[Holy Goat]

I am trying to determine how much more NaOH than Ca(OH)2 it would take to raise the pH of pure water the same amount. For example: how much NaOH (say a solution of 1 gram dissolved in 1 litre) would it take to raise the pH of 1 litre of pure water from 7 to 8. How much Ca(OH)2 (again say a solution of 1 gram dissolved in 1 litre) would it take to do the same? I'm not so concerned about the actual numbers there as I am the comparison of how much NaOH it would take compared to Ca(OH)2.

My initial assumption was that it would simply take roughly double the amount because it had half the number of available OH- ions. However, that explanation seemed too simplistic to be true. I then did the math for the molar concentration of each and it was again just about double the amount. I'm just trying to find an equation to do the calculation properly. It's been quite a while since my University chemistry classes. 

For context: I work at a small scale recirculating aquaculture system growing Rainbow Trout and we have been using Lime to maintain the pH (at between 7 and 7.4) and we are currently thinking about switching to Sodium Hydroxide instead, but are trying to determine approximately how much NaOH it would take to replace our Ca(OH)2 use. Obviously there are water quality considerations (like ammonia, nitrites, nitrates, etc.) but I'm just ignoring those things at this point.

[Corribus]

Your treatment is too simple because assumes complete dissociation,  which is certainly not the case for calcium hydroxide. To solve your problem would have to do some equilibrium calculations using the equilibrium constants. Or you could use pH calculation software. Borek, system administrator here, has a nice one with a free trial I believe.

[Holy Goat]

Ah, that totally makes sense. I figured it was more complicated than I thought. I'm just having a real bear of a time trying to find the proper equations anywhere.

[Borek]

The sad truth is that the best you can do is to determine the necessary amount of NaOH experimentally. Twice the amount of Ca(OH)₂ is not a bad starting point though, it is definitely not exact, but should give the right order of magnitude. Probably a bit higher than the real value, because of the Ca²⁺ + OH^-  CaOH⁺ equilibrium.

[Holy Goat]

I had a hunch this would ultimately be the case. We just ended up buying a pallet of Lye and are going to do exactly that, rig up a dosing system for it and just figure it out experimentally.

Thanks for the quick and informative responses!

[billnotgatez]

This discussion talks about the pH manipulation. What about the biological environment.
I would think changing the concentration of various ions would have an effect.
I know that (from WIKI)
One liter of Ringer's lactate solution contains:
130–131 mEq of sodium ion = 130 mmol/L
109–111 mEq of chloride ion = 109 mmol/L
28–29 mEq of lactate = 28 mmol/L
4–5 mEq of potassium ion = 4 mmol/L
2–3 mEq of calcium ion = 1.5 mmol/L

I wonder if you take ion concentration into account when making your aquaculture system?