Is there a good explanation for this?

[kb27973]

Hi,
A little background: My tank has a kalk reactor and a calcium reactor. For a long time I had the problem of low pH. At night it would get down to 7.6 sometimes. After reading a lot of posts I finally came across a guy who said he fixed his low pH problem by putting the output of both reactors into the same cup. I tried this and low and behold it worked. The lowest my pH would get would be around 8 with a high of 8.4 during the day. I cranked along like this for months until just a couple of days ago when my pH started to hit 8.7 during the day with a low of 8.4 at night. After doing some testing I found that my calcium was at 350, usually it would be around 440. Alk was around 9. Mg tested around 1100. Added Mg and Ca. The next day Mg still low Alk was at 8 and Ca was 420. pH still way high. I finally got Mg up to 1300 and magically Alk, Ca and pH are now stable. pH is back to normal 8-8.4 levels. I understand the Alk and Ca wackiness with low Mg but don't understand the high pH effect. Can somebody explain this?
Confused and bewildered,
Ken

 

[MCsaxmaster]

When you say that the effluent lines for both reactors go into the same cup, what is the residence time in that cup? Is the water from each getting flushed quickly into the system?

The calcium reactor effluent will have a ton of calcium and alkalinity, but a low pH, so there will be very little carbonate and little chance for calcium carbonate to precipitate abiotically. If you raise the pH of this water quickly (without letting it mix into the tank and thereby diluting the calcium and alkalinity) some of the bicarbonate in the water will dissociate to carbonate, raising the carbonate concentration substantially and making abiotic precipitation very easy. That can consume a lot of calcium and alkalinity, and magnesium will slowly get consumed as well since that tends to precipitate as MgCO3 with the CaCO3 to a small degree.

I'd say that for right now I'd keep the lines separate and ensure that both get rapidly mixed into the tank water (area of strong flow). Let's then adjust alk, Ca, and Mg to the proper levels and go from there.

cj

 

[bertoni]

There are a lot of factors that influence pH. I think it's going to be hard to track down why the pH shifted the way it did. A change in the evaporation rate, for example, could shift the effect of the kalkwasser reactor.

 

[kb27973]

I thought about evaporation rate as well Bertoni, but it hasn't changed much if at all. I was really just wondering if there was a known relationship between Mg and pH that I hadn't heard about.

Jc your explanation seems to be what is happening. I have both effluents dripping into one of those roll film containers. The calcium reactor is dripping into this all day (45ml/min) and then at night the kalk reactor dumps a few ml's every 20 minutes or so. The container is full of reactor effluent so there is some contact time before the mixture overflows into the sump. You were spot on about the precipitation, the container is completely caked in CaCO3. When I did drip both separately into the sump the pH at night would get down to 7.6 but once I put both into the same cup it wouldn't get below 8. It was stable like this for months: pH 8.4 day-8 night, dKh 10ish, Ca 450ish... Then when the Mg fell to around 1100 the pH shot up. Wierd.

Everything is back to normal now and I'm sure fixing the Mg had something to do with it. Don't know what though.

Ken

 

[MCsaxmaster]

The magnesium does have an effect on pH, but not a drastic one. The ionic strength of sea water affects the pH to a small degree. If there is more magnesium the ionic strength is higher and the pH tends to be a bit lower and vice versa. Major changes in the ionic strength will show up as changes in the salinity though. Even so, we're probably talking a difference of something like 0.01, which is not much.

Magnesium also forms ion pairs with a lot of the carbonate in sea water, keeping the pH from going real high. However, a reduction in the concentration of magnesium from 1295 ppm to 1100 ppm (S = 35) is going to have a pretty small effect on ion-pairing with carbonate and result in a pretty small rise in pH. Magnesium also pairs with OH- with similar but even smaller effect. It pairs with HCO3- too, and a reduction of magnesium would result in more free HCO3- and a very slight decrease (reducing the increase due to more free CO3-- and free OH-).

All of these effects are very small though, and wouldn't be enough to cause what you saw. The effect of magnesium as well as calcium on pH is negligible for our purposes (though often times get ideas otherwise...). The parameters that are primarily responsible for determining pH in sea water are alkalinity and pCO2 (CO2 partial pressure). Those have major effects while everything else is a fairly minor player.

Chris

 

[loc01]

You can't mix the 2 together, the precipitation of ca and mg will occur due to a high ph that kalkwasser gives. You can try to drip your ca effluent into a container and use a good air pump with an air stone to degas the exessive co2, this works well for me at least 4 years without any problem.

Loc

 

[jdieck]

IMO the Mg dropped because of the high PH not that the PH increased because of the Magnesium was low.
The high PH caused abiotic precipitation carring with it Magnesium from the water colum. What caused the high PH, beats me.
When the Magnesium was replenished it helped prevent further precipitation improving stability. What brought the PH back down, again beats me. A temporary overdose of Kalk that corrected itself? Any issues with the Kalk doser?