Unexpected manganese compound

Back again with another manganese-related salt post.

So I had managed to prepare some relatively pure MnCl2 with another method involving only manganese dioxide and hydrochloric acid. It formed the expected red-pink solution due to the presence of Mn2+ (sample image 1). Upon heating, the colour changed from this to green, and eventually yellow (image 2). Boiling the solution down, a tan precipitate formed which dropped out as a powder (image 3), which redissolved upon the addition of more water to reform the yellow solution. The solution subsequently produced yellow crystals of irregular morphology (images 4 and 5).

I've briefly trawled the internet and cannot find any reference to yellow manganese compounds. I can safely rule out the presence of chlorine because the crystals are also yellow, under the assumption that Cl2 would remain in the aqueous phase. There is a slight sweet smell not unlike that of aldehydes, but it is barely detectable and likely not too relevant. Any ideas on what compound I have formed?

Synthesised from the reaction between sulphuric acid and sodium chloride, bubbled into water at unknown concentration, but there's definitely a significant amount present.

I'll prepare a precipitate using hydroxide and carbonate, see what it forms; the first tube shows brown precipitate darkening on top, indicative of manganese hydroxide. The second contains the light tan precipitate of manganese carbonate.

[Edited on 8-5-2017 by LearnedAmateur]

Quote: Originally posted by LearnedAmateur

Synthesised from the reaction between sulphuric acid and sodium chloride, bubbled into water at unknown concentration, but there's definitely a significant amount present.

Quote: Originally posted by LearnedAmateur

I'll prepare a precipitate using hydroxide and carbonate, see what it forms; the first tube shows brown precipitate darkening on top, indicative of manganese hydroxide. The second contains the light tan precipitate of manganese carbonate. [Edited on 8-5-2017 by LearnedAmateur]

I've encountered this or a similar green stuff as well, though in a different form. I don't think I have the pictures left, but maybe they can be found in the manganese chloride thread.

Anyway, I was boiling down bubblegum-pink MnCl₂ solution a few years back. Everything was going fairly well, and I had no evidence before this to indicate at any time that there were impurities in the manganese, especially iron. The solid precipitates out, and I boil to dryness to remove as much water as possible (having been annoyed earlier by hygroscopicity and not having much in the way of secure storage containers).

Now, here's the part where the green stuff comes in. As soon as the last bit of the solution disappeared, the whole stock of bright pink, solid manganese chloride turned lime green. Almost instantaneously. I didn't even catch it until my dad pointed it out to me and I turned around again, and by that time it had begun shifting back to pink again.

I used an OTC source of hydrochloric acid - TransChem brand, if I remember right, and manganese dioxide from batteries that I had purified following NurdRage's video. Now, I know everyone says the OTC hydrochloric acid is contaminated by iron because it's green, but has anyone ever managed to prove it? Nobody I know has managed to get the iron back out of there, and you'd think it would be quite easily done with some NaOH.

That was the only time I ever saw manganese chloride turn green, as it stayed pink upon cooling and at all times thereafter. I kind of forgot about it until now, honestly.

Those precipitates of manganese hydroxide and carbonate look exactly like the ones I worked with, so I'm pretty sure they're of reasonable purity.

MrHomeScientist, air and heat do most of the work in changing the color of manganese hydroxides, from white Mn(OH)₂ to hydrated brown MnO₂. I've noticed that hot NaOH will solely form brown manganese dioxide, while cold, dilute NaOH will form the lighter, off-white hydroxide, which turns brown on exposure to air as it converts to the hydrated dioxide.

What could the green stuff be? I know different hydrates of salts can have completely different colors, cobalt's a good example of that. But if manganese chloride has a different hydrate that's yellow to green in color, how come nobody's noticed?

EDIT: Found the pictures I was referencing. Forgot I was actually quick enough to get a photo in, so you can see the green I'm talking about on the first one. The second is just seconds to minutes later.






[Edited on 5/8/2017 by elementcollector1]

@MrHomeScientist
It was bottled water, the analysis on the label doesn't show iron concentration and the lowest is potassium, at 1mg/L - the other metal ions are: Ca 80mg/L, Mg 26, Na 6.5. It was immediately dark with the OH solution, if it is iron then I can't see how it could've been introduced unless it was through the MnO2. Here's an image of iron hydroxide (left), with my sample of hydroxide. Albeit, even this doesn't explain why my solution changed colour so drastically from the original, I would've expected it to stay pink since I didn't alter the initial Fe2+/3+ concentration.

@elementcollector1
That's interesting, for me it occurred during the boiling phase when everything was still in solution. Almost immediately as it started to give off visible steam, it went the lime green then yellow, then it precipitated, never returning to the pink colour. I used hot (from the enthaply of solution), concentrated NaOH, so that would explain the dark colour as per your comment.

@JJay
I used all of the hydrochloric acid I synthesised, it was only a small amount of ~40mL, but I'll make some more using the same method and reagents as before when I get the chance.



[Edited on 8-5-2017 by LearnedAmateur]