Colored Ions of the Transition Elements See HAIGHT,G. P , JR, J. Chem Edue., 29, 296 (1952).
When an acidified solution of VSO. is titrated with a solution of KMnO,, the color of the titrated mixture changes from purple [due to VSO,) t o green to blue to vellow. and finally to orange when an excess of i ? ~ n 0 ,is added. All the colors except the last are due to the different colors of vanadium in its four nossible oxidation states. (The final orange color is due to a combination of the yellow vanadate ion and the purple permanganate
. -,
I"".,
Thesolution of VS04 is prepared by passing a sulfuric acid solution of VOa'or VO++ through a Jones reductor.
Questions:
Determine the oxidation state of vanadium in each colored form
Can the several oxidation states of vanadium be obtained in any other way, in addition to reaction with permanganate? Is each of the different colored vanadium-containing ions Positively Ions of m a w other transition elements are also differentlv colored, dependingupon the oxidation state of the metal. select another transition metal and demonstrate this property, identifying each oxidation state involved.
The Allotropy of Sulfur See BUTLER,S. B., J . Chem Educ., 31, 187 (1954) Allotropic forms of an element have different nhvsical DrODeI~ monoclinic and rhombic sulfur differ in their ties. F O example, solubility in carbon disnlfide. Also, these two allotropes have different melting points, 118 and I 1 3 T for monoclinic and rhombic sulfur, respectively, a t ordinary pressures. This difference can be demonstrated by placing samples of each allotrope in a bath of boiling n-butyl alcohol (bp 116'C). To demonstrate the difference in melting points, Butler suggests that a three-neck flask be used. An Erlenmeyer flask fitted with a three-hole stopper will serve as well. Two holes accommodate short, lengths of glass tubing with one end closed in which the allotropes are placed, and a condenser is fitted into the third hole.
Questions:
Determine the melting points of monoclinic and of rhombic sulfur and select a liquid with an intermediate boiling point to show that one allotrope melts while the other remains solid. Identify other differences in the properties of the sulfur allotropes. Sulfur is only one of many elements that exhibit the phenomenon of allotropy. Select another suitable element and identify differences in the chemical and physical properties of the allotropes of the selected element. (One interesting possibility would be to use tin, obtaining the necessary quantities of this element from the coating on used "tin cans".)
The Effect of Nitrous Acid on the Oxidizing Power of Nitric Acid See BRASTED,R. C , J Chem. Educ , 28, 442 (1951). Nitric acid is colorless; the yellow color of ordinary nitric acid is due to the presence of other nitrogen compounds, large]." NOa, NO, and HNOa. When sulfamic acid is added to ordinary nitric acid the liquid becomes colorless because these impurities are removed by the sulfamic acid. Pure nitric acid has properties that are noticeably different from those of ordinary yellowish nitric acid. For example, pure nitric acid slowly oxidizes dilute solutions of iodide ion, compared with the-rapid reaction of yellowish nitric acid. Pure nitric acid will, however, oxidize concentrated solutions of iodide ion about as rapidly% ordinary nitric wid. (The use of sodium aside to decolorize ordinary nitric acid is e r g dangerous. Do not attempt decolorizaticm by this procedure.')
Questions:
..
:.
From your own laboratory investigations, find other reactions for which the behavior of pure and of ordinary nitric acid are different.' Account. for the differences you observe and test the validity of your explanations by further laboratory work. When decolorized by sulfamic acid, other impurities are necessarily introduced into the colorless nitricacid. Are these substances perhaps responsible for the difference-in chemical properties between vellowish and colorless nitric acid? '-See, for example, KURZ,P. F., J. Cham. Educ., 33, 625-6 (1956), which discusses the role of NOSin the etching of silver by nitric acid.
A Secondary-School Course in Inorganic Preparations See WEAVES,E. C., J. Chem. Educ., 30,386-7 (1953).
Few activities in chemistry offer more stimulation and eventual satisfaction than the preparation of unique inorganic compounds, Some of these preparations require elaborate apparatus, but many can be performed with apparatus available in elementary laboratories. Often, also, a needed piece of special equipment can be fabricated by the student or a substitute devised. Today, several excellent laboratory manuals are extant; some are mentioned in this article and others are mentioned in GAYER, K. H.,AND ELKIND,M. J., J. Chem.. Educ., 30, 90-93 (1953) and HOLTZCLAW, H. F., JR., aid., 29, 95-6 (1952). Others that have been more recently published can be found by examination of the current catalogs of the publishers mentioned in these references.
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Journal of Chemical Education
Two points in connection with work of this type should be mentioned. In the preparation of these inorganic compounds, difficulties may be encountered; the product, once obtained, may or may not be what it is presumed to be. Hence, students who undertake this kind of project should keep a careful record of their difficulties and the methods they used to surmount them; any final product should be analyzed quantitatively to establish its composition. Select an inorganic compound listed in a book on inorganic preparations that is available to you and, after preparing this compound, certify its composition by analysis.
