What causes KH precipitation when added?

[Justfbilly]

I have been chasing KH for the last 2 months. It’s been hovering about 6.5DKH. I have always added the Sodium Bicarbonate to my sump to try and get my KH to rise. Today I decided to add it directly to my tank and use the flow of one of the return lines to mix it in. I noticed as soon as I started to add the Sodium Bicarbonate I could see it precipitate as soon as it started to mix into the tank? I checked my dosing pump and the calcium had not been dosed yet. I am running a 120 gallon tank with a icecap 30 sump.

tank parameters are:
KH 6.5
Ca 440
Mag 1400
NO3 .05
Phosphate .02
Salinity 1.025
Temp 79

salt mix is Aquavitro
Any advice is greatly appreciated

 

[Spare time]

From my understanding, it is because (in very simple terms), when you dose large amounts at once, you are creating a small space in the tank where the alk is insanely high, causing precipitation at that spot. I usually don't see this as much of a problem with bicarbonate compared to pure carbonate.

 

[Spare time]

This explains it well

http://reefkeeping.com/issues/2005-07/rhf/index.php#6

 

[mingc]

i am using brs pure carbonate but i diluted it with rodi water to 20% concentration then drip dose at night in higher flow area, it seems just ok without much precipitation issue, i don't know if i am diong wrong this way since my ph and kh was pretty stable by doing this.

 

[Randy Holmes-Farley]

I have been chasing KH for the last 2 months. It’s been hovering about 6.5DKH. I have always added the Sodium Bicarbonate to my sump to try and get my KH to rise. Today I decided to add it directly to my tank and use the flow of one of the return lines to mix it in. I noticed as soon as I started to add the Sodium Bicarbonate I could see it precipitate as soon as it started to mix into the tank? I checked my dosing pump and the calcium had not been dosed yet. I am running a 120 gallon tank with a icecap 30 sump.

tank parameters are:
KH 6.5
Ca 440
Mag 1400
NO3 .05
Phosphate .02
Salinity 1.025
Temp 79

salt mix is Aquavitro
Any advice is greatly appreciated

What exactly are you dosing?

As noted in the link above, it is normal to see precipitates when dosing high pH solutions. It is less likely with bicarbonate solutions, which is why I ask exactly what you are dosing.

This is from the above link:

http://reefkeeping.com/issues/2005-07/rhf/

Precipitates When Carbonate Solutions are Added
Many aquarists are familiar with the cloudiness that forms when high pH two-part calcium and alkalinity additive systems are added to marine aquaria. Figure 3 shows the initial cloud that forms, for example, when the alkalinity part of B-ionic is added to a relatively low flow reef aquarium. The initial cloud sinks and spreads out, eventually dissolving. A similar phenomenon is not observed when adding calcium or magnesium salts, but is observed when adding sodium carbonate solutions.

This cloudiness is, at least in part, magnesium hydroxide and is formed when hydroxide ions are added and the local pH rises. Unlike the addition of limewater, which is unlikely to form magnesium carbonate, this may, although I think it unlikely. The reason it might form here is that the addition of the carbonate ions may push the magnesium carbonate solubility product above saturation. The precipitation of magnesium carbonate can be kinetically slow, just as the precipitation of calcium carbonate can be slow, and since this cloudiness forms instantly, magnesium hydroxide is a much more likely candidate. However, if the additive is not rapidly mixed in, or worse yet, if solid globs of the initial precipitate settle out and are very slow to dissolve, then conditions may be ripe for magnesium carbonate (and calcium carbonate) to form.

In any case, any magnesium carbonate that does form will probably dissolve later as the pH returns to normal reef aquarium levels, so whether the initial cloudiness contains any magnesium carbonate or not is not a critical issue. It does not contain calcium carbonate if mixed in reasonably quickly (a couple of minutes or less), as CaCO3 would not dissolve when mixed with seawater (and this material is observed to dissolve).


Figure 3. The transient cloud of magnesium hydroxide that forms when high pH additives are added. In this case, the alkalinity portion of B-ionic was added to a fairly still portion of one of my reef aquaria.

 

[Justfbilly]

What exactly are you dosing?

As noted in the link above, it is normal to see precipitates when dosing high pH solutions. It is less likely with bicarbonate solutions, which is why I ask exactly what you are dosing.

This is from the above link:

http://reefkeeping.com/issues/2005-07/rhf/

Precipitates When Carbonate Solutions are Added
Many aquarists are familiar with the cloudiness that forms when high pH two-part calcium and alkalinity additive systems are added to marine aquaria. Figure 3 shows the initial cloud that forms, for example, when the alkalinity part of B-ionic is added to a relatively low flow reef aquarium. The initial cloud sinks and spreads out, eventually dissolving. A similar phenomenon is not observed when adding calcium or magnesium salts, but is observed when adding sodium carbonate solutions.

This cloudiness is, at least in part, magnesium hydroxide and is formed when hydroxide ions are added and the local pH rises. Unlike the addition of limewater, which is unlikely to form magnesium carbonate, this may, although I think it unlikely. The reason it might form here is that the addition of the carbonate ions may push the magnesium carbonate solubility product above saturation. The precipitation of magnesium carbonate can be kinetically slow, just as the precipitation of calcium carbonate can be slow, and since this cloudiness forms instantly, magnesium hydroxide is a much more likely candidate. However, if the additive is not rapidly mixed in, or worse yet, if solid globs of the initial precipitate settle out and are very slow to dissolve, then conditions may be ripe for magnesium carbonate (and calcium carbonate) to form.

In any case, any magnesium carbonate that does form will probably dissolve later as the pH returns to normal reef aquarium levels, so whether the initial cloudiness contains any magnesium carbonate or not is not a critical issue. It does not contain calcium carbonate if mixed in reasonably quickly (a couple of minutes or less), as CaCO3 would not dissolve when mixed with seawater (and this material is observed to dissolve).


Figure 3. The transient cloud of magnesium hydroxide that forms when high pH additives are added. In this case, the alkalinity portion of B-ionic was added to a fairly still portion of one of my reef aquaria.

I am dosing sodium bicarbonate (BRS pharmacy pouch) I am adding it in front of my gyre to make sure it’s in a area where there is a large amount of water movement.

 

[Randy Holmes-Farley]

I am dosing sodium bicarbonate (BRS pharmacy pouch) I am adding it in front of my gyre to make sure it’s in a area where there is a large amount of water movement.

If it is properly dissolved before dosing, then there should not be any problematic precipitation, and any initial cloudiness is not a concern.