I use 5 ml per gallon of prepared syrup. I got the idea from Randy Oliver (link to site with info) and it has served me well thus far.Some use bleach.
Organic acids like ascorbic acid (vitamin c), citric acid, malic acid, and lactic acid lower the pH of the foodstuffs they are added to, which slows down the growth of many microbes, more so bacteria than fungi, because most fungi tolerate lower pH environments than do bacteria. Vitamin c is also a potent anti-oxidant, which is why it helps to prevent cut fruit from browning.My i ask why you want it acidic? ty