Any chemists out there?

[Hodgkinson]

Hi all,
I've got a home chemistry-related project going at the moment and im wondering if there are any chemists here on A.org that could help me out ;-) - The results of the project aren’t what I expected to find (Purity issues).

Help appreciated, :help:
Hodgkinson.  

[bloodline]

I was a Chemist... But that was nearly 10 years ago now...

[JaXanim]

Yep. What's the problem?

JaX

[Hodgkinson]

Hi there!
As I've found with chemistry there's not much point "Beating around the bush" so I’ll just describe what im trying to do.
What im working on is a process to recycle isopropyl alcohol left over in the container from these and to reduce the water content for use with electrical contacts and electronics in general.

I have strung together a process where the alcohol is "Salted out" (Excess table salt added) to cause the alcohol and water to separate into distinct layers. Once the alcohol layer is carefully removed, it's distilled to remove the salt component, leaving what should be pretty nearly pure isopropyl alcohol (Well, according to the associated "Instructable" that is...Note that the distillation step used is somewhat more *appropriate* to that shown...).

The problem is that a density test on the distilled isopropyl alcohol showed 20.81g for 25cm3 of product, which, assuming that water is the impurity, would indicate about 20% water present :-?
In a seperate test, evaporating off about 5cm3 of the product left no appreciable residue, so by that I presume that the distillation was successful in removing all the dissolved salt.

Given the additives listed in the link, and the fact that the table salt being used contains, in addition to sodium chloride, magnesium carbonate and sodium hexacyanoferrate II as anti-caking agents, is there anything present that might upset the "Salting-out process" and lead to only some of the water being separated out?

Sorry about the huge list of info :-o

Theories appreciated!

Hodgkinson.

[JaXanim]

Assuming your density/water content data are correct, I'd say you haven't got enough water out at the salting out step. The basic principle is that though water and IPA are miscible, adding a water soluble electrolyte (NaCl) reduces the proportion remaining in solution. A small residual water content will spoil the results when you distil the product. IPA and water form an azeotrope, which means the water will co-distil and dilute the product. The salt isn't soluble in IPA at all, but water will partition between the IPA and brine layers depending on how much salt is used. I suspect you didn't use enough. Alternatively, you didn't allow long enough for the salt to 'find' all the water at the shaking stage. You need to get this mixture really well shook up for quite a while to ensure the salt does its job.

JaX

[Hodgkinson]

Thanks for the advice - I've just been and tipped yet more salt into the already salted-out (But not yet distilled) mix batch - /Me wonders how hard it could be to make an electric stirrer :-D
I'll have a look at the mixture in the morning.

Would gentle heating of the mixture help in any way (By allowing further salt to dissolve) just prior to separating the layers?

Regards!
Hodgkinson.

[persia]

I went to the Chemists this afternoon, needed some panadol...

[JaXanim]

@Hodgkinson

Heating won't help very much and you'll lose some IPA by evaporation. It's the contact that's important. I'm surprised that salting out will get the water down as low as suggested in the article. Making 99%+ IPA by this method is gonna be difficult I reckon. A better method for the final drying step is dehydration using something like anhydrous sodium sulphate or maybe silica gel. The IPA layer should be treated with this for a couple of days before distillation.

JaX

[Hodgkinson]

Well, there was some additional water in there - About 20mins of shaking and additional (Excess) salt yielded about 1ml of water right at the bottom of the container. :crazy:

Since silica gel is another form of sand (Am I right here?) then I assume that it’s unlikely to react with just about anything.
Would there be any detriment to adding the dried-out silica gel after the distillation step as opposed to before it (***And maybe leaving it in the storage container***) - I'm restricted on what I can do where in the process due to efforts to recycle salt/anti-bumping granules/left over alcohol etc ?

Besides, (Theorising here) isn't there a possibility of salt becoming trapped within the structure of the silica gel and either damaging the silica gel structure when drying it out, or at least "clogging up" the structure?

Any ideas how much silica gel might be required for a particular amount of alcohol?

Thanks!
Hodgkinson.

[JaXanim]

There needs to be excess salt undissolved in the mixture to ensure you get the maximum dehydration. Saturated brine is only around 27% salt so add enough to remain undissolved.
You may not need to add anything extra if you can get the last traces of water out using salt. Any sodium sulphate or silica gel is better added before distillation. It won't be affected by the salt.

JaX

[Hodgkinson]

There's piles of undissolved salt in the mixture, so no problem there. The plan is to pipette off the top alcohol layer as far as possible, leaving just a thin "boundary layer" of alcohol behind (Practical limitations), along with the undissolved salt and associated lower water layer. To save salt and to avoid cumulative alcohol losses, I'll add the next batch to this remaining mix and repeat the process.

Out of curiosity, why add the silica gel before the distillation? To prevent contamination?

Hodgkinson.

(PS. Thanks for the pmail)

[Cymric]

Hodgkinson wrote:
The problem is that a density test on the distilled isopropyl alcohol showed 20.81g for 25cm3 of product, which, assuming that water is the impurity, would indicate about 20% water present :-?

How did you establish the density, and what are the uncertainties in the reported values? Wikipedia gives a density of ~0.79 g/cm^3 for IPA, and the values you give indicate a density of ~0.83 g/cm^3. If you are just 1 ml off (as in 26 ml), the density already drops to 0.80 g/cm^3!

Second, if these values are indeed highly accurate, and precise to four decimals each (i.e., 25.00 ml---very unusual if you manage that!) then you really want to be using partial molar volumes. In other words: if you add a volume of water to a volume of IPA, I'll guarantee you that the resulting volume is not the sum of the two bodies of liquid. It can be more, or it can be less. In other words yet again: the density of the mixture does not change linearly with water content; linearity is only an approximation which in case of very sensitive measurements fails to hold up.

Third, if you can find some, I'd recommend zeolite granulate as a drying agent rather than silica gel. The later is mechanically very weak; zeolite is stronger. The stuff looks like off-white sprinkles, and in order to activate them you just roast them in an oven at a high temperature.

[JaXanim]

Hodgkinson wrote:
- - - why add the silica gel before the distillation? To prevent contamination?

No, simply to avoid distilling the azeotrope. You can do a final dehydration on the distillate if you wish. What you use for that depends on what's available to you. I supposed you were doing this in the kitchen sink, but if you're working in a lab, there's other alternatives/techniques. Anhydrous sodium sulphate takes some beating.

Watch the videos.

JaX

[Hodgkinson]

The density was established using a standard 25cm3 lab pipette (25cm3 of alcohol used) and a balance, accurate to 0.01g.

The ~1ml of water that was additionally observed was in a 500ml bottle of salted-out alcohol (Sorry about any confusion). I wasn't aware of the issues with regards to the volumes not adding up directly, so thanks for the info.

Oh, just for the record, this lot is in fact being done at the kitchen sink :crazy: , just with lab equipment; and along with taking the odd sample to school to use their *expensive* balances. I’m deliberately trying to limit myself to easily available chemicals/agents as a direct result, and so that the process is easily repeatable.

I think I'll give the process a try this afternoon without the drying agent for simplicity (Just as soon as tea's cleared up) and try to find chance to do another density test at school sometime soon. If the water content still *seems* to be high I might try the silica gel method since we’ve been collecting the stuff for some time now (Helps to keep all the electronics in the garage dry).
If I don't report back in the next week or so the kitchen has probably blown up or burnt down (Or both)! :insane:

Hodgkinson.

[Hodgkinson]

New thread started by WTF. Just thought I'd leave a note.

=> Kitchen not yet burnt down :crazy: