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I have a chiller bath, which is attached to a 150 ton cooling unit through 4" PVC pipe. The bath itself holds around 5000 gallons of water, and is made of 304 stainless steel. It constantly circulates through the PVC into the cooling units outside via a 20hp pump. The outside cooling units are made of steel, although I do not know the exact grade of the steel. It was however built for water circulation. I use a sodium hydroxide solution to raise the pH of the water, therefore preventing rust and keeping my water clean. The problem is that I constantly have to add the solution to keep the pH at a level that is acceptable (9-11). I have watched it enough to say that on average the pH drops by about 0.3 a day. I am wondering where all my sodium hydroxide solution is going. Thank you for the responses so far, I apologize for not being more clear on the problem.

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Usually, the biggest problems with preventing a sodium hydroxide solution from changing in $\ce{NaOH}$ concentration, or pH, over time are adsorption of $\ce{CO2}$ from the air, and evaporation of water via permeation through the container holding the solution.

If we can really say that it is inside a closed system as you've specified, then any degradation over reasonable time-scales (i.e. months to years) should be negligible.

However, you need to be sure about your "closed system". Is it glass, which could slowly react with the $\ce{NaOH}$? Is it a plastic that could be slightly permeable to water vapor, air (and thus $\ce{CO2}$), etc.? Is it, heaven forbid, a metal that would react and degrade your $\ce{NaOH}$? You need to think about these things at every contact point (vapor or liquid) throughout your system.

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Except for a very minute amount, generally solutes do not disappear (Evaporate) when left alone in solution. Your solvent, water, may evaporate over time depending on the conditions, however unless you repeatedly transfer container your are unlikely to lose much of your NaOH.

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