Is this acceptable pH down?

So, 1st timer on the hydro. I have been using vinegar up until now to decrease pH. Its simply not cutting it now and my DWC tank is fluctuating back to basic too quickly. What is the contents of pH down for hydroponics I have a gallon of leftover buffer solution at 4.00 that I had used a long time ago for an electronic ph meter. Its contents are 'potassium acid phthalate' - will this work?

Yes it will but obviously add slowly and check pH. BTW pH Down or Up is just $10 a bottle and contain more than a single acid or buffering agent. I suggest buying some when you can. Forget the pH Up. Tap water takes care of that free.

if you are running hydro you should have a ph meter and ph up & down solution.