The Oxidation States of Vanadium
The goal of this experiment is to prepare the four common oxidation states of vanadium, separate them using ion-exchange chromatography and characterize them using chemical and spectroscopic techniques.
Vanadium is a d-transition metal found in Group VA of the Periodic Table. Vanadium was first discovered by the Spanish Mineralogist Andres Manuel del Rio in 1801 while experimenting with a mineral obtained from a mine near Hidalgo in Northern Mexico. He prepared a number of colored salts from this "brown lead" which were similar to salts of chromium. He named his new element erythronium which means red after observing that most of his salts turned red upon heating. A French chemist named Collett-Desotils disputed his claim of discovering a new element insisting that his new element was nothing more than impure chromium. Del Rio, believing he was incorrect, withdrew his claim of discovery. Thirty years later, the Swedish chemist Nils Sefström isolated a new oxide while experimenting with some iron ores. He named this new element Vanadium in honor of the Scandanavian goddess of beauty, Vanadis, because of its varied beautiful, multicolored compounds.
Vanadium naturally occurs in about 65 different minerals, including vanadinite, the ore of lead and vanadium, from which Del Rio first isolated it. In the U.S. it is most commonly found in sandstones which are rich in uranium ores. Vanadium is also found carbon-rich deposits such as coal, oil shale, crude oil, and tar sands. While minor amounts of vanadium are found in the human body, its biological purpose is unknowns. Vanadium is an important element in sea squirts of the ascidian family. These marine filter feeders concentrate vanadium in their bodies to a level one million times higher than the concentration of vanadium in seawater.
Vanadium is used in making specialty steels, rust resistant and high speed tools.
Vanadium's ground state electron configuration is [Ar] 3d34s2. When transition elements ionize, they lose their valence s electrons before losing their d electrons. Vanadium has 5 valence electrons that can be lost. One of the characteristics of transition metal is their ability to adopt multiple oxidation states. Vanadium exhibits four common oxidation states +5, +4, +3, and +2 each of which can be distinguished by its color.
The ease with which the different oxidation states of vanadium can be interconverted has led to its usage in a vanadium flow battery. In the vanadium redox battery (VRB), each half-cell is composed of a vanadium redox couple. At the anode VO2+ ions are converted to VO2+ ions and when electrons are removed from the positive terminal of the battery. At the cathode, electrons convert the V3+ ions into V2+. The electrolytes of the two half-cells are separated by an ion exchange membrane. In this battery, half-cells are connected to storage tanks and pumps allowing very large volumes of the electrolytes can be circulated which allows them to be very high capacity batteries for large scale power storage. These batteries are being used to store electricity from solar energy collectors and wind mills. The battery has also been used to power vehicles. One interesting feature of the VRB is its ability to be recharged by either conventional means or by exchanging spent solutions for fresh ones at suitable refueling stations. A golf cart that runs on VRB power has been developed for test purposes. Learn more about the the Vanadium Redox Battery here.
In this experiment, you will start with the +5 oxidation state of vanadium, generate the other oxidation states by redox reactions, and separate them using ion exchange chromatography. Ammonium metavanadate, NH4VO3 will be the source of the +5 oxidation state. Treatment of the ammonium metavanadate with hot hydrochloric acid partly reduces the vanadium to the +4 oxidation state in the form of the VO2+ ion. The two species are separated using an ion exchange resin. The VO2+ ion is then treated with mossy zinc generating the V3+ and V2+ which also are separated by ion exchange chromatography.
Ion Exchange Chromatography (IEC)
Ion exchange chromatography
is a form of liquid chromatography in which retention of the materials
in a mixture is predominately controlled by electrostatic interactions
between the charges present in the molecules or ions of the solute and
opposite charges present in the stationary phase. In order to separate
a mixture of cations, the stationary phase must contain immobilized
anions with which the cations can interact. The stationary phase is
usually an organic polymer fabricated into small 1-2 mm beads onto which
sites where ions are easily trapped and released. The polymer used in
our separation is a divinylbenzene cross-linked polystyrene that has
been modified by adding sulfonic acid groups to the benzene rings by
sulfonation of the polymer and is classified as a strongly acidic resin.
The properties of the resin can be changed markedly by altering the
amount of cross-linking in the polymer. The amount of cross-linking
in these polymers is dependent on the proportions of the monomers used
in the polymerization step and typically ranges from 2-16%. The resin
used in our separation is AG50W-2X where the 2X refers to the
amount of cross-linking. In generating this resin, 2% divinyl benzene
is added to the styrene monomer used in the polymerization step. Sulfonation
introduces sulfonic acid groups (-SO3H) inside the polymer
particles as well as onto the surface. The more highly cross-linked
the polymer, the harder it is to add sulfonic acid groups into the particles.
The more highly cross-linked, the lower the capacity of the resin. In
it's native form the R+ is the hydrogen and the group is a sulfonic
acid group. During the chromatographic separation, the hydrogen is replaced
by other cations. The stronger the electrostatic interaction between
a cation and the SO3
Many natural water sources in the Willamette Valley are hard water sources meaning that they have significant amounts of magnesium and calcium ions along with dissolved iron. These ions react are responsible for deposits that build-up in pipes and that stain sinks, showers and tubs. If this hard water is passed through a column containing and ion exchange resin having sodium ions in its active sites, the undesired ions replace the sodium ions and are immobilized on the resin lowering the concentration of these ions in the water. Read more about ion exchange and water systems here.
Part I. Preparation of the Chromatography Column
Part II. Separation #1
Part III. Reduction and Separation #2
Part IV. Characterization of Products.
You will write a formal, typed laboratory report for this experiment. Guidelines for this report can be found here.
General informationon the chemistry of vanadium can be found in the following sources: