pH change with reduction?

If evaporation only, and the plant is taking up nothing, then the PH will go down linearly with the reduction in water.
 
Generally, the plants in a hydro system cause the PH to go up as they consume the nutrients more quickly than the water evaporates in most sealed indoor systems (with adult heavy feeder plants, like peppers and tomatoes.)
 
Outdoors, this is not always true. I tank up my hydro systems on plain water every day in mid summer. The plants really cut back on nutrients during high temps and use more water instead, so some of this is environmentally affected as well. Therefore, in that scenario, the PH tends to drop, even with huge plants growing out of the system.
 
All of this is highly dependant on your environment, you can take measurements twice a day and record them to see how your specific setup performs with the specific plants you are growing. As the plants grow, the rates will change.

shortbus said:

I know reducing leaves salt behind, thus concentrating its presence. What happens with pH as liquid is reduced (water evaporated)? Add acid to water and pH lowers the more you add. Seems taking water out should do same.

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If your talking about sauce reduction, then yes, from a strictly reduction point, without adding anything to it, if the sauce is already acidic, reduction of the sauce will become more-so acidic, lower ph.