Oxidation State Determinations for Some Reduction Products of Vanadium(V)
F. C. Hentz, Jr. and G. G. Long North Carolina State University Raleigh. North Carolina 27607
I
A multi-colored titration
R e d u c t i o n of v a n a d i u m ( V ) i n acidic s o l u t i o n i s the classic d e m o n s t r a t i o n of the m u l t i p l e oxidation s t a t e s characteristic o f t h e t r a n s i t i o n m e t a l s . T h i s h a s b e e n used i n a CHEMS film," h a s b e e n t h e s u b j e c t of articles i n this J ~ u r n a l , : 'a. n~d h a s s e r v e d as t h e b a s i s for g e n e r a l c h e m i s t r y l a b o r a t o r y exp e r i m e n t s . W e feel t h a t a n ideal experiment should allow t h e s t u d e n t t o o b s e r v e t h e f o u r o x i d a t i o n s t a t e s of v a n a d i u m obtainahle in aqueous solution a n d should permit accurate det e r m i n a t i o n of t h e o x i d a t i o n n u m b e r s o n o n e o r m o r e of the r e d u c e d v a n a d i u m suecies w i t h o u t u n d u l v c o m o l i c a t i n e the chemistry. N o n e of t h e a b o v e m e n t i o n e d e x p e r i m e n t s d o t h i s . T h e filmYi s a semi-quantitative d e m o n s t r a t i o n using a J o n e s reductor. T h e a r t i c l e b y Tietze:' calls for c o m b i n i n g t h e gene r a t e d V(I1) i m m e d i a t e l y w i t h e i t h e r excess Fe:'+ o r VO: to p r o d u c e VOY+;" these s t e p s f r e q u e n t l y l e a d to confusion a n d d o n o t allow t h e s t u d e n t to clearly observe t h e +3 and +4 o x i d a t i o n states of v a n a d i u m . The a r t i c l e b v D a v i s ~ n v o l v e s difficulties associated w i t h t h e d i r e c t t i t r a t i o n of v a n a d i um(I1). The e x p e r i m e n t b y Conroy, et al., like t h e a r t i c l e b y T i e t z e , i n t r o d u c e s t h e confusing s t e p w h e r e excess V(V) is added to t h e V(I1) and h e n c e involves t h e s a m e difficulties. Here we descrihe an experiment in which the student preparesand ohserves all four of the vanadium oxidation states obtainable in aqueous solution, hut uses the stable V(II1) and V(IV) states for determination of their oxidation numhers." This is accomplished in a single period, using simple equipment, by carrying out two separate reductions of VO: followed by permanganate titrations
vanadium species present in the hlue-green solution resulting from the aeration tn he determined; the student should deduce that the violet color of the solution prior t o the air oxidation must he due toavanadium species havingan oxidation number less than t3. 2) While waiting for the first sample to he reduced by the zinc amalgam, each student is assigned tocarry out asecond reduction of VO; with either Fe2+ or with SO:,'-. In both eases the sample is reduced to V(IV) (as the bright blue-colored Vo"+ ion) and is titrated with permanganate t o determine the oxidation numher of the vanadium species. Thus, each student directly observes the colors of sulutbns containing, respectively, V(II), ViIII), V!IV), and ViV) and also directly determines the oxidation numbers on two of the reduced species, V(II1) and V(1V). MnO; in acidic solution may he regarded as an "electron pump" with a capacity for accepting five e l e c t n m and an oxidizing strength sufficient t o cause the oxidation V"+
-
V(V)
+ (5 - " ) C
Both the reductions and the titrations are extremely colorful, the endpoint isvery sharp, and students can calculate oxidation numbers that are very close t o integers.
Experimental
I. Reduction o f Vanadium(VJ With Zinc Amalgam Prepare zinc amalgam by treating 5 g of 20 mesh granular zinc in a 50-ml Erlenmeyer flask with 20 ml of 0.1 M HgC12 in 1 M HCI. The contents af the flask shbuld be gently swirled for 3-5 rnin until the metal takes on a shiny, wetted appearance and the granules tend to
Student Results Sample Treatment
Reduction with suttite Reduction with ironlll) Reduction with Zn(Hg) followed by aeration
Oxidation Number Found tor Vanadium 3.99 1 0.01 4.01 10.01 3.03f 0.07
d i m t a ~ e t h e r Puur . ~ off m m t of the HeCI., solut~on.hut keeo the wash water. T h e zinc amalgam is now ready for use. Add 10 ml of 1 M H2S0., and 10.00 ml afO.lOxx VO! stock solution"'to the flaskcontaining the amalgam. Stopper the flask and allow it to stand for several minutes, swirling from time t o time and noting anv color changes." Proceed with the oortion af Part 11 assigned hv the instruetor.'~hereduction is eom;lete when the salutiok in thk flask has turned violet; the solution may he kept at this point until P a r t 11 has been completed. Set up a 250-ml Erlenmeyer flask fitted with a ruhher stopper amtaining a thistle tuhe which will reach almost to the bottom o f t h e flask and a short, right-angle shaped piece of glass tubing. Transfer the violet solutiod quantitatively to the flask by pouring it thmugh the thistle tube but without transferring anv of the amalgam. Wash the amalgam and flask 2 or 3 times with l & ~ l portions ;,f distilled water to ensure a auantitative transfer. Connect the elass tuhine t o
' T h e following ealar changes are involved in the titration of V 4 + : addithm of the deep purple-mrlored potassium permanganate wlution tu the hlue-green solution of V:'+ initially pmduces a hright hlue arlution (VWt), which as more MnO; is added. becomes pure hright green (mixtureof hright hlue Vo"+ and lemon yellow VO:), yellowgreen, hright lemon yellow (VO;), and finally the endpoint is indicated by the first permanent darkening of the lemon yelluw color. A drop of titrant heyond the endpoint produces a pinkish caloration (excess MnOl-l.. "rasted, R. C., "Vanadium-A Transition Element," Film No. 4172, Modern Learning Aids, 1212 Avenue ofthe Americas, New York, IAu~...., n n x t." ~fi? :'Tietze, H. R.. J. CHEM. EDUC., 40,344 (19fi:3) 'Davis, J. M., .I. CHEM. EDUC., 45,473 (19fi81. "See, for example,Cmmy, L. E.,Tohias, R.S., and Brasted, R. C., "General Chemistrv Laboratow 2nd Ed.. Macmillan. . Ooerations." . New York, 1971, p..191. 'Wanadium(I1) is readily oxidized hy air and if m e quickly titrates an acidic vanadium(l1) solution exposed to air with permanganate and uses the data tocalculate the oxidation numher of the vanadium, typically the unsatisfactory value -2.3 is obtained. T h e two approaches t o avoid this difficulty have heen ta try t o exclude air (Ref. ( 4 ) )or oxidize thevanadium(1l) tavanadium(lV1 (Refk. ( 9 )and ( 5 ) ) and then use vanadium!lV) as the basis of the titratim. Here, h o w ever, we focus on vanadiumllll) which is easily formed without ad^ ditim of interfering reagents, i.e. color associated with excess iron(ll1). eerium(IV), or vanadium(V). :Swinehart, J. H., Inorgan. C h m , 4.10fi9 (19651. "The reduction may he markedly speeded up if a magnetic stirrer is used. "It is important during the formation and use o f t h e zinc amalgam that the metal he pnrtected from the air hy heing awered with water or s r h t i o n . Even a very short exposure to the air can totally ur partially reduce its effectiveness.
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Volume 55, Number 1. January 1978 1 55
with distilled water. Add 10 ml of 6 M HrS04 and heat the solution just to boiling. Titrate the hot solution with standard 0 . 0 2 1 ~ M~ KMnOn. A series of color changes are observed during the titration' but the endpoint is signalled by the first persistent (30 si darkening that occurs after the solution has turned bright lemon yellow. The oxidation state of vanadium present after the air oxidation step can then he calculated from the number of mmoles of vanadium initiallv taken. the nu~nlrcr~ I ' m m d ue i~5ln0, .and the tact rho1 aittr thr t~tratismthe v . w d i , ~ m1s prvwn~ns \ ' ! V t .
Il-A. Reduction of Vanadium( V) With Bisulfite 'l'rdnilrr 3 !?500-ml s.mplc c,ilhp(, IOxx 3f YO: s t d a h l i o n '
Discussion
T. v.~ i c aclass l results are shown in the table. The data were from a first semester class of chemistry majors but are about the same as that obtained from other classes. The errors are somewhat larger for the Zn(Hg) procedure due to the failure of some students to obtain a quantitative transfer of the vanadium solution to the aeration flask.
"'The VOt stock solution may be prepared by weighing out 46.80 g (0.4000 mole) of reagent grade ammonium metavanadate and t u 2511-ml ~ Erltt~xnevtrfld*k \ d d 10 ml l l l G . \ I H.50,. I g d NsHiO, slurrying this in 1 1 of water. Thirteen-hundred milliliters of 3 M ftw ?La SO,,, .=grl t u d i w d ~ c . ~ nr d i w c s h d
