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My Hydrochloric acid and Hydrogen peroxide etchant goes red without any copper in it. How come? Answered

I recently tried making acid cupric chloride etchant; however, my batch always goes a darkish red colour as soon as i mix the two together even when i haven't added any copper. I've been using 3% hydrogen peroxide from a pharmacist and 30% HCl From Bunnings. All the stuff was fresh and in dark plastic bottles. The container i put it into is plastic. It still etches but it's nearly impossible to see if the copper is etched away without pulling it out. Can anyone tell me what's in the mixture that's making it red and how i can correct it. Thanks


I am having the same problem with a different brand of HCl (from bondall) and H2O2 (from a chemist).

I have noted that the red colour fades over time (presumably as the H2O2 decomposes). However the faint smell of chlorine suggests my acid is significantly losing potency...

In a separate use, I find that the red solution etches Al very quickly.

I'm using HCl to disolve Sn to produce SnCl2 (stannous chloride). Adding H2O2 makes the Sn dissolve much more rapidly, but I am not certain that the outcome is SnCl2 solution in HCl.

The suggestion that the red comes from ClO2 is interesting, but ClO2 is yellow rather than red in solution. I have noticed that just using this HCl with Sn produces a slightly purple coloured solution. That may be a further clue, but I'm not sure what it points to.

After both the Sn + HCl and Sn + H2O2 + HCl have been left for quite some time, both solutions become faintly purple. Adding a drop of H2O2 makes the latter solution go red again, but the colour quickly disappears.

The first image shows the 2 solutions:

1) HCl + H2O2 + Sn immediately after being prepared (left)

2) HCL + Sn after 48 hours

The second image shows the same 2 solutions about 24 hours later. Note that these started as 5ml ~10M HCl with 2g Sn. The Sn is "Aquasafe" solder labelled as containing 99.5% Sn and 0.5% Cu. The lighting is different and it is hard to see the solution to the left is pale purple. Te solution to the right appears unchanged in colour between the two images.


My mixture also turns clear to green and then black. I timed it and started mixing it vigorously and it turned black in a minute and 38 seconds. Is this good or bad. This all started because I didn't get all the copper out of my gold scrap.

when I do it it turns black after stirring it vigorously and the glass jar gets hot

i have an answer for the reason why the reaction turns red.

this is due to the fact there are alot of metal salts in the hcl in diggers brand, that act as a catalyst to allow hydrogen peroxide to react with hcl so that you end up with water with dissolved chlorine oxide in it.

i have done a little test that confirmes that there are mineral salts in diggers brand hcl, because hcl leaves behind no residue when it completely dissolves, and when i let 200ml of 60% pure diggers brand hcl evapourate, the glass dish i used was completely coverd in super hard salt, with a slight greenish colour to it, i suspect it may be copper dichloride, to help replace any lost chlorine that detaches from the acid, as to make sure there isnt any build up of hydrogen gas within the bottle. also its used to prevent people from createing the explosive, hmtd, as hmtd can be made very ifficiently with hcl, because unlike citric acid, it evapourates away, whereas citric acid must be washed away, still making unstable hmtd.

I am not entirely sure if it would work for this purpose, but you could try mixing in a teaspoon of oxalic acid to each cup or two of the mixture. Since oxalic acid acts to dissolve iron oxide, I imagine it also has a the ability to precipitate the iron oxide out of the solution, at which point you could simply skim off the resulting 'dust' at the bottom or pour out the clear portion which would be your 'clean' solution.

Again though, I am no master of chemistry and I am not entirely sure if this is a safe method, or even if it will work. However after a bit of research, I have seen no reports of adverse effects from using this method other than potentially diluting the acid mix so I am inclined to say it is safe to at least try.

 I have confirmed this. diggers (brand) HCL from Bunnings and peroxide from Coles/Woolworth's or chemist/pharmacies WILL YEALD RED solution due to contaminant in diggers HCL. Unfortunately that is the case. 

HCL that works beautifully which i bought about 10 minutes ago from a pool shop in 630 waterdale rd, Heidelberg, VIC  (5 minutes away from Bunnings in Preston) have "Pool Shop" branded on the clear bottle. it have a slight yellowish Color and fumed a little as soon as you open the bottle. It was 15 bucks for 5 litre. 

for anyone who lives around Heidelberg/ Bundoora area in VIC let me know as i still have around 3 litres of this stuffs that i might never use.

Good luck with all your electronic projects :)

Aww. Unfortunately i don't live in VIC so i guess I'll have to go to another pool store and try to find some uncontaminated HCL.

. If pharmacy means the same thing in AU/NZ as it does in the US (dispensary for prescription drugs), then the H2O2 shouldn't have much in the way of contaminants (you may want to ask, to be sure). HCl from a hardware/paint store, however, I would find to be a likely suspect.

This is an issue with the Diggers brand HCl. It contains some kind of trace impurity - maybe Fe2+ which the hydrogen peroxide oxidises to orange Fe3+ ?

Anyway, what you want is a different brand of HCl.

I see you're in Australia, by the way - I assume they don't have Bunnings in any other countries.

Try a pool chemical supplier for some hydrochloric acid.

Last time I checked, Clark Rubber had some good stuff - "Liquid pool pH decreaser" in a 5L drum, which cost about $15 for 5L. You should double check on the label in case what you got is not the same brand as the stuff I got, but this stuff is actually just concentrated HCl - and it won't give you the orange/brown colour.

You could tray a flame test of the acid.


You could use it to identify the contaminant.
The dark reddish brown solution, I suspect may be Iron oxide suspension, i suspect.

How would I do a flame test on a liquid?

Also, I thought about an iron suspension but i don't have any settling even after a long time (months). Does iron oxide particles form a colloid or will it eventually settle.

I tried adding sodium carbonate to the HCl to neuturalise the acid then precipitate out the (suspected) iron as iron carbonate. I got a whole load of white powder in the bottom of the mixture whilst the liquid turned clear (from green). I suspected that the white powder was excess sodium chloride forming from the reaction.
2NaCO3 + 2HCl -> 2NaCl + H2O + CO2
I added water to the solution and the white powder disappeared (I presume dissolved). This would suggest that it wasn't iron carbonate as it would have remained at the bottom of the solution.

Um, having just been in bunnings yesterday, are you sure that they supplied you with 30% HCl ? The only product the sales attendant was able to find for me, was a HCl and dissolved salts, for cleaning concrete. Those salts may well be your answer.

Ok, that is defiantly a different product to what I found. The one I found was in a white 2L bottle, with a white label. You are quite correct that according to the data provided by the manufacturer, there should not be additional salts in solution, however from the behaviour of the mix you described, it is obvious that there is contaminant somewhere. Either in the HCl, the H2O2 or possibly the vessel you are mixing in. For myself, I am still looking for a "clean" source of HCl, without having to go through laboratory outfitting companies ( i know a few of them, and they are somewhat expensive)

I've used two different brands of H202. One was in a glass bottle, one was in a plastic one. I think we can rule out the Hydrogen peroxide. I've also tried different plastic containers to mix the chemicals in, so i don't think that's the problem. I mananged to get some lab grade HCl from a friend who works at a lab and it works. The solution stays clear until copper is added and it goes green. However, as the HCl is a consumable i don't really want to have to rely on friends to get it for me, or buy it from a chem supplier when it's $50 minimum regardless of volume. Best option would be either to find a way to get the contaminant out or find a cheaper supplier. Unfortunately, it seems neither of these solutions is easy to find. :(

Thanks for sharing your findings! I goes to show that you can't trust manufacturer specs these days. Please do update us if you can find a cheap good source for HCl, I'll do the same. Another instructible used toilet bowl cleaner that apparently contained 20% hydrogen chloride. I've been looking for something similar whenever I went grocery shopping but haven't found any. Will try looking in hardware shops when I get a chance.

Will do, However the diggers stuff is the only HCl that I've been able to find that isn't from a lab supplier. Right now I'm trying to figure out what's in it and see if there's a way to remove it. I might be wrong but i suspect Iron. One of the Iron Chlorides is green and Iron Oxide is reddy-brown which would explain the colours and fit the colour change when the H2O2 is added. However, Iron oxide should be insoluble :( which doesn't explain why it doesn't come out of solution. Any chemist want to give me a hand here? Any easy tests for contaminants and methods of removing them?

Since we're both using the "Diggers" brand acid, maybe we can probably rule out the hydrogen peroxide as the cause for the initial darkish-red mixture by comparing the brand used? I'm using "Faulding Remedies" brand hydrogen peroxide. The bottle is semi-trasparent brown with brownish-yellow cap and labels. It clearly states 3% w/w. If you're using a different brand of hydrogen peroxide we can probably assume the acid is errr....contaminated?

Hi, I'm experiencing the exact same thing. I bought the very same hydrochloric acid from Bunnings. My etchant doesn't seem to be working very well. I've look though the comments on the cupric chloride Instructible and found that there are others with a similar problem but no solution. If indeed the acid is the problem, is there someone in Australia that has success with this and know where to source for the acid? I've tried Mitre 10 but they have the exact same brand as Bunnings.

A pharmacy is the equivalent to a drug store in the US. The H202 is sold to be used to clean cuts. I've only been mixing hydrogen peroxide and hydrochloric acid however I'm not sure as to the purity of the acid. It doesn't have an analysis on the side of the bottle and is from a hardware store. I don't really want to get it from a chemical supplier because i asked and it's something like $40 for 500mL!! Would there be a way that i could test what's in the acid and maybe precipitate it out or something?

Just hydrogen peroxide and 30% HCl?

Colours in solution tend to come from either metal-complexes (you're indicating that there's no metals in this) or organic molecules (but you've only got H, O and Cl).
No idea, unless the acid is "dirty" - what does the analysis say on the bottle?