Yes, it's possible by connecting 1st regulator output to 2nd regulator input.

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Hello again! Let's examine it thoroughly (seems i miss something) - you have 1 control element - valve, so the only thing you can influence is flow of concentrated acid. Now if your concentration would be constant you would need only 1 loop - input variable = current pH, manipulated = % of valve openness.

Next - your acid can have different pH and thats why you want to make cascade?

Hmmm, could you describe your object detailed?

Just how i see it:

1) If you acid flow has always pH much less then pH of your tank (i.e. inflow contains stronger acid) then you could operate variable 'amount of acid' = inflow_pH*Flow. Thus you have simple PID without cascade.

2) If 1) is not true then how your system shall react in situation of inflow_pH > tank_pH (i.e. acid in the tank is stronger) - close valve to 0%?

3) If inflow_pH is slightly less then tank_pH then what's reaction time you have? Do you have some sort of mixer inside or?