Reef Chemistry Question of the Day #78 Sodium Silicate

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #78

One of the few things that I dose to my aquarium is sodium silicate solution to help provide silica to sponges and diatoms.

Which of the following is not a consequence of this dosing?

A. There is local precipitation of sodium carbonate at the point of addition
B. There is local precipitation of magnesium hydroxide at the point of addition
C. The pH rises
D. The silicic acid level rises
E. The alkalinity rises
F. The silicate level rises

Feel free to pick more than one if you believe it appropriate.

Good luck!

























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[Cory]

IM not sure but I'd like to start dosing it for the sponges in my tank. I'll guess everything but F. Here's w poor pic of the sponge behind the corraline I want to have grow:

 

[Cory]

sorry I'm not sure why every pic I post turns out sideways!

 

[Wiz]

I thought I had this reefing down. My tank does well. I m one of the more knowledgable people I know. Including my lfs and local reef site. But you , Randy, have shown me that I know very little. I need to start reading on reef chem. In depth. I thought silicates were bad and that's why I have to bake the baking soda. To rid it of them. This site is the best thing I've done. I hope to be an expert someday. I've always been into animals. Like to have ecosystems. I was huge into terrariums until I got brave enough to do salt. Now I want my reef to have the best. Always. So more reading I guess. Blahhhhh lol. But the knowledge will be worth it. that being said, I have no idea. My thought process came up with the best guess of B.

 

[Randy Holmes-Farley]

Thanks, Wiz. I'll give a bunch more info on silicate in the answer.

On baking the baking soda, that's to drive off carbon dioxide to give the tank a pH boost as well as an alkalinity boost.

 

[Randy Holmes-Farley]

IM not sure but I'd like to start dosing it for the sponges in my tank. I'll guess everything but F. Here's w poor pic of the sponge behind the corraline I want to have grow:

Where is the sponge in the picture?

 

[Wiz]

Where is the sponge in the picture?

nog mine. But I think its the lighter purple on the bottom left of the main coraline growth.

 

[Randy Holmes-Farley]

Ah yes, I see it now. Nice color sponge.

I've had some that color, but it is not the most common type in my system.

 

[Cory]

Where is the sponge in the picture?

Sorry I knew it was hard to see. I've got lots more on the underside of the rock. Here it is:

 

[Randy Holmes-Farley]

Anyone else before I give the answer?

 

[Randy Holmes-Farley]

And the answer is...A. There is local precipitation of sodium carbonate at the point of addition (that is, A does not happen)

Sodium silicate solution is a very high pH additive, with a pH between 11 and 13.

When any high pH additive is added to seawater, magnesium hydroxide is often the first thing to precipitate. That happens because magnesium hydroxide is not very soluble, and the high pH increases the hydroxide concentration a lot locally.
n below:

When mixed into lower pH water (like a reef aquarium), it changes into mostly silicic acid [Si(OH)₄ ] and some silicate [Si(OH)₃O^-]. So the silicic acid cocentration rises.

The alkalinity rises because the silicate releases OH^- to form silicic acid:

SiO₃^-- + 3H₂O --> Si(OH)_{4 +} 2OH^-

Sodium carbonate won't precipitate from seawater at the concentrations that might be attained, even if the rise in pH converted all the bicarbonate into carbonate. It is just too soluble.

Hope that all makes sense!

 

[Randy Holmes-Farley]

FWIW, this article has more on silica and silicate, including dosing:

Feature Article: Silica In Reef Aquariums ? Advanced Aquarist | Aquarist Magazine and Blog

 

[jedimasterben]

I was just looking at my bottle of sodium silicate and wondering if I should keep dosing it now that I've removed my skimmer. I only have an ATS for filtration now (well, besides ROX carbon and Purigen).

 

[Randy Holmes-Farley]

I see no reason to not dose it. If diatoms become a problem, you can always stop.

 

[jedimasterben]

Sure thing. Anything to try and take nutrients away from the algae on the rocks

 

[Cory]

Can someone show me a good online source with adequate purity?

 

[Randy Holmes-Farley]

None that I've seen say, but the dose is pretty low and the material is very concentrated.

 

[Atu]

Hi, this is the first time I post, though I've been stalking the forums for years. First of all I want to thank you Randy in name of all of us who never write anything, your post and your articles are a light in the darkness of unfounded opinions that fills the internet.
Second, and the reason that made me came out of the shadows and finally create an account... I know that sodium silicate should affect alkalinity ever so slightly (that being the reason that I choose this thread to post), but thats not what I'm seeing when I'm testing with a salifert alkalinity test kit, I'm seeing a big spike that disappears after a few hours, and I would like to know why and if its dangerous.
I've been dosing sodium silicate according to your article with no ill effects, about 5ml in a 400 gallon system once or twice a week (well, maybe a little more than what you recommended) for months now.

I've a journal where I keep everything noted down, the first time I though I had had a bad reading, but now I've re-checked the results a second time and the same alkalinity spike appeared.

From the first time:
-15:30 Alkalinity at 8,9 dKh (note: this was unusually high already, but at the time I was trying to adjust my dosage of kalkwaser. The previous nigh was at 8,5dKh, so I don't know how true this value is)
-next morning at 9am I added 5ml sodium silicate in about 500ml RO water
-just after I tested alkalinity at 11,1 dKh, I couldn't believe this, so decided to retest after an hour blaming not enough time for mixing (added and tested at the sump)
-10:30am Alkalinity at 9,3 dKh
-15:50 Alkalinity at 8,6 dKh


The second time:
-12:40pm Alkalinity at 8,5 dKh
-16hs 5ml sodium silicate in about 500ml
-16:10 Alkalinity 10,9 dKh
-17:40 Alkalinity 8,6 dKh

So, my questions are:
1) Are this values even posible? And if they are, are they real (meaning, is the water really at that values at the time of testing) or an artifact created by the testing method? How fast they come back to normal seems to indicate the later
2)If this values are real, is this only because of the OH- being realized or are there other posible mechanism at play?
3)And finally, are this spikes dangerous or damaging for any living creature at the tank? Would it be better to slowly dose it through the week?

Thanks a lot!

 

[Randy Holmes-Farley]

Welcome to REEF2REEF!

Do you know the concentration of silicate in the 5 mL?

 

[Randy Holmes-Farley]

FWIW, the alk boost numbers look way to high to ascribe to the silicate dosing