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Non Life Sciences => Chemistry => Topic started by: AnkitaA on 20/06/2019 10:51:40

Title: How can I precipitate silicon from an acidic solution at room temperature?
Post by: AnkitaA on 20/06/2019 10:51:40
Adam asks:

"I am trying to precipitate Silicon / Silica from an acidic solution at ambient temperature, but I'm having no luck. The Si in solution was first extracted from a solid amorphous silica sample using a 0.2M NaOH solution (1:400 soil / solution ratio) at 85 degrees Celsius to give a Si solution concentration of 760 - 900 mg Si/L (27 - 32 mM). I then took a 75mL aliquot (of the cooled solution) and whilst the sample was being agitated I acidified it (by titration) using approximately 20mL of 2M HCl to give a resulting pH of around 1.1. Given the Si concentration appears to exceed the solubility of amorphous Si I was expecting it to precipitate into a white solid, but after 3 days of agitation the solution still remains clear and no precipitate has formed (no sol or gel either has formed).

What am I missing and what should I do to make the Si precipitate from the acidic solution?"


Can you help?
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: Bored chemist on 20/06/2019 19:19:41
I'd imagine you have a colloidal suspension.
Saturating it with salt and/ or boiling it might goagulate the silica gel.

What are you trying to achieve?
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: dirtH2O on 20/06/2019 21:02:42
Adam asks:

"I am trying to precipitate Silicon / Silica from an acidic solution at ambient temperature, but I'm having no luck. The Si in solution was first extracted from a solid amorphous silica sample using a 0.2M NaOH solution (1:400 soil / solution ratio) at 85 degrees Celsius to give a Si solution concentration of 760 - 900 mg Si/L (27 - 32 mM). I then took a 75mL aliquot (of the cooled solution) and whilst the sample was being agitated I acidified it (by titration) using approximately 20mL of 2M HCl to give a resulting pH of around 1.1. Given the Si concentration appears to exceed the solubility of amorphous Si I was expecting it to precipitate into a white solid, but after 3 days of agitation the solution still remains clear and no precipitate has formed (no sol or gel either has formed).

What am I missing and what should I do to make the Si precipitate from the acidic solution?"


Can you help?
I'd imagine you have a colloidal suspension.
Saturating it with salt and/ or boiling it might goagulate the silica gel.

What are you trying to achieve?

I am trying to achieve a white precipitate from the solution - which is likely to be an amorphous silica (this is what others in the literature have found). Another common way of precipitating amorphous silica is to neutralise a solution of sodium metasilicate with HCl, but I still can't get the amorphous silica to precipitate out of solution. In both of the above cases my Si concentration is >800 mg/L which is well above the solubility for amorphous silica of around 150 mg/L. I still don't know what is going on?
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: Bored chemist on 20/06/2019 21:21:01
I am trying to achieve a white precipitate from the solution
Why?
What do you plan to do with it?
I ask because, for example, if all you want to do is strip it out of solution you might achieve that by adding something else and co-precipitating it.
For example you could add calcium chloride and then sulphuric acid, the calcium sulphate that is precipitated would drag the silica  down with it.

Or you could simply boil off all the water.
That would get everything out of solution . Than you could (probably) redissove any other materials present, but leave most of the silica behind.

How you get it to precipitate depends on what you want it for.
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: dirtH2O on 20/06/2019 21:36:33
I am trying to achieve a white precipitate from the solution
Why?
What do you plan to do with it?
I ask because, for example, if all you want to do is strip it out of solution you might achieve that by adding something else and co-precipitating it.
For example you could add calcium chloride and then sulphuric acid, the calcium sulphate that is precipitated would drag the silica  down with it.

Or you could simply boil off all the water.
That would get everything out of solution . Than you could (probably) redissove any other materials present, but leave most of the silica behind.

How you get it to precipitate depends on what you want it for.
Thanks. I'm trying to get a relatively pure amorphous silica precipitate from the solution, which doesn't have any (or very little) Fe, Al, Ca, Mg, Na, which I then want to filter the solution off leaving the white precipitate which I can then dry. In order to successfully get a precipitate of amorphous silica I also tried neutralising a solution of sodium metasilicate (Si concentration around 1,000mg/L) with 2M HCl and still no luck, which is contrary to what the literature says.
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: chiralSPO on 21/06/2019 17:43:26
It's hard to troubleshoot chemistry from afar, so please bear with me as I ask some questions:

How do you prepare the sodium metasilicate solution, and how do you know the concentration of Si in it? (did you make it yourself, or buy it, and then did you titrate it, or use spectroscopy etc.?)

For the dirt extract--do you have a way of confirming that the Si is extracted?

After acidification, is the pH stable, or does it change slowly (sometimes, especially when dealing with solids, endpoints can be very hard to reach--you add acid, and the pH falls quickly, then creeps back up...)?

Are you working from a previously published proceedure to do the extraction? I am somewhat surprised that a 0.2 M solution of NaOH would be sufficient to dissolve silica at a reasonable rate (even at 85C)?

What temperature is the solution cooled to?

Do you have access to a laser pointer? (This can be used to determine whether a sample is a homogeneous solution or a colloidal suspension--no beam will be visible in the homogeneous solution, but you will be able to clearly see scattered light if there are suspended particles).
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: Bored chemist on 21/06/2019 18:22:45
Hydrolysis of metasilicate is fairly slow (days).
This can tell you if you have a colloid or not
https://en.wikipedia.org/wiki/Tyndall_effect

Rate of precipitation is dependent on degree of supersaturation.
Try using a higher concentration of silicate.
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: dirtH2O on 22/06/2019 04:19:36
It's hard to troubleshoot chemistry from afar, so please bear with me as I ask some questions:

How do you prepare the sodium metasilicate solution, and how do you know the concentration of Si in it? (did you make it yourself, or buy it, and then did you titrate it, or use spectroscopy etc.?)

For the dirt extract--do you have a way of confirming that the Si is extracted?

After acidification, is the pH stable, or does it change slowly (sometimes, especially when dealing with solids, endpoints can be very hard to reach--you add acid, and the pH falls quickly, then creeps back up...)?

Are you working from a previously published proceedure to do the extraction? I am somewhat surprised that a 0.2 M solution of NaOH would be sufficient to dissolve silica at a reasonable rate (even at 85C)?

What temperature is the solution cooled to?

Do you have access to a laser pointer? (This can be used to determine whether a sample is a homogeneous solution or a colloidal suspension--no beam will be visible in the homogeneous solution, but you will be able to clearly see scattered light if there are suspended particles).

Thank you for your response. Yep - I acknowledge that trying to answer a chemistry question is difficult and therefore I appreciate your time. I have provided a response to you questions below:

How do you prepare the sodium metasilicate solution, and how do you know the concentration of Si in it? I purchased a commercially available sodium metasilicate solid (lab grade) with a know Si content of 27 - 29% SiO2.

For the dirt extract--do you have a way of confirming that the Si is extracted? Yes - once I extracted the Si using the NaOH solution I had it analysed at a commercial laboratory. I'm actually using a radiolarite as my source for the Si (84 - 90% SiO2).

After acidification, is the pH stable, or does it change slowly (sometimes, especially when dealing with solids, endpoints can be very hard to reach--you add acid, and the pH falls quickly, then creeps back up...)? Yes - the pH is stable at around 1.3 and has not moved for 3 days.

Are you working from a previously published procedure to do the extraction? I am somewhat surprised that a 0.2 M solution of NaOH would be sufficient to dissolve silica at a reasonable rate (even at 85C)? The 0.2M was an error and it in fact was 8M NaOH solution with the reaction undertaken at 85 degrees Celsius. I couldn't actually find a published procedure for the dissolution of radiolarite, and I initially tried dissolving it in 2M HCl with no luck. I then saw that I should be using concentrated alkaline solution; hence 8M NaOH was used to dissolve the radiolarite material, resulting in 80 - 95% extraction of the Si. I now want to precipitate the Si out of solution so that I can filter the solid to obtain a relatively pure SiO2 material.
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: Bored chemist on 22/06/2019 11:51:53
Incidentally, "acid washed" silica is available.
The point of the wash is that it removes metals.
Silica of very high purity can be made by hydrolysis of SiCl4 or by oxidation of silanes with O2. Those are ( IIRC) the techniques used in the production of chromatographic grades of silica where even ppm levels of metals are best avoided.
Title: Re: How can I precipitate silicon from an acidic solution at room temperature?
Post by: chiralSPO on 22/06/2019 23:09:49
8M NaOH solution
That makes much more sense!

Perhaps you would want to neutralize with a more concentrated acid (neutralizing 8 M NaOH with 2 M HCl, would also result in a roughly fivefold dilution). You could cool the alkaline Si-containing solution in an ice bath and add concentrated HCl (~12 M) or H2SO2 (~18 M, and diprotic) very slowly (with a buret or dropping funnel, you could set it up to go drop by drop).