Sciencemadness Discussion Board

Failed attempt at dissolving zinc oxide in alkaline solutions

deltaH - 27-11-2013 at 05:16

This is [yet another] example of how the seemingly simplest of chemical reactions throws one a curveball :mad:

I wanted to dissolve zinc oxide in potassium hydroxide to yield a solution of zincates, so I purchased some pharma grade ZnO powder from my chemist and proceeded to mix it with two equivalents KOH and enough water to form a thin paste (but not too much).

After boiling and simply waiting, the ZnO is giving no indication of doing anything but remaining resolutely insoluble! I have covered it with clingfilm and will leave it overnight, however, I doubt very much this will achieve anything.

The problem could be that pharma grade ZnO is probably calcined and so too crystalline and inert for 'wet' dissolution. I presume this would work much better with freshly precipitated zinc hydroxide or using zinc metal and base solutions instead?

Sigh.

Since I have quite a bit of this zinc oxide, I really want to find a way to use it and not something else.

I thought maybe the best thing to try next would be to fuse it with molten KOH in a gas flame and then dissolve the product of the fusion in water. Presumably the molten KOH can and will attack even crystalline zinc oxide powder?

However, I believe the product of such a fusion would at best be a polymeric zincate of the type, K2ZnO(OH)2 as I don't believe a true Zn(OH)4 2- zincate can be formed under anhydrous conditions. Nevertheless, even K2ZnO(OH)2 should be soluble?

Any suggestions or thoughts would be most welcome!

[Edited on 27-11-2013 by deltaH]

shaheerniazi - 27-11-2013 at 05:33

Drop it in sulfuric acid for zinc sulfate
Drop it in nitric acid for Zinc Nitrate
" " " Hydrochloric acid for zinc Chloride
" " " Vinegar and boil for zinc acetate.

woelen - 27-11-2013 at 05:50

If you want to make zincate, then first dissolve the oxide in acid (such as dilute HCl). Then add a calculated amount of solution of KOH. Do not add too much, you don't want it to redissolve. Rinse the resulting precipitate of Zn(OH)2 with water to get rid of the KCl in your solution. Then add KOH, such that all of it just dissolves.

I expect the ZnO to dissolve more easily in dilute HCl than in aqueous alkaline solution. But you might need some heating and a slight excess amount of acid to get all of it dissolved.

Calcined oxides are a real nuisance. I have had similar experiences with quite a few oxides and also with sulfides. I myself have ZnS and this is remarkably inert. If I add this to cold 10% HCl, then only a weak smell of H2S occurs and I need to heat the liquid for tens of minutes to get all of it dissolved!

deltaH - 27-11-2013 at 05:58

Quote: Originally posted by shaheerniazi  
Drop it in sulfuric acid for zinc sulfate
Drop it in nitric acid for Zinc Nitrate
" " " Hydrochloric acid for zinc Chloride
" " " Vinegar and boil for zinc acetate.


Thanks Shaheer for the suggestion, I failed to mention that I require the zinc in basic form (as zincates), so can't simply use a zinc acid salt.

However, the thought did occur to me to first dissolve as the acid salt and then precipitate the hydroxide, wash out the salt formed and then add additional base to dissolve the hydroxide as zincates, however, this method is significantly less scalable and requires additional reagents and co-generates a salt, not good for my intended application.

Zinc metal powder would surely have worked, however, the oxide was readily available so I went with zinc oxide, alas, a poor choice in the end :mad:

EDIT: Ha woelen, Snap! :) You posted as I was replying to Shaheer with the same answer lol

Yes I was myself surprised at the inertness of this oxide, I suppose it's a bit like trying to dissolve sand in caustic, doesn't work unless fused!

I'm completely dumbstruck that even the sulfide reacts slowly, my gosh, shocking!

***
Okay, did some more experiments today with this frustrating system.

I tried a fusion with a small amount of water (one equivalent to two equivalents KOH and one ZnO) so that the mixture can start to melt at a lower temperature and perhaps with some luck, I could get K2Zn(OH)4.

After melting it soon started to boil off this water, but the boiling didn't produce any knocking so I was happy to continue. I assume temperatures climbed as the melt became more and more anhydrous. At some point the mixture started to turn a shade I believe is called 'buff' (a kind of yellow tan). I was concerned that my stainless steel pot was corroding and so stopped the experiment prematurely at this stage out of concern for my pot :)

Anyhow, deciding that fusions would not be as well behaved as I had hoped, I decided to begin tinkering with the dissolution in acid then precipitation route.

This was interesting!

The ZnO powder dissolved quickly in a little battery acid I had left over. I then added incrementally small amounts of 8% ammonia solution. Initially hardly anything happened, then all of a sudden white gels formed upon addition of the ammonia.

Up to here, this was as expected. I continued to add ammonia incrementally until I could just just notice the faintest wisp of an ammoniacal smell.

At this point I had copious amounts of gel. I then proceeded to add potassium hydroxide pellets incrementally with swirling. At first the gel started to disappear and yield a buff coloured solution (solution turned from white to buff), but then upon excess KOH, suddenly a heavy precipitate formed that settled quickly to the bottom as a white powder. Interestingly, two powders formed of different colour settles as two distinctly coloured layers, a completely white one and a buff coloured one. The buff one settles over the white powder layer. I've shaken this up again and leaving it to settle again to confirm as this is peculiar. I might take a picture of it tomorrow.

I think potassium zincates formed (heavy precipitate that settles quickly) and also that these are not very soluble, even in ammonia solutions.

Ok, so that's that for now.

[Edited on 27-11-2013 by deltaH]

deltaH - 28-11-2013 at 00:03

Okay, I've had a look in daylight at my precipitate and have to say that I cannot see the two layers that appeared last night, so I must conclude that this was an optical illusion of some sort. I have taken a photo and increased the image's saturation so that the image colour more closely resembles what I see in reality (the photo came out too white and doesn't do the buff colour justice).

If this is a potassium zincate, then it is fairly insoluble. This may be a reason why it's difficult to prepare it from ZnO and KOH solutions directly.

I will now try to see if sodium zincate is more soluble. If so, this may allow me a route to the potassium zincate by first preparing the sodium zincate and then precipitating the potassium zincate by cation metathesis... a similar trick as precipitating KClO4 from NaClO4 solutions.

possible potassium zincate precipitate.JPG - 184kB

I will report if this route is successful or not shortly.

****
Okay, I mixed 20g NaOH with 20g ZnO and about 200ml water. I then brought the slurry to a boil, waited for it to cool and then added 90ml of 8% NH3 solution (cloudy ammonia). I noted that the solids settled much faster in the slurry that had the ammonia compared to the slurry that didn't but the precipitate was not the same buff colour, much more white this time. However, this was not nearly as convincing as with the previous trial using zinc hydroxide gels.

It appears that sodium zincate is also not particularly water soluble either (if it indeed even formed here). I also don't like this method so much as I cannot be as confident that the particles have reacted all the way through and not simply on the surface. I was more confident of this when employing the zinc hydroxide gel as these first partially redissolve and then precipitated as I added KOH in the previous trial.

My current most successful method is therefore to dissolve the ZnO in battery acid strength sulfuric acid (no warming required) using just enough to make all go into solution, then add cloudy ammonia solution (8%) until a slight ammoniacal smell persists. At this point, a bluish-white gel has also formed in copious amounts. Then add potassium hydroxide incrementally with stirring until all the gel disappears and is replaced by a heavy buff coloured powder that settles quickly at the bottom. This can be decanted/filtered easily.

As a final note, the use of cloudy ammonia over pure ammonia may or may not be playing a role here. Possibly the small amounts of surfactants added to cloudy ammonia assist this process, but as I do not have pure ammonia to compare the results to, I cannot say either way. However, I hope that the precipitate is not simply an insoluble zinc soap :o

I think I will try an ammonia free route next (using only KOH to neutralise the acid salt) and see if there is an apparent difference!

***
Just tried the KOH only route added to a solution of zinc sulphate. This also produced a slightly off white precipitate as before upon excess addition, except this time it was much lighter, nearly white. Also, the precipitate formed a thick slime/gel and not a nicely quickly sinking product like the ammonia route did. This product is also much more voluminous. It looks suspiciously like a zinc hydroxide gel.

I will need to repeat my ammoniacal route again and add much more KOH (excess) and see if I don't form the same voluminous precipitate there as well, although last time it appeared to cleanly form this quick settling powder.

It could just be that the ammonia route produces a more denser final product for whatever reason, but this needs confirmation!

***
Ok, repeated the experiment with cloudy ammonia and then KOH as before, except I added excess ZnO powder to my acid in the first step and broke clots and made sure I had lots of undissolved ZnO (i.e. as concentrated a ZnSO4 solution as I could achieve).

I decanted the clear solution and then added ammonia with stirring which at some point produces masses of precipitate gel (presumably zinc hydroxides). I continued adding the ammonia until a faint ammoniacal odour persisted even with stirring.

I then added pellets of KOH directly to the solution with stirring, the gel breaks down, the solution becomes liquid again and a white precipitate (powder) forms that starts to settle if stirring stops. I added some more KOH for good measure (ensuring I am precipitating everything that wants to precipitate) and am letting it stand now.

Interestingly, the precipitate does not appear buff coloured this time, but will see when it settles out completely. It could be because I used much more ZnO to sulfuric acid this time (excess ZnO, then decanted).

This buff colour versus white may simply be an innocuous particle size effect and so conditions dependent on the precipitation.

[Edited on 28-11-2013 by deltaH]

kmno4 - 28-11-2013 at 04:47

Your motto is "Better to produce lengthy posts than make some literature research", isn't it ?

From some RSC article (DOI:10.1039/F29747001978)
"Although the solubility of ZnO in KOH solutions has been measured by many investigators [16-21] the results show much variation, and it has further been noted
that the solubility undergoes changes on standing (...)"

Do not tell you do not have access - it is boring.
Visit "References" instead.

deltaH - 28-11-2013 at 05:20

Thank you for the info kmno4. The lengthy investigation is because I wanted to study this system. In the end it proved useful as it demonstrated that using cloudy ammonia to precipitate before adding KOH yields a fast settling powder instead of gels that are hard to filter and wash, so I am not unhappy to have investigated the system :)

woelen - 28-11-2013 at 07:42

I have zinc nitrate and zinc chloride, I can check solubilities of zincates at different concentrations. I am quite surprised to read that you get a precipitate again on adding more KOH after the initial dissolving of the precipitate.

The color you report most likely is due to some impurity. Zinc salts are white or colorless, only at elevated temperature (a few hundreds Celcius), zinc oxide can be yellowish, but on cooling down, the color reverts to white.

I finally wonder what you mean with "cloudy ammonia". I have ammonia in 5% concentration, 25% concentration and 32% concentration. All of these are clear like water. The 5% stuff is plain household stuff.

Random - 28-11-2013 at 07:46

This yellow color reminds me of my manganese carbonate. Try to add bleach to soluble salt of this stuff to see if it produces black precipitate.

deltaH - 28-11-2013 at 08:06

@woelen

The colour is strange, the second time I did it, it did come out white but not the first time, although the concentrations were different between the two. The first time my system was more dilute. For a change, my chemicals are very pure, I'm using pharma grade ZnO and I purchased KOH pellets from a chemical supplier. My container was a cleaned glass jar (embarrassing) lol

Cloudy ammonia is store bought ammonia for cleaning. The brand I was using is cited as 8 wt.%. I suppose that isn't pure, but I was using the same bottle for both batches and got different results. Typically cloudy ammonia also contains a small amount of surfactant to aid cleaning. I don't have pure ammonia solution, so I can't compare if that behave differently in this reaction.

I would be very upset if the precipitates are nothing else than zinc surfactant salts, but I doubt it, why would it only precipitate upon adding the KOH then? No I think this is the right stuff, colour results may vary, as I said my last batch was very white.

@Random

I'll give it shot

@WGTR

Thank you for you idea with carbonates, I may play around with that :)

Agreed, the salts are not particularly clean by way of contaminating salts. From a heterogeneous catalytic viewpoint, salts are not nice. If I could, I would probably be dissolving my ZnO with nitric acid and then adding ammonia and then KOH. The only contaminant then is small amounts of ammonium nitrate (after washing) and potassium nitrate and all this should decompose in a mild calcination of the product (forming NOx and H2O).

But for my means, this is not possible, just saying this would be a better way to make a cleaner product if one has the means.

[Edited on 28-11-2013 by deltaH]

woelen - 28-11-2013 at 13:02

I have fiddled around a little with zinc salts, to check the properties of zincate.

I have zinc nitrate and dissolved some of this in a small amount of distilled water, so that a very concentrated solution is obtained. In a separate test tube I prepared a very concentrated solution of KOH. I added an excess amount of the solution of KOH to the solution of Zn(NO3)2. When this is done, then a white precipitate is formed, which on addition of more KOH does not dissolve or only partially dissolves.

I then distilled added water to the liquid with the white precipitate. I added water in small amounts and swirled the test tube after each addition. After each addition of water, part of the precipitate dissolved. Finally, I obtained a clear and colorless liquid.

Next, I prepared a little bit more of concentrated solution of KOH and added this to the clear solution, described above. When this is done, then no immediate reaction seems to occur, but after a minute or so, everywhere in the liquid very fine glittering needles are formed. These needles have a length of 2 mm or so and are very thin, like a human hair. These little crystalline needles slowly settle at the bottom.

On heating, the needle-like crystals dissolve again and a clear colorless liquid is obtained.

The conclusion which can be drawn from my experiments is that potassium zincate is soluble in water, but not in very concentrated solutions. The final experiment is a little bit less sure, the very fine needles could also be potassium nitrate. More investigations are needed here.

It might be that deltaH can achieve his goal by trying to dissolve the ZnO in a dilute solution of KOH instead of concentrated KOH. Try this on a test tube scale first. Heating does help.

deltaH - 28-11-2013 at 13:43

Thank you woelen for you fantastic help here. These observations are indeed extremely interesting, particularly in regards to the needle-like crystals you obtained.

I will certainly try the dilute KOH experiment with prolonged boiling.

From your observations and from what I have seen myself, I am now wondering if the powder that initially precipitates is not the mono potassium salt or something similar. I still wonder why this precipitates as a 'denser' material when ammonia is present than without?

Perhaps this needle-like crystals you obtained upon additional KOH added could be the well defined crystalline K2Zn(OH)4? I will experiment further, as you said, this could be potassium nitrate, though probably unlikely so.

Even if I cannot succeed with the direct route of reacting ZnO with KOH, I am still happy that I have an indirect route that is reasonably well behaved from a preparatory point of view. Filtering and washing gels for the amateur is seldom an option. To precipitate dense zincates is already a big help!

***

Tried the suggestion of employing dilute conditions with my ZnO powder without success:

Dissolved one teaspoon KOH in 500ml water and added one teaspoon pharma grade ZnO. Heated to a gentle boil maintained for 30 minutes. No perceivable dissolution evident.

Then added another teaspoon of KOH and continued gentle boiling for another 30 minutes with a top up of water to restore lost volume. Still no perceivable dissolution.

Allowed solution to stand and cool completely. White powder settled to the bottom, there was a lot of it. No crystals evident in solution.

Conclusion: pharma grade ZnO powder is too inert to be attacked by dilute caustic even in prolonged boiling.

Only acid dissolution (fast) followed by precipitation seems a viable route. Using ammonia solutions to affect hydroxide precipitation before employing KOH seems to work better than KOH alone as it yields a denser zincate intermediate that is easier to wash and filter.

Woelen and myself observed that this initial potassium zincate precipitate is partially water soluble.

[Edited on 29-11-2013 by deltaH]

woelen - 29-11-2013 at 07:41

Interesting observation that ZnO does not dissolve in solutions of KOH, while freshly precipitated Zn(OH)2 does. This behavior is the same with Cr2O3 vs. Cr(OH)3. The former does not dissolve in alkalies, nor in acids, while the latter dissolves easily in both. ZnO is not so inert as Cr2O3, but to some extent it also is. The calcined ZnO is not easily hydrated to Zn(OH)2, otherwise it would dissolve quickly in alkaline solutions.

----------------------------------------------------

In my experiment I boiled the liquid with the needle-like crystals and this gives a clear and colorless solution again. On cooling down, however, I did not get crystals anymore, the liquid remained clear and colorless! Tonight I will scratch the surface and see if that helps.

I also have ZnSO4.7H2O instead of Zn(NO3)2.6H2O, but I am afraid that experiments with the sulfate are less conclusive, because of relatively low solubility of K2SO4.

deltaH - 29-11-2013 at 08:13

Yes, these fine needle-like crystals of yours is hopefully the prize. kmno4 mentioned earlier that zincates solubility depends on aging as well and this may explain why you are struggling to crystallise you old batch. I can think of a reason for this but don't want to speculate about it.

According to wiki's article on zincates, both the monoalkali and dialkali salts are possible for sodium. For sodium, they say that NaZn(OH)3· H2O is polymeric, which makes sense, while I assume Na2Zn(OH)4 is more crystalline because it consists of discrete Zn(OH)4 2- ions.

So, this suggests that the first powdery precipitate we obtain with potassium may be a polymeric monopotassium zincate. This fits the observation that this material does not appear highly crystalline. I think the fine needle crystals you obtained upon addition of more KOH to clear solutions of the first precipitate is most likely K2Zn(OH)4, if the trend seen with sodium holds.

Time will tell if this hypothesis is indeed correct.

References:

1. Stahl, R.; Niewa, R.; Jacobs, H. (1999). "Synthese und Kristallstruktur von Na2Zn(OH)4". Zeitschrift für anorganische und allgemeine Chemie 625: 48.

2. R. Stahl, H. Jacobs (1998). "Synthese und Kristallstruktur von NaZn(OH)3· H2O und NaZn(OH)3". Zeitschrift für anorganische und allgemeine Chemie 624 (1): 25–29.

[Edited on 29-11-2013 by deltaH]

WGTR - 29-11-2013 at 11:20

I have a reference for you. The title is quite a mouthful, as you can see:

Production and Properties of Zinc: A Treatise on the Occurrence and Distribution of Zinc Ore, the Commercial and Technical
Conditions Affecting the Production of the Spelter, Its Chemical and Physical Properties and Uses in the Arts, Together
with a Historical and Statistical Review of the Industry (Google eBook)

Walter Renton Ingalls
Engineering and mining journal, 1902

It's a pretty interesting book overall, but I'll post some excerpts from it. Here, for example (page 160):

zinc_hydroxide.jpg - 48kB

It's interesting to note the observation that 1. Zn[OH]2 becomes less soluble once it has been dried. 2. Decomposes easily with
heat, and 3. ZnO does not unite directly with water. To be fair, perhaps ZnO technically does hydrate in water, but not
to a practical degree.

Zn[OH]2 is fairly unstable towards heat. Wiki lists it as decomposing at 125C, but this may be deceiving. For example, when
I react the solutions of sodium hydroxide with copper sulfate to precipitate copper hydroxide, if too much excess NaOH is
added, then the blue copper hydroxide precipitate decomposes in solution to black copper oxide, even at room temperature.
The solution actually gets darker as more NaOH is added, although this reaction may take several minutes to complete. This is
in spite of the observation that in neutral solution, copper hydroxide is fairly stable almost up to the boiling point of water (or
80C as per Wiki). If solid pellets of NaOH are added to the solution, the decomposition observed is immediate,
probably due to localized heating and high basicity. The reason that I mention this, is perhaps by adding solid pellets of
hydroxide to your precipitate, you might be reforming ZnO from your Zn[OH]2 precipitate, thereby making a more dense
precipitate. It might help to eliminate this as a possibility. This compact precipitate that you obtain, deltaH, are you able to
dissolve some of it in water? How about NaOH/KOH solution?

It would not surprise me at all if NaOH has more than one effect on Zn[OH]2. At very dilute concentrations of NaOH the Zn[OH]2
may not dissolve as completely as it would if there was more NaOH in solution, but at very high concentrations of NaOH, the Zn[OH]2
may be decomposing to ZnO, especially with some heating. Ammonium salts can form complexes with zinc hydroxide, so it
may even be catalyzing this decomposition. Most of this paragraph is speculative, however.

If you want to react ZnO directly with an alkali salt to form a zincate, this may work for you:

zinc_oxide.jpg - 49kB


Edit: to show what little I know about zincates, here's a link to a reference on the solubility of zinc oxide in NaOH over 25-100C:

www.ysxbcn.com/down/paperDown.aspx?id=32388

This other link may also be of interest:

www.minsocam.org/ammin/AM48/AM48_642.pdf

[Edited on 30-11-2013 by WGTR]

woelen - 29-11-2013 at 11:26

I have taken my cold and clear solution and shaken it vigorously for a few seconds and guess what? Needle-like crystals are formed again in the next few minutes. The effect was quite spectacular, it looked as if the liquid was filled with long hairs (some of them almost a cm length, while being thin as a little piece of hair as you sometimes see in dust from clothes). The crystals are like shining needles and are very delicate. I now have put aside the test tube and will allow the needle-like material to settle at the bottom.

WGTR - 29-11-2013 at 21:46

OK, if you look at the common ways of preparing sodium zincate, they almost always involve
reacting a soluble zinc salt with NaOH, and then dissolving the precipitate in excess NaOH. This
gives a mix of salts in solution. So along those lines of thinking, I decided to dissolve NaCl
and NaOH together in solution, and try dissolving ZnO into that.

5.84g of NaCl and 4.03g of NaOH were dissolved into 50.1g of DI water, and stirred to dissolve.
Some insoluble particles were noted, so the solution was passed through filter paper, leaving a clear
solution. This step negated some of the accuracy of the earlier measurements, due to some retention
on the filter paper.

Small (10-50mg) increments of ZnO were added to the stirred solution. When the solution
became transparent, more was added. The solution was stirred for about 1 hour at 25°C, and when
the solution was taking longer to clarify, the temperature was raised up to 37°C and stirred for
another hour. In all, 0.313g of ZnO was dissolved into solution. This is not necessarily
all that will dissolve, but at this point the ZnO was beginning to dissolve very slowly. Even though
transparent, the solution was somewhat cloudy. It is uncertain right now whether this is yet
un-dissolved ZnO, or some other contamination. The lingering cloudiness that remained after the
ZnO dissolved increased gradually with each addition of ZnO.

One thing that was noted while adding the ZnO was that it tended to remain in clumps of
powder that bounced around in the beaker. A very small drop of BacDown soap was added (for
the surfactant) in an attempt to improve wetting. It is not certain what effect this had, but it
didn't appear to hurt anything. Also, the stir bar was adjusted to a speed that would stir the
particles, but also allow them to get sucked under the bar and smeared against the bottom of the
beaker. This greatly facilitated the dissolution of the ZnO.

I'm not sure if I'm understanding the charts in the previous reference correctly
(www.ysxbcn.com/down/paperDown.aspx?id=32388), but it appears that about 0.5g of ZnO
should go into solution under the present conditions of the experiment. So things don't look
that far off.

The NaCl really shouldn't be needed, so the next thing to try would be the same experiment with
NaOH and surfactant only. If that works, then NaOH without surfactant.

According to the reference, the solubility of ZnO goes up dramatically with Na+ concentration,
and then drops above a certain point. It's possible that we've been trying to dissolve much
more ZnO than the given conditions will allow, giving the appearance of a negative result.

If you want to duplicate this, then I'd suggest starting with half the amount of ZnO, and verifying
that the solution goes transparent before proceeding. Even then, the dissolution is slow, and
it may take an hour or two for before the solution becomes transparent.

[Edited on 30-11-2013 by WGTR]

watson.fawkes - 30-11-2013 at 07:26

Kirk-Othmer has this to say; it's in the section "Zinc Compounds":
Quote:

[...] Zinc also tends to form stable covalent complex ions, e.g., with ammonia [Zn(NH3)2]2-, cyanide [Zn(CN)4]2-,and hydroxyl [Zn(OH)4]2-. The coordination number is usually 4, to a lesser degree 6, and in some cases 5. A good review of zinc compounds is given in Ref. 1.
    Zinc forms salts with acids, but since it is amphoteric, it also forms zincates, e.g., [Zn(OH)3⋅H2O]- and [Zn(OH)4]2-. The tendency of zinc to form stable hydroxy complexes is also important because some basic zinc salts are only slightly soluble in water. Examples are 3 Zn(OH)2⋅ZnSO4 [12027-98-4] and 4 Zn(OH)2⋅ZnCl2 [11073-22-6], which may precipitate upon neutralization of acidic solutions of the salts.

[...]

1. M. Farnsworth and C. H. Kline, Zinc Chemicals, Zinc Institute, Inc., New York, 1973.
It seems that, as usual, a small amount of research can save lots of time.
<hr>bfesser moderator expellendus est

WGTR - 30-11-2013 at 22:46

4.01g of NaOH and 0.310g of ZnO were added together into a beaker, and 50.0g of DI water was
then added. A slight exotherm was noted upon dissolution of the NaOH, and the solution was
allowed to come back down to 25°C while stirring for two hours.

After checking on the solution at this point, it was noted that it had become clear. Everything
had dissolved.

Upon adding an extra 0.020g of ZnO and stirring, the solution had still not yet cleared after
another two hours.

I didn't use any surfactant with this test, and I'll probably stop here unless someone has a question
about something. For now, I have the solutions stored in glass bottles.

deltaH - 1-12-2013 at 01:30

@WGTR

Thank you for the wonderful literature you found and volumes of it too, I will need to work through it all. I am curious what kind of ZnO you are using because it doesn't sound as inert as mine. As for your point of the localised heating, I will try to pre dissolve my KOH. That what initial precipitate I obtain is, however, partially water soluble (I noticed on decantings and washing that it was quickly disappearing :o ) I managed to recover a little washed powder which I have let air dry... I can experiment further with this by dissolving in water and adding KOH, hopefully I get some of woelen's hair like crystals :) Carbonate fusions are probably the way to go, but I don't have a furnace (I'm guessing that carbonates have to be fused at high temperatures in excess of 800C).

Thank you again for your wonderful contributions to this thread!

@woelen

I vaguely recall reading papers about zinc oxide nanowires and nanorods... seems zinc chemistry is rich in this kind of morphology. By the sounds of things, the potassium zincate is an exceptional examples of such 'wire' crystals, albeit not nano but macro!

I know that zinc oxide doped with aluminium is electrically conductive, so called aluminium zinc oxide (AZO) is supposed to be a replacement for the more expensive ITO coatings on glass. Now, I am very curious if you include a few mole percent aluminium salt at the start, if the potassium zincate wires obtained could also be made conductive... then we can truly call them 'wires'.

I hope to give this a shot as well when I catch up to you with these hair like crystals.

@watson.fawkes

Thanks for the Kirk-Othmer text. This is a possibility, but I'd be a little surprised if I prepared 3 Zn(OH)2⋅ZnSO4 in the presence of so much ammonia. After precipitating the ZnSO4 with ammonia, the addition of the KOH causes a strong ammoniacal smell to form as presumable K2SO4 is formed in solution releasing lots of NH3 (which previously was present as (NH4)2SO4 from the ammonia neutralisation of ZnSO4).

I add excess KOH to precipitate the potassium zincate intermediate of course, but the point is that at this stage the free ammonia concentration is quite high as well. The point of the ammonia is to solvate zinc cations by forming soluble metal ammine complexes. The only thing then that can precipitate are the anionic zincates that do not form metal ammine complexes.

Well that's the theory anyhow

Thanks for the info.

[Edited on 1-12-2013 by deltaH]

WGTR - 1-12-2013 at 02:36

Some of the speculations that I was making in that earlier post about the decomposition of Zn[OH]2 are probably not
correct. I hope that doesn't present too much of a distraction. But it would be interesting to see if pre-dissolving the KOH
makes a difference in the type of precipitate that you obtain.

The ZnO that I have is stuff that I got from a pottery store. I think that it is Aardvark brand. I've had trouble getting it to dissolve
too, and this is what initially got me interested in the subject. Looking at the reference that I posted earlier, if the
concentration of NaOH is too dilute, then ZnO becomes poorly soluble. This is obvious from the two tests that I just did.
With 4g of NaOH dissolved into 50g of water, I only got about 0.3g of ZnO to dissolve. With a higher concentration of
NaOH, ZnO solubility goes up dramatically, according to the literature. Past a certain concentration, though, the zincate
will begin precipitating out. It almost sounds like your experiments on dissolving ZnO into NaOH might have involved a combination
of too much ZnO, and both too little and too much NaOH concentration.

Just as a sanity check, I'd suggest stirring a very tiny amount of ZnO into NaOH solution (just enough to turn the solution milky)
for a couple of hours, just to verify that it turns clear. If it doesn't, then something is very wrong.

deltaH - 1-12-2013 at 03:20

Quote:
Just as a sanity check, I'd suggest stirring a very tiny amount of ZnO into NaOH solution (just enough to turn the solution milky)
for a couple of hours, just to verify that it turns clear. If it doesn't, then something is very wrong.


My experiment of one teaspoon ZnO to 500ml water was about these conditions... nothing happened. I am more or less happy to declare direct routes pointless, short of fusing with carbonate in a furnace at very high temperature.

The acid route works wonderfully well and is well behaved, so no real issue. I was originally worried about having to wash and filter slimes which is impractical and also the co-generation of salts, but that is no longer an issue if one employs ammonia as the neutralising agent (precipitates powders). Also, the resulting potassium ammonium sulphates in solution can just go on my lawn as fertiliser (once properly neutralised with addition battery acid off course) :) Zn is also a trace plant nutrient, so small amounts of that left in solution is also fantastic for the 'waste'.

I'm quite happy with this zinc chemistry now, after a bumpy start and with everybody's contributions, I think we've tamed this misbehaved beast!

Zephyr - 5-12-2013 at 23:11

Ummmm, what is this?????

deltaH - 6-12-2013 at 01:33

Quote: Originally posted by Pinkhippo11  
Ummmm, what is this?????
What do you mean, what is what?

This thread is about making potassium zincates, initially by the failed route of trying to dissolve ZnO powder in potassium hydroxide solutions, which doesn't work... and then goes on to discuss ways that appear to work.

[Edited on 6-12-2013 by deltaH]

deltaH - 9-12-2013 at 11:08

I was reading today about zinc oxide and came across this article about the synthesis of nanostructured zinc oxide:

http://www.hindawi.com/journals/jnm/2012/427172/

The authors carried out a 'trick' of sorts of employing urea in solution to slowly raise the pH of a zinc acetate solution by hydrolysis of the urea generating ammonia. Anyhow, they did this because they specifically wanted to slowly precipitate zinc oxide so as to grow nanostructures where particular morphologies/sizes would be favoured.

This got me thinking if the first powder precipitates I saw after adding potassium hydroxide to the ammonia precipitated zinc hydroxide gel (from zinc sulfate in solution) may in fact also have been some 'activated' zinc oxide and not a lower potassium zincate as I previously thought.

The difference in my case is that I rapidly raised the pH by addition of the KOH as opposed to the slow raise the authors did with urea hydrolysis.

Interestingly, their urea hydrolysis, nevertheless, progressed quiet rapidly, taking only several minutes to complete at 100C. Perhaps the zinc oxide here is acting as a catalyst since I had the impression that even at elevated pH, urea hydrolysis is otherwise quiet slow at these temperatures.

Just some food for thought. Reading this article made me realise that I should not discount the oxide forming in between.

So long story short, this intermediate may simply have been an activated oxide which goes on to slowly dissolve upon further addition of KOH and water.

EDIT: Just saw that this is what WGTR was saying before! I forgot about your post earlier... sorry :(

[Edited on 9-12-2013 by deltaH]