pH spiked after adding magnesium

[Newsmyrna80]

I'm starting BRS 2 part on my 80 gal. I mixed everything yesterday and let it sit over night. I tested magnesium this morning and it was at 1000. To raise it to 1350 I need to add 16.75 ml per day over 4 days. I added 7 ml slowly and I got a text from Apex saying pH spiked to 8.5. It was 7.9 prior to adding the mag. I know I mixed and marked the jugs correctly so what could be going on?

[tmz]

Magesium Mg2+ stabilizes carbonate alk conentrations which could effect the pH. I've never had mag at 1000ppm which is pretty low. I've never noted pH spikes when adding it though the effect you are seeing may be more pronounced given the initial low level. It might just be testing noise.

This article has more:

http://www.advancedaquarist.com/2003/10/chemistry



This is from it:

In the case of carbonate, for example, the ion pairing to magnesium so stabilizes the carbonate that it is present in far higher concentrations than it would be present in the absence of magnesium. This effect, in turn, makes seawater a much better buffer in the pH range of 8.0-8.5 than it otherwise would be. Without this ion pairing, seawater pH might be significantly higher, and more susceptible to diurnal (daily) swings.

What was the alkalinity before adding the magnesium? What is it now?

[Newsmyrna80]

Thanks Tom! Believe it or not that's kind of what I was thinking. But I would figure that if I had low mag that I would have precip from dripping kalk, which is what I was doing prior to starting the 2 part. But I didn't have precip. Alk before dosing was 6.5 now it's 8.

[bertoni]

The dKH changes sound more like measurement issues or other effects to me, depending on what might have been dosed. Adding magnesium won't raise the dKH. For that to happen, some sort of dosing must be done. I am skeptical about the pH reading, as well, although there's some reason for that. I ran tanks for years at 1100 ppm magnesium, because IO came at that level, although I don't think I ever measured a tank at 1000 ppm.

[Newsmyrna80]

Thought it was low as well since last week it was at 1350 but I did the test (Red Sea) twice. I'm going to retest everything tonight.

[tmz]

to be clear:

While magnesium won't add any alkalinity it will deter precipitation
which effects the carbonate alkalinity level, allowing a higher concentration to remain in solution and less of the dosing( ongoing) to be lost to precipitation.

stabilizes the carbonate that it is present in far higher concentrations than it would be present in the absence of magnesium.


I would figure that if I had low mag that I would have precip from dripping kalk, which is what I was doing prior to starting the 2 part.

Depends on how much kalk you were dosing in a given timeframe.At 6.5 dkh with 7.9 pH I wouldn't expect much observable precipitation .

BTW, kalk (calcium hydroxide ) dosing adds no magnesium. It precipitates out at the high pH in limewater( kalkwasser) as part of the slurry of impruites. Commercial two part mixes have some Mg. Calcium reactor media has some Mg.
Food and salt mixes also add Mg.

In my case salt mix and food are enough to keep it steadily over 1400ppm even with kalk dosing for calcium and carbonate.

[tmz]

Alk before dosing was 6.5 now it's 8.

Was the alk dosing via supplements or water changes the same before and after?

How are you measuring pH ? Many tests are error prone. Monitors are subject to electrical interference from time to time . Sometimes a stray asterina starfish or other organism or precipitation can mess up a probe giving a false reading.

[Newsmyrna80]

pH is measured via Apex. Typical range is 7.8 (5 am) to 8.3 (5 pm). I think the mag test may have been expired (a little over a year old) so I bought a new test. I retested mag yesterday and it was at 1420 ppm and alkalinity was 8 dkh. I also tested the magnesium in freshly made saltwater and it was at 1350 ppm.
So, in short, I don't know why there was a pH spike, if indeed there was one. It could have been noise, errant asterina or electrical interference as suggested. As of now, everything seems to be back on track.

[bertoni]

Well, we see a lot of measuring problems reported here, so perhaps the issue was an errant reading. As long as the animals are doing well, I wouldn't worry that much, personally.