Description of experiment
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Needed compounds:
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hydrogen peroxide : H2O2
ascorbic acid : C6H8O6
disodium phosphate : Na2HPO4
sulphuric acid : H2SO4
ammonium molybdate : (NH4)6Mo7O24 . 4H2O
Class:
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elem=Mo,P
coordination
redox
Summary:
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Molybdates and orthophosphates give a yellow complex, which is reduced more
easily than molybdates alone.
Description:
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Dissolve some ammonium molybdate in water and add some dilute sulphuric acid
(appr. 1 mol/l): Liquid remains colorless.
Split up the liquid in two equal parts. To part I some solid disodium phosphate
(disodium monohydrogen ortho phosphate) is added, to part II nothing is added.
Both liquids are heated to almost boiling:
- part I becomes yellow, liquid remains clear
- part II remains colorless.
Let both liquid cool down to appr. 15 C: Both liquids becomes colorless again.
Add some ascorbic acid to part I and to part II:
- part I becomes light blue, it becomes darker slowly
- part II becomes light green.
Heat both parts:
- part I quickly becomes dark blue.
- part II also becomes dark blue, but this must be heated for a
much longer time.
Add an excess amount of hydrogen peroxide to the blue liquids: No immediate
change, but after some heating, the liquids become very light yellow, almost
colorless.