1 General PLC has AD conversion module, the input is mostly 4-20ma, 0-5V. so, your sensor needs to choose the one with the above outputs to be good. Even if not, it does not matter, you can modify their own, you can also choose other methods to solve.
2 The valve problem depends on the volume of your system. Smaller systems can use flow meter control. We commonly use gas flow control flow meter, called "mass flow controller" (MFC). It is a small instrument for precise flow control. You need to purchase one that is resistant to acid and alkali will be fine. Its input signal is also 4-20ma or 0-5V.
3 See earlier answer.
The key problem of your design is accuracy, if the accuracy requirement is not high, it is very simple, add alkali when there is more acid, and vice versa. If you have precision requirements, you can introduce PID control, you can write your own program, you can also buy PLC PID module. In short, it is not difficult.
Good luck.
Good luck.