Sciencemadness Discussion Board

HCl Generator Tune-Up

nimgoldman - 1-9-2018 at 09:53

I am trying to produce near quantitative amounts of HCl gas but my generator seems to underperform and I am not sure why.

I have been inspired by this tutorial: Best Method for Hydrochloric Acid Gas.

The generator setup (see image) consists of a 100 ml RBF with claisen adapter, addition funnel and a hose adapter. The bottom RBF contains 50 ml 96% sulfuric acid. The addition funnel contains 25 ml 35% hydrochloric acid (0.8 mol). The excess amount of sulfuric a. ensures dehydration of all the HCl(aq).

The gas goes through a wash bottle (serving as a suckback trap) and gas inlet tube going into the receiver RBF.

Both flasks are stirred to promote even mixing. The generator flask is also heated as this pushes more HCl gas out of the sulfuric acid and generates more pressure towards receiver (less chance of abrupt suckback).

All joints are greased and clamped and I haven't observed any leaks.

The generator yield (according to Rhodium) should yield over 90% HCl from the 35% solution. Since I used 25 ml, this should be over 0.72 mol of HCl.

The receiving flask contains 250 ml of 5% NaOH (0.3 mol) with nitrazine indicator (blue = basic / yellow = acidic).

The solution stayed basic even after all the HCl has been dripped, meaning that the generator effectively produced less than 0.3 mol of HCl instead of the proposed 0.7+ mol. And I haven't even used bubbler or dryer as proposed in Rhodium's writeup, to minimize air space.

Could significant amount of the gas stay in the wash bottle? It was indeed filled with HCl (it was sizzling when neutralizing it on cleanup).

I used just water on my first trial, which caused a violent suckback just after 12 or so drops of hydrochlorid acid - this is why I made the water basic, to make sure it will absorb more of the HCl gas.

DSC_0513.JPG - 141kB

[Edited on 1-9-2018 by nimgoldman]

Ubya - 1-9-2018 at 10:33
Quote:

I used just water on my first trial, which caused a violent suckback just after 12 or so drops of hydrochlorid acid - this is why I made the water basic, to make sure it will absorb more of the HCl gas.


you made HCl gas, but not enough, so if this technique really has a 90% yield, maybe the starting chemicals were not as concentrated as you thought, are you sure your acids were 35% and 98%?

nimgoldman - 1-9-2018 at 12:09

Quote: Originally posted by Ubya  

Quote:

I used just water on my first trial, which caused a violent suckback just after 12 or so drops of hydrochlorid acid - this is why I made the water basic, to make sure it will absorb more of the HCl gas.


you made HCl gas, but not enough, so if this technique really has a 90% yield, maybe the starting chemicals were not as concentrated as you thought, are you sure your acids were 35% and 98%?


Yes I am sure about the concentration as the chemicals are lab grade and taken from original bottles. The sulfuric acid is 96% (fresh, barely opened) and the hydrochloric acid is 35%.

I have just weighed the hydrochloric acid and 100 ml weighs exactly 118 g so it is surely 35%.

The wash bottle volume is 250 ml so even if full, it should contain no more than 0.3 g of hydrogen chloride (based on its density 1.48 g/L). The additional space in tubing and flask is also very small and should not change the result, as there should be over twice as more HCl generated then actually was.

Okay now I see where I made the (stupid and common) mistake:

25 ml of HCl 35% contains 10.3 g HCl(g) (since 1 000 ml contains 413 g), i.e. 0.28 mol

250 ml of NaOH 5% contains 12.5 g NaOH(s), i.e. 0.31 mol

This shows amount of HCl cannot overturn that of NaOH so it cannot neutralize the basic solution even at 100% yield.

I simply forgot to multiply HCl weight by 35% to adjust for the actual concentration ... the bottle says 1180 g per liter, but this is indeed the total weight (water+HCl), not just the dissolved HCl gas.

I will repeat the experiment and report to make this thread at least a little bit useful.

I have also experimented with the calcium chloride method not mentioned by Rhodium, but even though this is very easy, safe and foolproof method, it does not provide quantitative results, depends on calcium chloride source, needs to be heated just right to improve absorption etc.

[Edited on 1-9-2018 by nimgoldman]

nimgoldman - 4-9-2018 at 17:20

I have looked in Brauer for HCl generation and one improvement should be adding HCl(aq) to the bottom of H2SO4 solution. This might work better against pooling of lighter HCl(aq) on top than just by stirring.

Yet another improvement might be dripping H2SO4 on a slurry made of HCl(aq.) and NaCl.

Several people in forums advised for this "inverse" method rather than adding HCl to H2SO4 but I haven't found it in literature or don't find any reasons for it being better. Smoother stream of gas? Taking advantage of H2SO4 reacting with NaCl giving additional yield of HCl?

This process is interesting but since the composition of this system of three components in the reactor changes according to several rules, it would be nice to have a solid theoretical understaning of it.

Any advice?

Doc B - 8-9-2018 at 18:07

Does your gas inlet tube extend below the surface of the hydroxide solution? If not then you might be letting a significant amount of generated gas escape without reacting. Introducing the gas below the surface of the hydroxide solution and fitting a Hg/oil bubler to the open neck of the receiver flask would likely ensure that more of the HCl(g) reacts and doesn't escape before it has the chance to do so.
Adding HCl(aq) to H2SO4 is a more vigorous reaction, one much more prone to suck back, that may produce a drier stream of HCl(g). I would suggest x3 the wash bottles. One containing con'c H2SO4 to dry the evolved HCl(g), placed between the two other empty wash bottles which serve as traps for the generator and the receiver.

nimgoldman - 9-9-2018 at 13:27

Quote: Originally posted by Doc B  
Does your gas inlet tube extend below the surface of the hydroxide solution? If not then you might be letting a significant amount of generated gas escape without reacting. Introducing the gas below the surface of the hydroxide solution and fitting a Hg/oil bubler to the open neck of the receiver flask would likely ensure that more of the HCl(g) reacts and doesn't escape before it has the chance to do so.
Adding HCl(aq) to H2SO4 is a more vigorous reaction, one much more prone to suck back, that may produce a drier stream of HCl(g). I would suggest x3 the wash bottles. One containing con'c H2SO4 to dry the evolved HCl(g), placed between the two other empty wash bottles which serve as traps for the generator and the receiver.


Yes I already resolved the problem. It was an error in stoichiometry. Indeed enough gas was generated.

Yes the tube was submerged of course and the solution stirred to promote more even dissolution of the gas.

This is an experimental setup and I still wait for a second wash bottle I use as a trap. Drying was not necessary as the solution is aqueous and the extra water won't skew the result.

Suckback could be quite and issue and the only solution now it to cool the receiving flask and heat the generator (it should be heated anyway).

But at least the suckback is an indicator that no more gas can be consumed and therefore gassing can finish.