Lab 7-ammonium decavanadate PDF

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Lab report for the "Synthesis and Analysis of Ammonium Decavanadate"...

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Synthesis and Analysis of Ammonium Decavanadate Your Name and Your Partner’s name: Randi Baker and Tyler Boone WSU ID: k757u452 Time of this lab: 5:30 1. Introduction

Polyoxometalates are complex molecules made up of three or more transition metal oxyanions linked together by oxygen atoms, into a three-dimensional structure. POMs are chemically interesting because they form spontaneously in solution through self-assembly. The reactions that occur during the self-assembly process are still largely a mystery to researchers. Chemists do know that the species of POM formed is pH dependent. (Figure 1) In this lab we will control the concentration, pH, and temperature of a solution, to isolate a decavanadate salt in which Vanadium exists as an oxovandate in the oxidation state 5+. Figure 1. Phase Diagram

Ammonium decavanadate can be synthesized from ammonium metavanadate in a solution. Acetic acid is added to solution to lower the pH, creating the conditions for the formation of V5+. Ethanol is added to solution to increase solubility because solid ammonium metavanadate is only partially soluble in water, but soluble in ethanol and ether. The solution must also be warm because ammonium metavanadate only decomposes when the solution is heated but not boiled. Above the pH of 6.5, vanadate turns bright orange, self-indicating the presence of decavanadate (a precipitate). These titrations are unique to previous experiments, in that an indicator is not needed for the detection of the end point. 1

After synthesis, the chemical composition of the salt can be analyzed through redox titration. When the salt is added to solution with a weak acid, like H2SO3, V5+ is reduced to V4+. This reduction reaction creates a blue color in solution. Potassium manganate dissociates in solution, leaving MnO4 - ion in solution. When added to our analyte solution, the titrant MnO4-, acts as a reducing agent, removing an electron from Vanadium to bring the oxidation state back to 5+. This oxidation reaction produces a yellow color. The end point is indicated when the yellow solution darkens. This titration will allow for the data needed for the calculation of the concentration of Vanadium in the decavanadate salt. During this titration, V4+ is oxidized back to V5+. Safety is important in this lab, as we are working with acids and other toxic materials. It is crucial to always remember that acid is added to water, never the other way around. A portion of this lab calls for the lab hood to be used to reduce the likelihood of toxic fume inhalation. Reactions 1. Formation of Ammonium Decavanadate

2. Standardization of titrant solution

3. Reduction and Oxidation of Vanadium

2. Purpose

The purpose of this lab is to properly synthesize a decavanadate salt under strictly determined conditions and then calculate the percent weight of Vanadium in the salt using the endpoint of a redox titration. 2

3. Procedure 3.1

Synthesis of (NH4)6V10O28 * H2O

Add 3.0 g of solid, white NH VO (ammonium metavanadate) to 100 mL of hot water. Constantly stir the 4 3 solution until the majority of the solid is dissolved. Next, filter the solution through a filter paper into a flask. Next, while stirring solution, add 4.0 mL of 50 vol% acetic acid to the solution. Still stirring, add 150 mL of 95% ethanol and cool the solution in an ice bath. Once cooled, vacuum filter the solution, washing with 2 15 mL aliquots of cold 95% ethanol. Solution should be bright orange. Set analyte solution aside to dry in desiccator until next lab period.

3.2. Dilution of H2SO4 1L of 0.9 M H2SO4 is needed for this experiment. Add 50 mL of concentrated H2SO4 to a volumetric flask, dilution with 900 mL of deionized water, up to the calibration line of the flask. 3.3. Standardization of MnO4Obtain three samples of solid sodium oxalate, adding each sample to a beaker. Under the fume hood, add 250 mL of 0.9 M Sulfurous acid to each solution. Titrate each solution using KMnO 4. The end point will be indicated when the solution turns colorless. Reheat the solution and repeat the steps in 3.3 for all three oxalate solutions. A blank titration will be necessary in calculation to subtract from each titration volume. Titrate a 250 mL solution of 0.9 sulfurous acid using KMnO4. 3.4. Vanadium analysis Weigh out three samples of (orange) ammonium decavanadate. Dissolve each in 40 mL of 1.5 M Sulfuric acid. Add 50 mL of deionized water and 1 g of NaHSO3 to each analyte solution. Stir to dissolve. After 5 minutes, boil the solution gently for about 15 minutes, ensuring that any excess SO 2 is released. Titrate each solution while still warm. The color indicator will be a yellow that darkens slightly. Record each titration volume. 4. Original data

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5. Data calculations and observations Observations

In the second titration, we allowed to solution to cool too much before beginning. Once we started the titration, the color changed immediately, indicating that we did something wrong. After a minute or so the solution went clear again. We reheated the solution and continued titrating, considering the couple of mL that had already been released into solution. Our blank titration is likely inaccurate. I didn’t realize the titration would happen so quickly. I should have been titrating a half drop at a time into solution for accuracy. I believe we ended our titrations slightly early. After examining the color plate in the Harris textbook, I realize that we most likely did not let our solutions darken enough. I recorded a lightyellow color produced. If the data is skewed, this may indicate that there was still unreacted V4+ in solution. 3) Calculations Example follows the calculations of trial 1. The rest of the calculations were done in Excel. The standardization of KMnO4 allows for the calculation concentration of potassium manganate using the experimental end point and the theoretical end point. Once this value is obtained, weight composition of chemical elements can be analyzed.

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6. Results and Discussion 6.1 Titration results The data below summarizes the experimental results and the calculations involved with determining the weight of Vanadium present in ammonium decavanadate. By comparing the experimental percent weight (44.76%) of oxovandate to the reference (47.80%) the data indicates that a level of accuracy was achieved. Random and systematic error can account for the difference in the results and the reference. The analytical scales could have been slightly off or dirty, some compound may have been lost in transfer, and inexperienced students may have made mistakes along the way. I believe we stopped the titration slightly early and did not allow the yellow color to darken enough. This assumption is reasonable because the experimental percent is lower than the reference, indicating that a few more mL of titrant would have resulted in a darker solution and a more accurate result. The standard deviation and the percent RSD indicate that our data collected is precise and reproducible.

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trial

Na2C2O4 (g)

1

0.2722

2

0.2803

3

0.2516

trial

experimental end point (mL)

1

38.31

2

39.01

3

33.99

moles Na2C2O4 2.0314E03 2.0918E03 1.8776E03

molarity KMnO4 (M) 2.1210E02 2.1449E02 2.2096E02

moles KMnO4

end point (theoretical) (mL)

8.1254E-04

40.63

8.3672E-04

41.84

7.5105E-04

37.55

Vtitrated (L)

Blank titration (mL)

0.02249

0.0003

0.02301

0.0003

0.02401

0.0003 average

moles KMnO4

moles VO2+

moles V10O

4.7064E04 4.9354E04 5.3053E04 4.9824E04

2.3532E03 2.4677E03 2.6527E03

2.3532E04 2.4677E04 2.6527E04

ammonium decavanadate (g)

% weight V10O

0.2647

46.71

0.2863

45.29

0.3295

42.30

average standard deviation % RSD

44.76

7. Conclusion

The experiment was successful in that ammonium decavanadate was synthesized and its chemical composition was analyzed to a level of accuracy that is acceptable. The color plate was observed in the redox titration of Vanadium. The titration was successful because it led to accurate calculations. The accuracy of this experiment could be improved by experience and practice with redox titrations. For example, if the experiment was repeated, the mistake of titrating the second solution while it was too cool could be avoided. Also, titrating the analyte to the true end point (a dark color) would improve the accuracy of the results. Overall, the experiment successfully demonstrated the purpose and concepts of this lab.

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2.25 5.03

References: Harris, D.C. Quantitative Chemical Analysis, 8th Ed, Freeman: New York, 2002.

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