pH adjustment of Spirulina culture via dilution

I often use citric acid as a mild acid for acidifying sensitive solutions.  The spirulina might also be able to use it as a food source. 

 I don't know whether the identity of the counter-ion is or is not important in terms of growth of the cells, but you could also use sulfuric acid.  If you are worried that the local pH around a strong acid will be too low before mixing is complete, you could use a somewhat weaker acid such as KHSO₄ or acetic acid.  There is also cost to consider.

You have a good point that baking soda is not a strong base. It has a pH of 9, so it should not have - by itself - managed to bring the pH above 9. There are nutrients in the culture as well, but none that should have been any more alkalizing. So then what could have caused this? I could see how the pH would gradually reduce since the Spirulina could consume the carbon, but what would cause it to increase like this? I've never seen this before, so assumed it a miscalculation on my part. Now I am not so sure.

In any case, that mystery aside, pH is currently at 12 and I need to correct that.

I realize that I can add a strong acid, but I would rather use dilution, and even if I was going to use a strong acid to correct I would still want to know exactly how much to use rather than randomly try. The strain can survive a high pH, but it won't survive a low pH and I figure that through dilution the absolute worst I'd get is a pH of 7.

 C1V1=C2V2 wouldn't seem appropriate here anyway. That's used to calculate unknown quantities where two solutions/mixtures are proportional. That would be useful if I was trying to figure out how much baking soda is in a cup, given how much I put in a tank, but I don't really need to know that.

Isn't there a formula for the amount of dilution with water required to lower the pH by 1? If there isn't I could take varying sample sizes and dilute until I find a good dilution ratio for what I have and hope it scales up properly.

EDIT: Is it possible that carbon dioxide from the air could be causing this via too much aeration? I just read an interesting article that asserts that in nature the carbonic acid is broken down by algae (maybe in this case cyanobacteria?) to raise the pH.

https://principia-scientific.org/carbon-dioxide-makes-alkaline-water-experiment/ 

However, what's the highest such an effect could go?

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Isn't there a formula for the amount of dilution with water required to lower the pH by 1?

For a simple solution of a strong (preferably monoprotic) acid or base - yes, it is possible to derive such a formula. Won't be trivial to use, and changing pH by 3 units (12 9) will require diluting the solution 1000 times (doesn't have to be the same number for other starting pH values).

But you don't have a simple solution, with a well defined composition. Your solution contains buffers, and pH of a buffer solution doesn't change during dilution (it is actually more complicated, but will do as a first approximation here).

Just like hypervalent_iodine I don't see other reasonable approach than neutralization with a strong acid.

Baking soda is not a strong base, so it is not as simple as C1V1=C2V2. Moreover, you will have other components in there that will alter the pH. The best way would be to add HCl and monitor the pH as you go.