modifying randy's two part for even higher pH? NaOH?

[rwb500]

Hey guys,

From carefully reading Randy's articles (mainly this one: http://reefkeeping.com/issues/2006-02/rhf/index.php), I can tell that Kalk is so good for the tank's pH because the hydroxide ion has to pull a dissolved CO2 molecule in order to become carbonate. In contrast, sodium carbonate addition from two-part is dosed as carbonate, so it does not pull any CO2 from tank water.

Could the Alk part of the two part be changed to a blend of NaOH and Na2CO3? The blend would have be done carefully, to avoid overdosing NaOH. And it would have to be added slowly. But if I did the math, and designed a solution so that I would be adding about the same amount of hydroxide as if I were adding a reasonable amount of kalk, would it work to give me the same pH benefits as Kalk?

I've got a scale and access to whatever chemicals I need. I bet someones done this before, but if not, i'm happy to be the guinea pig.

My system is too small for a full-blown kalk reactor and every other method of kalk dosing is kind of a pain IMO.

p.s. Randy you should write a book. I'd buy it.

 

[rwb500]

one more question: I was trying to calculate how many moles of CO2 are pulled from the water when one liter of saturated limewater is added. There are two reactions in Randy's limewater article (http://reefkeeping.com/issues/2005-01/rhf/index.php).

Fortunately, when limewater is used in a reef aquarium, it quickly combines with atmospheric and dissolved carbon dioxide and bicarbonate to form bicarbonate and carbonate:

4. OH- + CO2 -> HCO3-

5. OH- + HCO3- -> CO3-- + H2O

are these reactions 1:1? Or even close to it? If they are 1:1, then only half of the hydroxide in kalk ends up sucking up CO2. If #4 is much more frequent than #5, then basically all of the hydroxide sucks up CO2. My end goal here is to determine how much sodium hydroxide is necessary to mimic the CO2 reduction of a known-to-be-safe amount of kalk.

 

[bertoni]

Lye (NaOH) is fine for an alkalinity supplement if you're very careful in dosing it. It's basically Kalk without the calcium, so I'd drip it.

The equations you gave are correct. I'm not sure what you mean by 1:1. We can compute the amount of NaOH that's equivalent to a given amount of Kalk fairly quickly.

 

[disc1]

one more question: I was trying to calculate how many moles of CO2 are pulled from the water when one liter of saturated limewater is added. There are two reactions in Randy's limewater article (http://reefkeeping.com/issues/2005-01/rhf/index.php).



are these reactions 1:1? Or even close to it? If they are 1:1, then only half of the hydroxide in kalk ends up sucking up CO2. If #4 is much more frequent than #5, then basically all of the hydroxide sucks up CO2. My end goal here is to determine how much sodium hydroxide is necessary to mimic the CO2 reduction of a known-to-be-safe amount of kalk.

The first reaction goes basically to completion. The second reaction which forms carbonate happens on a much smaller scale.

Only about 1% or so of the carbonate in seawater is present as carbonate. About 99% of it is there as bicarbonate. Once you have that ratio, then the second reaction won't go anymore. To be more correct would be to say that it goes backwards as fast as it goes forwards, or that it reaches equilibrium.

 

[rwb500]

thanks guys. i'll post back with my recipes, in case anyone is curious.

i realized my last comment about the two reactions is a moot point, because anything that is happening to the hydroxides from Ca(OH)2 will also happen to the hydroxides from NaOH. Don't know why I didn't think of that. I'll just match molar amounts of hydroxide.

I dose about 40 ml of two part (each part) a day, and my tank evaporates a gallon of water a day. I'll get a gallon of kalk worth of hydroxide into 40mls of my alk part. Maybe a bit more, because I can dose very slowly throughout the day, and I have a pH probe and log through my apex so I can make sure that the carbonate/bicarbonate equilibrium reactions can keep up with the additions of hydroxide to keep pH stable.

 

[rwb500]

well, to get 1 gallon worth of kalk's hydroxide into 40mls would require 3.75 molar NaOH. Randy's alk part is only 1.86 molar sodium carbonate. So, the alk part of Randy's recipe could be entirely replaced with sodium hydroxide, and the tank would still not be getting as many hydroxide ions as is possible with kalkwasser (this is all based on my tanks dosing and evap rates).

So lets say I replace Randy's 1.86M Na2Co3 with 1.86M NaOH. The only difference (after the carbonate system equilibrium) is the sodium. Randy usually adds Na and Cl in a 2:1 ratio. My recipe combined with Randy's 0.93M CaCl2 would yield 1:1 Na:Cl. Am I correct to assume that this change in sodium is negligible and harmless?

With my NaOH recipe, 10mls contains the same pH boosting effect (aka quantity of hydroxide) as 465mls of saturated kalk (0.02M Ca(OH)2).

Edit: 1.86M NaOH is pretty dangerous isn't it. any thoughts on this? would it react with CO2 in the air to become Randy's recipe, or would it form a film on top like kalk, or would there be no reaction?

 

[bertoni]

The sodium and chloride balance will be fine with the lye. You might need a few more water changes per year. The lye solution will react with carbon dioxide in the solution to form sodium carbonate. It's the same situation as Kalk, without the calcium, so I don't think you'll see much precipitation. Loosely covered, the solution should last a long time, I think.

 

[rwb500]

thanks!

 

[bertoni]

You're welcome!