Mixing Kalk in predictable amounts

[sen5241b]

I notice a lot of people mixing kalk end up with a white sediment in the bottom of the mix container. Is that the best way to mix kalk? Does the dissolved calcium hydroxide stay the same each time you mix leaving a predictable amount of dissolved, dosable kalk? Should I add a small pump to keep any kalk from settling to the bottom of the mix container?

[Origami]

The solubility of Kalk is dependent upon temperature and, in fact, is so predictable that it can be used as a (temperature dependent) pH standard.. It is only slightly soluble and, by putting a slight excess of kalk in the container, you are trying to create a saturated solution that has the most kalk in it that can be dissolved. That's why you'll see stuff settle to the bottom. It's the stuff that's not dissolved because the solution is saturated. You can put a pump in your container but only run it intermittently - don't run it continuously. And don't dose it while the solution is cloudy with suspended solids. If you do that, you'll be dosing a variable amount of kalk depending upon the density of the suspended solids. Instead, you only want to dose the clear, saturated solution so that a known amount of kalk is being dosed.

[astroboy]

So, the next question, Tom, is, if I have a bunch of precipitate on the bottom of my top off reservoir, when I add RO/DI water to it, will the precipitate dissolve into the water so I don't have to add any more kalk mixture? Or is the precipitate not soluble enough, or doesn't contain what we want to dissolve into kalkwasser? I've never been clear on this....

 

Thanks,

 

Mark

[Origami]

So, the next question, Tom, is, if I have a bunch of precipitate on the bottom of my top off reservoir, when I add RO/DI water to it, will the precipitate dissolve into the water so I don't have to add any more kalk mixture? Or is the precipitate not soluble enough, or doesn't contain what we want to dissolve into kalkwasser? I've never been clear on this....

 

Thanks,

 

Mark

 

If it's precipitate, no. If it's simply undissolved Kalk, yes. However, a quick stir can help bring it into solution faster and more effectively.

 

Here's the thing: If it's precipitate, that implies that a reaction has taken place. And, if it has, then it's most likely with carbon dioxide

 

Ca(OH)2 (kalk powder, aka calcium hydroxide or lime) + CO2 (carbon dioxide)  ==> CaCO3 (calcium carbonate) + H2O (water).

 

Calcium carbonate is insoluble at high pH. Some of the stuff at the bottom is undoubtedly calcium carbonate but it shouldn't be too much (unless it's accumulated over time). Most of it should be undissolved lime or, possibly, other impurities in the kalk powder.

 

Here are some notes that I have pertaining to Kalkwasser:

• pH: 12.54 @ 25 deg C (12.627 @ 20 C, 13.00 @ 10 C, 12.289 @ 30 C)
• 808 ppm Ca, and 40.8 meq/l alkalinity at 25 deg C.
• Solubility: 0.185 gm per 100 cc water (@ 0C) down to 0.077 gm per 100 cc (@ 100C)
• Conductivity: 10.3 mS/cm @ 25 C

So, if the pH of your kalkwasser is around 12.5 and there's some powder at the bottom of the reservoir, then the solution is saturated and you've likely got undissolved kalk on the bottom. If, on the other hand, the pH of your kalkwasser is considerably less than 12.5 and there's some powder at the bottom of the reservoir, then that powder is unlikely to be undissolved kalk and the powder can be siphoned out and discarded as it's probably just junk.

[sen5241b]

All very good info.  In Autumn, spring and the bizarre weather we see now when temperatures swing evaporation rates swing widely. Isn't this going to risk a kalk overdose?

[Origami]

All very good info.  In Autumn, spring and the bizarre weather we see now when temperatures swing evaporation rates swing widely. Isn't this going to risk a kalk overdose?

 

Because of the low solubility of kalk in water, in many tanks (SPS or LPS dominated) kalk won't be sufficient to offset consumption. That is, you'll be supplementing with something else. So you won't overdose kalkwasser in most cases - at least, not in the manner that you're speaking of. (If your tank is really new and you don't have much in the way of hard corals, clams, LPS, coralline, or other things that consume calcium and alkalinity, then you will want to limit how much kalk you use.)

 

Instead, most kalk overdoses occur when something in the setup fails (e.g. ATO failure, kalk stirrer turns over, something else). Using kalk carries with it a risk of overdose and thinking through and managing that risk has been subject to a lot of discussion. Off the top of my head, these include: 1) limiting the amount of kalk powder in a kalk stirrer so that, in the event of a spill or ATO failure, it's not a "fatal" dose. 2) Limiting the freshwater reservoir size to limit the max amount of kalkwasser that can be delivered by the ATO. 3) Limiting the rate of dosing using a peristaltic pump to limit the amount of kalk that can be delivered daily. 4) limiting the max dose time by using a peristaltic pump and a timer (i.e. fixing the total dose). 5) Monitoring pH and shutting kalk off when the pH gets above a certain level (which presumes a kalk system failure).