Rising alkalinity after using Sodium Bisulfate (NaHSO4) in salt mix tank

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TL;DR I am experiencing rising alkalinity without dosing additives after using Sodium Bisulfate during salt mixing

I have a supply of Instant Ocean that is mixing to 11 dKh for a FOWLER that I am transitioning to reef. In an attempt to use up my last two boxes of this before switching, I have gone the route of dosing Sodium Bisulfate to the freshly mixed saltwater to bring the alkalinity down to the 8 - 9 dKh range, as described by @Randy Holmes-Farley in this thread: https://www.reef2reef.com/threads/s...in-new-salt-water-or-in-display-tanks.362825/

I have slowly brought the alkalinity in the tank down to 9 dKh using water changes with this IO plus Sodium Bisulfate mix (after aerating the water and bringing the pH back up).

What I am experiencing is rising alkalinity in the tank. I have used both the Trident and the dKh Hanna checker to do the testing, and both units show the same trend of rising alkalinity of about 0.18 dKh per day.

No dosing is happening, just the water changes to bring the FOWLER down to 9 dKh and then the rising trend. Again the alkalinity has been tested with both the Trident and a Hanna checker.

With that laid out, are there are any chemical mechanisms that would bring the resultant bicarbonate (HCO3-) from the addition of sodium bisulfate (NaHSO4) back into solution, thus raising the alkalinity in the tank?

If I didn't provide some information that it relevant, I am happy to provide it.

Thanks for your time if you've read this far!
 

Randy Holmes-Farley

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No.

When you add sodium bisulfate to seawater, it immediately becomes sulfate and H+. There is already a ton of sulfate in seawater and there's no going back to create alkalinity.
 
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well shoot, I was hoping for that to explain the rise.

I have also been checking salinity the last week to make sure that this isn't a simple problem with evaporation since I assume that as salinity goes up, so does alkalinity.

Any other common reasons for rising alkalinity, after ruling out salinity rising and a reversed sodium bisulfate reaction?

I know that there's some interplay between alkalinity and pH, so here's the trident alkalinity data (orange) against the pH (green). I don't think such little changes in the daily pH swings would result is such a steady rise in alkalinity, but of course I'm not a chemist.

1635178411285.png


Two weeks ago I worked with Neptune support to double check the Trident (that's when I also started secondary tests with the Hanna checker). The two move in sync with each other.

I made it through AP Chemistry 20 years ago, so the curiosity of this has really hooked my brain
 
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At the end of the day, thank you for ruling out the HCO3- reversing back into alkalinity
 

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At the end of the day, thank you for ruling out the HCO3- reversing back into alkalinity

You're welcome.

If it keeps rising, there must be another source, such as dissolving sand, additives you are using, declining or dosing nitrate, or top off water that has alk in it.
 
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You're welcome.

If it keeps rising, there must be another source, such as dissolving sand, additives you are using, declining or dosing nitrate, or top off water that has alk in it.
to these points,

* no sand, just a lot of rock for the volume (the tank started as a rock cycling tank, added some fish, now corals).
* no additives, just water changes and RO/DI top off
* i can up my nitrate testing game, using the newer Hanna Nitrate HR checker, it was 35pmm on 10/17 and 48.5ppm on 10/25. These are my only two data points and I suspect some problems with my testing method consistencies since its my first testing trying to get all the reagent powder out of the sachet.

Regarding alkalinity in top off water. Am I able to use my dKh Hanna checker with freshwater?
 
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I'm guessing not, since there is a different Hanna checker for Alkalinity in freshwater
 
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yeah, 0ppm coming out of the DI resin

I need to do a water change to bring the alk back down. Thinking that I'll put some of the freshly mixed saltwater in a bucket with a pump, test the alk and let it sit for a week, topping off and testing the alk along the way. See if it matches the trend line of the tank in the same time.

I'm not sure this will tell me anything by isolating the water away from the rock, fish and livestock, but at least it'll feel like i'm testing something to distract from the alk rising
 

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yeah, 0ppm coming out of the DI resin

I need to do a water change to bring the alk back down. Thinking that I'll put some of the freshly mixed saltwater in a bucket with a pump, test the alk and let it sit for a week, topping off and testing the alk along the way. See if it matches the trend line of the tank in the same time.

I'm not sure this will tell me anything by isolating the water away from the rock, fish and livestock, but at least it'll feel like i'm testing something to distract from the alk rising

That's a fine experiment, just be sure the low pH is not redissolving old calcium carbonate that may be in the mixing container
 
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That's a fine experiment, just be sure the low pH is not redissolving old calcium carbonate that may be in the mixing container
a ha! that's an idea. The mixing is done in a Brute (i leave the lid off while off-gassing the CO2). The Brute is in out laundry room which is fairly walled in and in the basement. I try to remember to leave the door open to help out.

I dump the salt in, let it mix overnight, do some adjustments to get to 35ppm. At this point the pH of the mix is about 7.9-8.0

I then dose the acid, tanking pH down to 6.3-6.5. I utilize a large pump to create some rippin' turbulence at the surface of the open-topped brute. with this though, I can never get the pH higher than 7.6-7.7. I figured that's the equilibrium that it likes and do the water change. I had been doing large 30% changes to bring the tank Alk down.

I'm pretty sure that 7.6 pH water wouldn't have a dissolving effect on the rock in the tank right? My naïve 20-year-old memory of AP Chem says that it wouldn't have a dissolving effect until the pH is below 7. right?
 

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a ha! that's an idea. The mixing is done in a Brute (i leave the lid off while off-gassing the CO2). The Brute is in out laundry room which is fairly walled in and in the basement. I try to remember to leave the door open to help out.

I dump the salt in, let it mix overnight, do some adjustments to get to 35ppm. At this point the pH of the mix is about 7.9-8.0

I then dose the acid, tanking pH down to 6.3-6.5. I utilize a large pump to create some rippin' turbulence at the surface of the open-topped brute. with this though, I can never get the pH higher than 7.6-7.7. I figured that's the equilibrium that it likes and do the water change. I had been doing large 30% changes to bring the tank Alk down.

I'm pretty sure that 7.6 pH water wouldn't have a dissolving effect on the rock in the tank right? My naïve 20-year-old memory of AP Chem says that it wouldn't have a dissolving effect until the pH is below 7. right?

Some aragonite can dissolve at pH 7.7 and below (at 7 or less dKH), which is why I am concerned by pH at those levels in the tank.

If you add it slowly to a larger volume at higher pH, pH will not get that low, but if you pour it over any solids before getting diluted, it may dissolve some.
 
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yeah, the water changes were very much, drain 30%, exposing rock, very large rush of fast water being pumped in (no coral at the time, just rock and fish)

If that 7.6 water change water had dissolved some of the rock, would I still be seeing the rising alkalinity 2 weeks later? I should also add that it was 3 separate 30% water changes over 3 consecutive days with that 7.6-7.7 water.

I would think that I would see the alk spike right after the water change and not a consistent, steady rise over 2 weeks. am I off base with this thinking?
 
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well, i'll still do the bucket experiment to keep my brain busy and feel like i'm working toward a cause :)

i'll report back afterward

thanks again for the conversation about this!
 
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