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olgakurt

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silica, nearly always identified as silicon dioxide,when dissolved in water is actually hydrated to silicic acid (SiO2 nH2O) and associated anions. Below a pH of 9, silica is present mostly in the silicic acid form. As the pH of water exceeds 8, silicic acid increasingly dissociates into the bisilicate anions H3SiO4- and H2SiO42-. It is my understanding that this will increase the solubility of the silica unless divalent and polyvalent cations (calcium, magnesium, iron, and aluminum) are available in significant concentrations.

My question is will the Ca concentration and high pH of kalk solution increase the solubility of silica or cause the precipitation of insoluble silicate salts (and loss of available Ca)?

I've seen an increase in diatoms since I upped my Kalk dosage. The diatoms decrease when I cut back on the kalk (I've been using tap for 10 years on this tank/1 year on this setup).
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In a word, yes, for both questions. You pretty much answered your own question, lol.
 

olgakurt

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galleon,
I'm not sure both could happen and I hadn't heard about either in reference to dripping kalk. If you increased insoluble silica salts, it would seem that available silica would be reduced. The protonation effect would increase the avialable silica.

I'm not sure which process would be dominant with a Kalk solution given the high pH level and available calcium ions.

IME, this (pH effect on silica) may be much more important than silica in the tap water (w/out kalk) or from the substrate. I never had diatoms (in 11 years with tap and 10 years with natural silica sand); but I have them now after increasing the kalk drip to completely replace evap.
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BReefCase

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olgakurt - What you are seeing in your tank is Silicon/Calcium chemistry. You start off with Silica sand (Silicon Dioxide) in your tank, and add water:
SiO2 + 2(H2O) <==> H4SiO4

Then you add Kalkwasser (Calcium Hydroxide):
Ca(OH)2 <==> (Ca++) + 2(OH-)

which absorbs CO2 from the water:
(Ca++) + 2(OH-) + 2(CO2) <==> (Ca++) + 2(HCO3-)

Now the stage is set for
H4SiO4 + (Ca++) + 2(HCO3-) <==> CaSiO4 + 3(H2O) + 2(CO2)

Notice that all of these equilibria can go both ways, driven to one or the other state as a function of solubility, relative ion concentrations, and (importantly), pH. The solubility of Silica is very pH dependent, but in an odd way. At pH=2 Silica has a solubility of 150 mg/liter, which DECREASES with increasing pH until it bottoms out at about 100 mg/liter at pH=8.3. Then, the solubility of Silica starts to INCREASE RAPIDLY with increasing pH until it reaches 1000 mg/liter at about pH=10 and keeps on climbing out of sight.

Note that these solubility numbers are in the ABSENCE of Calcium and Magnesium ions (divalent ions with charge "plus 2"). This is the key to your problem. Normally, since we have plenty of Ca++ and Mg++ floating around - literally -- in our tanks, Silica stays put as solid SiO2 (sand). This is because the solubility of Silica is LOWEST at normal tank pH (pH=8.3) and because the divalent Calcium and Magnesium ions act as precipitation nuclei (much like dust is necessary for raindrops to form in the atmosphere).

However, when you drip Kalk in your tank too fast, faster than it can find CO2 to react with to the Bicarbonate endpoint, you are both decreasing Calcium ion concentration and boosting pH. (This can even happen locally in small volumes of water at the Kalk entry point instead of tank-wide, if you drip Kalk too fast with poor mixing.)

Decreasing your Calcium ion concentration is doubly bad here, because not only is it acting as precipitation nuclei, but it also acts to hold down Silica solubility through the "Common Ion Effect" (Le Chatelier's Principle). All this gives Silicic Acid the opening it needs to dissociates into the Bisilicate anions (H3SiO4-) and (H2SiO4--), which are the biologically available forms that cause Diatom growth.

Note also that even as H4SiO4, Silica is generally not highly ionized at reef tank pH levels. The ionization can be shown as:

H4SiO4 <==> (H3SiO4-) + (H+)

For example, at a pH of 8.5, only 10% of Monosilicic Acid is ionized, and even as the pH reaches 9 to 10, it still is only about 50% ionized.

So, you must be really hiking up your pH as you add Kalk to be seeing bio-available Silica.

To make a long story short if it's not already too late, I'd have to guess that either you are dripping Kalk way too fast and/or you don't have enough CO2 in your tank to make the Kalk go to (Ca++) and 2(HCO3-) as it should. It's spiking your pH, dropping your Ca++ levels, and making the Silicic Acid dissociate to bio-available form, causing Diatom growth. To stop this, drip slower, increase bioload to provide more CO2, or use Acetic Acid (distilled white Vinegar) to provide CO2 for your Kalk. There is along thread going around on using Vinegar in Kalk. (Search on "Kalkwasser" started by llowwell.)

Good luck.

For more info, see also:
http://www.wqa.org/glossary.cfm?gl=1941
 

olgakurt

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Breefcase,
Thanks, for your time and explanation.

Perhaps did not phrase my thoughts clear enough, but I think your post concurs with what I was thinking.

The pH of the kalk solution increases the bioavailable form of silicate in the water I used to make the Kalk solution (I hope not in the tank)(i.e your 10x solubility and 50% ionized form #s). The variable is what effect do the Ca ions have on these #s.

If this is the case then it is much more important what water is used for the kalk solution than used for top-off or water changes. The available silica in one gallon kalk solution dripped in the tank would equate to the available silica in about 25 gallons of water used for water change at pH 8.0 or so

I currently have Ca base aragonite and southdown sand in the tank. Previously with this tank and before I moved and changed substrates (10 yrs with natural silica sand) I had not had any problems with diatoms using tap water (and either silica or Ca sand) but was using less kalk solution (perhaps 1/4).

[ August 22, 2001: Message edited by: olgakurt ]
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randy holmes-farley

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olgakurt:

Calcium silicate is quite insoluble. Consequently, it is not obvious to me that silica in tap water is made worse by adding Ca(OH)2. In fact, you may precipitate out some of the silicate, making the silica less likely to get into the tank. Personally, I think that effect is more likely to be important than the form that the silica takes before hitting the tank water.

When you say up the limewater dose, are you raising the concentration, or the amount of limewater?
 

randy holmes-farley

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Breefcase:

Do you have any evidence that seawater is saturated with respect to calcium silicate? If it is not (which I believe is true), then the calcium concentration will have no impact on silicate solubility. Hence, the connection to rapid limewater addition seems suspect to me.
 

BReefCase

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RHF -- You're right, there are lots of forms of Calcium Silicate Hydrates (not just Wollastonites as I suggested in my equilibrium equations, but more complex Tobormorites, Gyrolites, and others) and once formed they are all highly insoluble in seawater at reasonably high pH.

However, I was just taking olgakurt at his word that his Kalkwasser feed water has amorphous Silica contained in it, and was trying to help him out by postulating a possible mechanism by which Kalkwasser additions might make Silicic Acid dissociate more into its bioavailable ionized forms and thus spur the growth of Diatoms, rather than get deposited on the bottom as insoluble precipitates of Calcium-bearing minerals.

Locally high pH and/or Calcium precipitation during Kalk addition seemed to fit olgakurt's observations about what's going on in his tank, and offered the added advantage of suggesting a range of readily achievable remedies to try -- adding Vinegar in the Kalk mix, slower dripping, better mixing, and more CO2.

I'm sure olgakurt would welcome any other suggested mechanisms that better explain what he is seeing, especially if accompanied by useful suggestions on how to proceed, other than by "dropping the bomb" -- getting a full-fledged RO/DI system for his water.

Olgakurt -- I see that you have another related thread going on the board, rightly suggesting that judicious use of Calcium and Silica test kits might shed more light on exactly what's going on. I'd be interested to have you post the results of that sometime in the future.

Reefers complain a lot about Diatoms, but not much gets published on the board or elsewhere about what chemistry makes the outbreaks come and go, other than blaming new tank syndrome.
 

llowwelll

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Breefcase,
Dang! You seem to have quite a grasp on the bio-chemical processes involved in this hobby. You should look into putting a together an anthology of your replies to questions on this site and publishing it. Thanks again for all the time and thought you put into your postings.
 

tatuvaaj

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BReefCase and RHF,

My silicate test doesn't work with limewater (alk > 7.14 meq/l).

Is there a method to lower the alkalinity of limewater without affecting the silicate test?
 

olgakurt

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tatuvaaj, Thanks, I appreciate you trying the test.

RHF & Breefcase,

Perhaps this kalk/silicate thing is a shot in the dark explanation at the diatoms I experienced, but the potential interaction interests me and the diatoms have subsided since I stopped the kalk additions. I was doing a slow drip and as I ramped up the dosage amount (completely replacing topoff water) my Ca levels climbed and the coralline and diatoms exploded. I thought the increased dissociation of silicic acid (in the kalk solution) might be one of the factors, especially since I am using tap water. I wasn't really thinking amorphous silica (I mentioned solubility only trying to think which way the equilibria may be pushed with the salt formation which I am more clueless about).

I may try making up the kalk solution with purified water and see if there is some difference.

I may also try to contact some of my profs from grad school (Marine Science)and see what they say about this and testing. I haven't had much inorganic since college so I am real rusty. Thanks for everyone's comments.
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randy holmes-farley

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I don't doubt that the lime and/or limewater might contain either silica, phosphate, or both that could be adding to a diatom problem. FWIW, my tap water is loaded with silica, and I wouldn't use it as is.

Before getting worked up about what's going on, I'd test the tap water and the limewater for both.

If your kit is unsuitable for testing phosphate and silica in limewater, you can add a bit of vinegar or muriatic acid (HCl) to drop the pH to 6-9. As a control, I'd test the diluted vinegar or muriatic acid to make sure it did not contain silica or phosphate impurities itself (if, in fact, you see any in the limewater).

AND, like everything in reef tanks, it could just be a coincidence that you got some diatom growth at the same time that you increased limewater usage.

[ August 23, 2001: Message edited by: Randy Holmes-Farley ]

[ August 23, 2001: Message edited by: Randy Holmes-Farley ]

[ August 23, 2001: Message edited by: Randy Holmes-Farley ]
 

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