Zinc Sulphate Monohydrate stock solution turning brown

Charterreefer · Mar 13, 2018

I mixed up a stock solution using Zinc sulphate Monohydrate (from Alpha chemical) to dose zinc. I mixed two batches already and have had the same thing happen i.e. the solution turning slightly brown tint with what looks like a flaky precipitate laying on the bottom on the glass container the stock solution is stored in. I use one gram of the zinc compound dissolved into 250 mL of DI water.
The first time I thought I cross contaminated it with another additive. I mixed a second batch and after 2 weeks have the same problem. I use the same eye dropper but rinse it in between the Iodine and Manganese that I also dose daily.

Randy Holmes-Farley · Mar 13, 2018

I think it must be some other chemical that is an impurity. Maybe iron. Or the manganese.

There are no colored forms of zinc that are likely to form.

Christoph · Mar 13, 2018

Since imo all Zinc salts are colorless i would assume its some contaminant in your zinc sulfate. - Possibly iron (II) that oxidized in sokution to iron(III) giving you the brownish coloration.

Strange is the precipitate since also iron sulfates should be readily soluble in water.

cheers,
Christoph

Edit: Randy was quicker

Randy Holmes-Farley · Mar 13, 2018

Charterreefer · Mar 13, 2018

Christoph said:
Since imo all Zinc salts are colorless i would assume its some contaminant in your zinc sulfate. - Possibly iron (II) that oxidized in sokution to iron(III) giving you the brownish coloration.

Strange is the precipitate since also iron sulfates should be readily soluble in water.

cheers,
Christoph

Edit: Randy was quicker

Thanks for the reply.
Where would the iron come from? How would it get into my solution?

Randy Holmes-Farley · Mar 13, 2018

Charterreefer said:
Thanks for the reply.
Where would the iron come from? How would it get into my solution?

Contamination in the zinc.

Charterreefer · Mar 13, 2018

Randy Holmes-Farley said:
Contamination in the zinc.

Can that be a contaminant in the zinc sulphate?

Charterreefer · Mar 13, 2018

Randy Holmes-Farley said:
Contamination in the zinc.

Is there a specific grade that is free of contaminants? I bought it from Alpha chemicals. I believe they only had one grade. Can you tell me of a better place to get chemicals at a reasonable price?

Randy Holmes-Farley · Mar 14, 2018

There is nothing you can afford that is "free" of contaminants. All materials have contaminants. The question is how much you dose and how much is present in the additive.

In this case, you are dosing a trace element which implies quite a small amount of dosing. Let's suppose it is 98% pure. Then at most, if all of that other 2% was some other contaminant, you are dosing 50x as much zinc sulfate as anything else. Is there any ion that we would worry about if dosed at 50x lower than the amount of zinc appropriately added to a reef tank? I doubt it. Iron certainly isn't a concern.

So I would not worry about it if you bought any decent grade of zinc sulfate.

By comparison, you dose vastly much more calcium and alk additives, and those all have contaminants in them too. Maybe ten thousand times more calcium (10 ppm per day??) is added than zinc (1 ppb or less per day, likely, I never added any and maintained NSW levels) to maintain a reef tank. So contaminants in those big additives, even if far lower than in your zinc additive, will be adding much more.

Charterreefer · Mar 14, 2018

Randy Holmes-Farley said:
There is nothing you can afford that is "free" of contaminants. All materials have contaminants. The question is how much you dose and how much is present in the additive.

In this case, you are dosing a trace element which implies quite a small amount of dosing. Let's suppose it is 98% pure. Then at most, if all of that other 2% was some other contaminant, you are dosing 50x as much zinc sulfate as anything else. Is there any ion that we would worry about if dosed at 50x lower than the amount of zinc appropriately added to a reef tank? I doubt it. Iron certainly isn't a concern.

So I would not worry about it if you bought any decent grade of zinc sulfate.

By comparison, you dose vastly much more calcium and alk additives, and those all have contaminants in them too. Maybe ten thousand times more calcium (10 ppm per day??) is added than zinc (1 ppb or less per day, likely, I never added any and maintained NSW levels) to maintain a reef tank. So contaminants in those big additives, even if far lower than in your zinc additive, will be adding much more.

Thanks Randy