If you're talking about hardware store acid, it is yellow because it's contaminated, iron I believe. Pure HCl is clear. Keras - 2-10-2022 at 23:43
32% hydrochloric acid is clear. If yours turns yellowish, just add a crystal of sodium thiosulphate, it should clear up immediately.Tsjerk - 3-10-2022 at 08:49
32% hydrochloric acid is clear. If yours turns yellowish, just add a crystal of sodium thiosulphate, it should clear up immediately.
What does thiosulfate do?
It reduces Cl₂ to Cl⁻. I suppose the yellow colouration is caused by dissolved Cl₂. Bedlasky - 3-10-2022 at 09:42
Hardware store HCl is usually contaminated with iron. Yellow is from Fe3+ ions, not from chlorine.unionised - 3-10-2022 at 11:18
Reduction of Fe(III) to Fe(II) will remove the colour.Bedlasky - 3-10-2022 at 14:05
Yeah, but iron is still there. You introduce more contamination by reducing agent.j_sum1 - 3-10-2022 at 14:14
Distillation is the way to go.
I have found that addition of water and azeotropic distillation of my hardware store HCl has been extremely helpful. I get a non-fuming product. It
is consistently clean. The concentration is pretty consistent at 20.2%. It is useful for most purposes. And on the occasions when I do need
something stronger (which is actually not that often), I can always concentrate it by bubbling some gaseous HCl through it.
Iron is not the only possible contaminant btw. Around here I have had a few purchases of HCl that were very low in iron but contained titanium (of
all things).
[edit]
I will add, it is not advisable to distil to dryness. Or even close. There can be a surprising amount of rubbish in here. And my experience is that
it is difficult to get the flask clean again.
[Edited on 3-10-2022 by j_sum1]Mateo_swe - 4-10-2022 at 05:52
I have bought some about 32% HCL several years ago and it was clear when i purchased it.
But after many years now it has got a slightly yellowish color after long storage.
I see many new batches at the store got same slightly yellowish color and i think its because of long storage.chornedsnorkack - 4-10-2022 at 23:17
I have bought some about 32% HCL several years ago and it was clear when i purchased it.
But after many years now it has got a slightly yellowish color after long storage.
I see many new batches at the store got same slightly yellowish color and i think its because of long storage.
Maybe it is because it originally had (weakly coloured) FeCl2 impurity, which slowly oxidized into (strongly coloured) FeCl3 impurity?
Other colourless impurities which might slowly oxidize into yellowish ones would include HI and HBr.
Also: FeCl2 and FeCl3 are not volatile on the distillation of HCl (in the region of up to 110 C). HBr, HI and Br2 are. I2 less so, but more than
FeCl3.
[Edited on 5-10-2022 by chornedsnorkack]
[Edited on 5-10-2022 by chornedsnorkack]Rainwater - 5-10-2022 at 04:35
By placing a container of dirty hcl, next to a container of 1/2 volume h2o, all inside a sealed container. The water will draw out the hcl, for 500ml
it takes about a 24hours at 25c, longer when cold. Density shows about 20%
