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Author: Subject: Hydrochloric acid impurities, where do they come from?
DrScrabs
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Hydrochloric acid impurities, where do they come from?

A few days ago I was walking trough my local hardware store and they changed their aq HCl (20-23%) to a slightly yellowish product (it was clear before). Although I never bought it, as I have access to reasonably pure aq HCl, I was wondering about the origin of impurities of hydrochloric acid. As far as I know it should be mostly Fe salts, wich makes it even weirder for me. Why use Fe in contact with HCl? Where else can it come from? How is HCl made industrially except from the elements? I don´t expect them to make it from NaCl and H2SO4.
Can somebody please give me a hint?

DrScrabs
Ubya
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wikipedia says that there are various routes, one is the direct reaction of hydrogen and chlorine (made from the electrolysis of brine to make NaOH), but this gives very pure HCl so it is even used in the food industry. the other route is as byproduct of chlorinations, for example in the manifacture of PVC, HCl is a byproduct. i don't know where iron could cone from, maybe other processes give much lower quality HCl, or the transport and stocking in vessels could contaminate it with the iron of the pipes (not ideal for the plat though)

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teodor2
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I did some query in russian-language sources but found only manufacturing process dated of the beginning of XX century. And the problem of Fe contamination was connected to the material of "boxes". The system of "boxes" was used to absorb HCl - it went by the surface of water from one box to another. The material for "boxes" was usually stones, depending on available building stones in place where manufacture was located. In russian plants they used granite - that was the source of Fe (as well as many other metals having not so bright colored salts).

So, it is ridiculous, either they use the same material till now or they didn't sell a stock from beginning of the centure

fusso
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Storing a chemical in container materials that WILL react with the chemical?! Seriously?!

DrScrabs
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 Quote: Originally posted by fusso Storing a chemical in container materials that WILL react with the chemical?! Seriously?!

Exactly my thought!

AJKOER
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 Quote: Originally posted by Ubya ..... don't know where iron could cone from, maybe other processes give much lower quality HCl, or the transport and stocking in vessels could contaminate it with the iron of the pipes (not ideal for the plat though)

The answer I suspect is from transport in stainless steel containers!

It is assumed that H+ and a chloride ion in the presence of air (likely O2 is the active agent) is linked to the attack of stainless steel (see https://www.justmfg.com/corrosion noting more steel corrosion in marine environments).

So, I would speculate no problem in transport as long as there is no/limited oxygen presence (for example, when a stainless container car is filled with HCl), else possible some electrochemical based metal corrosion leading to iron (and other steel alloy metals) contamination.

[Edited on 13-7-2019 by AJKOER]
Swinfi2
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I've been trying to replicate nurdrages copper chloride process for H2SO4 and every time I distil off the HCl it's green... I presume either CuCl2 is misting over or the hot HCl is reacting with oxygen (but no smell of chlorine).

Anyone have a way to test? Or more experience to say?
Amos
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It should if anything be a commercial dye (unlikely, the color doesn't increase on concentration and clears completely on dilution), leachate from plastic containers (they often have a red lid) or more plausibly leftover chlorine or perhaps chlorine oxides from the reaction process. Myself and a few other members here have repeatedly proven that the 2 most common HCl brands here in the US (Jasco and Klean-Strip, the kinds you find as hardware store muriatic acid) DO NOT contain any iron impurity. They have zero residue on evaporation and show no formation of either prussian blue or the brilliant purple iron(III) salicylate complex when concentrated by evaporation and tested with the appropriate reagents.
wakatutu
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I personally know a retired chemical engineer who worked for one of the major world-famous chemical companies, who knew of a major client who purchased a significant proportion of the total amount of HCl produced by the company. When the manufacturing process was improved and there were no longer iron salts contaminating the HCl, the client called them up and complained that the product was not yellow! They couldn't believe it. Their response was to send a technician around to all of the barrels destined for that particular client and have them drop a nail in each barrel. The client was happy with the result.
AJKOER
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Actually, it may have been also the reactivity of the FeCl2/FeCl3/HCl was improved!

FeCl3, as I have previously noted on SM, being trivalent, appears to be even more effective on impacting the activity coefficient. This means that weaker/cheaper HCl can behave like stronger/more expensive hydrochloric acid! Also, ferrous, oxygen and H+ could lead to some fenton chemistry based radicals for an added punch.

Or, it could just be that many people like colored solutions over clear, as it is very common to add dyes to household cleaning products (like, for example, vinegar based window cleaners). After all, who wants to pay  for cleaning solutions that look just like water?

[Edited on 16-7-2019 by AJKOER]
Antiswat
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no, it wouldnt be stainless steel, this is HCl specification from APCpure, however this is food grade, cant find any worse quality on there it seems

Minimum Assay: 35.6-36.6%
Molecular Formula HCl
Molecular Weight 36.46 g/mol
Sulphate (SO4): <0.6%
Iron (Fe): <10ppm
Arsenic (As): <2ppm
Copper (Cu): <2ppm
Total organics: <100ppm
Colour: Clear - Pale Yellow
Storage Temperature: 20-24C

im more curious about arsenic?? because this is a pretty common contaminant, 10ppm wouldnt be from actual steel in contact with the HCl, that would be much higher ppm then, mind you tap water has as much as 300ppm Total Dissolved Solids
it does list ORGANICS.. it could be dye? im feeling a bit weird about 10ppm iron would be capable of dying HCl that much, its 10 milligrammes per litre

~25 drops = 1mL @dH2O viscocity - STP
Truth is ever growing - but without context theres barely any such.

https://en.wikipedia.org/wiki/Solubility_table
teodor
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 Quote: Originally posted by Antiswat Arsenic (As): <2ppm

Arsenic is listen as a common hydrochloric acid contamination in various (old) books. Has no idea except that one I already wrote - usage of HCl absorption reservoirs made from stone. Just one comment: now I understand why in school book of experiments they suggested to wash H2 going from Kipp's apparatus (as a result of reaction between hydrochloric acid and Zn).
AJKOER
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One possible explanation, not considered to date, is the presence of even small amounts of polyhalogen ions, which apparently are intensely colored!

To quote from Wikipedia (https://en.wikipedia.org/wiki/Polyhalogen_ions ):

"Most polyhalogen ions are intensely colored, with deepened color as the atomic weight of the constituent element increases. The well-known starch-iodine complex has a deep blue color due to the linear [I5]− ions present in the amylose helix.[4] Some colors of the common species were listed below:[3]
>fluorocations tend to be colorless or pale yellow, other heteropolyhalogen ions are orange, red or deep purple[4]
> compounds of [ICl2]+ are wine red to bright orange; while that of [I2Cl]+ are dark brown to purplish black
>[Cl3]+ is yellow"

So, how could say [Cl3]+ be formed in HCl? Yet another unmentioned causative factor could be light, which may produce many species, per below:

HCl + Light = •H + •Cl

where there is, apparently, an increase in chlorine radicals especially with ultraviolent light (see, for example, http://www.vias.org/genchem/hydrogen_chloride.html ). Other reactions:

•Cl + Cl- = [•Cl2]-

•Cl + •Cl = Cl2

Cl2 + Cl- = [Cl3]- (see https://www.sciencedirect.com/science/article/pii/S002207286... )

[•Cl2]- + [•Cl2]- = Cl2 + 2 Cl- (see https://pubs.acs.org/doi/full/10.1021/jp036211i?src=recsys )

•Cl + Light = Cl+ + e- (see https://www.chegg.com/homework-help/questions-and-answers/io... )

Cl2 + Cl+ = [Cl3]+ (yellow)
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Also, even though the conc of Fe and Cu ions is low, there could be cycling (which, with time, could become evident):

Fe(ll) = Fe(lll) + e-

Cu(ll) + e- = Cu(l)

Net redox couple:

Fe(ll) + Cu(ll) = Fe(lll) + Cu(l)

The species Fe(ll) and Cu(l) in the presence of any O2 and H+, could via fenton (or fenton-like) reactions create the hydroxyl radical, •OH. Light could also assist via a photo Fenton reaction. At low pH in the presence of Cl-, this leads again to •Cl and [•Cl2]- species (see my expanded comments and references at https://www.sciencemadness.org/whisper/viewthread.php?tid=98... ).

Also, any of the above chemical activity, including from light application, may increase the temperature of the solution resulting in the release of hydrogen chloride gas, which is described as "colorless to slightly yellow" (see https://pubchem.ncbi.nlm.nih.gov/compound/hydrochloric_acid ).
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Interesting to me is if any of my hypothetical presented pathways above, are even partially verified to be correct, then the very topic of explaining the coloration of HCl with minor impurities is apparently too advanced for an elementary chemistry course!

[Edited on 17-7-2019 by AJKOER]
draculic acid69
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I've had clear HCl turn yellow when put in a clear bottle and kept in the closet for a few months.
AJKOER
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 Quote: Originally posted by draculic acid69 I've had clear HCl turn yellow when put in a clear bottle and kept in the closet for a few months.

Wow! The above observation by draculic acid69, after exposure of the HCl to air and light over time, if verified, seemingly supports the its complex conjecture.
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I just did a search and this comment at https://www.sciencemadness.org/whisper/viewthread.php?tid=15... by Renge Ishyo appears to be supportive of my comment, to quote:

"The yellow color in HCl is due to trace amounts of impurities such as chlorine gas, iron, or organic molecule impurities. Even if you start off with a "pure" clear HCl solution, over time it will turn more and more yellow as it is exposed to the air and as it eats into the container you are storing it in. I see this all the time as the HCl we use for the swimming pool is now quite yellow after having sat around for a while."

where the sitting around in the sun together with the reflected UV from the pool does point to possible chlorine radical formation and associated species therefrom.
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The answer could also be the creation of the yellow [FeCl6]3- species (color noted at https://www.youtube.com/watch?v=GKZgdfg7ZMg ). This may form from the consumption of some of the H+ (leaving an excess of Cl-) by the action of light releasing hydrogen gas which escapes;

HCl + uv --> •H + •Cl

•H + •H --> H2 (g)

or perhaps, fenton (or photo-fenton) chemistry, which consumes H+ in the presence of O2 with time in the presence of iron.

Or, most likely, in my opinion, the following electrochemical reaction in the presence of an electrolyte (FeCl2, FeCl3, CuCl2,...) consuming H+ via:

4 Fe(ll)/Cu(l) + O2 + 4 H+ ---> 4 Fe(lll)/Cu(ll) + 2 H2O

where the ferric or cupric could be recycled in the presence of light and/or a redox couple:

Fe(ll) + Cu(ll) = Fe(lll) + Cu(l) (complexed with chloride)

Note, consistent with the transition metal, dissolved oxygen and HCl path to a yellow complex is this report on SM detailing a bright yellow solution created upon adding HCl gas to tap water (rich in ferrous, oxygen and dissolved salts serving as an electrolyte) at http://www.sciencemadness.org/talk/viewthread.php?tid=13029 .

[Edited on 19-7-2019 by AJKOER]

 Sciencemadness Discussion Board » Fundamentals » Chemistry in General » Hydrochloric acid impurities, where do they come from? Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues   » Detritus   » Test Forum