Bedlasky
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HCl gas generator for making (NH4)2[MoOCl5]
Hi.
A found article about preparation of (NH4)2[MoOX5] (X = Cl; Br).
I want to try prepare at least (NH4)2[MoOCl5] (I don't have any HBr or destillation setup currently for making (NH4)2[MoOBr5]). As reductor I'll use
SnCl2 instead of HI, because from my experience iodides can't reduce molybdates. For crystallization is needed to introduce some HCl gas in to the
solution. So I have an idea - reaction between NaCl and concentrated H2SO4. But I am not sure if this reaction doesn't release some chlorine gas too.
I also read that dissolving anhydrous CaCl2 in concentrated HCl also leads to releasing HCl gas. What method is better?
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njl
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Probably the latter, and if you do try that method for HCl generation a quick write up would be nice too.
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Fery
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Those at room temperature - dehydrating 37% HCl using CaCl2 you mentioned or conc. H2SO4 works too. It is good practice then to bubble the gas evolved
from the generator through conc. H2SO4 in a washing bottle to remove traces of water if you need the gas very dry. The dropping funnel should be
pressure equalizing - if you do not have such one, then better to heat NaCl + conc. H2SO4.
48% HBr is sold by e.g. pentachemicals.eu - perhaps funchem can buy it for you if you do not have business ID ? Funchem is very kind supplier (ask
them via an email, they respond very quickly).
Just remember that for producing HBr gas you can't use H2SO4 as it oxidizes HBr to Br2.
HI could be generated in situ from I2 and recycled using tetrahydronaphtalene (it can be used to synthesize HBr from Br2 too).
These experiments with HCl, HBr gases must be done in fumehood and disassembling the apparatus is best to do outside when there is a slow wind, these
gases have higher density than air so they stay at the bottoms.
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Bedlasky
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I tried today prepare (NH4)2[MoOCl5]. I mixed 0,5 of (NH4)6Mo7O24, 0,5ml of 1+1 ammonia, 5 ml of 31%HCl and 0,4g of SnCl2.2H2O. I obtained brown
solution. So I add some HCl until solution turned colour in to the green. Then I solution cooled in saturated NaCl from freezer and added HCl gas from
31%HCl/anhydrous CaCl2 HCl generator. But I obtained only some white precipitate on the bottom of test tube (probably NH4Cl).
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Fery
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Why didn't you use HI as suggested in the book? There are some possibilities to prepare it (it is too overpriced to buy it).
KI + H3PO4
H2S + I2
P + I2
tetrahydronaphtalene + I2
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Bedlasky
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From my experience - iodides can't reduce molybdates. I tried it some time ago, and it didn't react. I can try it again and change little bit
conditions, molybdenum chemistry is complicated. But I am sure that SnCl2 or TiCl3 can reduce Mo in to V oxidation state, so that's reason why I used
SnCl2.
Edit: I quickly tried this - I dissolved Na2MoO4 in concentrated and 1+1 HCl and add some KI in to it. Both solutions for a while turned in to very
dark brown, so reduction truly occurred. When I tried it first time, I use only 10% H2SO4 for acidification - for reductions by thiosulfates or
ferrous salts is it enought, but for iodide stronger acid is needed.
[Edited on 22-2-2020 by Bedlasky]
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woelen
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Are you sure it is the molybdate, which oxidizes the iodide? Could it be oxygen from air? Maybe you should try the experiment again (with the 1+1
acid), by boiling the solution first. This drives off oxygen. You should put the iodide in a small vial with a cap. Then you should add the solution
to the small vial, which you fill to the rim. Then cap it tightly to avoid introduction of new oxygen into the solution on cooling down. Especially
for experiments of this kind I have purchased glass test tubes, with plastic screw caps.
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Bedlasky
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Fe in HCl can oxidize iodide too. But when I add iodide in to my HCl, solution is yellow. This was much much more intense colour. I plan repeat this
reaction and boil off all iodine to see what colour have solution without it.
I also want to try again preparation of (NH4)2[MoOCl5] with some modifications.
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Bedlasky
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It is confirmed, that iodides can reduce molybdates in to V oxidation state. I mixed together ammonium heptamolybdate, potassium iodide and HCl,
boiled off all iodine and I obtained green solution.
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woelen
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Interesting result and good experimentation. Nice to see your reasoning behind the experiments and how you use the observations to come to your
conclusions.
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