Which kind of reactor are you using? 0.2 is not so big so you could use a small pH electrode. pH meter for small sample are available on the market. Do you need to control the pH or do you just need to measure it?
Sadib Bin Kabir, this is just a suggestion. Depending on the precision required you can use a "pocket" pH tester. Is just a needle that can you insert on the bottle. You have to take care to seal the bottle cap every time you finish.