Yellow solid on copper sulphate crystals

[jhato200x]

Hello everyone, about 4 years ago I bought copper sulphate from a pool store. As you may imagine, it wasn't completely pure, but it worked for treating the pool.

 

A month ago I decided to make some crystals from it, and dissolved it using hot tap water (I know I should've used distilled water but wanted to see what would happen). After filtering the initial saturated solution, I then left it on a closed glass bottle for some days (so there couldn't be any evaporation). As expected, I didn't see any crystals forming, but I noticed there was this yellow solid stuck on the bottom of the bottle.

 

I thought it could be calcium or sulphate carbonate, but I wasn't sure. I filtered this mixture again and proceeded with the crystallization in a red plastic recipient; I obtained really beautiful (but rather small) copper sulphate crystals after about one week. However, I noticed that the next growing crystals had again a yellowish solid on their surface. I continued with the process until all the water was gone. The yellow solid had cristalized as thin filaments.

 

Is there any way of telling what compound could it be? Any suggestions?

 

 

[OldChemE]

Since you used tap water, there are many, many possibilities. But, my first guess is that something in there reduced the sulfate ions, and you now have traces of pure sulfur-- which is yellow.

[jhato200x]

Thank you for your answer! I've been thinking on it, and have been checking the ways it could occur. Since the biological route is highly improbable (no rotten eggs stink), the sulfate were to be reduced by inorganic means. The potential would need to be enough for doing so; for instance, we would need find the reducing agent that was around the water or the glass bottle. One possibility could be copper(I), as it's oxidation potential is quite low:

 

Cu²⁺(aq) + e⁻ → Cu⁺(aq) +0.16

 

I don't know if there is enough Cu⁺ for allowing this. O could it be other ion, such as Fe? If Fe³⁺ was formed indeed, I don't think it could displace the Cu²⁺ ions in the solution. Maybe it formed FeCl₃?

 

I would love to share your ideas, maybe I'm completely wrong.

[Sriman Dutta]

What could be the reducing agent?

[John Cuthber]

Reduction to sulphur in the presence of air is very unlikely.

 

Based on the colour, I would guess that there are traces of iron present (from the copper sulphate or from the tap water).

The colour of that yellowish stuff is about right for basic iron(III) sulphate.

[jhato200x]

Thank you again for your answers! I think there could indeed be trace (or quite high) amounts of iron, both in the copper sulphate and the tap water (specially as it was boiled in a rather old steel pot). Is there any way to confirm the identity of that solid? Iron(III) sulphate seems like a fairly good possibility, but as I don't remember seeing it before, I couldn't relate to it.

[John Cuthber]

Confirming Fe(III) is relatively easy.

I presume you don't have a chemistry laboratory.

Scrape some of the yellowish stuff off the crystals.

Put it in a little vinegar to dissolve it.

 

Make some very strong tea: essentially soak a teabag with hot water and squeeze out the extract.

Mix a few drops of the tea with the vinegar extract.

Iron will give a black colour/ precipitate.