Alk precipitation

hllb

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I sometimes need to dose alkalinity to my tank. Current using the Red Sea foundations liquid. I’ve never needed to dose Ca or Mg, just the alk on occasion.

I’ve noticed it precipitates. I did a little reading and it said it likely will slowly dissolve and be fine, and since it raises my alk the amount I expect, that appears to be true. Is there any issues with this? A reason it happens? I don’t think it always does it but will admit I haven’t paid attention in the past.

Today’s water parms before dosing and after 15% wc :
Salinity 1.026
Ph 8.1
Alk 7.7
Ca 450

I didn’t test mg today but last week it was 1600. It’s normally between 1400-1600.

I dosed 4ml of the Red Sea alk. I dose it in the third chamber of my biocube - with the return pump. A couple hours later I retested alk and it was 8.2.

any issues?
 

redfishbluefish

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It's common to get a "poof" of precipitate that should quickly dissipate. This is the reason you want to dose in a high flow area. If you watch the early part of this video from BRS, you'll see the typical "poof" you get from alk additions.

 
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hllb

hllb

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Thanks that video was helpful. I think I’ll dose it right into the return stream next time. I dose it in the chamber with my return pump but I think there’s enough water in that chamber that the surface isn’t high enough flow.
 

Randy Holmes-Farley

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The precipitate on dosing a high pH additive is magnesium hydroxide:


from it:

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.

1587383748121.png


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).
 

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@Randy Holmes-Farley

I use RedSea Foundation B (alk) and in my case when I does into high flow I get exactly the image above which is presumably Mg(OH)2. This cloud lasts for about two seconds and then flaky precipitate (snow) forms where the cloud is. These flakes wander around for a bit and then eventually disappear. It is not clear if they go back into solution or settle to the bottom.

Do you have any idea what these flakes are? Is this just another stage of the Mg(OH)2 precipitation process? Is it likely going back into solution?
 

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@Randy Holmes-Farley

I use RedSea Foundation B (alk) and in my case when I does into high flow I get exactly the image above which is presumably Mg(OH)2. This cloud lasts for about two seconds and then flaky precipitate (snow) forms where the cloud is. These flakes wander around for a bit and then eventually disappear. It is not clear if they go back into solution or settle to the bottom.

Do you have any idea what these flakes are? Is this just another stage of the Mg(OH)2 precipitation process? Is it likely going back into solution?
They redissolve. :)
 

Randy Holmes-Farley

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I expect that is correct, but if you do see flakes collect on the bottom somewhere and stay there, it might be calcium carbonate. It too is supersaturated locally where you dose high pH additives, but it is slower to form, so usually doesn't form before the high pH area disperses.
 

Tobin VP

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Thanks guys.

I have been having a hard time getting alk to go above 6.5 dKH regardless of my dosing efforts and have been investigating reasons. I use Coral Pro salt which starts at 11.5 dKH. I wanted to be sure that the flakes were not a sign of something else really wrong - like super fast CaCO3 formation. Glad to hear that this is unlikely.

It sounds like the flakes (some are pretty large) are then likely MgCO3 and then they are reabsorbed pretty quickly. This would be consistent with the fact that about 20-30 seconds the water column clears again ... and I only see an occasional flake on the bottom. It may be that my supersaturation conditions are showing a bit more exaggerated effect than what I have observed in other videos.
 

Randy Holmes-Farley

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Thanks guys.

I have been having a hard time getting alk to go above 6.5 dKH regardless of my dosing efforts and have been investigating reasons. I use Coral Pro salt which starts at 11.5 dKH. I wanted to be sure that the flakes were not a sign of something else really wrong - like super fast CaCO3 formation. Glad to hear that this is unlikely.

It sounds like the flakes (some are pretty large) are then likely MgCO3 and then they are reabsorbed pretty quickly. This would be consistent with the fact that about 20-30 seconds the water column clears again ... and I only see an occasional flake on the bottom. It may be that my supersaturation conditions are showing a bit more exaggerated effect than what I have observed in other videos.

You might try dosing baking soda (sodium bicarbonate). It has a lower pH than most alk additives (including the one you are using) so will have less potential for precipitation.

Here's a calculator to gauge amounts:

 

merkmerk73

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I expect that is correct, but if you do see flakes collect on the bottom somewhere and stay there, it might be calcium carbonate. It too is supersaturated locally where you dose high pH additives, but it is slower to form, so usually doesn't form before the high pH area disperses.
Hey Randy,

This is exactly what has started happening in my system - and the flakes are significant and don't seem to be dissolving.

I have to go blast them with a baster to try and get it to mix up.

I am dosing a pretty low amount into my return chamber - it's the only area of my sump that the tube holder will go, since I'm using a Trigger systems sump and they all have huge lips around the edge.

What's the solution? Tons of micro doses throughout the day? Dose the calcium at a different time of the day?
 

merkmerk73

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IMG_1292.jpeg
I’ve relocated the doser to here - this area gets a lot of flow - I wonder if this will be better than the return chamber which is extremely low flow?
 

merkmerk73

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Low flow is a bad place, so hopefully that will help. Smaller doses will also help.

It still globs on as a paste that needs to be stirred/agitated to disperse

Am I supposed to repurpose a powerhead just to be in the simp to stir this water up for the doser?

I have a little nero down there but it's for the chaeto fuge to give it some flow

Looking at your article from other threads:

This precipitation takes place primarily because the limewater has pushed the CaCO3 supersaturation very high by converting much or all of the bicarbonate into carbonate. Since the precipitation of calcium carbonate can be slow to occur, rapid dispersal of the limewater doesn't lead to much or any precipitation of calcium carbonate. But if a region maintains high pH for long enough, calcium carbonate will precipitate. How long this process takes depends on the degree of supersaturation, but can be on the order of minutes to hours.

This sounds like you need a dedicated powerhead wherever you're dosing your Alkalinity

I'm curious how most people solve this issue
 
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merkmerk73

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Red Sea Alk and Calcium

6 ml Calcium per day (for now) and about 30-40ml of Alk
 

Randy Holmes-Farley

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Red Sea Alk and Calcium

6 ml Calcium per day (for now) and about 30-40ml of Alk

I’d switch to baking soda for the alk part if you are getting excessive precipitation.
 

merkmerk73

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I ended up putting a small Nero powerhead in there and just blasting the area things dose at inside the return chamber
Did the job
 

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