Synthesis of ammonium decavanadate, and analysis via IR, UV-Vis and quantitative titration. Time: 1.5 lab periods
The purpose of this lab experiment is to expose students to the synthesis of a colored POM, and to connect the use of standard analytical techniques to this new type of compound. It introduces the use of IR spectroscopy of inorganic materials.
Ammonium decavanadate ((NH4)6V10O28•6H2O) is an example of a polyoxometallate (POM). POMs are a large and diverse group, containing various metal cations, including, but not limited to, vanadium, molybdenum, chromium and iron. They are formed spontaneously in solution through what is called ‘self-assembly’. Essentially, they come together in solution through unknown mechanisms. In the lab, we can isolate various POMs by controlling conditions to favor one form over another, by varying concentration, pH or counter-ion. In the phase diagram of vanadates (Figure 1), one can see how varying pH and vanadium concentration can yield monomeric vanadate (VO43-), or POMs containing 2-10 vanadium atoms.
Decavanadate in Geology
The decavanadate anion occurs naturally in minerals, huemulite Na4Mg2V10O28•24H2O, hummerite (KMg)2V10O28•16H2O, lasalite (NaMg)2V10O28•20H2O , pascoite Ca3V10O28•16H2O, magnesiopascoite Ca2MgV10O28•16H2O, and rauvite Ca(UO2)2V10O28•16H2O, which differ in the cations and amount of hydration. As you can see from Figure 3, the dominant color is yellow-orange. This is due to the presence of the decavanadate anion, which is yellow-orange. All of the vanadium POMs and vanadate are yellow-orange due to charge-transfer bands in the 200 – 500 nm range.
Decavanadate in Biology
Vanadate (VO43-) is isostructural and isoelectronic with phosphate (PO43-). For this reason, vanadate has found interest with researchers studying the role of phosphate in biological systems, such as in insulin uptake in cells. In addition, decavanadate has been used to bind to biologically relevant molecules, such as gelatin, cytosine, estrogen receptor, and myosin.
In this experiment, students synthesize ammonium decavanadate, and analyze the resulting product via UV-Vis and IR spectroscopy as well as by permanganate titration.
- The student will define polyoxometallates (POMs).
- The student is able to interpret a concentration-pH diagram to determine conditions suitable for formation of the desired product.
- The student will apply IR spectroscopy to identify inorganic compounds.
- The student will use Beer’s Law to analyze the purity of a colored compound.
- The student will use redox titration to analyze a compound for purity.
Materials for a class of 20:
Ammonium metavanadate (NH4VO3, CAS#7803-55-6, Aldrich 99%): 6.0g
50% acetic acid solution: 100 mL
95% ethanol: 360 mL
Oxalic acid dihydrate (H2C2O4•2H2O, CAS#6153-56-6, Aldrich 99%): 2.5g
1.0 M sulfuric acid: 1.2 L
Sodium bisulfite (NaHSO3, CAS#7631-90-5, Aldrich): 2.0g
0.10 M potassium permanganate (KMnO4): 100 mL
50 mL beakers, 100 mL volumetric flasks, funnels and erlenmeyers
50 mL burets, 10.0 mL and 20.0 mL transfer pipettes
UV-Vis (Spec 20 is fine) and IR spectrophotometers
Because the synthesis is quick, but the crystals should be allowed to dry, I pair this experiment with another synthesis and analysis experiment, and spend the first lab period on the two syntheses, then the subsequent lab periods on analyses. If you have another experiment that takes 1.5 lab periods, then you can pair appropriately. The 0.5 lab period part is the synthesis. See Instructor Notes for other comments.
Student performance on Preliminary Questions (PQs) and reports are assessed
Percent yield and percent purity from UV-Vis and titration analyses
Accuracy in completion of table of IR bands/assignments
Answers to Thought Questions
Percent Yield across two years: average 80%+/-11% for 21 students
Percent Purity via titration analysis in first year: -12% error (less than theoretical); implemented improved estimation of endpoint – see Instructor Notes
Added IR Spectroscopy Tables (given values, and a blank one to fill in on report). Without these 'lead by the nose' guides, students did a poor job demonstrating understanding of the meaning of the IR spectra. This is most likely due to the fact that this is their first exposure to IR spectroscopy (second-semester freshman level).