Kalk + magnesium chloride mix

[ZaneTer]

Hi everyone
I apologise as chemistry is not something I use every day but I would appreciate some help from the chemists out there.
I am wondering if it is possible to mix kalkwasser and magnesium chloride together and dose it that way.
What would the ratio of weight of Kalk:mag be?
Would it be possible to use this mix to cover all/calcium/mag all in a single mix or would it precipitate out?
Thanks

 

[william.sting]

Assuming that your lime water is fully saturated and at a lower pH than your magnesium chloride solution I think the primary effect would be a pH change. This would result in a super-saturated solution of calcium hydroxide that would precipitate fairy quickly depending in nuclear site availability. The magnesium itself would be incorporated into the growing crystals, which would somewhat impair crystal formation leaving the solution more concentrated in calcium hydroxide than it otherwise wound be.

Now I haven't actually done this experiment, and so there is some guesswork here. Take minute quantities of each and combine them. What happened?

 

[william.sting]

.

 

[Randy Holmes-Farley]

No, it is not possible.

The magnesium will all precipitate as magnesium hydroxide, which is insoluble at the pH of limewater/kalkwasser.

 

[william.sting]

No, it is not possible.

The magnesium will all precipitate as magnesium hydroxide, which is insoluble at the pH of limewater/kalkwasser.

Thanks Randy. I was about to post a correction when I remembered that the pH of lime water was so high and that magnesium precipitates at 9. Should have walked 20 feet over to my bookshelf and grabbed one of my chemistry books first. Oh well, little rusty I guess.

 

[Randy Holmes-Farley]

Thanks Randy. I was about to post a correction when I remembered that the pH of lime water was so high and that magnesium precipitates at 9. Should have walked 20 feet over to my bookshelf and grabbed one of my chemistry books first. Oh well, little rusty I guess.

lol

No problem.

FWIW, I show some graphs on solubility of certain metals as a function of pH here:

Chemistry And The Aquarium: Metals In Limewater ? Advanced Aquarist | Aquarist Magazine and Blog
https://web.archive.org/web/20030623032014/http://www.advancedaquarist.com/issues/may2003/chem.htm

and

Aquarium Chemistry: Magnesium And Strontium In Limewater ? Advanced Aquarist | Aquarist Magazine and Blog
https://web.archive.org/web/20060212023721/http://www.advancedaquarist.com/issues/dec2003/chem.htm

from the first one. Note that limewater solubility is above 12, so the magnesyum solubility has dropped a lot:

Figure 4. The total solubility of several metals in water as a function of pH.

 

[Randy Holmes-Farley]

Just as an aside, folks should not be fooled by the misleading kalk+2 product by Brightwell.

here are my copied and pasted statements from older threads:

Magnesium is not soluble in limewater, so it will just sit on the bottom and worse, the amount present in kalk+2 is not nearly enough to meet the demand for magneisum, and far worse yet, is LESS than is present in many bulk brands of calcium hydroxide as an impurity.

Here's the Brightwell kalk+2 claim:

http://brightwellaquatics.com/products/kalk2t.php

Guaranteed Analysis
Calcium (min) 54.20% (542,000 ppm), Strontium (min) 1.00% (10,000 ppm), Magnesium (min) 0.15% (1,500 ppm)

So that is a ratio of magnesium to calcium of 1:361.

I show in this article that bulk calcium oxide (dried calcium hydroxide) has more magnesium as an impurity (1:263) and that in the actual limewater, the magnesium ratio does not exceed 1:36,000, with the remainder ending up undissolved on the bottom.

Magnesium and Strontium in Limewater
http://www.advancedaquarist.com/issues/dec2003/chem.htm

I show in this article that a typical coral or coralline aglae incorporation rate is 0.1 to 4%by weight, for a ratio of about 1:400 to 1:10.

http://www.advancedaquarist.com/2003/10/chemistry

 

[ZaneTer]

Wow gents.
Thank you so much for the in depth response. You have taught me a few things.
Please excuse another ignorant question: If solubility of magnesium is so heavily affect by the pH of limewater then what were to happen if I were to use an acid, of some variety, to lower the pH to around 8. I would add the magnesium afterwards.

A little background this is for use on a 70l I am playing with while I am in China for the next 8 months. I am not going to set up a dosing system or waste money on it. It will be mainly GSP and Xenia. I can get chemicals with surprising ease though. I will not be using Kalk for its high pH properties so I am perfectly happy to lower it.

Is this a possibility? Or am I just wasting my time?

 

[Randy Holmes-Farley]

If you add a mineral acid (muriatic, sulfuric, etc.) to limewater, you eliminate the alkalinity.

You can reduce hydroxide and pH with vinegar, and get the alkalinity potency back later when the acetate is metabolized by bacteria, but that takes a lot so its like a lot of organic carbon dosing, and bacteria can grow in the solution if it is near neutral pH.

 

[ZaneTer]

Thank you
I think it best for me to give up on this one.
Thank you very much for your help and teaching.
I will instead just bring back half a kilo of Kalk and 250g of magnesium chloride and I will just dose separate.
Thanks for the lessons again.

 

[Randy Holmes-Farley]

Sounds good. I used limewater only for 20 years. I boosted my new salt water with magnesium, but never dosed the tank.

 

[Nubbs01]

I have never used it this way however now I am curious. If the ph is too high and the magnesium precipitates what is the intended use of the BRS magnesium sulfate and magnesium chloride? I have alwaysed dosed according to their "2 part" instructions but I noticed they have a "Kalk and calcium reactors" option to their instructions.

 

[Randy Holmes-Farley]

That part of the directions relates to using the magnesium additive in a system using limewater/kalkwasser or a CaCO3/CO2 reactor, but not putting it into the limewater/kalkwasser or the reactor.

The reason the recipe is different is because when using a two part that itself uses calcium chloride, you are adding a lot of chloride. SO when designing the magnesium part of a two part, you use less of the chloride form and more of the sulfate form of magnesium.

When using as a stand alone magnesium additive (such as in a tank that uses only limewater or a CaCO3/CO2 reactor), you would want to use chloride and sulfate int eh ratio present in natural seawater, but when also using other forms of chloride (i.e., the two part calcium portion), you have to adjust that ratio to keep chloride and sulfate balanced to match NSW.

 

[Nubbs01]

Ahhh, thanks!

 

[Randy Holmes-Farley]

You're welcome.

Happy Reefing.