Carl H. Snyder, Jackson P. Sickelr, and Clara J. Del Valle The University of Miami Coral Gables, Florida 33124
Calcium Oxide-Zinc Fusion The successor to sodium fusion in the undergraduate laboratory
As a conseauence of several curricula revisions in this Department over the past few years, the teaching of qualitative organic analvsis has been moved from its traditional position in the senior year to the introductory organic lahoratory normally taken by sophomores. With this move a number of laboratory procedures used in the course have been modified t o make them more suitable t o the introductory organic laboratory and t o students with diminished laboratory experience. Chief among the procedures requiring modification was the decomposition used for qualitative elemental analysis. Sodium fusion, the procedure described in greatest detail in two excellent and probably in greatest use, proved too hazardous and generally impractical in the introductory organic laboratory. T h e hazards involved in the handling and decomposition of metallic sodium and the uncertainties often encountered in t h e nitrogen test prompted us t o examine other approaches to qualitative elemental analvsis. W ~ r hrespect to laborarory safety, the must appealing of these is the calcium oxide-z~ncfusion."Co our knowledre neither calcium oxide nor zinc nor a mixture of the twoys particularly hazardous under the conditions of t h e decomposition. Furthermore, t h e addition of water t o t h e cooled fusion mixture poses n o hazard comparable t o t h a t encountered with metallic sodium. The procedure is mentioned only briefly and in general terms in S h i n e r , Fuson, and C u r t i q 2 while t h e operations described in t h e original article8 are entirely too complex for use in any h u t the most advanced undergraduate laboratories. With extensive modification of the original procedure we have developed a calcium oxide-zinc fusion technique which is remarkably convenient, reliable and, t o the best of our knowledge, safe. I t shows promise of replacing sodium fusion as the method of choice for qualitative elemental analysis in the undergraduate organic laboratory. Details of the fusion and of t h e test for nitrogen follow.
Preparation of the Organic Sample for Fusion. If the organic s a m ~ l eis a liquid. dace 0.25 ml in a clean. drv test tube or simi. . lar tmtnincr and prepare d dropping piper t o rranifar rh? liqu~d ro the rube containing the cslcum uxide-zinc mixrun.. If the sample ia n d l d . place tuo purtinnt, each a h u r half thc
volume of the calcium oxide-zinc mixture, on separate pieces of filter paper. Finally, for both solid and liquid samples, moisten two strips of universal, continuous range pH paper with water in preparation for the nitrogen test. Fusion and Nitrogen Test. Heat the calcium oxide-zinc mixture, in a hood, vigorously hut cautiously with a hot, well-defined burner flame until the zinc granules just begin to glow or until the calcium oxide just begins to turn from white to pale yellow. Set aside the burner and hold a strip of moist pH test paper near, hut not over, the mouth of the test tube. Liquid Sample. With a dropper add one or two drops of the sample to the hot calcium oxide-zinc mixture. Use care! Alkyl halides react vigorously; other classes of compounds may also react vigorously or explosively. As the liquid reacts with the hot calcium oxide-zinc mixture hold the moist pH paper aver and about 1 cm above the mouth of the test tuhe. The presence of nitrogen in the sample is indicated by a color change to the basic region of the pH paper. False positive tests occur when calcium oxide is splattered upward in a vigorous fusion reaction and when the pH test paper comes in contact with calcium oxide particles adhering to the lip of the test tuhe. Differentiating between tNe and false positive tests for nitrogen is quite simple if the result is compared with those obtained with a blank and with a nitrogencontainingsample. While absence of any color changes in the pH paper indicates the absence of nitrogen, the paper will change color to the acidic region on decomposition of some acids and halogen-containing compounds. If an acidic color change occurs, the presence or ahsence of nitrogen must be determined by a procedure described under "Preparation of Stock Solution." Reheat the fusion mixture, remove the burner flame and complete the decomposition of the liquid sample hy adding the remainder dropwise or all at once, depending on the vigor of the initial decomposition. Repeat the nitrogen test for confirmation. Now heat the fusion mixture vigorously for a few seconds to remove any liquid which may remain. Proceed to "Preparation of Stack Solution." Solid Sample. Add one of the portions of solid unknown all at once to the hot calcium axide-zinc mixture. Caution! A violent reaction may occur with some classes of compounds. As the solid reacts with the hot mixture place the pH paper over and about 1 cm above the mouth of the tube. Interpret color changes in the pH paper as described for liquid samples. Reheat the fusion mixture to glowing, remove the flame and add the second portion of the solid sample all at once. Repeat the nitrogen test for confirmation. Heat the fusion mixture to glowing briefly and proceed to "Preperation of Stock Solution." Preparation of Stack Solution. Allow the fusion mixture to cool and add 3 ml of distilled water. Heat the mixture gently until it just begins to boil, remove the flame and test bath the vapor and the liquid with pH paper. Normally the liquid will be basic because of the calcium oxide. If it is not. add 6% sodium
'Pasta, D. J., and Johnson, C. R., "Organic Structure Determination," Prentice-Hall, Ine., Englewaod Cliffs, N. J., 1969, pp 31620. ZShriner, R. L., Fuson, R. C., and Curtin, D. Y., "The Systematic Identification of Organic Compounds" (5th ed.), John Wiley & Sons, Inc., New York, N. I,., 1964, pp 62-6. 3Bennett, E. L., Could, C. W., Jr., Swift, E. H., and Niemann, C., Anal. Chem., 19,1035 (1947).
acidic vapors on decomposition as discussed above. Dissolve any sulfide or halide salts which may be present in the fusion residue by boiling the mixture gently for a few minutes. Fusion residues which appear as solid crusts should he broken up with a stirring rod to insure that all sulfides and halides dissolve. Add distilled water if necessary to maintain a volume of about 3 ml. Centrifuge the cooled mixture and use the liquid piuse far sulfur and halogen tests.
Experimental Procedures h-eparation of the Fusion Mixture. Add a mixture of 0.04-0.05 g of powdered calcium oxide and 0.3-0.5 g of granular zinc (20 mesh) to a clean, dry 10 em Pyrex test tuhe. Clamp the tube vertically to a support in a hood.
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/ Journalof Chemical Education
Representative Compounds Used in Calcium Oxide-Zinc Fusions
No Halogen, Nitrogen or Sulfur solid anthracene bend dextrose naphthalene liquid acetone benzene cvclohexane
Halogen Alone
Nitrogen Alone
solid and m-bromobenzoic acids 4-bromobiphenyl p-chlarohenzaldehyde p-dibromobenzene iodoform liquid bromobenzene I-hromobutane hromoethane I-bromonaphthalene tert-butyl chloride carbon tetrachloride chlorobenzene iodobenzene I-iodohexnne .~..
solid acetamide acetanilide ambenzene henzamide cyclohexanone mime 3,5-dinitrohenzoic acid ethvl . .o-aminobenzoate liquid nitrobenzene pyridine
o-
cjxlahexanol cyclohexanone dibutvl nhthalate . . dioxane methanol propionic acid
~~~
Sulfur Alone solid I-naphthalenesulfonic acid liquid dimethyl sulfoxide
~~~~
Halogen and Nitrogen
Nitrogen and Sulfur
Halogen, Nitrogen and Sulfur
solid p-chloroaniline p-chloronitrobenzene ethyl glyci"ate hydra-
solid methyl orange thiaacetamide thiourea liquid phenyl thia-
solid p-nitmhenzenesulfonyl chloride thiamine hydrochloride liquid (a mixture of chlorohenzene, dimethyl sulf-
p-iadoaniline tetra-n-butylammonium iodide
cyanate
oxide and ni-
Testa for Sulfur and the Halogens. These are performed as described for sodium fusions.',2 Both the calcium oxide and the zinc used in this study were reagent grade and were used a s received. Effects of variations in the ratio of these inorganic reagents were not examined. However, after a few initial fusions the reagent quantities were assessed by visual inspection alone with no noticeable effect on analytical results. A particle size of 20 mesh was chosen fur the zinc to avoid possible hazards involved in using zinc dust.
The temperature a t which the first visual change in the appearance of the calcium oxide-zinc mixture appeared was chosen as the appropriate point to discontinue heating prior to fusion. This was reached with either a barely perceptible glowing of the zinc granules or a slight change in the color of the calcium oxide from white to pale yellow. The origin of this color change was not investigated. The decomposition itself converts sulfur and the halogens to sulfide and halides, as does sodium fusion. Detailed descriptions of tests for these anions appear elsewherel,2 and need not be repeated here. Naturally, the basic stock solution must be acidified before the tests are carried out. The test for nitrogen is normally carried out during the decomposition itself and depends on the pH of the fusion nivaoors. Basic vaoors result from reduction of organic " trogen to ammonia during the fusion or from vaporization of volatile, undecomoosed amines. In either case the detection o f h a s i c vapbrs during fusion indicates the presence of nitrogen in the organic sample. Acidic vapors have been obtained on decom~ositionoCp-nitrobenze~esulfonyl chloride, 3,5-dinitrohenzoic acid and carbon tetrachloride. With some samples close observation of the pH paper reveals the formation of acidic and basic vapors a t different stages of the fusion. In any event, the detection of acidic vapors in the fusion requires a test of the pH of vapors of the stock solution. as deevolved durine - oreoaration . . scribed above. All stock solutions prepared in this study were found to be basic and none required addition of sodium hydroxide solution. Alternatives to some of the procedures used in this study may be desirable in certain circumstances. Litmus paper, for example, can replace the universal pH paper used here. With this modification, however, the nitrogen test on fusion should he carried out with two strips of litmus paper, one red and one blue, held in conjunction over the mouth of the tube. In another alternative, isolation of stock solutions by filtration should he as effective as the centrifugation used in this study. With the exception of hemoglobin all compounds examined (see table) gave satisfactory analyses. Difficulties encountered in testing for nitrogen in hemoglobin may reflect the exceptional stability of the porphyrin ring or may he related to the i;on cornplexing. However since porphyrins, complexed or not, are not normally used as unknowns in the undergraduate organic laboratory, the matter was not pursued.
Volume 50. Number 1, January 1973 / 73
