PH Down question - using Battry Acid

I'm sure this has been answered a million times...sorry! I could not find what I was looking for though....

So I bought some battery acid (35.15% sulfuric acid) and am wondering how to mix it....actually I'm wondering WHY I have to mix it (yes I understand it's strong acid)....
I have a 7 gal DWC system and have to use PH down daily (because I use tap water to refill the system) so instead of using 5ml of regular (9.6%) why can't I use a dropper (2ml) of battery acid?

Thanks!!

That sounds like a terrible idea.

lemon juice.

from Carthoris at RIU (not my idea)....HOW TO MAKE IT:

Buy sulfuric acid at the auto parts store for 7 dollars. Just ask them for battery acid. Get the 33% kind (that means its already partially diluted)

Get a gallon of distilled water, pour out 2 cups.

Put 1 cup of battery acid in your gallon jug of water. Wait a minute or two. Put top back on. Lightly agitate to mix.

Thats it, you have just made a gallon of PH down for a maybe a dollar or so. BUT.. then I read this:::Carthoris= the name of the disease thats going to be caused by smoking plants grown with Sulfuric Acid.

Squeeze and strain a troll

I prefer to use Food Grade Phosphoric acid

If you want to just save money and keep it simple this product is cheap and convenient too ..

If we look at SCIENCE! we see you have 37 g of H2SO4 (a strong acid) and 63 g of water. Considering one mole of sulfuric acid = 98 g you (assuming 1000g of water make 1 liter, which it does) would have ~ 3 moles per liter would have a molarity (mole concentration per volume) of 3M. (verified according to http://answers.yahoo.com/question/index?qid=20120221070403AAshhVJ )

Which means you can calculate the ph of anything you need to grow by knowing the volume and pH of the solution to which you are adding the acid by using a scientific calculator.

If you want to get to a 5.7 pH you would take your water pH, lets say 8.0 (mine is a little high in pH over here) that would be scientifically notated as 1*10^-8 and divide that by that your desired ph, (10^-5.7=2*10^-6) 2*10^-6 you would get .005 meaning you need a concentration of .005M for your acidic solution which means you need 1mL acid per 600ml water to get your pH from 8.0 to 5.7.

That was a lot of math rather fast so I may have made some errors, always test and keep a pH meter on hand and ALWAYS pour ACID into WATER never the other way around. (side note a 10% solution doesn't mean 1 part _ to 10 parts _, it means one to nine. I have seen enough people getting that confused on a variety of different forums.)

^ Or you could just buy some pH down and save yourself the hassle.