How to stabilize pH of the solution that containes silica?

[k_amy21]

How to stabilize pH of the solution that containes silica?

Hello,

I have a problem with adjusting the pH of my solution to value 7.
It contains these chemicals: Na2SiO3, CaCl2, KCl, Na2SO4, NaHCO3 and NaCl. The initial pH (after adding all of these stuff) is around 10. The problem occurs when I start adding HCl (probably due to the present silica). I can not for some reason stabilize the pH. It is constantly rising!
I suppose I could use some buffer solution in order to adjust the pH, but the thing is I can not add any other cations or anions other then these.
Can someone help and tell me what to do? Is there an acidic buffer solution that contains some of these cations/anions?
I would appreciate very much any suggestions, thoughts, anything that could help with solving this problem...

Thnx
A.

 

[Borek]

My guess is that after you acidify the solution, carbon dioxide evolves, so pH goes up. Is it an open system? Can you replace air over the solution with carbon dioxide?

 

[k_amy21]

Hi Borek,

first of all thank for such a quick respond.
I was thinking about the CO2 effervescence as one of possible things that happens, it´s just that I have only 0.15 g/L of HCO3-. How much of a CO2 can evolve from that concentration?
And, yes, unfortunately my system is open and it would be too complicated to close it and replace the air above with CO2.
This solution is meant to be just for exposure test of the metals in order to investigate their corrosion behavior. That is why I am looking for some very simple solution – if it exists, of course...

 

[Borek]

0.15g/L is in a mmol range, typical for buffers. Than it is simple stoichiometry - 1 mole will give around 22.4L, milimoles will give 1/1000 of that volume.

 

[k_amy21]

Wow, you are really fast. ;)

My question “How much of a CO2 can evolve from that concentration?” was actually rhetorical ;), but now that you´ve mentioned it I did some calculations.
According to these equations:

HCO3- + H+ -> H2CO3 -> H2O + CO2

where:

γ(HCO3-) = 0.15 g/L => n(HCO3-) = 2.46 x 10-3 mol

and therefore

n(H+) = 2.46 x 10-3 mol (H+ came from HCl – while acidifying),

pH = -log c(H+)

during the evolution of CO2, the pH would change for 2.61 units. Is that correct?
If so, does that mean that after the pH changes for 2.6, it will stabilize? Maybe this question sounds a bit silly but I´ve made this solution 2 weeks ago and the pH is still rising (though very slow...). Can I somehow make this reaction go faster – to achieve the equilibrium faster??

 

[Borek]

Could be what you see now is hydrolyzing silicates. They are reasonably stable only in high pH.

 

[k_amy21]

I assume these silicates cause the major problem in this solution. As soon as I start adding the salts in the solution of sodium silicate, the silica precipitates. And that´s all right, I don´t need to have a homogenous solution where everything is dissolved in side. I just need to have a STABLE solution, with a stable pH but I don´t know how to achieve it.

Some other suggestions... ? Maybe you know someone who has an experience with silica solutions?

 

[Borek]

No more ideas. Now I started to wonder if the solution composition makes sense... you mix calcium chloride and sodium silicate, sulfate and carbonate - seems to me like precipitation is unavoidable, no matter what the pH is. Unless concentrations are low, but if they are in milimole range like bicarbonate, if memory serves me well you are above Ksp for all three salts.

 

[k_amy21]

Yes, I know it seems a bit irracional, but this is a simulation of a geothermal fluid. It needs to be the same composition as in the nature in order to conduct my experiments (corrosion tests) in the laboratory.

Anyway, thanks a lot for this discussion. Although we didn´t not solve my problem, it showed me some other directions in which my thoughts could go. ;)

Thnx
Amy

 

[chemisttree]

Without knowing how much of each of your ingredients or your procedure, it is a difficult question to answer. You mentioned that you formed a ppt. Did it form when you added a particular ingredient? Did it form immediately? If it is a silica/calcium silicate floc, that might be your problem. The procedure I would use would be to add a dilute solution of the CaCl2 dropwise with vigorous stirring to a dilute solution of your sodium silicate, sodium sulfate, KCl, NaCl. Do this dropwise. When you have combined all of these, add your solution of sodium bicarbonate last, dropwise with vigorous stirring. You might find that you want to add the CaCl2 solution last and the bicarb solution first. In either case you want to add these as dilute as you can. Ex, if you want to make up a liter of solution, dissolve your silicate, KCl, NaCl and Na2SO4 in 350 mL of DI water and place into a 2L flask. Dissolve the bicarb in 350 mL of water. Dissolve the CaCl2 in 300 mL water. Combine these solutions dropwise and sequentially with vigorous stirring. I wouldn't add the CaCl2 and the bicarb together before you add them to the silicate solution.

Try to avoid forming a ppt for as long as you can! Flocs play havoc with stoichiometry and trap solution in their interiors. CO2 is a reactant in this solution so I would not blanket the solution with CO2 since you will change the properties of your solution. If that isn't a problem in your procedure then it might not be such a problem. If you are simulating a down hole solution, keep in mind that the fluid will be under pressure and at a certain temperature. A solution kept uncovered on your bench at RT will very likely be a different animal from a down hole fluid once all is said and done.

I would be very suspicous of a silicate floc in this application. If the geothermal fluid itself is a turbid solution of silicates then perhaps this is no problem. If this simulated solution is based on an analysis of soluble components of a geothermal fluid then your problems might be much more difficult than you think. Soluble silicates reeeealy affect corrosion since they can passivate steel so removing a soluble silicate from solution (by producing a floc) is problematic.

 

[k_amy21]

Thank you, chemisttree, for such a comprehensive response.
I will sure try to do what you suggested me. But I have to say that I tried making the solution in 2 ways.

1st way:
I diluted Na-silicate in DI water (clear, transparent solution was made) and after that I added salts (unfortunately I don't know in which order). Then the precipitates started to form.

2nd way:
I diluted all of the salts in DI water (again, clear transparent solution was made). As soon as I started adding Na-silicate (dropwise with pipette) precipitates started to form.

As I said, I will try making the solution according your logic. Maybe it really helps somehow. Hopefully..
If you have anything more to add, please feel free and sare you knowledge/assumptions with me.

And yes, if it'll help here are the concentrations in g/L of each ion in the solution:
SiO2 0.55; Cl 1.2; SO4 0.2; HCO3 0.15; Ca 0.2.
Unfortunately, I don't know the concentration of Na and K because I don't have now the data with me but I remeber they were both below 1 g/L (or Na was a bit more??).


Regards

 

[Borek]

After some more thinking... why do you want pH to be 7? Was it measured at 7, or is it just a guess? If we are talking natural water of such composition, I would expect it to be slightly alkaline.