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Author: Subject: Reducing sodium molybdate by nascent hydrogen
wg48temp9
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[*] posted on 13-6-2020 at 13:16


Quote: Originally posted by Bedlasky  
I posted article about this reaction on my website in czech and english.


I just checked out your link, its a good compliment to this thread and interesting display of the various colours of molybdeum solutions.




i am wg48 but not on my usual pc hence the temp handle.

Thank goodness for Fleming and the fungi.
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Bedlasky
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[*] posted on 14-6-2020 at 07:05


Thanks.

I am preparing article about Mo complexes (but I don't have much time right know). This is also colourful and spectacular chemistry. Molybdenum is really chameleon among elements.




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[*] posted on 19-6-2020 at 15:29


Quote: Originally posted by Bedlasky  
wg48temp9: Thank you very much for your post! You help me to explain one mystery. A few days ago I want to take a picture of sodium phosphomolybdate solution. I dissolved some Na2MoO4 in water, add tiny amount of K2HPO4 and add few drops of conc. HCl. I obtained yellow solution. But when I added more phosphate, I obtained yellow precipitate, which was surprise for me. Now I know why. I used POTASSIUM hydrogen phosphate. When I repeated this experiment today, I used NaH2PO4 instead of K2HPO4. I added just a tiny amount of phosphate and obtained yellow solution. When I added more phosphate, solution became colourless - which corresponds with my previous observations.

So sodium phosphomolybdate is interesting reagent for K+, NH4+ and Rb+ test.

Btw - when you add K+, NH4+ or Rb+ salt in to the solution of sodium silicomolybdate, you obtain also yellow precipitate. Reaction with Rb+ is immediate, while reaction with NH4+ and K+ requieres heating.

Yesterday I prepared ammonium phosphomolybdate. This was a test to check the colour of this compound.
A crystal of 1x0.5cm of ammoniumparamolybdate (NH4)6Mo7O24.4H2O and ~10ml of water were gently heated until a completely clear and colourless solution was formed. The solution was split in two parts of similar volume.

1. To one part, 1ml 85% (ortho)phosphoric acid (H3PO4) was added in a test tube. The mixture was shaken and heated, but no change in colour took place. 2 ml of 10% HNO3 solution were added without change of colour. When it was heated shortly to ~80C, the solution stayed clear but turned yellow; when cooled, it became colourless again after a few minutes. This turning yellow on brief heating and discolouring on cooling was repeated a few times - the reaction seems reversible.
When heated for a few minutes to boiling, a very fine yellow precipitate was formed and the solution turned yellow. After a few hours, a fine precipitate had settled. The supernatant liquid was then colourless, with a some microscopically fine yellow powder on its surface.

2. Likewise, the other part was put into a small flask and converted to ammonium phosphomolybdate by adding 85% H3PO4 and 10% HNO3 solution and boiling briefly. To this an equal volume of 10% HCl solution was added. This was boiled for ~5 minutes and water was added as needed. No changes were observed.

Results 1 and 2 after standing for 1 day are shown here:
IMG_2793_adj_small.JPG - 73kB

I did another experiment. A strong solution of MoO3 in 36%HCl is green in colour. From this solution, 1ml was taken, and 1.5ml of 85% H3PO4 were added. The mixture was shaken. No reaction or change in colour was observed. When water was added (~5ml), the colour immediately turned deep blue. When heated, no changes were seen. The colour disappeared when a soda solution (Na2CO3) was added. No further changes took place when boiling the result. Adding 2ml of 10% HNO3 made no visible changes, neither after boiling
Extra H3PO4 and more HNO3 were added, but no changes were observed, neither after boiling or standing for a day.

Question: Is the presence of chloride known to prevent the formation of phosphomolybdate?


[Edited on 19-6-2020 by Bezaleel]
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Bedlasky
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[*] posted on 19-6-2020 at 22:49


Interesting observations!

I made phosphomolybdate solutions with H2SO4, HNO3 and HCl without any problems, so chloride ions doesn't interfer.

With sodium molybdate and sodium dihydrogenphosphate I made two different phosphomolybdate solutions - one yellow and one colourless. If you add small amount of sodium dihydrogenphosphate (it must be sodium salt, with potassium or ammonium salt you obtain yellow precipitate) in to acidified sodium molybdate solution, you obtain yellow solution. If you add more phosphate, solution become colourless. If you add more molybdate, solution become yellow again. This probably affected your experiment. But it's interesting that ammonium salt didn't precipitated out. I don't know why, but it is probably caused by conditions.

It's also interesting that this equilibrium also depending on temperature. I'll definitely try it.

MoO3 solution in conc. HCl is green? That's something new to me :D. Deep blue solution is mistery. Normally I guess molybdenum blue, but there wasn't any reducing agent and soda solution doesn't affect molybdenum blue.

Thanks for sharing your observations. :)

[Edited on 20-6-2020 by Bedlasky]




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

"An old friend once told me something that gave me great comfort. Something he had read. He said that Mozart, Beethoven and Chopin never died. They simply became music." Dr. Robert Ford, Westworld
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[*] posted on 19-6-2020 at 23:29


Interesting stuff!

I am interested in an analytical test to identify/check rubidium and caesium salts.

I bought some ‘cheap’ rubidium chloride years ago to make rubidium metal - never quite got round to it - but was going for high temp reduction with lithium.

I have been looking for a suitable test and found this paper that uses phospho and silico molybdic acids

I have have ordered some phospho molybdic acid and looking for the silicon version

Attachment: 7209121.pdf (657kB)
This file has been downloaded 21 times

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[*] posted on 20-6-2020 at 05:25


Look at woelen's webpage. There are some interesting tests for caesium.

https://woelen.homescience.net/science/chem/exps/CsCuCl3/ind...

Sodium silicomolybdate is in my opinion better reagent for distinguish K and Rb, because K salts precipitates out only after heating. Sodium silicomolybdate solution can be easily prepared in this way: In to the solution of sodium molybdate add small amount of sodium silicate and then add 1ml of conc. HCl at once (ideally with stirring). If too much silicate is present then hydrous SiO2 precipitates out, so add only small amount!

Another way is flame test. Colour of K, Rb and Cs flames are simmilar, but Rb have more bluish flame than K (I never saw Cs flame, but I suppose that it will be even more bluish).




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

"An old friend once told me something that gave me great comfort. Something he had read. He said that Mozart, Beethoven and Chopin never died. They simply became music." Dr. Robert Ford, Westworld
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[*] posted on 23-6-2020 at 15:02


Quote: Originally posted by Bedlasky  
Interesting observations!

I made phosphomolybdate solutions with H2SO4, HNO3 and HCl without any problems, so chloride ions doesn't interfer.

With sodium molybdate and sodium dihydrogenphosphate I made two different phosphomolybdate solutions - one yellow and one colourless. If you add small amount of sodium dihydrogenphosphate (it must be sodium salt, with potassium or ammonium salt you obtain yellow precipitate) in to acidified sodium molybdate solution, you obtain yellow solution. If you add more phosphate, solution become colourless. If you add more molybdate, solution become yellow again. This probably affected your experiment. But it's interesting that ammonium salt didn't precipitated out. I don't know why, but it is probably caused by conditions.

This well explains what I ran into: too much phosphate forms a Mo complex which is richer in phosphate. Such complexes are colourless and their salts are normally well soluble.
Here are page 660 and 661 from Mellor, vol XI, about the various phosphomolybdates, which supports your observations.
Attachment: Mellor vol XI - phosphomolybdates p660-661.pdf (1.9MB)
This file has been downloaded 19 times
This table is taken from it and gives a synopsis of the ratios between Mo and P at which complexes have been found:
Mellor - phosphomolybdic acids p661.jpg - 150kB

Quote: Originally posted by Bedlasky  
It's also interesting that this equilibrium also depending on temperature. I'll definitely try it.

MoO3 solution in conc. HCl is green? That's something new to me :D.

If concentrated HCl solution is used, and copious amounts of MoO3 are added, it is indeed green:
IMG_2804_adj_small.JPG - 71kB
It's really striking how much MoO3 can be dissolved in concentrated HCl solution.

Quote: Originally posted by Bedlasky  
Deep blue solution is mistery. Normally I guess molybdenum blue, but there wasn't any reducing agent and soda solution doesn't affect molybdenum blue.

Thanks for sharing your observations. :)
[Edited on 20-6-2020 by Bedlasky]

This has to do with the HCl concentration, I think. I only observed the green complex in concentrated HCl with larger amounts of MoO3 added. I think that when the green complex is broken down, some molybdenum blue is formed.
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[*] posted on 25-6-2020 at 16:20


Quote: Originally posted by Bezaleel  

This has to do with the HCl concentration, I think. I only observed the green complex in concentrated HCl with larger amounts of MoO3 added. I think that when the green complex is broken down, some molybdenum blue is formed.


I don't think so. MoO3 is only weak oxidant, it can't oxidize HCl. But it forms few chloride complexes - maybe is one of them blue?

Book 1

Book 2

These two books mentioned [MoO2Cl2(H2O)2], [MoO2Cl3(H2O)]- and [MoO2Cl4]2- complexes. Book 2 also mentioned that [MoO2Cl2(H2O)2] is formed in 6M HCl and [MoO2Cl4]2- is formed in 12M HCl. But they didn't mentioned colour. (Book 1 - page 734, Book 2 - there aren't number of pages, just find title Mo(VI)).

[Edited on 26-6-2020 by Bedlasky]




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

"An old friend once told me something that gave me great comfort. Something he had read. He said that Mozart, Beethoven and Chopin never died. They simply became music." Dr. Robert Ford, Westworld
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[*] posted on 27-6-2020 at 11:22


Bezaleel: I try today make this Mo(VI) chloride complex. I poured small amount of concentrated HCl in to the test tube and dissolved in it some ammonium heptamolybdate. I obtained yellow solution. On direct sunlight looked yellow, but when I put it in to the shadow it looked blue or bluish-green depending on angle of view.

When I dissolved ammonium heptamolybdate in hot acid, some solid precipitates out after some time (probably MoO3). It was almost insoluble in water. Dissolved in NaOH and even better in H2O2.

I'll probably try make some MoO3 from ammonium heptamolybdate and dissolve it in HCl if there will be some difference.




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

"An old friend once told me something that gave me great comfort. Something he had read. He said that Mozart, Beethoven and Chopin never died. They simply became music." Dr. Robert Ford, Westworld
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[*] posted on 28-6-2020 at 11:18


So there are another observations:

With MoO3 there isn't any change in colour. If you dissolve molybdate or MoO3 in commercial 31% HCl, there is formation of light yellow solution, in pure 36% stuff it's bright yellow. Under UV light it's green with yellow tinge. If you dissolve too much molybdate in HCl and place solution in hot water bath, after some time there is formation of MoO3 precipitate which dissolves after adding more HCl. After dilution with water solution lost his colour. Few times there was partial reduction in to molybdenum blue (which can be oxidized by two drops of KMnO4 solution), but I don't know why. Maybe some contamination?

Wiki says, that MoO2Cl2 is yellow compound.

https://en.wikipedia.org/wiki/Molybdenum_dichloride_dioxide

But there is also mentioned green MoOCl4.

https://en.wikipedia.org/wiki/Molybdenum_oxytetrachloride

I don't know why your observation is different from mine. There are two possible reasons:

1. Your MoO3 is contaminated with some reducing agent and there is molybdenum blue formation. Yellow + blue = green.

2. Or you prepare different chloride complex.




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

"An old friend once told me something that gave me great comfort. Something he had read. He said that Mozart, Beethoven and Chopin never died. They simply became music." Dr. Robert Ford, Westworld
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