Sciencemadness Discussion Board

Yellow calcium chloride

mrtimelord007 - 21-11-2017 at 08:42

Hello,
I was making calcium chloride by mixing hydrochloric acid (~36%) and calcium hydroxide. When I poured HCl on Ca(OH)2 it made yellow. After all calcium hydroxide dissolved it was still yellow.
I used pure reagents, and repeated this few times - it was still yellow.

Could someone help me? Why this is yellow?
Thanks

PS. I am sorry for my English

DraconicAcid - 21-11-2017 at 08:50

I'd guess a slight iron contamination in the hydrochloric acid.

aga - 21-11-2017 at 08:59

Home made CaCl2 is almost always yellow, for the reason Draconic stated.

Works OK as a drying agent, so do not worry about it.

Making 100% pure chemicals is really hard to do.

Sigmatropic - 21-11-2017 at 09:14

I too have noticed a yellow color when mixing sulfuric acid and commercial calcium chloride. I presumed some of the chloride was oxidized by the sulfuric acid, but did not bother looking up the oxidation potentials. IIRC The color faded when I diluted it with water.

happyfooddance - 21-11-2017 at 09:18

If you need CaCl2 of higher purity, you can convert your Ca(OH)2 to the carbonate and rinse well, and HCl is easily purified if you have the skills. I have had varying success as well, but using distilled HCl always gives a white finished product for me.
Sometimes when you add the acid the solution will turn yellow, until all the CaCO3 has reacted, then it turns clear.
Alternatively, you could probably reflux your CaCl2 solution with activated carbon to clear it up a bit.

unionised - 21-11-2017 at 10:18

It's not a bad guess that the problem is iron.
If it is in that acid there's a simple, but slow way to deal with it.
Get a large container than you can seal and which will not react with the acid. A plastic sandwich box would do, and two small glass containers which will fit in the box.
Put acid in one small container and put Ca(OH)2 and a little water in the other.
Then put both of them in the box, and wait.
The HCl is volatile and will evaporate. It will then dissolve in the water and react with the hydroxide. The CaCl2 formed can't easily "lose" HCl so the reaction goes virtually to completion.
Any iron present in the acid will be left behind.

It may take a long time.

There's another thought that may be useful here. If the issue is iron then adding an excess of Ca(OH)2 will precipitate iron as Fe(OH)3. You can filter that off and (hopefully) get a clean white product.

mrtimelord007 - 21-11-2017 at 12:34

The problem is that when I add few drops of NaOH precipitated white Ca(OH)2.

unionised - 21-11-2017 at 12:43

OK, probably not Iron.
Have you considered recrystallising the CaCl2 ?
That should get rid of the impurity, whatever it is.

aga - 21-11-2017 at 13:21

The colour is usually iron - the most likely culprit usually is.

I tested some of my acids and found traces of iron in each.
Unsurprising seeing as they were bought as agricultural products - side products from industrial processes conducted in steel containers.

Purity all depends on what you want calcium chloride For.

If it needs to dry gases like chlorine, yellow is OK.
It will be .2H2O anyway, unless you got it up to around 300+ C for a long while and cooled it under vacuum.

If it needs to be 100% pure as an actual Reagent, then you got to balance $ against time, which generally ends up as a Buy decision.

Recrystallising CaCl2 is impractical - it is so hygroscopic it deliquesces.

No chance of it ever crystallising at all in 'normal' conditions, it just ends up as a puddle.

happyfooddance - 21-11-2017 at 13:28

Quote: Originally posted by aga  
Recrystallising CaCl2 is impractical - it is so hygroscopic it deliquesces.

No chance of it ever crystallising at all in 'normal' conditions, it just ends up as a puddle.


Which is why I suggested converting to carbonate: it is much easier to purify than either the hydroxide or the chloride.

Also I am not convinced that NaOH would precipitate iron before calcium: I would still suspect your acid. What color your acid?

aga - 21-11-2017 at 13:42

Please detail/demonstrate that process, with photos.

Every time i ever made calcium chloride, it came out yellow.

Put your home-made calcium chloride colour where your mouth is ;)

Edit :

The only process i have used is to dissolve the Carbonate in HCl then filter, evaporate down, then blast with as much heat as possible to get it anywhere near dry.

If converting it back to carbonate works better, i would very grateful indeed.

Show us.

[Edited on 21-11-2017 by aga]

happyfooddance - 21-11-2017 at 15:19


15113014289301679613661.jpg - 1.9MB

I had written a small novel, but it didn't manage to post. That is good, I am disheartened so I won't be so longwinded (saving in notepad, fml).

Aga, you said you add HCl to the carbonate? I only make the carbonate as a byproduct of making NaOH solution. I rinse the left over filter cake well, it filters slowly and sits neglected in a corner for days. I rinse it until it has no sodium taste. I use clear, distilled azeotropic HCl. It requires heat and stirring and time to dissolve all the calcium carbonate without adding too much acid.

The other main thing is drying, I finish drying in a glass dish, first on a hotplate until it looks dry, then in a small electric oven set to 200°C. When it starts to thicken up it is best to break it into clumps, so that they dry faster and especially don't stick to the glass. This is pretty much an art. I also dry my "recieving" jar and lid in the oven, you can see that the very dry salt has coated the jar which has been opened several times, likely in a humid kitchen.

If there is a way to properly crystallize calcium chloride, I wouldn't know it. It is extremely hygroscopic, has an ass-backwards solubility curve in water, complexes with alcohols, and is insoluble in polar solvents AFAIK... But I suggest using pure HCl. I use my calcium chloride for drying but also for making pickles, there's no way I would HCl that hasn't been distilled carefully.

[Edited on 11-21-2017 by happyfooddance]

mrtimelord007 - 21-11-2017 at 22:41

My acid is colorless and reacts with everything normally, so I guess that the problem is not with the acid. Calcium hydroxide should be also pure. I bought reagents in shop, so it should be pure.

Now I don't need calcium chloride, but I am curious why it's yellow.

unionised - 22-11-2017 at 13:43

Quote: Originally posted by aga  
The colour is usually iron - the most likely culprit usually is.

I tested some of my acids and found traces of iron in each.
Unsurprising seeing as they were bought as agricultural products - side products from industrial processes conducted in steel containers.

Purity all depends on what you want calcium chloride For.

If it needs to dry gases like chlorine, yellow is OK.
It will be .2H2O anyway, unless you got it up to around 300+ C for a long while and cooled it under vacuum.

If it needs to be 100% pure as an actual Reagent, then you got to balance $ against time, which generally ends up as a Buy decision.

Recrystallising CaCl2 is impractical - it is so hygroscopic it deliquesces.

No chance of it ever crystallising at all in 'normal' conditions, it just ends up as a puddle.


Put the puddle in a closed bottle and cool it in the fridge.
The xtals are white and the mother liquor yellow.

Worked fine when I did it.

unionised - 22-11-2017 at 13:45

Quote: Originally posted by happyfooddance  

Which is why I suggested converting to carbonate: it is much easier to purify than either the hydroxide or the chloride.


How?

happyfooddance - 22-11-2017 at 13:49

When I convert to the carbonate, it is refluxed in what is essentially a highly concentrated sodium hydroxide solution, for about half an hour. I would imagine this would dissolve most organics and oils (dust?), which then are easily washed away. I agree also that your problem is likely not the acid; I am wondering if food grade CaO is more pure, it might get additional treatment to remove arsenic.
Quote: Originally posted by unionised  
Quote: Originally posted by happyfooddance  

Which is why I suggested converting to carbonate: it is much easier to purify than either the hydroxide or the chloride.


How?


Reflux with equimolar sodium carbonate.

[Edited on 11-22-2017 by happyfooddance]

aga - 22-11-2017 at 16:26

Amateur Chemistry is more about what an Amateur can do in their lab.

Working with 99.9% pure reagents is not common.

Not too unusual for some, just less Normal for most.

aga - 22-11-2017 at 16:35

Quote: Originally posted by unionised  
Put the puddle in a closed bottle and cool it in the fridge.
The xtals are white and the mother liquor yellow.

Worked fine when I did it.

Sounds like a plan.

From my experience it sounds like a bad one, but there is only One way to find out.

Some sea shells and hydrochloric acid and a bit of filtering will show if it works.

if anyone else cares to mix calcium carbonate/white sea shells with hydrochloric acid for themselves, then stick it in their fridge, please do so.

Confirmation is always good, which is how Science happens.

Then again, complex chemical procedures like this are difficult for most people, so don't feel bad if it is beyond your current capabilities.

Edit:

changed freezer to fridge

[Edited on 23-11-2017 by aga]

happyfooddance - 22-11-2017 at 16:42

Calcium chloride is far more soluble in water at cold temperatures than hot. When I put my damp crystals in the fridge or freezer, they turn into a bigger puddle. Unless we are talking about different calcium chlorides? :)

aga - 22-11-2017 at 16:44

Nope. Same Calcium Chloride.

unionised has stated that you just stick it in the fridge and it worked.

Worth the small effort to find out if that is true,

Heating the bejeezus out of it takes more effort.

happyfooddance - 22-11-2017 at 16:56

I have tried it. More than a few times. I have also tried to skim crystals as they form in a boiling saturated solution, it is not worth the burns as it bumps badly, and they melt unless you manage to keep them hot, and you still have to put them in the oven to dry. The crystals anyone gets from an aqueous solution are still going to have to be rigorously dried, I don't see you doing that in any way that doesn't require heating the bejeesus out of it. We are talking about the same extremely hygroscopic calcium chloride, right?

A nice chart and article, from others who have also done this apparently:
http://www.phasediagram.dk/binary/calcium_chloride.htm

[Edited on 11-23-2017 by happyfooddance]