J. 0. Shrk
The Oxidation States of Molybdenum
Merchant Taylors' School Liverpool, England
T h e following experiment is suggested as a suitable exercise for college students involving a titrimetric determination of the oxidation states of the transition element molybdenum. A similar exercise using vanadium has already been published in THIS
+
2MoOzQ+ 2Ilg
MOO"
Approximately 3 g of ammonium molybdate, (NH&MoiOn.4Ha0, is weighed out accurately and dissolved in about 25 cma of water. Ten cma of concentrated hydrochloric acid are then added, giving a. white precipitate which redissolves to give a clear solution. This is then diluted to 250 cma with water in a graduated flask. 1 ) Reduction of Mo(V1) to Mo(V). A 25 ema aliquot of the molyhdate solution is shaken vigorously with about 20 cma of concentrated hydrochloric acid and s. few cm5 of clean mercury for approximately 5 min in a stoppered bottle. The precipitate of mercury(1) chloride is allowed to settle and the remaining solution decanted through a plug of cotton wool. The residual mercury and the precipitate are then washed twice with a mixture of .5 cma of concentrated hydrochloric acid and 20 cma of water, shaken vigorously, and finally the washings are added to the decanted solution. The solution of molpbdenum(V) is then titrated with standard cerium(1V) sulfate solution (0.1-0.12M) using 0..5 cm3 of N-pbenylanthranilie acid a5 indicator to the appeaance of apnrple color. 8 ) Redudion o j Mo(V1) to Mo(ll1). A 25-cm%liqoot of the molyhdate solut,ion is shaken vigorously with about 50 em8 of dilute hydrochloric acid and 20 cm3of liquid zinc amalgam until a green color is obtained, using a stoppered bottle as in (1). The green solution is then decanted rapidly into an erlenmeyer flask and the amalgam washed with dilute hydrochloric acid as in (1). The solution of molybdenam(II1) snd weshingq are then titrated immediatelv with standard cerium(IV) sulfate solut,ion
then
The following results were obtained in a typical experiment: 3.045g of ammonium molybdate were made up to 250 cm3 of solution. Titers of 0.105 M cerium(1V) sulfate solution: (1) 16.25 cm3; (2) 48.50 cm3. The ratio of the titers in experiments (1) and (2) is 16.25:48.50, or 1:2.95, and this corresponds to the reduction of R'Io(V1) to b[o(V) by mercury alone, and the further reduction to Mo(I11) by zinc amalgam. Theory
The changes taking place may be summarized in the following equations -MO(V)
-
red-brown
MO(VI) colorless
(1)
+ Hg?+ + 2Hs0
Mo(JII)
Mo(1V)
MOO+
+ Cea++ 2 H t
- - - brown
green
+ 3Zn + 4Hf
Experimental
colorless
MOO^^+
Mo(VI) 2MoOzP+
Results
2Mo0"
+ Ce4++ 8 0
oolarless
compounds are very unstable.
-
then
JOURNAL.'
M&I)
+ 4H+
-t
-
+ 3Ce4+ + HH.O
Mo(V)
red-brown
Mo(V1) (2) colorless
2MoO+
+ 3ZnP+ + 2H20
MOO."
+ ace'+ + 2Hf
illolybdenum has a great tendency to form complex ions and none of the solutions prepared in this exercise contains a simple molybdenum ion. The colorless, acid solution of hlo(V1) consists of the soluble IVlo02CIZ, which is reduced by mercury to the red-brown solution of Mo(V) containing R1oOCl3, and by zinc amalgam to the green solution containing the complex MoOCl. 4H20. The various colors corresponding to the different oxidation states of molybdenum may he observed during the reoxidation of Mo(I11) to Mo(V1) by Ce4+ion in experiment (2). Preparation of Zinc Amalgam. Fifteen grams of pure zinc are heated for one hour with 25 em3 of mercury and about 15 em3 of dilute hydrochloric acid on a water-bath. This must be done in a well-ventilated fume cupboard as mercury vapor is poisonous. The amalgam is then cooled, washed several times with dilute hydrochloric acid, and the liquid portion separated from the solid in a separating funnel. Preparation of N-Phenylanthranilie A& (0.005M). One gram of the acid is dissolved in 48 cm3 of 0.1 M sodium hydroxide solution and diluted to 11 with water. Preparation of a Standard Solution of Cerium(IV) Sulfate. A standard solution of cerium(1V) sulfate cannot be prepared by direct weighing. An approximately 0.1 M solution may be prepared by dissolving 35 g of pure cerium(1V) sulfate, Ce(SO4)r (formula weight 332.2) in dilute sulfuric acid and diluting to 11 with distilled water. The most reliable method for standardizing this solution uses pure arsenic(II1) oxide with an osmium(VI11) oxide c a t a l y ~ tbut , ~ ammonium iron(11) sulfate is sufficiently accurate for the present determination, with N-phenylanthranilic acid as indicator. Alternatively, a standard concentrated solution, as supplied by the leading manufacturers, may he used. DAVIS,J. M., J. CHEM.EDUC.,45,473 (1968).
' VOGEL,A. I., "A Textbook of Quantitative Inorganic Analysis," (3rd ed.), Longmsns, Green and Co. Ltd., London, 1961, pp. 314-20.
Volume 46, Number 8, Augusf 1969
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