Richard Cornelius Wichita State University Wichita, KS 67208
Ion Exchange Separation of the Oxidation States of Vanadium
The hright and distinctive colors so characteristic of the four oxidation states of vanadium in aqueous solution make vanadium an ideal transition metal for experiments through which students learn about oxidation-reduction reactions. The value of vanadium solutions has been recognized in reports of experiments'-"n which the oxidation state of one of the species is quantitatively determined. The experiment described here is desiened to emnhasize the discrete nature of the different oxidatim states by givingstudenrsanupportunits to separate species of all four of the dit't'errnt oxidation states by ion rxchange chnmatography. Ammonium metavanadate is used as the source ot'vanndium. The vanadium is reduced partly to vanadium(N) by hot hydrochloric acid, and the VOz+ and V02+ ions are separated by cation exchange chromatography. Some of the V02+ is then reduced to a mixture of V2+ and V" which are also readily separated by Hy roml)ining infonnntion cation exchange chromatogn~phy.~ from the method of preparation, f r m qualitative tests, and frum the chrumatugraphic hehaviur of the different speric;i. students can deduce nllt only the oxidation state of each species, hut also the charge on t h ~ions . V2*.V:lr, V02+,and
are darkest yellow hut discard any that are yellow-green or hlue-green. Add the remaining two thirds of the blue solution to about 3 g mossy zinc in a small flask. Swirl the flask for 5-10 min until the color has become a light lavender and no longer changes. Decant the solution from the zinc and nour it onto the chromatography culumn. Allow the level of thc liquid to fall iust to the to^ of the resin and then add 1.0 A1 HCI as necessary to keep the huret full. Ihring the separation distinct lavender and green hands will h r m on the column. Begin collecting fractions of roughly 5 ml as the solution coming from the column hecomes lavendrr. After the lavender hand is off the column, use 3.0 M HCI to drive off the green species. Place 1ml of each of the four, colored solutions in test tubes. T o the yellow and hlue solutions which came from the first column chromatography add dropwise 0.001 M KMn04, and to the lavender and green solutions from the second column chromatography add iodine water dropwise. Note which of the vanadium solutions decolorize the solution being added. Discussion This experiment has a low cost per student due to the limited amount of chemicals per student consumed: 0.2 g NHdVOa. . ... 3 ,e, mossv zinc. and hvdrochloric acid. The ion exchange resin is ready fbr &se Gthuut funher treatment.'l'he use of nlossv ~ i n in r nlacp of a Jones redurror eliminatrs hoth a troublesome preparation and the cost of expensive mercury salts. The oxidation of vanadium(I1) by atmospheric oxygen heeins as soon as the zinc is removed from the lavender solution &mtaininr predominantly vansdium(l1,. This"s1uw" reaction can beseparated from this is fast enuugh that \~anadium(IlIJ solution hut slow enough5 that the color and reducing properties of vanadium(I1) remain through the laboratory period. If the student knows the charge on any one of the ions, the charee on the other ions mav he deduced. The two ions with a plus two charge, V2+ and vo2+are hoth eluted with 1.0 M HC1. The ion with a +3 charge requires 3.0M HCIfor elution, while the +1 ion can he driven off with only 0.4 M HCI. The oxidation states are determined from the qualitative testing, so the formulas V2+, V3+, V02+ and VOz+ follow from a knowledge of the charge on the individual ions. A time period of 2%3 hr should be allowed for this experiment. When time and facilities permit, students may record the visible spectra of the different oxidation states. The concentration of all of the solutions is such that their visible spectra may he taken without dilution or concentration.
V02'.
Procedure Prenare a chromatoeranhv .. , column in a 25-ml huret. Use a glass rud to push a smnll wad of glass wool down to the stoocock. and then fill rhr buret with 3 slurrv uf AG50W-X? cation exAhange resin, 1W200 mesh, H+ for; (obtained from Bio-Rad Laboratories. Richmond. California). . Ooen . the stopcock and continueadding the &rry of cation exchange resin until the level of settled resin is between the 0- and 5-ml marks on the huret. Close rhe stuprock 111p r e v e ~ the ~ t level of the wlution from falling helow the topuf the resin. In a fume hood add 5 0 ml concentrated hydrochloric acid to 0.2 g NH4V03 in an 18 X 150 mm test tube. Heat the mixture in a boiling water bath for 5 min, add 10ml distilled water, and mix well. The solution should he bright green. (To prevent a bottleneck due to the limited number of fume hoods, half of the student group may be assigned to first prepare the ion exchange columns and half to first prepare thevanadium solutions.) Pour the hrieht ereen solution onto the cation exchange column. Allow ;he i h of the liquid tu fall just ro thc topbf thi, resin and thrn add 0.4 M HCI as necessary to keep the huret full. Two hands will form on the column. The upper hand is detected readily hy its blue ( ~ h rhut , the yellow color of the lower l ~ i u ~isddiffic~~lt to ut~servedue to the oranae culor of the rriin. When the .;nlution cwning from the column Incomes vellow. n)llect fractions of at~out5 ml each in 13 X IUO ml tes
