Sciencemadness Discussion Board

Help troubleshooting a simple double replacement... (Mercury Thiocyante Preparation)

LifeisElemental - 28-12-2015 at 14:09

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]

Volanschemia - 28-12-2015 at 16:01

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! :D

[Edited on 29-12-2015 by Volanschemia]

Etaoin Shrdlu - 28-12-2015 at 16:18

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]

LifeisElemental - 28-12-2015 at 16:45

Quote: Originally posted by Etaoin Shrdlu  
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]


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?

Volanschemia - 28-12-2015 at 18:40

It is odd. Maybe it is forming some kind of soluble mercury/thiocyanate/chloride complex?

LifeisElemental - 28-12-2015 at 18:52

Quote: Originally posted by Volanschemia  
It is odd. Maybe it is forming some kind of soluble mercury/thiocyanate/chloride complex?



Any thoughts on altering concentrations to prevent this? Or destroying the soluble complex?

Volanschemia - 28-12-2015 at 19:01

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).

Bert - 28-12-2015 at 19:37

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.


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.

LifeisElemental - 28-12-2015 at 20:15

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?

Volanschemia - 28-12-2015 at 20:21

Are both your HgCl2 and KSCN solutions saturated?

LifeisElemental - 28-12-2015 at 20:26

Quote: Originally posted by Volanschemia  
Are both your HgCl2 and KSCN solutions saturated?


Yup!

Etaoin Shrdlu - 28-12-2015 at 20:46

Are you sure both your compounds are what you think they are? Mercury thiocyanate has ridiculously low solubility compared to the reactants.

Volanschemia - 28-12-2015 at 20:48

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.

LifeisElemental - 28-12-2015 at 20:49

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.


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)?

Etaoin Shrdlu - 28-12-2015 at 20:51

Are you sure you have potassium thiocyanate and not potassium cyanide?

Volanschemia - 28-12-2015 at 20:51

Hg2Cl2 has a solubility of 0.2mg/100mL.
I think you'd know it you had that :P.

[Edited on 29-12-2015 by Volanschemia]

Volanschemia - 28-12-2015 at 20:55

Quote: Originally posted by Etaoin Shrdlu  
Are you sure you have potassium thiocyanate and not potassium cyanide?


Or Potassium Cyanate for that matter.

LifeisElemental - 28-12-2015 at 20:58

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.

Volanschemia - 28-12-2015 at 21:02

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]

LifeisElemental - 28-12-2015 at 21:10

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.

Volanschemia - 28-12-2015 at 21:22

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.

LifeisElemental - 28-12-2015 at 21:44

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.


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]

Volanschemia - 28-12-2015 at 21:49

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.

Boffis - 29-12-2015 at 13:20

@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.

LifeisElemental - 29-12-2015 at 20:51

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]

Etaoin Shrdlu - 30-12-2015 at 10:29

The image in the Popular Science magazine shows the precipitation happening instantly as the thiocyanate is poured. I'm finding mentions that the solution should be stirred, but no indication of any lengthy reaction times anywhere. Very odd.

Nice that you got crystals.

LifeisElemental - 30-12-2015 at 13:40

Quote: Originally posted by Etaoin Shrdlu  
The image in the Popular Science magazine shows the precipitation happening instantly as the thiocyanate is poured. I'm finding mentions that the solution should be stirred, but no indication of any lengthy reaction times anywhere. Very odd.

Nice that you got crystals.


Yes very odd. I actually decanted the solution a few hours after and found that crystals formed overnight in that decanted solution even.

LifeisElemental - 30-12-2015 at 22:05

Another Update

I upped the scale. Keep in mind I am doing this in a cold environment of approximately 16-18C. In a 500mL Erlenmeyer flask I dissolved 23.675g of mercuric chloride in 350mL of water with heating of ~50C. I then dissolved 11.998g of KSCN in 250mL of water and added this to the 500mL flask containing the dissolved mercury salt. As always, nothing appears to happen. Even within 10 minutes, nothing.

Within 50 minutes crystals formed in solution. I will check on this again in the morning as I know they will continue to grow. Very beautiful, needle like crystals. At some point I will certainly setup a time-lapse of this for my videos.

Absolutely puzzling. Does anyone have mercuric chloride/potassium thiocyanate that they may be willing to attempt this on a hundred milligram scale?

To anyone wishing to reproduce this in large scale as I have - keep mercuric chloride in excess and keep the thiocyanate in relatively the same molar concentrations.

[Edited on 31-12-2015 by LifeisElemental]