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Making Zinc Oxide

MrHomeScientist - 17-5-2013 at 05:48

A co-worker of mine is interested in making her own sunscreen using zinc oxide. She'd rather not buy pounds of the stuff online, so I suggested making it herself from very readily available zinc metal. I want to make this process as simple as possible so she can follow it with a minimal kknowledge of chemistry. In researching this, I found two potential pathways. Both involve producing the hydroxide, then decomposing to the oxide:

(1)
Zn + 2HCl --> ZnCl2 + H2
ZnCl2 + 2NaOH --> Zn(OH)2 + 2NaCl
Zn(OH)2 --heat--> ZnO + H2O

This is the aqueous method, and is straightforward. Metal is dissolved in acid, reacted with hydroxide, and the precipitate is filtered, washed, and heated to ~125C. My concern with this route is contamination from hardware store HCl (mainly iron, which would also drop out as the hydroxide).

(2)
Electrolysis of a sodium bicarbonate solution with zinc electrodes:
Zn + 2H2O --> Zn(OH)2 + H2
Zn(OH)2 --heat--> ZnO + H2O

This is the electrolysis method. A saturated solution of baking soda is electrolyzed with a zinc anode, yielding a white scum of zinc hydroxide. This is filtered and heated as in (1). For this method, I am unsure of the proper voltage, current, and time needed to run the setup.

=========================
My questions are:
For method (1), is there a way to avoid excessive iron contamination? In method (2), what are the correct parameters (V, I, t) to run the electrolysis at? And finally, how do I know I've decomposed all the hydroxide into ZnO? How do we distinguish these two white, insoluble solids?
=========================

Looking around a bit, I found two promising things.

1) Zinc hydroxide is amphoteric. To eliminate iron (or most other) impurities from method (1), I could add excess hydroxide until the precipitate dissolves, filter off other hydroxides, then acidify to drop the hydroxide back out. Several water washes would take care of any excess NaOH.

2) Zinc oxide is thermochromic. This video shows that heating ZnO reversibly turns it yellow. So if I instructed her to heat the hydroxide (from either method) until it all turns yellow, would that be sufficient to ensure it was fully decomposed?

Thanks everyone for your input.

DubaiAmateurRocketry - 17-5-2013 at 06:08

Can i make Zinc hydroxide by using electrolysis ?

12AX7 - 17-5-2013 at 07:31

The *simplest* is simply heating it in air, of course, this isn't very safe to do, either.

Electrolysis is overkill, it'll corrode itself quite nicely given the chance (hence alkaline cells with zinc for the active ingredient!). The main trouble is keeping the oxide off the surface.

If you insist, simply use enough voltage to deliver a useful current. Current defines reaction rate, which also defines heating. You don't want to overheat it, else you'll boil as much as electrolyze. A medium size jar might dissipate, oh, 20W or so at a toasty temperature. Figure the cell dissipates ((V + 0.8) * I) watts, the 0.8V addition being because the zinc itself burns exothermically. If you have a voltmeter and ammeter, you can use this equation to estimate power dissipation.

A neutral salt (like sodium chloride or sulfate) would be better than carbonate, because some carbonate will precipitate as ZnCO3, leaving NaOH behind, dissolving some zinc and plating metal on the cathode. If left untended (not scraping off the metal dross that forms), it will eventually short out. (This is also true of copper, with some differences: although copper is also slightly amphoteric, it more readily forms a carbonate complex, so a bright blue solution forms, and metal plates onto the cathode that way.)

I don't suggest using a base step. Zinc isn't all that acidic, and it takes a lot of NaOH to dissolve a little bit. So you waste a lot of NaOH and HCl (or H2SO4) for not much byproduct, which still contains aluminum impurity (aluminum is a stronger acid than zinc, so it dissolves more readily in base).

The hydroxide (or carbonate) of course needs to be calcined, and yes, if you see it turning yellow, it's certainly hot enough to be crystalline ZnO.

Tim

vmelkon - 17-5-2013 at 08:00

If you put zinc in an iron can, put it in the fireplace, let it heat up until it glows, about 600 °C, the zinc will burn on its own like magnesium but much slower.

This is going to be as time consuming as your chemical method but on the plus side, it doesn't use HCl, NaOH, NaHCO3.

MrHomeScientist - 19-5-2013 at 09:31

Tim,

Thanks for all the suggestions! Using table salt instead of baking soda makes sense, I should have thought of that. I think electrolysis with boating zincs as electrodes in a table salt solution sounds very simple, and won't introduce much impurities. (I'm extra concerned with contamination since this is intended for use in a skin lotion.) I'll play around with it and see what kind of current/voltage works for me, and also see how quickly I can produce my product. I'll keep everybody posted.

Metacelsus - 19-5-2013 at 13:48

A word of caution:
Zinc often contains arsenic impurities, depending on the source. With electrolysis, it shouldn't be much of a problem, but if you do decide to use HCl, you could end up making some arsine gas, which is quite toxic.

MrHomeScientist - 20-5-2013 at 05:51

I tried this electrolysis last night, and it seems to work nicely. I used both electrodes as zinc, in tap water with some MgSO4 sprinkled in for conductivity. The current was set to just under 1A, which put the voltage at about 20V (quite high!).

After running this for about 30 minutes, lots of white Zn(OH)2 was falling off one electrode. The other was soon coated in some black substance, which flaked off and mixed with my nice white hydroxide! After running for an hour, my recovered product is now somewhat grey from this. Any ideas what this black material might be?

Sublimatus - 20-5-2013 at 06:00

<strike>Might be fine magnesium? I've noticed that electrolytically reduced metals often plate out as dark powders or nodules that one wouldn't first recognize as an elemental metal.

Is there still some of the black stuff on the one electrode? Might try scraping it off and lighting it.</strike>

Now that I think about it, that's a stupid suggestion. Don't mind me.

[Edited on 5/20/2013 by Sublimatus]

12AX7 - 20-5-2013 at 15:26

Probably zinc particles reduced at the surface. You'll probably end up with a gray to dull white product after calcination.

Tim

MrHomeScientist - 20-5-2013 at 19:51

In my first trial, the black particles were produced at the positive electrode (cathode). I did a second trial tonight with the same electrolyte, but replacing the positive zinc with an MMO mesh. Strangely, the black precipitate appeared on the anode this time! After running this cell at about 0.5A for an hour, this is what it looked like:

I think zinc particles are a good candidate for this, but why would they be made on the same electrode as the Zn(OH)2?

12AX7 - 20-5-2013 at 20:18

Ah, corrosion is eating away the tastybits and leaving impurities. It could be a mixture of zinc, copper and precious metals (if any). Or if it's pure zinc, then... just zinc.

Weird that it looks added on... not corroded away. Hmm...

Tim

[Edited on 5-21-2013 by 12AX7]

Oscilllator - 21-5-2013 at 00:55

The electrolytic method seems to be a huge overkill, especially for someone not versed in chemistry. And I fail to see the problem with iron impurities, as the iron hydroxide will be decomposed to iron oxide which is completely non-toxic.
The simplest ways are almost always the best.

vmelkon - 21-5-2013 at 05:04

Quote: Originally posted by MrHomeScientist

In my first trial, the black particles were produced at the positive electrode (cathode). I did a second trial tonight with the same electrolyte, but replacing the positive zinc with an MMO mesh. Strangely, the black precipitate appeared on the anode this time! After running this cell at about 0.5A for an hour, this is what it looked like:

I think zinc particles are a good candidate for this, but why would they be made on the same electrode as the Zn(OH)2?

The positive electrode is called an anode, not a cathode.
The anode attracts anions in an electrolytic cell.

MrHomeScientist - 21-5-2013 at 06:59

Edited to remove a mistaken implication on my part.

@vmelkon:
Damn, I knew I was going to get that wrong. I can keep most naming conventions in chemistry straight, but for some reason anode/cathode always gets me. There's surprisingly some conflicting information on the naming online. I'm definitely not very experienced with electrochemistry

===========================================

In reading another thread on zinc compound preparations (here), watson.fawkes brought up a good point:

Quote: Originally posted by watson.fawkes

There's no need for high-grade zinc in a sacrificial anode, and there are two relevant, common commercial grades. One is 98% grade zinc, with Pb added: Pb 1.4%, Cd 0.20%, Fe 0.05% and Al 0.05%. If Pb is cheaper than Zn at the source, expect this alloy. The other is 99.9% grade, with Pb 0.03%, Fe 0.02%, Cd 0.02%. All impurity percentages are maximum level permitted to meet grade requirements. No guarantees on what grade is actually there (or even if it's one of these two), but that should give you an idea.

I'd be concerned with lead in my product, particularly. With that in mind, I think perhaps a good route to try would be the following:
1) Dissolve the Zn anode in sulfuric acid, leaving behind any lead as the sulfate.
2) Filter and recrystallize for purity.
3) Neutralize with NaOH to precipitate Zn(OH)2, filter and wash.
4) Calcine to ZnO as mentioned in my other methods.

At least with this route I can recrystallize to exclude other impurities besides lead, which I couldn't do when producing hydroxide directly. Might be worth a try some just to see how much lead sulfate comes out, to give me an idea of my zinc's composition.

[Edited on 5-21-2013 by MrHomeScientist]

[Edited on 5-21-2013 by MrHomeScientist]

12AX7 - 21-5-2013 at 17:18

This comes to mind for some help in explanation:
http://mysite.du.edu/~jcalvert/phys/elechem.htm

Tim

Making zinc oxide

annaandherdad - 2-6-2013 at 10:33

mrhomescientist, I recently posted on another thread some of our recent experiences with making zinc compounds from pennies. You might find it useful. Here is the link:
http://www.sciencemadness.org/talk/viewthread.php?tid=23059#...

Obviously if you were going to make zinc oxide for sun screen, you'd want it to be pure (no lead certainly). One point is that zinc from recent pennies seems to be very pure (99.995%, according to the supplier), except for the copper, which is removed if you use sulfuric acid.

As for the sulfuric acid, here is point I've been thinking about, and I can't find it mentioned in all the threads on OTC sulfuric acid. That is that there are are least two OTC sources of sulfuric acid, drain cleaner and battery acid. And the manufacturers of battery acid have a motivation to make it pure, since you don't want gunk in your battery. Of course that doesn't mean it *is*pure, but it seems to me you've got a better chance than with drain cleaner. I did a little googling, and it seems that Kragen and Napa auto parts suppliers both sell sulfuric acid at 98% concentration, and it's also available on Amazon. I haven't bought any to test it for purity (not sure how I'd do that anyway, apart from looking at the color and titrating it), but it would be interesting to know how pure it is. Maybe the manufacturers state it somewhere.

We haven't made any zinc oxide yet, but have made a lot of zinc carbonate, and I want to make both zinc oxide and sulfide while we're on the kick.

MrHomeScientist - 4-6-2013 at 09:58

Thanks annaandherdad, I was following that thread too. I didn't know that penny zinc could be so pure! I would have thought they would use a very cheap and impure grade of zinc, to make it cost less than 1 cent to make each penny!

=========================

I've been slowly dissolving a slug of zinc in sulfuric acid over the past couple weeks, from a small boat anode, to see what happens. The slug now looks like a little sponge, as I believe the sulfuric has dissolved all it can but left other metals and lead sulfate behind. Gentle swirling broke this up into a rough powder. Interestingly, my drain cleaner acid was orange to begin with but the reaction has completely removed the color. Once this is fully finished reacting, I can weigh the remaining material and determine (roughly) the purity of my boat anode.

annaandherdad - 5-6-2013 at 05:10

How does your friend plan on converting zinc oxide into sunscreen? Is there some kind of oil or base she plans to mix it with?

I noticed recently that Coppertone Waterbabies brand sunscreen, which my wife favors, uses zinc oxide. I would have thought that to be effective as a sunscreen, zinc oxide would have to be applied rather thickly (so that your skin would look white). But evidently not, this brand of sunscreen is white when you apply it, but you rub it around and the whiteness seems to disappear.

I imagine the zinc in pennies must be electrolytic, to be so pure. Perhaps the mint insisted on purity, since toxic contaminants (like lead) would be bad for currency. Old Roman silver coins had a fair amount of lead in them, I don't know about American silver coins (when they were still making them).

MrHomeScientist - 5-6-2013 at 05:45

I think she simply plans on mixing the oxide with a carrier. She mentioned trying coconut oil, shea butter, etc. Zinc oxide is an excellent UV reflector and is photostable and nontoxic - that's why it's used for sunscreen. To get it to rub clear, the particles need to be tiny. Apparently this makes it transparent optically, but still retains its UV reflective properties. I suspect anything we produce will stay white because it's hard for me to control particle size, but we'll see.

hyfalcon - 5-6-2013 at 07:06

You might find chapter 4 in this book ," The manufacture of chemicals by electrolysis" interesting.

http://babel.hathitrust.org/cgi/pt?id=uc1.b32013;view=1up;se...

I think this one is even in the SM library.

[Edited on 5-6-2013 by hyfalcon]

annaandherdad - 5-6-2013 at 12:21

Thanks for the book, I spent a pleasant couple of hours browsing through it!

Random - 5-6-2013 at 15:32

I wouldn't apply anything made from common zinc metal, could contain quite a few toxic impurities. Lead for example.

MrHomeScientist - 6-6-2013 at 08:30

Quote: Originally posted by Random

I wouldn't apply anything made from common zinc metal, could contain quite a few toxic impurities. Lead for example.

That's the advantage of the sulfate method I mentioned - the zinc is dissolved while the lead precipitates and can be filtered off. Cadmium would dissolve, though. A recrystallization or two of the ZnSO4 would give acceptable purity, I would think. See my quote above of watson.fawkes' comment to see the usual impurities present.

[Edited on 6-6-2013 by MrHomeScientist]

ElectroWin - 17-6-2013 at 09:20

Quote: Originally posted by Oscilllator

The electrolytic method seems to be a huge overkill, especially for someone not versed in chemistry. And I fail to see the problem with iron impurities, as the iron hydroxide will be decomposed to iron oxide which is completely non-toxic.
The simplest ways are almost always the best.

yes, iron impurities, while non toxic, will cause the sunscreen to be coloured; iron(iii) hydroxides yellow ochre, oxides red ochre, iirc.

maybe that isnt so bad?

annaandherdad - 7-7-2013 at 14:58

Mrhomescientist, I've recently read a couple of articles in the NYT about sunscreen:

http://www.nytimes.com/2011/06/21/health/21brody.html?ref=su...
http://consults.blogs.nytimes.com/2009/06/10/what-to-look-fo...

and some others too. These refer to "micronized" zinc oxide, which is apparently zinc oxide treated in some manner to make the particles especially small. This is favored in sun screens because the particles are so small that they do not make your skin look white when rubbed in. I don't really understand this, but I haven't found a technical description. In principle I can see why smaller particles would provide better coverage and better UV absorption for the same number of gms/cm^2, and I wonder how a home chemist could micronize his/her zinc oxide.

I found an amusing reference that tries to argue that zinc oxide is not a chemical; apparently, customers don't want to buy anything with chemicals in them. This is it:

http://www.wisegeek.com/what-is-micronized-zinc-oxide.htm#di...

Supposedly zinc oxide is best at absorbing UVA. I'd like to know more technical details, also about titanium dioxide. If you have any references on these questions, I'd be interested.