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LifeisElemental
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Help troubleshooting a simple double replacement... (Mercury Thiocyante Preparation)
Hey guys,
Would really appreciate help troubleshooting the reaction of mercuric chloride with potassium thiocyanate. This obviously should just be a simple
double replacement reaction where mercury (II) thiocyanate crashes out of solution.
What I've done so far. I made a solution of ~5g mercuric chloride in ~200mL store distilled water. I added in ~20mL of a saturated
KSCN solution. Absolutely nothing happened.
On PrepChem it is stated that nitric acid should be added to prevent the "formation of a basic salt". I did not do this.
I added in ~4 drops of concentrated hydrochloric acid to see if acidifying the solution would change anything. It did not. pH tested to be ~2.
I then added in enough sodium bicarbonate to bring the solution to ~12. Nothing happened.
Any thoughts? Will try as they come in. I can prepare new solutions without this contamination.
EDIT: A video of what I'm seeing happening.
https://www.youtube.com/watch?v=0HUXPqF9e6g&feature=youtu.be
[Edited on 29-12-2015 by LifeisElemental]
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Volanschemia
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I think your problem is that you are using the chloride salt, whereas the reaction is usually carried out with the nitrate. Metathesis reactions
won't always do what you expect them to (I've had a few do strange things).
I'm not certain on this, and I'm sure someone more experienced will be able to answer it better, but I am thinking it would be because the chloride
ion is much more electronegative than the nitrate ion. It wants to stick around with the Hg(II) and not be replaced with the thiocyanate ions.
Would it be possible for you to make some Mercury(II) Nitrate and give it a try that way?
[Edit] 250th post! 
[Edited on 29-12-2015 by Volanschemia]
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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Etaoin Shrdlu
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Volanschemia is right, chloride liberates thiocyanate from mercury.
EDIT: Actually, I was remembering a test that has iron present, which likes the thiocyanate. This indicates what you're doing should be working fine.
https://books.google.com/books?id=AiEDAAAAMBAJ&pg=PA202
[Edited on 12-29-2015 by Etaoin Shrdlu]
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LifeisElemental
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So weird! I don't have mercuric nitrate on hand but can get a small amount after New Year.
Any suggestions to mix things up? Maybe in the conditions I used somehow mercuric thiocyanate is soluble?
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Volanschemia
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It is odd. Maybe it is forming some kind of soluble mercury/thiocyanate/chloride complex?
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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LifeisElemental
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Any thoughts on altering concentrations to prevent this? Or destroying the soluble complex?
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Volanschemia
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If there is a complex, diluting the solution may work. It works for crashing some compounds out of solution dependent complexes
(CuI/I- complex for example).
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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Bert
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Old fashioned Pharaoh's serpents?
Been a LONG time since I have seen these. Never tried it myself, I had "louder" products in mind for whatever Mercury compounds were available to me.
One of several consumer fireworks ingredients specifically banned in USA, along with white Phosphorus.
Per Weingart (who actually made these things, apparently):
| Quote: |
PHARAOHS SERPENTS EGGS.
This remarkable substance consists of small pellets of sulphocyanide of kmercury which has the remarkable property of swelling 25 to 50 times its
original size when lighted, producing a long snakelike ash. To prepare, make a concentrated solution of mercuric chloride and add little by little a
solution of potassium sulphocyanide, stirring constantly. A greyish precipitate will be formed and when the last drop of sulphocyanide added no longer
produces cloudiness permit the mixture to settle. Drain off as much as possible of the clear supernant liquid, remove precipitate to a paper filter
placed in a glass funnel and wash slightly. When thoroughly dry reduce to a fine powder. When ready to form the eggs moisten very sparingly with a
weak solution of gum arabic to which may be added a pinch of saltpeter and form into cones.
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To hazard a guess, Weingart specifies "concentrated solution" but your Mercuric chloride solution is nowhere near saturated. I find a figure of
7.4g/100ml H2O solubility @ 20 C on a quick Google search, vs. 5g/200ml from your report.
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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LifeisElemental
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Quote: Originally posted by Bert  | Old fashioned Pharaoh's serpents?
To hazard a guess, Weingart specifies "concentrated solution" but your Mercuric chloride solution is nowhere near saturated. I find a figure of
7.4g/100ml H2O solubility @ 20 C on a quick Google search, vs. 5g/200ml from your report. |
Gave it a try in a test tube at a smaller scale with a concentrated mercuric chloride solution. Nothing still. 
I recorded myself adding the thiocyanate salt if you'd like to see what I'm seeing.
https://www.youtube.com/watch?v=0HUXPqF9e6g&feature=youtu.be
Ill try crashing this into a large excess of water as suggested previously.
Does anyone have any thoughts as to if any additives may help?
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Volanschemia
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Are both your HgCl2 and KSCN solutions saturated?
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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LifeisElemental
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Yup!
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Etaoin Shrdlu
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Are you sure both your compounds are what you think they are? Mercury thiocyanate has ridiculously low solubility compared to the reactants.
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Volanschemia
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Hmm, it seems like the only explanation is either a complex, impurities (doubtful) and what I said first up, about the thiocyanate ion being unable to
replace the chloride (although that looks to not be the case).
If you want to convert the chloride salt to the nitrate, I imagine it could be done by reacting the HgCl2 with a hydroxide (like NaOH) to
precipitate the insoluble yellow HgO. Then you could react that with concentrated Nitric Acid to produce Hg(NO3)2.
This will definitely react with KSCN to precipitate Hg(SCN)2.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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LifeisElemental
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| Quote: Originally posted by Etaoin Shrdlu | | Are you sure both your compounds are what you think they are? Mercury thiocyanate has ridiculously low solubility compared to the reactants.
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My only thought is that somehow I was shipped mercurous chloride but from what I can find that has a lower solubility and what I'm dealing with
dissolves in water in accordance with the state 7.4/100mL for mercuric chloride.
Is there a simple way I can test to ensure I'm dealing with mercury (II) rather than mercury (I)?
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Etaoin Shrdlu
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Are you sure you have potassium thiocyanate and not potassium cyanide?
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Volanschemia
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Hg2Cl2 has a solubility of 0.2mg/100mL.
I think you'd know it you had that  .
[Edited on 29-12-2015 by Volanschemia]
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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Volanschemia
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Or Potassium Cyanate for that matter.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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LifeisElemental
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| Quote: Originally posted by Volanschemia | Hmm, it seems like the only explanation is either a complex, impurities (doubtful) and what I said first up, about the thiocyanate ion being unable to
replace the chloride (although that looks to not be the case).
If you want to convert the chloride salt to the nitrate, I imagine it could be done by reacting the HgCl2 with a hydroxide (like NaOH) to
precipitate the insoluble yellow HgO. Then you could react that with concentrated Nitric Acid to produce Hg(NO3)2.
This will definitely react with KSCN to precipitate Hg(SCN)2. |
Reaction of the Mixture with 5N NaOH https://www.youtube.com/watch?v=IVnBDgS3jBk&feature=youtu.be
Well... this works like it should - so confusing!
The mercuric chloride is from Fischer so I'm very comfortable with its purity. The thiocyanate is from Home Science Tools but I've never had issues
with them before.
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Volanschemia
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The precipitate is Mercury(II) Oxide, formed from the decomposition of the Mercury(II) Hydroxide formed by the reaction of Hg<sup>2+</sup>
and OH<sup>-</sup>.
I'm really not sure what's going on re the chloride and thiocyanate. I think your best bet would be to convert some of your chloride to the nitrate
in the way I mentioned above and then react that with the thiocyanate to get your Mercury(II) Thiocyanate.
[Edit] Typos.
[Edited on 29-12-2015 by Volanschemia]
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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LifeisElemental
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| Quote: Originally posted by Volanschemia |
I'm really not sure what's going on re the chloride and thiocyanate. I think your best bet would be to convert some of your chloride to the
nitrate... |
Thoughts on if the oxide will settle? Would prefer to dirty as little as possible and would rather decant off the waste solution than filter.
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Volanschemia
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I don't know. Just gauge how it's going after a few hours. If you can see a clear spot at the top then it should have settled after a day or two.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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LifeisElemental
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| Quote: Originally posted by Volanschemia | | I don't know. Just gauge how it's going after a few hours. If you can see a clear spot at the top then it should have settled after a day or two.
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Well it's late here on the east coast and I just decided to go for the little bit that had settled so we could have some answer.
New video of this mercuric nitrate preparation.
I dissolved the solid, which still had some liquid in it, in 70 percent nitric acid and then diluted this to the volume shown in the video. I added in
KSCN and saw the following.
Something brown forms... I will come back to it later and repeat the preparation in a more reproducible manner and without the initial mixing of HgCl2
and KSCN prior to HgO formation.
[Edited on 29-12-2015 by LifeisElemental]
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Volanschemia
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Wow, I don't know what that was. Definitely do it with purified Mercury(II) Nitrate next time. Be aware too that in concentrated acids KSCN can
react to form HCN.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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Boffis
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@Lifeiselemental; Over the years I have made a considerable amount of mercury II thiocyanate and it is easy BUT requires care and precision. I use
mercuric nitrate solution that I prepare myself by dissolving an accurately known amount mercury in HOT conc. nitric acid, the later in excess, and
then removing most of the excess nitric acid by vacuum distillation. The resulting solution still contains significant nitric acid but this is
essential to keep it stable (isolating crystalline mercuric nitrate is very difficult as it is extremely soluble, delequescent and readily hydrolyses
to insoluble basic nitates in the absence of free nitric acid). I then dilute this to a convenient volume in suitable volumetric flask. I then
calculate based on the original weight of mercury the molarity of the solution. From this I can calculate the amount of potassium thiocyanate (2 molar
equivalents) required to precipitate the mercury. I usually add about 0.3 to 0.4 molar equivalence of potassium acetate to the thiocyanate solution to
reduce the affect of the excess nitric acid in the thiocyanate ions (it oxidizes them). I add the thiocyanate to the mercury nitrate solution and
towards the end of the precipitation I test the solution by allowing the suspension to settle and then add a drop of fresh mercury nitrate solution,
no precipitate =more thiocyanate required, creamy white ppt =already an excess of thiocyanate.
A small excess of thiocyanate dissolves a lot of mercury thiocyanate because of the formation of complex thiocyanatomercurate ions but these ions do
not have a simple intiger ration of thiocyanate to mercury but rather depend on concentration and ionic rations in the solution and also on the
presence of competing ligands (i.e. chloride ions). I always use an excess of mercury nitrate and then precipitate the remaining mercury with sodium
hydroxide. I avoid chloride and ammonium ions altogether.
So your problem almost certainly stems from the fact that you have no idea what ratio of mercury to thiocyanate you are using. For instance how strong
is "concentrated" KSCN solution. My recollection is that this salt is exceedingly soluble and that a saturated solution is near 50% in which case you
have added for too much thiocyanate and all the mercury will remain in solution.
One further consideration is that mercuric chloride is almost completely un-ionised and exists in solution as HgCl2 molecules and it is quite possible
that the mercuric ion concentration in a 5% mercuric chloride solution is too low to precipitate mercury thiocyanate inspite of the fact that is
should do so theoretically (based on solubility product). When you add thiocyanate ions to this solution it takes time for the ions to disassociate
and combine with the thiocyanate, therefore the rate of precipitation may be controlled by the rate at which mercuric chloride splits into mercury and
chloride ions and this may be slow. You need to check out the chemistry of mercuric chloride, there is plenty of published data on it on the SM
library site on this compound.
So you need to use a different mercury salt or develop greater patience and precision in your experimentation. Always plan your experiments, even
simple ones, particularly ones that involve expensive or hazardous reagents. If in doubt try it on a small scale first.
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LifeisElemental
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| Quote: Originally posted by Boffis |
So your problem almost certainly stems from the fact that you have no idea what ratio of mercury to thiocyanate you are using. For instance how strong
is "concentrated" KSCN solution. My recollection is that this salt is exceedingly soluble and that a saturated solution is near 50% in which case you
have added for too much thiocyanate and all the mercury will remain in solution.
One further consideration is that mercuric chloride is almost completely un-ionised and exists in solution as HgCl2 molecules and it is quite possible
that the mercuric ion concentration in a 5% mercuric chloride solution is too low to precipitate mercury thiocyanate inspite of the fact that is
should do so theoretically (based on solubility product). |
Tried again carefully measuring with an analytical scale. Added ~3 equivalents of saturated mercuric chloride to 0.045g of KSCN in 3mL water. Still
no precipitation.
My current thoughts.
Is there anyway to easily increase the ionization of the mercuric chloride solution?
Thank you!
EDIT somewhat interesting. Over a few hours a few crystals formed on the bottom of the test tube. I will isolate these and see if
they will ignite to test if they are mercury thiocyanate tomorrow.
EDIT EDIT: it expands on heating into an orange solid. I think this is relative confirmation I've created the thiocyanate! Only a
hundred milligrams at this scale - I will up the scale a bit more and see how it turns out.
This is one weird double replacement but it does seem that it just slowly precipitates out as needle-like crystals if left sitting with
mercuric chloride in excess.
[Edited on 30-12-2015 by LifeisElemental]
[Edited on 30-12-2015 by LifeisElemental]
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