Sciencemadness Discussion Board

Stoichiometry for HCl gas generator?

monolithic - 6-11-2018 at 19:16

I need to generate, let's say, 0.75 mol of gaseous HCl. I plan on using an HCl/CaCl2 gas generator, which is basically:

HCl*(H2O)x + CaCl2 --> HCl (g) + CaCl2*(H2O)x

I want to calculate the amount of 31% HCl (roughly 10M) and CaCl2 I need to generate the 0.75 mol of gaseous HCl. Assuming an unrealistic 100% conversion of aqueous HCl to gaseous HCl:

0.75 mol HCl x (1 L solution / 10 mol HCl) x 1000 mL / L = 75 ml of 31% or 10M HCl.

Is this correct? It seems like I'm doing something wrong, but it's been a long time since I've done gas calculations. In reality, what would be a realistic assumption for conversion of aqueous HCl to gaseous HCl, if I wanted to generate 0.75 mol of the gas?

[Edited on 7-11-2018 by monolithic]

[Edited on 7-11-2018 by monolithic]

Sulaiman - 6-11-2018 at 21:00

The volume of 10M hydrochloric acid is correct,

As the molecular weight of CaCl2 is 110.98 g/mole
you theoretically need 83.235 g anhydrous calcium chloride,
or 0.75 x (110.98 + 6x18.01528) = 164.3 g if calcium chloride hexahydrate.
The actual hydration level will be unknown, from zero to more than six.
So if you want to work stoichiometrically you need to dehydrate your CaCl2 before weighing.

P.S. With HCl gas you do need a suck-back trap.

[Edited on 7-11-2018 by Sulaiman]

monolithic - 7-11-2018 at 04:25

Quote: Originally posted by Sulaiman  
The volume of 10M hydrochloric acid is correct,

As the molecular weight of CaCl2 is 110.98 g/mole
you theoretically need 83.235 g anhydrous calcium chloride,
or 0.75 x (110.98 + 6x18.01528) = 164.3 g if calcium chloride hexahydrate.
The actual hydration level will be unknown, from zero to more than six.
So if you want to work stoichiometrically you need to dehydrate your CaCl2 before weighing.

P.S. With HCl gas you do need a suck-back trap.

[Edited on 7-11-2018 by Sulaiman]


Thanks for pointing out that CaCl2 needs to be dehydrated, forgot all about that. If anyone else cares, this is the short write-up I'm following: http://curlyarrow.blogspot.com/2009/03/how-to-make-dry-hcl-g...

Sulaiman - 7-11-2018 at 05:48

For anhydrous HCl, if available to you, consider concentrated sulphuric acid (drain unblocker grade should be OK)
plus sodium chloride,
using the anhydrous calcium chloride to ensure dehydration of the gas flow.
... only because I'm not familiar with the HCl + CaCl2 route :P
(after posting I realised that I do not know the stoichiometric amount of CaCl2 required to completely dehydrate the 36% HCl)

You could also drip conc. sulphuric acid onto 36% HClaq for dry HCl gas,
CaCl2 will again be required to dry the gas stream as some water will come over as a very fine mist.

So, given that I do not know how much CaCl2 is required to dehydrate 36% hydrochloric acid to either azeotropic or zero concentration,
you should completely ignore my stoichiometry calculations in my earlier post :o
Similarly I do not know exactly how much conc. sulphuric acid is required for dehydration,
I just use as much as required.
sorry for any confusion caused :(

[Edited on 7-11-2018 by Sulaiman]

nimgoldman - 7-11-2018 at 06:45

I tried the HCl + CaCl2 route but it really sucks. It's easy for small amounts, but the calcium chloride quickly saturates and the generation of gas is not steady, it pulses with every drop. The yield was very low in my case, less than 50%.

My favourite method is dripping conc. H2SO4 on conc. HCl containing some NaCl, the method is described here:
Quote:
Taylor, Moddie D., and Louis R. Grant. "Preparation of anhydrous pyridine hydrochloride." Journal of Chemical Education 32.1 (1955): 39.


Dripping HCl on H2SO4 works for me great, too. The gas is already fairly dry - the drying tube is not needed unless your application is really water sensitive.

Depending on your apparatus, there is some air to be displaced and some empty space, so one way to estimate amount of HCl truly generated, you can use a known amount of base (i.e. 200 mL of 5% NaOH), add indicator in it and measure how much acid is need to be used to neutralize that amount of NaOH.

Another method is using a second bubbler after the receiver flask that will indicate no more gas is being dissolved in the receiver. Measuring the density of the water in receiver, you get an idea how much gas has been absorbed.

To reduce suckback, you can heat the generator flask and cool the receiver flask. This will also improve absorbtion of the gas.

For cleanup, I use spray bottle filled with carbonate solution to neutralize the nasty HCl.