Vanadium

Experiment 2 2/1 Inorganic Chemistry II THE OXIDATION STATES OF VANADIUM
References
1. Folder entitled "The Chemistry of Vanadium" on your bench.
2. Cotton and Wilkinson "Basic Inorganic Chemistry" sections 24-9 to 24-12.
3. Huheey, "Inorganic Chemistry" (3rd edition) Appendix F.
4. Shriver, Atkins, and Langford, "Inorganic Chemistry" chapter 8 Background
Most of the transition metals can exhibit a variety of oxidation states. Vanadium for example can form the oxidation states +2, +3, +4 and +5 under various conditions in aqueoussolution, and solid compounds containing vanadium in each of these oxidation states are alsoknown. Of these oxidation states +4 is the most "stable", stable in this context meaning withrespect to oxidation or reduction in aqueous solutions.
In this experiment solutions of the lower oxidation states are prepared by reducing an acid solution of ammonium metavanadate(5+). These lower oxidation states are thenreoxidised by various oxidising agents. Under these circumstances the strong oxidising agentspermanganate (KMnO4) and cerium(4+) will oxidise vanadium to the highest oxidation state possible for that element, vanadium(5+).
From the quantitative titrations of reduced vanadium solutions with standard permanganate in parts B and C, you should be able to calculate the oxidation states ofvanadium in the reduced solutions. You are expected to be familiar with the formulation ofredox equations and calculations involving volumetric solutions, and the exercises givepractice in the application of these concepts.
The various oxidation states of vanadium are characterised by their different colours in acid solution. From your titration results you should be able to correlate the observedcolours with the various oxidation states. These deductions will be checked in the latersections of the experiment.
These investigations of the oxidation states of vanadium essentially involve analytical methods, so that analyses accurate to within 1% are required.
The aqueous chemistry of vanadium is somewhat complicated. An outline of the solution chemistry is given on the accompanying diagram which shows a variety of oxyanionsand oxycations, aquo species, and polymeric species. This chemistry is the result of theseveral possible oxidation states, and the different acidic or basic behaviour associated witheach of these. Where amphoteric behaviour is involved, the nature of the species in solutionwill depend on the pH of the solution.
The exact nature of the ions in solution need not concern us greatly in this experiment, as we will be thinking mainly in terms of oxidation state: V(5+), V(4+) etc. But the followingnotes are helpful in understanding what species are present and how they arise.
In the low oxidation states vanadium ions are present in the form of aquo complexes e.g. [V(H2O)6]2+ and [V(H2O)6]3+. However, as the ionic charge increases, the coordinatedwater molecules become more acidic and some of the water molecules may lose a proton. Forexample: [V(H2O)6]3+ • [V(H2O)5(OH) ] + H+(aq) pKa= ~ 3.5 and +5 the species present in acid oxycations can be considered as beingformed from the simple aquo ions by extremely stable, and can exist inconcentrated acid solution withoutbeing reprotonated.
The species which are formed in alkaline solution need not concern us too much in this experiment except that in very basic, dilute solutionscontaining vanadium(5+) the tetrahedral vanadate ion, VO 3- ammonium metavanadate, which is the starting material used in Section A,the metavanadate consists of infinite chains of tetrahedral VO together by corner oxygens as shown below (See Figure 21-3 in the folder "Vanadium Chemistry" on the Reference Book Shelf) in which each VO4 ammonium metavanadate rapidly,converts the vanadate to VO + Redox Reagents
A number of reagents are used in this experiment which are common reagents for Sodium sulfite, Na2SO3, is a reducing agent which becomes oxidised to sulfate. In acid solution the sulfite ion is protonated and exists as sulfurous acid, H2SO3 which can be thought of as aquated sulfur dioxide, SO2 (aq). Boiling a solution containing H2SO3 expels SO2 as a gas so excess sulfite can be destroyed by acidifying and boiling the solution.
Zinc amalgam is a solution (alloy) of metallic zinc in mercury. It acts as a reductant with the Zn being oxidised to Zn2+ which then passes into the solution which is beingreduced. After the reaction is complete the amalgam can be separated from the solution as itis insoluble in water.
The oxidising action of cerium(+4) sulfate is described in Section D. Potassium permanganate is a well known oxidant and its reaction in acid solution is described in Huheey3rd Edition page 581, and Shriver, Atkins, and Langford, pg. 654.
Aim of the Experiment
To investigate some of the oxidation states of vanadium in acid solution and to note the colours associated with the oxidation states.
Outline of the Experiment
The experiment is undertaken in pairs.
Answer preliminary questions. All the information needed is covered in theintroductory section of the experiment.
Prepare a standard solution of NH4VO3 which contains vanadium in the oxidation state+5.
Reduce a portion of the vanadium(5+) solution with sodium sulfite and determine theoxidation state which results by titrating with potassium permanganate.
Reduce a portion of the vanadium(5+) solution with zinc amalgam and determine theoxidation state which results by titrating with potassium permanganate.
Reduce a portion of vanadium(5+) solution with amalgamated zinc in a JonesReductor, reoxidize with 1, 2, 3 etc. equivalents of cerium(4+) sulfate and note thecolours of the various oxidation states of vanadium in acid solution.
Preliminary Questions
Figure 1 on the following page will be helpful in answering these questions.
What is the maximum possible oxidation state formed by vanadium? How is thisrelated to the electronic configuration of vanadium? Write down the likely molecular species which exist in acid solutions for the variousoxidation states of vanadium. Which of these would you expect to be stable towardsaerial oxidation (i.e. O2 in air) and which would be unstable? It may be helpful toconsult the background information.
Write down the half equations for the reduction in acid solution for: the oxidising actions (in acid solution) of: permanganate; cerium(4+); and air.
the reducing actions of sulfite and zinc amalgam.
(Section A) What is the oxidation state of V in NH4VO3? Why is NH4VO3 dissolved inalkali, followed by addition of excess acid, rather than added directly to acid? Suggesta reason why the acid should be added rapidly as instructed in the script. Giveequations for the reaction with (1) one molar equivalent of acid, and (2) excess ofacid.
Experiment 2 2/4 Inorganic Chemistry II Experimental
A. Preparation of a Standard Solution of Vanadium(5+)
Dissolve an accurately weighed amount, of about 1.8 g, of ANALAR ammonium metavanadate (formula weight 117.0) in about 30 mL of sodium hydroxide solution (2 M).
While stirring this solution vigorously, add rapidly about 100 mL of dilute sulfuric acid (1 M).
Transfer this solution quantitatively to a 250 mL volumetric flask and make up to the markwith deionised water.
Calculate the concentration of vanadium in this solution.
B. Reduction of Vanadium(5+) with Sulfite
Pipette 20 mL of the standard vanadium(5+) solution into each of two 250 mL conical flasks. To each add dilute sulfuric acid (about 20 mL), excess sodium sulfite (use about 0.2g), and two anti-bumping granules. Gently boil the solution on the hotplate for about 10minutes to remove excess sulphur dioxide then titrate the still hot solution with the standardKMnO4 solution provided (this will be about 0.02 M). The end-point is given by the first permanent appearance (red-brown) of the permanganate.
Calculate the number of moles of vanadium taken in the 20 mL aliquot. Take the mean of the two permanganate titres†, and calculate the number of moles of permanganate added. From the half-equation for oxidation by permanganate calculate thenumber of moles of electrons transferred.
Calculate the ratio (moles of electrons transferred)/(mole of vanadium).
Deduce the oxidation state of the reduced vanadium solution.
Suggest reasons why your calculated oxidation state may deviate from being an exact integral number (i.e. discuss sources of error).
C. Reduction of Vanadium(5+) with Liquid Zinc Amalgam (Zn-Hg alloy).
All transfers involving amalgam should be done over the safety tray provided.
Pipette20 mL of the standard vanadium(5+) solution into each of two 100 mL conical flasks.
Add to each about 20 mL of dilute sulfuric acid and 2% liquid zinc amalgam (10 mL), andstopper the flask.
† An aliquot is a sample, usually of a solution. We titrate an aliquot of known volume with a titrant of knownconcentration. The volume of titrant added is known as the titre.
Experiment 2 2/6 Inorganic Chemistry II Shake the stoppered flasks by hand, observing carefully the series of colour changes.
When you observe no further colour change (the solution should finally be violet) shake theflasks for a further five minutes to ensure reduction is complete.
Decant as much as possible of the reduced solution carefully into an excess (use about 50 mL) of the original vanadate(5+) solution (from A) in a 250 mL conical flask. Take carenot to pour droplets of the amalgam into the vanadate solution (do this over the trayprovided). Introduce dilute sulfuric acid (10 mL) into the amalgam flask, stopper it, and shakevigorously with the amalgam before again decanting into the vanadate solution. A secondwashing of the amalgam in the same way with a further portion of dilute sulfuric acid shouldsuffice to quantitatively transfer the fully reduced product to the vanadate(5+) solution.
Heat the solution to 70-80°C, and titrate with the standard KMnO4 solution. Take the mean of the two titres and calculate the number of moles of permanganate required to oxidisethe V(4+).¤ Determine the oxidation state of the reduced vanadium solution as in Section B.
Suggest reasons why your calculated oxidation state may deviate from an integral D. Reduction of Vanadium(5+) in a Jones Reductor Followed by Stepwise
Oxidation with Cerium(4+).
The Jones reductor consists of a column of amalgamated zinc (a column of zinc granules that have been in contact with metallic mercury so that their suface consists of asolid zinc-mercury alloy) which offers a large surface area to provide efficient reduction.
When acid solutions of reducible elements (e.g. Fe, Ti, V, Cr, Mo, W, U) are passed downthe column quantitative reduction occurs, for most metal ions, to the lowest oxidation statestable in aqueous solution. The reduced compounds can then be estimated by titration withstandard solutions of suitable oxidising agents.
In this experiment, the Jones reductor is used to effect the same reduction as in Section C but more efficiently. Since the reduced vanadium solution is sensitive to aerialoxidation, the experiment is arranged to exclude air from the reduced solution. Thesubsequent oxidation is to be carried out semi-quantitatively with a standard cerium(4+)solution.
First calculate the number of moles of vanadium in a 20 mL aliquot of the standard solution prepared in Section A. After reduction in the Jones reductor this will be subsequentlyoxidised stepwise by adding equivalent moles of the oxidising agent. A standard solution ofcerium(4+) sulfate is provided (about 0.1 M). Cerium is a lanthanide element, and in the +4oxidation state is strongly oxidising. The half-equation is: Ce4+(yellow) + e- → Ce3+(colourless) E° = +1.44 V (in 1 M H2SO4) § The reduced vanadium solution V(n+) is readily oxidised by atmospheric oxygen, and this is minimised byreacting it immediately with an excess of the original V(5+) solution to produce an equivalent amount of thestable V(4+): From this equation it can be seen that the amount of KMnO4 required to oxidise (5-n) moles of V(4+) toV(5+) is the same as the amount required to oxidise one mole of V(n+) to V(5+). For example if the reducedproduct contains V(3+) then two equivalents of V(4+) are produced, one from the oxidation of V(3+) and thesecond from the reduction of V(5+).
Experiment 2 2/7 Inorganic Chemistry II Calculate the volume of the standard ceric solution required to oxidise the vanadium aliquot by one oxidation number (i.e. one mole of oxidant/mole of vanadium).
Connect up the 250 mL receiving flask and place about 50 mL of dilute sulfuric acid in the cup at the top of the column. Allow the acid to pass slowly through the column until thelevel is just above the zinc. (The demonstrator will assist should gentle suction be required).
Close the tap. This percolation of acid through the column activates the reductor. The liquidin the cup must never be allowed to fall below the surface of the amalgamated zinc. Detachthe collecting flask, rinse, and add to it the above calculated volume of cerium(4+) solution;measuring cylinder accuracy is sufficient here, since this part of the experiment is semi-quantitative only. Dilute with about 10 mL of water. Reconnect the flask to the column, andensure that the tube from the reductor dips below the level of the cerium solution.
Transfer 20 mL (measuring cylinder) of the vanadium(5+) solution from Section A to the cup at the top of the column. Allow this solution to pass slowly down the column. Thenpass through the column two 20 mL portions of dilute sulfuric acid. Collect the runoff fromthe column until it is colourless, indicating that all the vanadium has been washed off. Duringthe elution procedure rotate the receiving flask to ensure thorough mixing of the solutionsand remember to exclude air as much as possible.
You should have just sufficient cerium(4+) present to oxidise the vanadium solution V(n) + Ce4+(yellow) → V(n+1) + Ce3+(colourless) Because Ce3+ is colourless, the colour observed can be ascribed to the V(n+1) Detach the flask, and add successive similar amounts of the cerium(4+) solution. At each addition swirl the flask to mix and observe any colour changes before adding the nextportion. Add a sufficient number of such portions until you are satisfied that the vanadium hasbeen completely oxidised.
1. Your report should list your observations for each section of the experiment, your calculations, and the deductions made from these.
2. Summarise all the analytical results, and the colours assigned to the oxidation states in 3. Answer briefly the questions in the text.
4. (Section B) Why is it necessary to boil the solution before titrating with KMnO4? 5. (Section C) In decanting the reduced solution from amalgam: (i) why is the solution decanted into a solution of excess vanadium(5+)?(ii) why should the operation be carried out quickly?

Source: http://www.chemistry.adelaide.edu.au/cdrom/PracCDROM/2nd_Year/REFER/LEVEL2/INORG/EXPT2/VANADIUM.PDF