( The Preparation and Characterization of Lawrence E. Conroy
university Minnesota Minneapolis. 55455
II
a Sodium Tungsten Bronze An inorganic experiment
The development and production of solid state materials such as semiconductors, ceramics, magnetic materials, lasers, phosphors, glasses, and superconductors is a significant part of the chemical industry and an important source of careers for chemists. The techniques of solid state and high temperature chemistry are widely utilized in these fields. However, most undergraduate (and graduate) programs in chemistry provide little or no introduction into the concepts, methods, and equipment that are required in this type of work. The development of any laboratory program in undergraduate chemistry is limited by a number of constraints. Suitable experiments must he reasonably safe, brief, reliable, and inexpensive. In addition to illustrating chemical principles, the program should provide experience with a variety of materials and techniques. Particularly in the more advanced laboratories, the experiments should be representative of the problems that are encountered in modern industry and research. This article describes an experiment comprising the synthesis, analysis, and characterization of a sodium tungsten bronze. Such an experiment has been a regular exercise in the Inorganic Chemistry course a t the University of Minnesota for the past 15 years. Several aspects of the experiment provide experiences for the student that are unusual, or a t least atypical, for an undergraduate laboratory. First of all, the experiment demonstrates the phenomena of (1) a reaction in amolten salt mixture, (2) a "uonstoichiometric compound,'' and (3) an inorganic oxide that exhibits metallic properties. Secondly, the experiment utilizes the techniques of (1) high temperature (8W10IXl0C) synthesis, (2) crystallization from a molten salt, (3) oxidation-reduction in a molten salt, (4) powder X-ray diffraction, and (5) analysis by high temperature volatilization andlor a specific ion electrode. In addition, a provision is made for the student to make an "internal" corroboration of his analytical results by a t least two independent methods, one chemical and one physical. T h e physical analysis method, utilizing X-ray diffraction data, also provides the instructor with a very convenient way to check his students' analysis results against unsullied original experimental data. The experiment described below is the latest version that has evolved from experience with approximately 500 junior and senior undergraduate chemistry students over the past 15 years. Some evidence that the tungsten bronzes are still of current research interest is the appearance of a t least seven publications on these substances in the scientific literature since 1974, while four review articles are available in the past eight years
bronzes are crystalline solids having a metallic luster. The name "hronze" was applied by Wijhler, who first made these suhstances (51, because the typical preparation exhibits a metallic orange color like bronze. At least three different crystal structures are found in the sodium tungsten bronze system. If x in Na, WOa is less than 0.1, approximately, the structure is essentially that of pure WOa (Fig. 1). This diagram shows the idealized Won structure, consisting of W06 octahedra that share corners (oxygen atoms) with adjacent octahedra. Thus each tungsten shares each of its six oxygen nearest neighbors with another tungsten atom to yield the overall atom ratio of one tungsten to three oxygen atoms. The actual WO.2 structure is a sliehtlv distorted version of this structure. In the composition range 0.1 < x < 0.35 the bronze crystallizes in a tetraaonal structure which is re~resentedschematicallv in the lower part of Figure 2. This figure is a top view of thk tetragonal structure where the Woe octahedra are represented as octahedral units with the sodium atoms in the indicated A interstices of the WOs lattice. The tetragonal unit cell contains ten tungsten atoms with various numbers of sodium atoms, depending upon the value of I. For compositions in the range 0.35 < x < 1 the crystal structure can he described as a defective form of the perovskite structure, shown in Figure 3, where sodium atoms occupy a fraction of the 12-coordinated positions in the W01 structure. Stoichiometric NaW03, i.e., with x = 1, would have the true perovskite structure. For comparison, the cubic perovskite structure is shown schematically in the upper part of Figure 2. The sodium content, or x value, of a particular crystal is determined by the conditions under which it is synthesized. The sodium content can also he changed by allowing sodium to diffuse into or out of the lattice a t high temperatures. The tungsten bronzes are truly beautiful substances, ranging in color from gold for x values near unity, through orange (bronze), red, violet, and blue for compositions with
(1-4). Description of the Experiment
The type of substance to he prepared in this experiment is usually formulated as Na, WOs where 0 < x < 1.This formula is intended to indicate that the substance, or series of suhstances, deviates from fixed atom ratios, and it may he described as a non-stoichiometric compound or solid solution. Solid solutions in WORare also known for the other alkali metals, and for Ca, Sr, Ba, Al, In, T1, Sn, Ph, Cu, Ag, Cd, the rare earth elements, H+, and NH4+. The sodium tungsten
Figure 1. me idealized WOs crystal structure.
The material in this article was presented in part in the Symposium
Figure 2. Stylized representations of the cubic (tap) and tetraganal (bonom) tungsten bronze crystal snucture. me squares with diagonal crmses represent a top view of Woe octahedra.
on Teaching Solid State Chemistry at the 1974 Annual Meeting of the American Chemical Society held in Atlantic City, N.J., September 1974.
Figure 3. The cubic perovskite structure. This is the same structure as is represented at the top in Figure 2.
Volume 54. Number 1, January 1977 / 45
progressively lower sodium concentrations, and exhibiting a brilliant metallic luster throughout this range. The crystals usually form as more or less perfect cubes, depending upon the cnnditions of the crvstallization process. In addition to the luster the bronzes exhibit other typically metalltc properties such as high electrical condurtivity and weak temprratureindependent paramagnetism. Sodium tungsten hronzes can he prepared by reducing molten mixtures of sodium tungstate, Na2W04, and tungsten trioxide, WOs, a t 8C%120O0C. Reduction can he accomplished by hydrogen gas, electrolysis, W02, or metals. In this experiment electrolysis is the most convenient method, although alternative procedures using W metal or WOz are also described later. The electrolytic reduction can he described by the equation ~
~~
The reaction with tungsten metal can he formulated as 3xNanW04+ (6 - 4x)WOa + rW = 6Na,W0> and that with tungsten dioxide as
(2)
+
xNazW01+ (2 - 2x)W03 xWOz = 2Na,W0~ (3) The sodium content of the hronze, i.e. the value of x, is a function hoth of the quantity of reducing agent and of the Na:W ratio in the melt. In practice an excess of sodium over that indicated by eqns. (1-3) is necessary to achieve a given value of x, so that those equations are only approximations of the overall reactions that yield the hronzes. The Na,WOs is only. slightly . soluble ~ in the melt and crystallizes as the nmcentratim increaws or as the melt is cooled. The ~~nreacted material in the coded melt is ioluble in dilute alkali or oxalic acid solution and can he leached from the bronze crystals. For this experiment all assigned compositions are designed to yield only the cuhic perovskite crystals (i.e. 0.35 < x < 1)so that the analysis of the X-ray diffraction pattern is made relatively s~mple. Alter the crystab we prepared and separated from the melt, the student is reuuired to analvze his product hoth t)y chemx - r a y diffractometry. The ical techniques and by chemical analysis is carried out first, and the results must he approved by the laboratory instructor before the X-ray analysis is begun. Solution analyses of tungsten hronzes are not easy to perform. The hronzes are notoriously difficult to take into aqueous solution. Only strongly alkaline solutions are capable of dissolving Na, WOs, and the subsequent determination of sodium is complicated by the presence of large amounts of another alkali metal such as potnssium or lithium. Alternatively, the hronze may he decumposed by fusion with i 4 KNO ,-KS'O , mixture to ~ r o d u c ea Water soluhlr rlroduct, -,, hut this method also suffers from the large excess of potassium. However, the awdahtlity of speritic sadium ion electrodes makes ~ossihlethe determination of iodtun~in tht, presence of p o t a k n n , and this method is now utilized in the bronze analysis procedure. Alternative analytical procedures avoid the use of aqueous methods and resort to high temperature gravimetric techniques. The oxidation method utilizes the fact that the sodium tungsten hronze will be oxidized by heating in air a t 700°C. -
-
&
~
2Na,WOa +?O2-zNa2WO4 2
+ (2 -x)WOB
n = 3.7845 + 0.0820x (6) where a is the lattice constant in Angstrom units. An accurate determination of a thus permits the calculation of x . The student must submit his original X-ray data (film or diffractometer tracing) along with his analytical results and hronze sample. The lahoratory instructor can then calculate the true x value from the original X-ray data and use this information to grade both the student's chemical analysis and his lattice constant calculations. Thus the student is provided with an internal check on his analysis, and the instructor has, in the form of the X-ray data, a reliable analysis upon which to evaluate the student performance. It can he noted that, aside from the X-ray diffraction apnaratus. the eauioment reauirements for this experiment are .. modest. The most exotic items are the specific ion electrode and meter. a crucible furnace. and. oossihlv. a tube furnace. The X-ray data, while essential, neeh nut beohrotnrd by the student himself. The data can he rerorded in another 1at111ratory, even one outside the institution. Only the analysis of the X-ray data by the student is required in the experiment.
Procedure
The svnthesis nrocedure that is described here makes use of electrhytic reduction. Alternate procedures for reduction with W metal or WO? are described later. Similarlv the HC1 analysis procedure is described as an alternative for analyzing ,the bronze. Preparation of a Sodium Tungsten Bronze Approximntt.ly PO :: of Na.W03.2H .O i.i dried nr 110' fin 1 hr. .4 mixture i l i SniM'O; and M'Oi, wrighng appnlximnrelv RO 8.1spreonrrd aremime totht Na:M'rati.,thdt ia3sirnrd hvthc,nsrrurtm N ~ : Wratios between 0.85 and 1.40 are appropriate. The dry ingredients are ground together in a mortar to provide thorough mixing. The mixture is transferred to a porcelain crucible (4 cm, or larger, diameter),and this crucible is inserted in a second, identical crucible to contain the melt, should the first crucible break. The assembly is placed in the electric crucible furnace and heated to 850°C. A clear melt should result. The cathode is positioned horizontally within a few millimeters of the bottom of the crucible as shown in Figure4. A a 1-em2sheet satisfactorv cathode can be made (1) . . hv .soot-weldine . of 5-mil Nichrome sheet to a length of no 20 AWG Nichrome wire, or (2) by twisting Nichrome wire into a flat spiral approximately 1.3cm in diameter. The best anode material is sheet platinum. A square of roughly 1.5 X 1.5 em is spat-weldedto a 5-em length of platinum wire. he olatinum anode is suooorted horizon tall^. .iust below the surface .. of the melt. Electrolvsis , is carried out. wine direct current at a current r l m r q d X1(1 5 . 0 nn.4 rm!nr r h ~ c ~ 1 h & ; ( i ~ h 1 0 ( 1hrthe l m ~ 2rm rlwtnldr cwnplrtely immeriedl. The elcm~derlrruir i h d d he fused l i l r 1 .4. If lar,wr ,ryirnls ere sought. a l u w r nlrrmr ~ L n s l ia ~ \nwdrd. ~~
~~
~
~
(4)
This equation will he recognized as the reverse of (1). A value for x can he calculated from the weight change due to oxidation. However, this method is subject to considerable error because of the relatively small change in weight per gram of bronze that is oxidized. Another high temperature, nonaqueous analytical method involves the volatilization of tungsten oxides by HC1 gas. If the oxidation products from the hronze (eqn. (4)) are heated a t 650°C in a stream of dry HCI, sodium chloride, water, and volatile tungsten, oxychlorides are formed 46 / Journal of Chemical Education
IxNazWOI + (2 - x)WOs + (4 + 2x)HCI(,, = 2xNaCI+ 2W02C12,~)+ (2 + ~ ) H 2 0 1 ~(5) ) The oxychlorides and water are volatilized, and the residue is weighed as NaC1. The purity of the NaCl residue may he verified hv titration for chlorine. The X-ray diffraction procedure for analysis of the hronze is dependent upon the fact that the perovskite cuhic unit cell constant is linearly related to the sodium content of the bronze: i.e. the hronze svstem ohevs Vezard's Law. The lattice constant dependence upon x hasbeenwell established (6,7) as
Anode
Ceramic ln5Ulolor ) IP*
+2
Crucible
Figure 4. Diagram of
the electrolysis cell
Cathode
INichromel
Alternative Synthesis Procedure A sodium tungsten hronze may be prepared by reducing a NanW01-WOa melt with tungsten metal (eqn. (2))or W02 (eqn. (3)). The following procedure for the tungsten metal reduction may he easily modified to use WOz. Tungsten dioxide is available from commercial sources hut is quite expensive; however, i t can be readily prepared by heating W0a and W metal (2 mole WOs t o 1 mole W) under vacuum or in areon a t 950PC for 40 hr. (Reference (81 eives details for this orocedu~e.1The oroduct shouldbe " eolden drown. A purple product indicates the presence of W180d9and is evidence of insufficient heating. Prepare 60-80 g of a mixture of Na2W04,W03, and W in one of the mole ratios listed in the table. A particular mole ratio is assigned to each student by the laboratory instructor so that a wide range of compositions will be prepared in the class. Note that the given mole ratios all contain an excess of NazW01 over that required for eqn. (2). The excess salt serves as a flux or solvent for the mixture. The drv ingredients are thoroughly mixed, preferably by grinding in a mortar, and themixture is transferred to a 4 cm diameter,or larger, porcelain crucible fitted with a cover. This crucihle is inserted in a second, similar crucible to contain the melt, should the first crucihle break. The assembly is then heated in a covered crucible furnace (or, less conveniently, in a muffle furnace) to 1050DC.An atmosphere of nitrogen or argon over the crucible willsomewhat improve the yield of hronze. but this measure is not essential. The temoerature should first he held near IXKV"': furnt least R hr; rnen rne trmperatuw ii; gradually reduced in increments of 10' w a r y 5 rnin, tu 900°C. l'hc fdrnare power is then shuc irff, nnd the melt isallwred to cwl m4UiJSCwnhmt opening the furnace. Any nitrogen or argon atmosphere may be terminated when the temperature reaches 700DC. The procedure for cleaning the bronze crystals is the same as that described above for the electrolytic The hronze crystals will . svnthesis. . be found, usually as a mass of intergrown cubes, a t the bot&m of the crucihle.
During the electrolysis,bubbles of oxygen will be evolved a t the ancde, and this process is convenient evidence that reaction is proceeding. Continue electrolysis for 1.hr. During the electrolysis, there occasionally will aeeur growth of dendritic crystals that rapidly bridge the eao between the electrodes. Because the bronze is a eoad conductor. t h ~ prmew s willshort-circuit the electrod~s.Often the only evidence will he the cessatiun uf oxygen wolutlcm, but orca*ronally s massive .;hurt will blow the fuae. Ira eithercnse, the remedy rs todislodge the crystals witha quartz or alundum rod. At the conclusion of the electrolysis, the electrodes are removed from the melt and allowed to cwl. Many bronze crystalswill he found adhering to the cathode, but many more will have been dislodeed and fallen to the bottom of the crucible. The crucibleis removed f&m the furnace with tongs, and the melt is allowed to cool. The solidified excess reactants can he removed from the crystals and crucihle by heating them in 200 ml of 15%oxalic acid solution a t 80-lW°C for 2 hr. A 10%NaOH solution or 10% NasC03 solution will also dissolve the unreacted material though a little more slowly than oxalic acid. The hronze crystals are found, usually as a mass of intergrown cubes, on the cathode and a t the bottom of the crucihle. The crystals are rinsed with successive portions of water, dilute HCI, and dilute NHs. if the sample is contaminated with porcelain chios from the crucible. i t mav be treated with 48% H F in a prdyrrhvlene henker (Caution: Rubber gloves and fare protection a r e accrrsary, a n d t h e HF treatment must be carried out in a ventilating hood). The product iidried at 1 1 0 T and wrighed.
~..
~
~
Chemical Analysis of the Bronze Composition Three finely-powdered samples of the hronze, 0.2-0.4 g each, are weighed into three separate glazed porcelain crucibles. T o each erucihle, is added 2 g of a mixture of K&03 and KNOS ( 1 g K2C03to 1 g KNO:3. Each crucible is heated on a clay triangle with asmall burner flame until the potassium salts melt. On continued heating the hronze will dissolve in the melt to yield a clear liquid. The crucible is allowed to cool, and the outside surface is rinsed with water. The crucihle is then placed on itsside in a250-ml beaker, along withsufficient water to completely immerse it. The water is acidified with 6 M HNOa takine m e to avoid a t w vieorous evolution of Con and loss of samole hy rpatrenng. Exresa acidity ir, netrtralized hy adding I Af KOH dropwlse until the pH = LO. approximately. The solution is rhrn quantitatively transferred to a 250-ml volumetric flask and diluted to volume. This solution may he transferred to a bottle for storage until the sodium analysis. Sodium is determined with a sodium specific ion electrode. A standard 0.0100 Na+ solution is used to calibrate the electrode and meter. The standard solution should also be 0.09 M in KNO~ and . . 0.008 .U in K.\VO, toappmximnte any interferenre by those ions in the analyria aolutron. The d i u m mnrentrntions for the three parallel samples are determined and the mean value is used to calculate the x value for the Na, W08 preparation.
.
~
X-Ray Diffraction Analysis of the Bronze Composition The X-ray diffraction p a t t e r n of the sodium tungsten hronze preparation is obtained from a finely powdered sample, using either a diffractorneter o r a photographic technique IDehve.Scherrer. Cuinier). T h e s t u d e n t is instructed t o det e r m h e the t rag^ angles from the d a t a and to calculate the cubic cell constant, a. For t h e purposes of this experiment, a reasonably precise value of a c a n usually be obtained b y averaging the values from all reflections. However, if the results show a systematic trend o r considerable scatter, it is preferable to e m ~ l o van e x t r a ~ o l a t i o ntechniaue (e.e. Cohen e x t r a ~ o l a t i o n ) . F r i m t h e celiconstant, the s c u d e n t i h e n calculates x i n his Na,WOn o r e ~ a r a t i o nbv usina ean. ( 6 )and assumine t h a t t h e W O ra& remains 1:3;as i t does in t h i s preparatioi.
~
Optional Analysis Procedure I: Oxidation A 1-2-g sample of fmely powdered bronze is accurately weighed into a porcelain combustion boat (6-8 cm length is suitable). The powder should be spread in a thin layer over the bottom of the boat to expose maximum surface. The boat is placed in a quartz tube in a cold tube furnace, and the furnace is heated to 700'C toconstantweight (about 3 hrl. The samole will he oxidized t o a vellow-white oowder of Na2\V04and \\%I ieqn. (4,). The boat and sample are remked from the t u b furnace, nllowed to cool to room temperature, and rrwclghed. The value oi r in Na,\VO? is then calculated !rum the weight gain. Optional Analysis Procedure 11: Voiatization of Tungsten as WO*Cl, T h i ~analysis prucedure is r r p r e w m d hy eqnr. (41 and 151.Two finely powdered jamplei of the hnmze, weighmg 2 U 0 0 mg pnch are
Male Ratios for the Synthesis Procedure Mixture
Mole
Mole
Mole
Number
Na.WO.
WO,
W
Properties of the Sodium Tungsten Bronze The chemical properties of the bronze are examined by testing with a,mmon rragentr. A crystal of the hronne ia boiled for 10 mi" in each of the rsagrntr, runcd HCI, cvnrd HAO4, ronrd HKOI, aqua r w n . rmcd NH ,. and l u n NaOH. and the effects are observed and re. corded. as pro xi mat el^ 1g of NazC03 is melted in a crucible and a hronze crvstal is added to the melt. This orocedure is reoeated with '
resistivity of a bronze crystal is measured with a sensitive ohmmeter.
I
HCI Gas
Bubbler
i
Figure 5. Diagram of the apparatus for analysis by volatilization with HCI. Volume 54. Number 1, January 1977 / 47
weighd inrotwuctmhu~tionh a w . 1Mrmlen~thboatzaresuiuhlel. The w,wdt.r is~preadin a thin lnycr uver the hutum of each lloar. l'he boats are placed in a quartz tube in a cold tube furnace, and a mixture of air and dry HCI gas is passed through the tuhe. The temperature of the furnace is gradually raised, over a period of 20 min to 650DC. A flawof roughly 50ml HClImin is maintained for the first 2 hr; then the rate is reduced to roughly 25 mllmin for two additional hours. A convenient arrangement for the HCI train is shown in Figure 5. A bubbler, containing coned HzSOaor halocarbon oil serves as e ratemonitoring device. The water aspirator provides a slight vacuum (-5 Torr below ambient) to draw air through the tube to oxidize the bronze (eqn. (4)) and to provide the pressure differentialtocause the volatilized W02Clz(eqn. (5))to move out of the hot zone and be dewsited on the cold walls of the mart2 tube. The water asoirator also brovides a convenient method of disposal for the excess HCI. A glass or polyethylene aspirator should be used. A manometer, filled with halocarbon oil. is used to monitor nressnre. A t the completion o i the \.olatil~zation,the furnace is allowed to mol. nr~dI ht. H('I flwis stopped u hen t h temperature ~ has dropped to :lOO'C. The res~duein the hoxs should be pure white NaCl. A n v yellow or green coloration indicates incomplete conversion to ~ a c i , and, in such a case, the sample should be reheated in HCI at 650°C for 2 hr. A black or brown residue may sometimes occur when eommercial HCI is used and is apparently caused by hydrocarbon impurities. Ignition in air at 650°C will eliminate the colored substance. After weighing the boat and residue, a final check is made on the purity of the NaCl by dissolving it in water and titratingwith standard silver solution by the Mohr method. From the NaCl weight the bronze formula can be calculated from eqn. (5). Student Results and Conclusions This experiment proved to he one of the more successful exercises in our undergraduate laboratory course in inorganic chemistry. Since it is a quantitative experiment, its utility must be judged in part by students' success in determining the composition of their pre~arationhv both the chemical and x-ray diffraction meihods. The data presented in Figures 6 and 7 are taken from the records of our classes during 1973, 1974, and 1975. Figure 6 represents the results of student determinations of the value of x in Na,WOs hy the X-ray method in which they determine the cuhic cell constant and calculate x from the Vegard's Law equation (eqn. (6)). The ordinate shows the value reported by the students, and the abscissa shows the "true" values of x determined hv the instructor from the original diffractometer data obtained by the student. It is evident from this plot that most students are reasonably successful in this part of the experiment. It shwlld he pointed out that the data in Figure 6 do not reoresent all preparations by our students d u r h g the three-year period, ~~
~
~~
~
~
~.~ ~~
~~~
~
hut only those preparations that had the cuhic perovskite bronze structure. A small fraction of each class (5-lo%, roughly) will prepare samples that are in the tetragonal phase region because of errors in preparing their original melt compositions. Usually in such cases the student will discover the fact that his crystals are not cuhic in time to repeat the synthesis. Figure 7 shows the results obtained by students in the chemical methods of analysis of their bronze samples. Again the abscissa represents values of x in Na, W03 calculated from the original X-ray diffraction data hy the instructor. T h e ordinate represents thex values reported by the students from the oxidation method (circles) and the fusion method using the specific sodium ion electrode (crosses). The considerable scatter evident here is, of course, the result of all possible sources of error in the two analysis procedures. However, i t is encouraging and significant, I think, that the values do cluster around the line of slope = 1. Figure 8 is a representation of values obtained several years ago by students using the HCI volatilization method, described ahove under the optional procedures. I n this plot all data are those reported by the students. T h e abscissa shows values of
"Trui'x value
Figure 7.Plot of x values in Na.WOz obtained by student chemical analyses (ordinate)against vaiuees obtained by X-ray diffraction (abscissa).The circles represent results of the oxidation method: lhe crosses represent results from the fusion-ionelectrode method.
"True"x value
a40
a50 a60 a70 I
I
ago ago I
I
LOC
x from lattice constant
r -0.40
Figure 6. Plat of xvalues in Na,W03 obtained by student analyses of tungsten bronzes preparations by X-ray diffraction.The ordinate represents results reported by students: the abscissa represents results calculated by the instruo tMS.
48 / Journal of Chemical Education
0.50
0.60
0.70
0.80
Q90
LOO
Figure 8. Plat of x values in Na,W03 obtained by student analyses using HCI volatilization (0rdinate)againstxvalues obtained from X-ay diffraction analyses (abscissa).
x in Na,WOs calculated by the student from his X-ray diffraction data and using eqn. (6). The ordinate represents, in each case, the same student's calculated value of x from the HCI volatilization determinations. It is evident that. assumina the X-ray data to be more likely correct (see Fig. 6), the HCI method tends to vield low results for the sodium concentration. Turning now to a more subjective assessment of this experiment, I believe that i t has shown itself to be a very worthwhile laboratow exercise in inorganic chemistry. All of the objectives that we set out in the beginning paragraphs of this article are met hv this ex~erimentto a considerable degree. It may be argued that the equipment requirements for this ex~erimentare not trivial, but of these only the X-ray diffraction apparatus is a really major item and, as I have already pointed out, this service may be available in another location. Student response to this experiment has been highly favorable. Every year our students have rated this their favorite among theexperiments performed in our laboratory.
(The term "favorite" as used here includes both the "mostliked" and "least-disliked" ratings by students.) Among the reasons moat often cited for preferring this experiment are the pretty colors, the crystal shapes, the employment of high temperature and molten salt synthesis, the unusual analytical procedures, the use of X-ray diffraction, the opportunity to check their analvses bv two indenendent methods. and some confidence thaithe eiperimentcan be fairly graded by the instructor. Obviouslv the facultv likes this exneriment. We have used it regularly for 15 years. Literature Cited (1) (21 I:il 141 151
S.,Uuort.
llirkens. P. L a n d Whittinghsm. M. Re". C h e m S,,c.. 22.30 119681 Rijeida.5.. H U ~ P I10.4% . (19591. H1@nmaller, P., Pure A p p l
