Calcium Chloride vs Calcium Hydroxide?

[xcross]

Hi,
I'm trying to understand Randy's two-part Calcium and Alkalinity Supplement System and I'm confused by the difference between his recipe for the Calcium part compared to Kent Kalkwasser.

My naive chemistry question is this: why the huge difference in amount, about 2 cups of Dow flakes/gal vs 2 tsp Kalkwasser mix/gal? Even allowing for the water in the dihydrate, the amount of CaCl₂ in Randy's recipe is an order of magnitude larger than Ca(OH)₂ in the Kent Kalkwasser recipe. I know I must be missing something.

The wiki article on Calcium Chloride says it is "faster" than Kalkwasser, which I suppose means that it supplies more Calcium. Why is this so?

Chris

 

[HighlandReefer]

It's all about how much of the two chemicals you can get to dissolve in a given amount of water. You can only get 2 teaspoons of kalk to dissolve in pure water and any extra you add will sit on the bottom, unless you decide to add vinegar to the kalk which will allow more to dissolve.

 

[bertoni]

It's 2 tsp of kalk per gallon of pure water. No more will dissolve. Remember that kalk contains calcium and alkalinity, which combine fairly readily (with carbon dioxide) to form calcium carbonate (sand). Calcium chloride contains no alkalinity, and won't precipitate that way. It's far more soluble. Kalk also has a very high pH, so the rate at which it can be dosed is limited. The two-part chemicals are much lower in pH, and the recipe 2 actually drops pH a tiny bit, so they can be added much more rapidly.

I didn't check on the Wiki article, but I suspect they're referring back to the pH problem or the dilution problem.

 

[xcross]

Should I be able to see the precipitation as its happening? When I watch the kalk dripping, it appears to diffuse and I don't see anything precipitating. On the other hand today I made measurements before and after infusing kalk and saw Ca²⁺ drop, so the calcium is clearly going somewhere.

Starting levels:

pH 8.57, Ca²⁺ 380 ppm, Mg 1395 ppm, KH 3.38 meq/L

after infusing kalk (using vinegar, 4 tsp/gal )

pH 9.00, Ca²⁺ 360 ppm, Mg 1395 ppm, KH 4.0 meq/L

I've been trying to get Ca²⁺ above 400 ppm but with kalk I'm just raising pH and alkalinity. (I used soda water to bring the pH back down to 8.4.) So would I be able raise Ca²⁺ by using the calcium part of the two part alone? I've been reading Randy's articles on this but I haven't learned enough to understand where I'm at in the calcium/alkalinity/pH space and why the kalk isn't "working".

Chris

 

[HighlandReefer]

I would not use kalk water to try to raise your calcium level since it will increase your alk 1 dKH and only raise your calcium level by about 7 ppm. Calcium chloride (two-part) is the way to go to raise your calcium level.

 

[Chris27]

Agreed, a single dose of Calcium Chloride will raise only the calcium to the desired level, from there, daily dosing of kalk may maintain both the alk and calcium. Also, I'm thinking you had a little test discrepancy issue, adding kalk should not lower the calcium, if it precipitated, you would know simply because the tank would look like skim milk.

 

[bertoni]

If a lot of precipitation were happening at the site of the drip, you'd be able to see it, but there still might be precipitation occurring on heaters or inside pumps.

I agree that using kalk to raise calcium won't work unless the alkalinity also is very low, and the tank evaporates enough water.

 

[reefgeezer]

I got caught up in the chasing parameters loop once. My problem was that I was trying to keep all of the parameters near the top of the acceptable range with stuff from the LFS. I'd add calcium one day until the levels were in 500 ppm level, then add buffer the next day in an attempt to get the dKh up to 12 or so. I'd check calcium the next day and find it low. I'd assume magnesium was low and that it was limiting my calcium and alkalinity levels and add it to 1500 ppm.

Turns out that I was causing precipation as calcium carbonate each time I tried to max out a parameter. It got to the point that my sand bed began to become a solid! The guys at RC helped me understand my error.

My advice is to get your magnesium to 1300 or so first. Then get you alkalinity to 8-10 dkh using baked baking soda. Then, using calcium chloride, get your calcium to a little over 400. Randy's two-part works well for maintenance after that, but you'll definitely want to test a lot to determine the correct ratio and volume required for your system.

 

[jorgeldelapaz]

You are making a saturated solution.
A saturated solution is one that is incapable of absorbing anymore solubles. Example, you add enough salt to a cup of water that all the salt dissolves. You continue to add salt until the water can not dissolve even one more grain of salt. All the available water molecules are storing salt. The water is now a saturated solution. If you add more salt... No matter what you do it will not dissolve because you have reached the saturation.