Class Note for CHEM 242 at UMass(3)
Class Note for CHEM 242 at UMass(3)
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Date Created: 02/06/15
Lecture Experiment 3 Separation of the Oxidation States of Vanadium The objectives of this experiment as stated on slide 1 of the Powerpoint lecture slides is l to make complexes of the multiple oxidation states of vanadium and 2 to separate those complexes via ion exchange chromatography Why should we want to access multiple oxidation states of an inorganic element The transition metals characteristic chemistry arises from their multiple oxidation states No other group of elements displays so many stable oxidation states and no other group can change easily from one oxidation state to another as transition metals can This is because the dorbitals are the valence outermost orbitals of transition metals There are relatively many dorbitals they are closely spaced in energy and electrons within them in the various oxidation states can adopt a variety of stable con gurations rendering that particular oxidation state stable Other groups of elements main groups lanthanides and actinides do not have dorbitals as their valence orbitals and cannot undergo this chemistry The reactivity of transition metals39 di erent oxidation states is m differentnMnOz Mn4 for example is a stable compound used as a solid support while KMnO4 Mn is such a rapid oxidizing agent that it can even be explosive with organic solvents The very different reactivity of different oxidation states makes it important to choose correct one to obtain the reaction or reactivity that you desire Correct oxidation states are vital for all kinds of catalysis both biochemical and industrial the Pd 2 Pd0 oxidation states for example are very widely used in organic synthesis especially in the synthesis of highlyfunctionalized drugs and pharmaceuticals plastics and specialty organic chemicals Accessing the correct oxidation state of metals is also crucial in bioinorganic chemistry For example small clusters composed of iron and sulfur clusters Fe4S4 in cubic shapes act as electron shuttles in hundreds of enzymes Each iron is initially in oxidation state 3 Each iron can then accept an electron taking it to Fe2 The electrons are then passed along to enzyme sites where they can participate in reactions leaving the irons as Fe3 again ready to pick up more electrons and shuttle them into the reaction site by changing between the 3 and 2 oxidation states Humans and mammals use a suite of over 100 ironbased enzymes such as hemoglobin and cytochrome c to bind transport and use 02 Every enzyme depends on maintaining speci c oxidation states of iron and of other metals within the enzymes Each metal must be in the exactly correct oxidation state for the enzyme to perform its particular biochemical transformation of 02 However a class of sea creatures known as ascidians or sea squirts actually use the multiple accessible oxidation states of vanadium in an analogous suite of enzymes for the same purpose of binding transporting and using 02 Again having the vanadium metal centers in the correct oxidation states for the particular biochemical transformation of 02 by a particular enzyme is crucial In these mammalian and ascidian enzyme suites multiple oxidation states of the respective transition metals are accessed and used to bind O2 carry it to cells react it with compounds and transform it into usuallyunstable oxygencontaining ions The metals39 different oxidation states allow for the enzymes vital abilities to transform O2 to vital unusual oxidation states including H20 0 in 2 oxidation state H202 O in 1 oxidation state the peroxide ion and 0239 O in 12 oxidation state the superoxide ion All these unusual oxidation states of oxygen are essential to life and can only be generated by reaction of oxygenspecies with transition metals in variable oxidation states So the reactions of transition metals in different oxidationsstate complexes could be models for inorganic Oztransport systems Therefore an important part of inorganic chemistry is to learn to synthesize different oxidation states of the transition metals then to separate them to isolate the particular one we want That is objective of this lab Vanadium is in Group 5 in periodic table slide 2 and is a firstrow transition element It has five valence electrons this is a consequence of its being in Group 5 can therefore lose up to five electrons and therefore has a maximum oxidation state of V Lower oxidation states can be obtained and are stable down to V Other states even negative oxidation states are accessible but easily react with air and water So these states are not relevant to bioinorganic chemistry which takes place in air and water and we will not attempt to access them in this lab They can be synthesized by the use of airsensitive techniques The vanadium complexes of different oxidation states we will use in this lab are aquo complexes slide 3 VH206 VH2063 VH2050 V0H03239 quot0quot denotes the oxo ligand 02 which is doublybonded to the vanadium with a VO structure VOH03239 therefore has tetrahedral geometry while the other complexes are octahedral Note that the different oxidation states from 5 to 2 are all present To calculate these oxidation states use the following procedures VOH03239 one negative charge from OH39 ligand six from three 0392 ligands total charge on complex is 2 therefore V oxidation state is 5 VHzO5O2 two negative charges from O392 ligand none from neutral water ligands Total charge on complex is 2 so V oxidation state is 4 VHzO53 VHzO5z no negative charge from neutral water ligands total charges on complexes are 3 and 2 therefore V oxidation states are 3 and 2 respectively Remember that oxidations states are not necessarily equal to charge on complex There are shorthand notations for the aquo complexes of V which we will use given on slide 4 Note also that V0339 is also called the quotvanadatequot ion and VO2 is called the quotvanadylquot ion To synthesize different aquo complexes in different oxidation states we will start with highest oxidation state VO339 5 and use sequential reductions to access the other oxidation states The reducing agents we will use are shown on slide 5 HCl is the reducing agent that will take V5 to V and Zn amalgam will be use to reduce V4 to V3 and V quotZn amalgamquot is a quotsolutionquot of solid Zn in liquid mercury The mercury is said to quotamalgamatequot the zinc These sort of mercury amalgams can be made with many metals for example sodium or cobalt The different metals form solutions or solids when quotdissolvedquot depending on the ratios of metal and mercury used Amalgams exist as small even atomic particles of metal in this case zinc dispersed in mercury This gives the reactive metal in this case zinc very large surface area making it an excellent reducing agent As we will see zinc reacts very quickly from the amalgam to reduce the various vanadium complexes The chemistry of reduction is Zn0 lt gt Zn2 2 e39 There is no change in the mercury in any reaction the mercury is a carrier only It will not appear in any reactions that you write or redox potentials that you calculate in this experiment The standard redox potentials E0 for converting one oxidation state of V to another are given on slide 6 In this experiment we are again far from using standard conditions but the standard potentials work closely enough for our purposes For these syntheses two reducing reactions are needed for VT5 to V and for VT4 to V3 and V These are shown on slide 7 The reducing agent in the V5 to VT4 synthesis is actually chloride ionit goes from the l to the zero oxidation state DO NOT breathe the fumes of this reaction or take it out of the hood It will be evolving C12 gas which is extremely corrosive For other two reductions Zn from the amalgam is the reducing agent Because this reagent reacts very quickly it is sometimes dif cult to obtain the V3 complex as all the V4 may be rapidly reduced to V This may happen in first reduction as well with HCl reducing all the VT5 complex to V4 We use HCl and ZnHg in this experiment however because they are not airsensitive reagents ie they do not react with water or dioxygen The reagents that more slowly do this reduction and with which different oxidation states can be quantitatively synthesized are airsensitive But in a research lab using airsensitive techniques one can obtain the desired oxidation states quantitatively and selectively Once the vanadium complexes in different oxidation states are synthesized they must be separated This will be done using ion exchange chromatography slide 8 This is a very convenient very efficient separation method for many kinds of ions so we use it here But note that compounds must be ionic for this separation method to work The column is packed with an ion exchange resin which consists of an insoluble polymer base quotresinquot means an insoluble very hard tough polymer in bead form The top surface of the resin is functionalized with bound SOg39 sulfonate groups see slide each with a counter cation Resins are usually supplied in what is called quotHformquot mean the countercations of the sulfonates are H In first step of our separations we prepare the column by passing acid over it ensuring that resin in completely in the Hform Solution of the ions to be separated are then added In high concentrations they will replace H as the bound counter cations and H ions will be removed from the sulfonate groups while the vanadium complexes are bound to them The highest charged metal cation complex will be the most tightly bound as it can bind to and bridge many sulfonate anions The next highest positively charged complex will be the next most tightly bound and etc So differently charged cations can be separated by strong or weak binding to the column Now more His added in the form of HCl to the column In high concentrations equilibrium conditions will cause the least weakly bound vanadium complex the lowest positively charged complex to be displaced from the sulfonates and that complex will elute from the column This displacement of one cation by another by manipulation of the relative concentrations of the two species is called quotcompetitive bindingquot At first there is not a high enough concentration of H to break up the binding of highest charged vanadium complex cations so these remain bound on the column Therefore the various vanadium ions are separated the least highly charged complex comes off the column and can be collected while the most highly charged complex remains bound on the column We then increase the concentration of H to the point where the next most highly charged ion elutes and can be selectively collected Then the next and the next etc until the acid concentration is so high that most tightly bound vanadium cation is displaced and comes off the column We will use this method to separate for example V which will come off column first 3 Since it has a lower pos1t1ve charge from V What happens to anionic complexes such as VOH03239 They alway elute first before any cation because they are repulsed by the negativelycharged sulfonate groups and are therefore not attracted at all to the column Anions will elute very quickly as explained in this experiment s EP The Experimental Procedures shown on slide 8 are discussed at great length in this experiment s EP so read it thoroughly These are the bases and procedures for separating all kinds of ions The concentrations of three of the vanadium complex ions can be found by titration with an oxidizing agent KMnO4 potassium permanganate slide 9 Permanganate ion is a good oxidizing agent because manganese is in the 7 oxidation state its highest and therefore its least stable There is a driving force to remove electrons from other species and go to lower oxidation states As seen in slide 9 a stable oxidation state for manganese is Mn2 and our vanadium complexes will reduce permanganate to this oxidation state Using the half reactions shown in the EP for vanadium you will write balanced redox reactions showing Mn7 oxidizing V4 to V3 and V3 to VH Write these as separate reactions V4 to V3 to V not as one overall step from say V4 to V You will calculate the number of moles of permanganate used in the titration from the number of drops used and the concentration of permanganate given in the EP This is equal to the number of moles of vanadium complex present in the correct ratio to manganese as given in the balanced redox reaction In conclusion this lab is an example of how one can synthesis separate and characterize the many valuable oxidation states of transition metals an important goal in inorganic chemistry
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