[Terry Gallentine]

I am an artist who has been using metal salt solutions to tone ceramic frits and glazes.  I have been using mainly chlorides (cobalt chloride and manganese chloride) in a saturated solution.  I have been adding these salts to distilled water until saturation occurs to get the solutions that I need.  The problem is that I would like to increase the Ph of these solutions to around 6.5 without dropping much of the metal salts out of solution.  Is this possible and how should I go about it?

[Mitch]

What is the current pH of your solutions?

[Tetrahedrite]

Is the metals dropping out a real or inferred problem? You can't stop metals dropping out at high pH's because of the formation of oxides and hydroxides. Are you using Co(II) or Co(III), Co(II) is more soluble at higher pH's. I have a feeling that the amount of metals ppting out at that pH will be quite small. If you are going to adjust the pH using something such as sodium hydroxide, make sure it is quite dilute to prevent the instantaneos formation of metal hydroxides

[Terry Gallentine]

Thanks for the responses.  In answer to the two questions posed, I have encountered Ph's as low as 2.5.  I am working on ways to use these solutions in an inkjet printer but at these low Ph's the printheads corrode much too fast.  What I need is to get the solutions to just a slightly acidic  Ph.

As for the other question, I have been using Cobalt (II) chloride.  It seems to be very soluble but at least when I mix it with water to a saturated state, I get a very low Ph.  Do you have any suggestions for buffering it?

Thanks again
Terry

[Mitch]

You might want to try adding some lye(sodium hydroxide) that should increase the pH. You will also have a problem that the metals might precipidate out, but it's a place to start.

[Tetrahedrite]

I think you will have a lot of trouble buffering the solutions and maintaining the solubility. It doesn't matter what you use as a buffer, as a general rule the solubility of metals decrease rapidly with increasing pH because of the formation of oxides and hydroxides. If you want to sacrifice some of your solubility a lower pH you could use the metal sulfates instead of chlorides, the sulfate ion has some buffering capacity. There may be some other ligand that allows the solubility to be maintained at higher pH's but I'm not aware of one. Also, the the nitrates, especially of cobalt, are much more soluble than the chlorides although you still have the acidity problem

[Demotivator]

I'd be wary about changing the cobalt salts, like to sulfate or nitrate.  The Chloride is a ligand that gives the cobalt a blue color : CoCl₄2-.  Without chloride, cobalt would be hydrated and turn to pink.

[AWK]

Ph of your solution is low because of salt hydrolysis.
You can safely adjust pH to 6.5 with ammonia. Note, cobalt solution can change color.

[Terry Gallentine]

What form of ammonia would be the best to use to alter the Ph of the solutions?  The resulting color of the solution is not a problem.  The concept is to get the metal itself into a dried, clear glaze coating so that it will tone the clear glaze when fired.  

[Tetrahedrite]

It is true an aqueous ammonia solutions will increase the pH to the desired level, however adding a dilute solution of sodium hydroxide will do exactly the same thing and you don't need as much of it. (probably cheaper as well)

[AWK]

Use water solution of ammonia. Ammonia during neutralization forms salts that hydrolyse giving slightly acidic solution (without excess of ammonia) so its easy to stabilise pH using ammonia instead of NaOH (of course NaOH is cheaper).

[billnotgatez]

When the final result of the neutralization is going in a high temperature oven to bake away the unwanted stuff and leave behind the wanted metals, I would think that ammonia is better than sodium hydroxide. The Nitrogen and Hydrogen in ammonia would breakdown and leave no residue, while the sodium in lye would not break down in the kiln. Aqueous ammonia may be found in as a household cleaning agent. Finding a pure version as a household cleaning agent is the trick. Although, the other stuff in household cleaning agents may not have an adverse effect on your neutralization / firing process.
Regards,
Bill
 

[Terry Gallentine]

I went to the hardware store and got some "extra strength ammonia" to increase the Ph of a close to saturated solution of manganese chloride that I made up.  It seemed to work well.  I increased the Ph of the solution from 4.5 to 5.75 without any dropout.  The only problem that I had was that the ammonia that I got was probably a fairly dilute mixture and because of that it took more than I would want to bring up the Ph to where I need.  I need to keep as strong a solution of metal salts as I can and the dilute ammonia that I used diluted the overall solution too much.  
I will go to the lab supply to see if I can get a stronger solution of ammonia to offset the dilution problem.  Hopefully the stronger ammonia will not cause a drop out problem.
Thanks for the suggestions.  

[movies]

Perhaps you could add some NaOH to the ammonia solution.  I'm not certain that would help, but it might.

[billnotgatez]

Movies –
Look at my post above to understand why Sodium Hydroxide may not be the best solution to this problem.
Bill