative, e.g., to find water) or the reactions of a metal ion present in the compound. These extensions are freouentlv not completed althoueh most students dosome. d he emphasis is ongetting the aAysesright, and there is time available to repeat that have - procedures . gone wrong. The provided laboratory scripts give detailed instructions. There are some 20 experiments onoffer, and the class numbers are such that in each five.week period any one experiment is heine nerformed bv no more than two students. This -~ has tangible benefits withihe students talking to each other ahout common oroblems without feeline- part - of a r e ~ e t i tious exercise. Within the framework of the course, new experiments can be added and others withdrawn-a static course will rapidly decline. Suitable compounds covering a range of elements and types have proved quite elusive. Compounds should be (1) air-stable and easy (and inexpensive!) to prepare in high yield, (2) capahle of being analyzed by three methods a t the beginner level with the apparatus available. Examples from the literature that we use include a copper oxalato complex (5)and a cobalt ammine (6).T o illustrate these ideas we give below three syntheses of chloroanions and brief notes concerning the analyses. e.
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Potassium Hexachlorostannats(lV) (7a) Prepare a solution of tin(1V) chloride hydrate (SnC4.5H20,l.O g) in dilute hydrochloric acid (2M, 10 mL) by warming. Dissolve potassium chloride (KCI, 4.5 g) in the minimum af hot water (-15 mL). .. mix the two solutions. add conc. hvdrochloric acid (-2 mL) and cool to room temperat&e. Stand th; mixture m rce,water for -30 min, and, if not much solid hna formed, add a little more conr. hydrochloric arid. Filter off the white crystals an a sintered glass crucible using suction. Suck off as much water as possible on the pump, and place the solid on a wateh~lassin a desiccator uuntsining silica gel until dry (1-2days). Record the yield ( ~ a m sand ) place the omduct in a labelled soeermen hottle. ITwical student yields 4.0Ammonium HexachlorotelIurate(IV)(7b) Using a beaker, dissolve tellurium dioxide (TeOn, 4.0 g) in the minimum amount of cane. hydrochlorie acid (-16 mL) by heating. Prepare a separate solution of ammonium chloride (3.5 g) in the minimum volume (-5 mL) of hot water. Slowly add the hot ammonium chloride solution to the hot tellurium containing solution. (If s precipitate forms at this stage, gently reheat until it dissolves.) Add a few drops of cane. hydrochloric acid to the hot solution until there is a permanent precipitate, cover with a watch glass, and allow to cool to room temperature. Slow cooling often gives nice crystals in rather low yield. If not much solid has formed, carefully add eonc. hydrochloric acid, and leave the mixture to stand. Suction-filter off the bright yellow product using a sintered glass crucible (no. 3), wash with concentrated hydrochloric acid (3 x 5 mL), and continue the auction for another 5 min to remove the maioritv .~~~ . . of the liquid. Transfer to a watch glass, and store for wversl days in a desiccator containinga small heaker of potassium hydroxide pellets inaddition ~
to the silica gel. When dry and free from HCI, record the yield (grams),and transfer to alahelled specimen bottle. [Typicalstudent yields 5.57.0 g.]
Cesium Pentachloroaquoindate(III)(8) Weighout 1.1g of indium metal, and cut into small lumps. Usinga beaker (50 mL),dissolve the indium in conc. hydrochloricacid (-15 mL) by warming gently on a hot plate. Take cesium chloride (CsC1, 3.4 g), dissolve in about 4 mL of distilled water again warming gently, and then add conc. hydrochloric acid (-8 mL). Add the cesium chloride solution to the indium solution, and allow to cool to room temperature. Filter off the solid product using a sintered glass crucible (no. 3), wash with conc. hydrochloric acid (3 X 5 mL) followedby ethanol (3 X 10 mL).Air-dry the product after transferring to awatch glass. Finally transfer to alahelled sample bottle, and record the yield (grams). [Typical student yields 4.5-5.5 g.] Analyses T h e analytical procedures have in t h e main been taken from standard texts. KfinC16 Tin is determined gravimetridy using theN-benzoyl-N-pbenylhydroxylamine reagent (9); after hydrolysis, chlorine is determined volumetrieallyby titration against silver nitrate using an absorption indicator (dichlorofluoresceiu); potassium is determined using a flame photometer. One extension consists of the reactions of Sn(I1) and Sn(1V). [NHd JeCle Tellurium is determined gravimetrically by reduction (Sod to the element (9); using a Kjeldahl apparatus, ammonia is determined hy boiling out after the addition of NaOH, collecting the vapor in an
aqueous boric acid solution and titrating against standard acid; chlorine is determined after hydrolysis by titration with silver nitrate using a patentiometric method. One extension consists of the reactions of the tetraoxotellurate(1V) ion. Cs2[lnCI~Hfl] Chlorine is determined gravimetrically as AgCL; indium is estimated by a volumetric titration against standard EDTA using 1-(2pyridy1azo)-2-naphthol (PAN) indicator in the pH range 2.3-2.5 (10); cesium is determined using a flame photometer (Corningmodel 400, no filter used, calibration solutions 0-40 ppm Cst). In the extension the presence of water is shown hy IR spectroscopyand by quantitative weight loss experiments. Unlike other parts of our practical work, this course is not linked directlv to anv lecture material a n d can thus be undertaken in a& partof t h e teaching year. It is also conventional. b u t t h e satisfaction is verv real for student (and staff) when t h e results are good. copies of more detailed analytical instructions are available from the authors. ~~
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Literature Cited
163. 5. Dsr1ey.J. R.; Hoppe, J. I. J. Chem.E&c. 1972,48,36b366. 6. Schlessinger, G. G. Inorg. Synfh. 1967.9. 16CL163. 7. Brauer. G., Ed. Handbook of P ~ p o r o f i u alnorgonic Chemistry, 2nd od.: Aeedemic: New York. 1965; Vol. I , (a) pp 731.732. (b)pp 44P448. 8. Cislason. J.;Lloyd,M. H.;Tuek. D. G.Inorg. Chem. 1971.10,1907-1910. g anm o liyi r i 8 , 4 t h d . ; L o n g i i i : L ~ ~ d d d , 9. VogeI,A.I. T e i l b o ~ k ~ / Q ~ ~ n t i f o f i i i I ~ r A 1978. lo. west,T.S.Comple~omefry.3dd.;BOHChemicals:Pwle.England, 1 9 6 %186. ~ See a h Cheng, K. L. Anolyf. Chem. 1955,27,15821583.
Volume 67
Number 4
April 1990
343
