NEELIMA Nathawat,
Centre Number: - 10508
Candidate Number: - 2485
The preparation of ions containing vanadium in two oxidation states
Aim: - To prepare the two oxidation states of vanadium ions.
Introduction:-
Vanadium has the chemical symbol V with an atomic number of 23. Vanadium is transition metal and has 4 different oxidation states ranging from 5+ to 2+. The usual source of vanadium in the 5+ oxidation state is ammonium metavanadate (NH4VO3). Ammonium metavanadate can be reduced using zinc and an acid, usually moderately concentrated acid. In the case of my experiment, I will be using sulphuric acid (H2S04) with zinc to reduce ammonium vanadate to an oxidation state of 2+.
The exact vanadium ion present in the solution is very complicated and it varies with the pH of the solution. The reaction is done under acidic conditions when the vanadium present is VO2+ and this ion is called dioxovanadium (V) ion.
Safety and Precautions:-
Actions taken to increase accuracy of the procedure:-
Method 1:-
Preparing the solution for the changes in oxidation states of vanadium:-
- Take a conical flask and pour 25 cm3 of Ammonium Vanadate (V) and 25 cm3 of Sulphuric acid using a pipette filler. Then add 40cm3 of distilled water using measuring cylinder. Measure 5.00g of ...
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Actions taken to increase accuracy of the procedure:-
Method 1:-
Preparing the solution for the changes in oxidation states of vanadium:-
- Take a conical flask and pour 25 cm3 of Ammonium Vanadate (V) and 25 cm3 of Sulphuric acid using a pipette filler. Then add 40cm3 of distilled water using measuring cylinder. Measure 5.00g of zinc on a weighing scale and add this using a spatula in to the conical flask.
- Take a glass funnel and place it on the neck of this conical flask to minimise the entry of air and allowing aerial oxidation.
- Set up a Bunsen burner and gently boil the components in the conical flask for 30 minutes.
- While the solution is boiling, set up another Bunsen burner at a distance and boil some distilled water in a conical flask to be used in the further experiments.
- Notice the colour change that occurs when the vanadate solution is boiling and record the observations.
- Fill a burette with aqueous potassium manganate (VII) and run exactly 25cm3 of KMnO4 into another conical flask.
- Take a glass funnel and place it on the mouth of the conical flask containing KMnO4. With the help of tongs take some glass wool and place it in the glass funnel covering the conical flask with KMnO4 in it.
- If the amount of vanadate solution decreases while boiling, add some boiling water to the solution and record the number of times water has been added to the solution at the end.
- When the vanadate solution has turned pale purple after 30 minutes, take the glass funnel out of the reduction conical flask and filter its contents in to the conical flask with the KMnO4 through the glass wool. Make sure heatproof are worn at all times to protect from hot conical flasks and its boiling contents.
- Rinse all the contents of the reduction conical flask into the conical flask with KMnO4 and also pour some boiled water on to the glass wool. This allows all the vanadium ions to be transferred to the titrating mixture
- Allow KMnO4 from the burette to run into the titrating mixture drop by drop until an end point is reached and the colour changes to pale pink. This step should be carried out while the mixture is still hot.
- Record the total titre used to titre the vanadate solution. This would be 25cm3 plus the additional volume of KMnO4 used to titrate the vanadate solution.
Method 2:-
Preparing the solution for a change in oxidation state:-
- Take a conical flask and pour 25 cm3 of Ammonium Vanadate (V) and 25 cm3 of Sulphuric acid using a pipette filler. Then add 40cm3 of distilled water using measuring cylinder.
- Swirl the mixture properly and in the fume cupboard, gently pass sulphur dioxide gas into the solution for a few minutes. Notice and record the colour change that occurs. The solution will turn blue as vanadium will be reduced to a new oxidation state.
- In the fume cupboard, set up a Bunsen burner and boil the contents of the conical flask to exclude any sulphur dioxide gas present. To check this, dip a piece of paper into aqueous potassium dichromate (VI) and pass it over the vapour of the contents in the conical flask.
- If there would be any sulphur dioxide gas present, the potassium dichromate will change to a pale green colour. Therefore the solution must be boiled until all the sulphur dioxide is excluded from the solution.
- Bring the conical flask out of the fume cupboard and measure the temperature of the solution using a thermometer. Gloves must be worn at all times during this stage. Swirl the solution continuously until the temperature reaches approximately 70oC.
- Whilst the mixture is cooling down, fill the burette with aqueous KMnO4 .
- When the temperature of the mixture has reached exactly 70oC, allow aqueous KMnO4 to run into the vanadate solution drop by drop until an end point is reached which is when the reaction mixture turns pale pink.
- Record the titre values and the observations.
Observations:-
Method 1:-
Observation of the change in oxidation states of vanadium and the colour changes.
Results for method 1:-
Results for method 2:-
The ratio between the two titre values for method 1 and method 2 is
27.25cm3:10.65cm3
2.56 : 1.00
The ratio between the two values if almost 3:1 which is due to the difference in number of stages that occur in each method. In method one, the reduction of vanadium occurs in three stages whereas in method 2 this happens in only one stage and thus the ratio between the results from method 1 and 2 is 3:1.
Observing colour changes (Method 1) :-
While boiling the reducing mixture containing vanadium ions, sulphuric acid and zinc, the following colour changes can be observed.
Yellow Pale green Blue Blue green Green Grey Blue Violet
VO2+ VO2+ V3+ V2+
Intermediate colour change
Significant colour change
Method 1:-
While carrying out the titrations:-
MnO4- Mn2+
Purple Pale Pink
Whilst titrating the vanadium mixture, another redox reaction oxidises V2+ back to VO2+.
V2+ VO2+.
The half equations for the titration carried out in method 1 are as follows:-
1. MnO4- + 8H+ + 5e- Mn2+ + 4H2O
As the electrons are gained, this is a reduction reaction.
2. V2+ + 2H2O VO2+ + 4H+ + 3e-
As the electrons are lost in this reaction, it is an oxidation reaction.
1. (MnO4- + 8H+ + 5e- Mn2+ + 4H2O) x 3
2. (V2+ + 2H2O VO2+ + 4H+ + 3e- ) x 5
3MnO4- + 24H+ + 5V2+ + 10H2O 3Mn2+ + 12H2O + 5 VO2+ + 20H+
Simplifying the above equation gives:-
3MnO4- + 4H+ + 5V2+ 3Mn2+ + 5 VO2+ + 2H2O
Method 2:-
When sulphur dioxide is passed through the vanadium mixture, the colour change that can be observed is as follows:-
Yellow Blue
VO2+ VO2+
]
While carrying out the titrations:-
MnO4- Mn2+
Purple Pale Pink
Whilst titrating the vanadium mixture, another redox reaction oxidises VO2+ back to VO2+.
VO2+ VO2+
Experimental errors:-
There are a few reasons due to which the average titre for both methods was not perfectly in the ratio of 3:1. Some of the reasons are as follows:-
- The main experimental error that I experienced was deciding the colour change from yellow to blue when the sulphur dioxide was passed through the vanadium mixture. So, the entire sulphur dioxide may not have been removed from the mixture while boiling it even though the vanadium mixture was boiled for consistent 10 minutes. Thus, while titrating the vanadium mixture, a redox reaction took place between sulphur dioxide and manganate, where sulphur dioxide acted as the reducing agent. This means that the manganate reacted both with sulphur dioxide and vanadium ions and thus a higher volume of MnO4 was required to titrate the vanadium mixture and reach the end point as sulphur dioxide acts as a reducing agent. This resulted in the inaccuracy of the results.
- Also, some of the equipments used in the experiment were clumsy and unsteady such as the clamp stands holding the burette due to which it was difficult to read the titre values from the burettes accurately. For example, if the titre value on the burette was 26.70 cm3, it could have been 26.65cm3 but due to the unsteadiness of the burette, inaccurate titre values might have been recorded.
- Other experimental error was reading the accurate values off the measuring cylinders. The graduations in the measuring cylinders were quite large and thus reading the measurement values at the eye level was not the best way to measure volumes to the maximum accuracy and precision. Also the values were recorded according to the lower meniscus and therefore it was quite difficult to decide the exact point of the lower meniscus. This could have lead to inaccurate amounts of distilled water to be added to the vanadium mixture.