can left over precipitate in Kalk/ATO reservoir increase eventually lead to increased concentration if not cleaned.

[MoshJosh]

I mix 1 teaspoon of kalkwasser per gallon then add to my ATO (I let it settle/clarify first). I still get residue/build up over time (precipitate the right word?) this is fairly normal as I understand it. My question is, can this stuff be brought back into solution when more solution is added to the reservoir (when the top off is topped off).

Is my 1 teaspoon per gallon solution becoming more concentrated if I am not cleaning the reservoir every time?

 

[TangerineSpeedo]

In my experience, and this is only my opinion. You can have two types of settlings.
1. After your solution is mixed, what isn’t saturated falls to the bottom.
2.Other stuff that will not mix in that is part of the kalk. Kinda like what's in the bottom of your mixing container if you use some of the more affordable salt mixes.
Every few times, I just throw some RODI in there just to get some of the excess kalk absorbed. I will also rinse the container and the pump out every month-ish just to get rid of the stuff at the bottom.
Remember the RODI will only hold so much of the kalk powder, the max is a pretty fixed number as far as concentration I believe, unless you super saturate it.

 

[slickreefing]

I mix 1 teaspoon of kalkwasser per gallon then add to my ATO (I let it settle/clarify first). I still get residue/build up over time (precipitate the right word?) this is fairly normal as I understand it. My question is, can this stuff be brought back into solution when more solution is added to the reservoir (when the top off is topped off).

Is my 1 teaspoon per gallon solution becoming more concentrated if I am not cleaning the reservoir every time?

Very slightly, most of the settled powder is quite inert at that low of a saturation concentration. When you mix in the new water and additional powder you may notice it doesn’t curve upwards by much if at all. Corrective measure is to aim for the same PH reading of your clear solution after each batch has settled. Also good practice is to check how much the hydroxide solution’s ph has dropped towards the end of the batch.

 

[Randy Holmes-Farley]

There are two major types of precipitate happening in limewater (kalkwasser), and some very minor ones. None are a problem to worry about increasing potency long term.

1. Undissolved calcium hydroxide from the previous batch. Since you are adding less than saturation to begin with, this is going to be a minor component and won't change the potency much if you dot he same thing every time you refill.

2. Precipitated calcium carbonate. This material cannot dissolve in your limewater solution.

This has more:

What is that Precipitate in My Reef Aquarium? by Randy Holmes-Farley - Reefkeeping.com

Precipitates on the Bottom of Limewater Reservoirs
The solids on the bottom of a limewater reservoir (Figures 5 and 7) contain everything that did not dissolve, or that dissolved and later precipitated from solution. Such solids could contain magnesium hydroxide and carbonate, calcium hydroxide and carbonate and a variety of other impurities such as copper salts, alumina, silica, etc.

In order to determine what is in these deposits, I tested a sample of the white solid material that had been collecting for months on the bottom of my limewater reservoir, and detailed the results in a previous article. I removed the white sludge along with some limewater. The mixture of solid and liquid was acidified to dissolve it, and it was tested for calcium, magnesium, and strontium. The results are shown in Table 1. Only relative concentrations are shown as no effort was made to dry the sample prior to analysis, making absolute concentrations meaningless.

As anticipated, based on the very low solubility of magnesium hydroxide and the high concentration of hydroxide ion in limewater, the solid material on the bottom of the limewater is enriched with magnesium. Relative to calcium, magnesium is enriched by a factor of 13 in the sludge compared to the solid starting quicklime. This magnesium may be present as both magnesium hydroxide and magnesium carbonate, but because magnesium carbonate is fairly soluble compared to calcium carbonate, it is most likely that the primary magnesium salt is magnesium hydroxide. It may also be mixed calcium and magnesium carbonates.

Interestingly, strontium is actually depleted by a factor of two relative to solid starting quicklime, indicating that it is less likely than calcium to end up on the bottom of the reservoir. While strontium carbonate is somewhat less soluble than calcium carbonate, the strontium concentration in the limewater is so low that SrCO3 may not actually be saturated, so it may not precipitate at all. The strontium that is there may simply be copreciptiated with calcium carbonate.

The solids on the bottom of a limewater reservoir or the residue left in a limewater reactor can also contain other materials. Phosphate, for example, would be insoluble in limewater, precipitating as calcium phosphate. Many toxic metals, such as copper, are also insoluble in the high pH of limewater, forming carbonates or oxides. These metals can also bind directly to undissolved lime or to calcium carbonate precipitates, as I showed in a previous article. In a sense, this precipitation can purify the limewater so that in some cases it may be even purer than the starting water or lime.

This purification is also seen in practice by many aquarists who have noticed the solids on the bottom of their limewater containers discolor, often to a bluish/green color suggesting copper. For these reasons, I recommend that lime solids not be dosed to aquaria when it is possible to avoid it. Letting the limewater settle for a few hours to overnight will permit most of the large particles to settle out, and whether it looks clear at that point or not, it is likely fine to use. In general, it is a good practice to leave residual solids on the bottom of limewater reservoirs rather than cleaning them out every time, as they may actually help purify the water by these precipitation mechanisms. Once the solids discolor, or have been collecting for 6-12 months, however, they should be discarded.

 

[Randy Holmes-Farley]

Also good practice is to check how much the hydroxide solution’s ph has dropped towards the end of the batch.

FWIW, pH is a very crude measure of potency, with a difference of 0.3 pH units being about a factor of 2 in potency.

Conductivity is a much better way to gauge potency, with saturation around 10.3 mS/cm as 25 deg C.