Sciencemadness Discussion Board

Determining the Concentration of Dimethylamine in Anhydrous Ethanol

Newton2.0 - 18-4-2021 at 13:21

I have produced a solution of DMA in EtOH using the DMF/NaOH method. I used 3A mol sieves to remove the H2O from the solution so I am assuming that I have a mostly anhydrous solution. I don't have accurate starting measurements of anything, so I can't use that to do a "before and after" measurement.

I have determined that the density of EtOH is about 0.78g/mL at 30C and my solution weighs about 8.32g/10mL. Subtracting the weights gives me 0.52g DMA. One of the issues, however, is that I am neglecting the density contribution of DMA which confounds my actual measurement.

Does anyone have a formula or something? I apologize for not being able to figure it out right now. My brain is off. I am sure there's a way to do this.

If I had a table to correlate the density of DMA in EtOH as DMA varies, then this would be easy.

Antigua - 18-4-2021 at 13:25

Did you consider salting out the amine with anh. HCl and then evaporating all the ethanol to see how much you've actually got?
That would be the first thing that comes to my mind.

Newton2.0 - 18-4-2021 at 13:37

I had not considered that. So, I could probably try to produce DMA HCl using the 10mL volume I have, then evaporate the EtOH under vacuum and from there I have a rough estimate of the concentration (subtracting the weight of the HCl, of course) of DMA in my solution?

I don't have any anhydrous HCl.

Antigua - 18-4-2021 at 14:08

Yes. You can prepare anhydrous HCl by slowly dripping conc. HCl into conc. sulfuric acid and passing it through a drying tube. Alternatively You can use any other strong acids eg. phosphoric etc. but you'll have to evaporate the water you introduce and small amines tend to be very deliquescent.

karlos³ - 18-4-2021 at 14:12

Simply titration?
Just take an aliquot of the solution, titrate with your acid of choice, and then do the math.

You have 10ml's, take 0,1 of them and after neutralisation, do the obvious.

But next time, I would recommend you simply weigh the solution before and after gassing.

Density isn't actually helping you with that matter.

Antigua - 18-4-2021 at 14:15

Yup, karlos is right, titration will suffice actually. If you do it carefully and with low concentrations of acid and your analyte you'll get even better results than with evaporating.

karlos³ - 18-4-2021 at 14:44

Quote: Originally posted by Antigua  
you'll get even better results than with evaporating.

Definitely, just keep in mind how hygroscopic dimethylamine salts are(probably not all of them, but the HCl is definitely obnoxious with this property).
With titration you don't need to evaporate the solvent, you can do it all in the liquid with bretty gud results.

Damn I have to go in bed, but Antigua, didn't you told me the same just a little bit ago? :D
Good night! :P

Newton2.0 - 18-4-2021 at 15:13

I know what titration is but don't understand how that tells me what I need to know. Can you explain what I am looking for? Feel free to get technical or even refer me to a link.

Weight before and after is meaningless as a good amount of water is passed through, right? So I won't get good numbers, anyways.

I really do appreciate the help! I need an additional tool in my belt.

karlos³ - 18-4-2021 at 15:24

Well if you add a certain amount of your solution and neutralise it with an acidic solution of a known strength, you can deduce how much amine is in there as soon as you hit the neutralisation point and then calculate it back.
Thats a standard technique and there are even automatic machines for it.

Look at that fisher ad:
Its a simple math issue, you just need to know your acid concentration well(might require another initial titration in the worst case).

Newton2.0 - 18-4-2021 at 15:29

Okay, I was assuming you meant acid-base titration but couldn't figure out how it translated to how I envisioned my problem. That's so much easier than the incredibly stupid thing I just did.

zed - 18-4-2021 at 20:51

I've never done it that way. I might have hydrolysed that DMF with HCl, distilled off the water , Formic Acid, and excess HCl, perhaps under reduced pressure.

Thus leaving me with a residue of DiMethylamine HCl Neat. React the HCl salt, with concentrated NaOH. Liberating the gas. Dry the gas by passing it through a tube packed with NaOH pellets.

Absorb the gas into An anhydrous weighed solvent of your choice.

Might be a better way. Here, Vogel is your guy. Though, he does make the Dimethylamine from scratch.

It might be easier to acquire the makings. DMF is a real Chemical. Won't find that at Home Depot.

karlos³ - 19-4-2021 at 10:44

I did both of acidic and basic hydrolysis of DMF, and I can tell that gassing from the basic hydrolysis works much better than then quite lengthy acidic hydrolysis.

Otherwise, there's still formaldehyde and ammonium chloride, just need to adjust the stoichiometry for the secondary amine.

Newton2.0 - 19-4-2021 at 13:14

Now I can finally use my buret! It was the first piece of glassware I ever purchased and I am just now finding a use for it!

Oxy - 19-4-2021 at 13:28

It's just a pain in the ass when you don't use it frequently as it requires using standardised solutions.
And many of them can't be stored indefinitely so they have to be made on demand. Fixanals are great for that but they are too pricey... But for rough titration it's possible to make some shortcuts and make it less accurate but faster.

karlos³ - 19-4-2021 at 14:37

Yes amines, together with aldehydes, can't be stored for a long time, those two are the worst.
I prepared a solution of what, 0,2mol or such, in a quart liter of alcohol for immediatey use, but now that stuff sits like a half year around, I guess I have to increase the amount for the planned reactions at least.

For long-term storage, of course choose the salt instead.
I got lazier than usual and simply bought myself 100g's of Me2NHCl so I don't need to run through a quart liter of DMF in repeated experiments.
DMF is more often of value I think, than is making the amine from it.

Reminds me of me having to make a preparation of Böhme's salt for SM soon :D
Bretty gud!