woelen
Super Administrator
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Registered: 20-8-2005
Location: Netherlands
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Molybdenum colors
Yesterday I did an experiment with molybdenum and I think the results are quite surprising. I have no explanation for the observations.
I dissolved some MoO3 in an as little as possible amount of solution of NaOH. This gives a colorless solution of Na2MoO4. This is what I expected.
Next, I mixed the material with a large excess amount of 10% HCl. This again gives a colorless and clear liquid.
I electrolysed this colorless liquid with graphite electrodes, applying a current of approximately 1 A (I used a current source instead of voltage
source). The measured applied voltage was 3.9 Volts.
The distance between the anode and the cathode was approximately 1.5 cm. Both at the anode and the cathode, a gas is evolved. At the anode, there is a
sharp and choking smell of chlorine. At the cathode, I assume that H2 is formed.
What puzzles me is the formation of a red compound at the cathode as well. This red compound slowly forms and mixes with the entire liquid. On
dilution is looks yellow. The red compound is not a solid, it dissolves into the liquid, immediately on formation.
When the electrolysis is allowed to run for a few minutes, then the liquid becomes brown/yellow and it remains clear.
As a counter experiment, I also took some of the colorless liquid with Na2MoO4 and excess HCl and added a pinch of reagent grade zinc metal to this.
The zinc dissolves, giving hydrogen gas bubbles. However, here also a red (on dilution yellow) compound is formed.
I left the liquid standing for a long time (with air contact). After one day, the glass was covered by a blue layer and the top layer of the liquid
also was much darker and had a bluish hue.
As a final test, I took some of the dark liquid and put some sodium persulfate in this and heated slightly. This results in formation of chlorine
(smell) and the liquid becomes deep yellow. Here, I expected the colorless liquid again with MoO4(2-). A faint yellow/green color I can explain, due
to formation of chlorine, but not the deep lemon-yellow color.
If someone could explain all the colorful things, then I would be very pleased. This really puzzles me.
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Eclectic
National Hazard
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Registered: 14-11-2004
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Google isopolyanions, heteropolyanions. The colors come from mixed oxidation states with the charges spread out over large complex metal-oxygen
molecules. The chemistry is VERY complicated.
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JohnWW
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Oh yes. Ammonium molybdate exists as a complex polyanion containing 7 Mo atoms, and forms an intense yellow color when phosphate is added, the
phosphomolybdate anion. This is the basis of the standard colorimetric method for the determination of phosphate in water.
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guy
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Just a guess
MoO<sub>4</sub><sup>2-</sup> + 2H<sup>+</sup> ---> MoO<sub>3</sub> + H<sub>2</sub>O
MoO<sub>3</sub> + H<sup>+</sup> + Cl<sup>-</sup> ----> some chloride complex
Reduced by hydrogen to form MoCl3 (dark red) http://www.webelements.com/webelements/compounds/text/Mo/Cl3...
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