[Murphy]

Hello, been a while since I've been here but I have a problem and need some help since my knowledge of Chemistry is almost zero.

I bought a quart of battery acid from my local auto store and I need to use it to lower the pH of a gallon of rainwater to around 4.5.    Other than adding some acid to the water and using trial and error, I have no idea how much I'm going to need. 

Can someone calculate this for me?    My research says that battery acid is about 35% sulfuric acid and 65% water by weight which produces a Molarity of about 4.2M and a mole fraction for sulfuric acid in this solution is approximately 0.39.   While I understand that moles are basically a count of protons, I have absolutely no clue how to work with this information to formulate an answer. 

So, if I have a gallon of rain water, how much battery acid do I need to add to drop the pH to around 4.5  ? 

Thanks,

[Borek]

No way to calculate exact amount, as it depends on things dissolved in water. Rain water is typically rather soft and clean, so it shouldn't be a serious factor, but one never knows. In the end it is not about using calculated amount of water, but about adding acid till you get to the desired pH.

Assuming pure water and no interference to get pH of 4.5 you need sulfuric acid concentration in range of 10^-5 M. Adding a single drop of the concentrated acid will probably overshoot by orders of magnitude. I would start by diluting the concentrated acid 100 times, then the volume needed to acidify the rainwater would be more manageable, in mL range. It can be still too much for precise adjustment, but then getting exact pH with just an acid (and not a buffer) is always tricky.

That is, unless I didn't make any mistakes in my math at 9 a.m.

[Murphy]

No way to calculate exact amount, as it depends on things dissolved in water. Rain water is typically rather soft and clean, so it shouldn't be a serious factor, but one never knows. In the end it is not about using calculated amount of water, but about adding acid till you get to the desired pH.

Assuming pure water and no interference to get pH of 4.5 you need sulfuric acid concentration in range of 10^-5 M. Adding a single drop of the concentrated acid will probably overshoot by orders of magnitude. I would start by diluting the concentrated acid 100 times, then the volume needed to acidify the rainwater would be more manageable, in mL range. It can be still too much for precise adjustment, but then getting exact pH with just an acid (and not a buffer) is always tricky.

That is, unless I didn't make any mistakes in my math at 9 a.m.

Well, it's not quite a single drop, but pretty close.     I filled up a gallon milk jug (Jug A) with rain water and squirted some battery acid into it.   I didn't measure anything but I estimate it was probably 3 to 5 ml worth of acid.     It dropped the pH down into the 2 range, which surprised me.     Next, I filled up 5 more gallon milk jugs but left a bit of room in them and added about a coffee cup from Jug A to each..    Ended up with a pH in each jug of about 3.8 to 4.0. 

Took the jugs and filled up the watering can and watered the area around the blueberries very lightly.   I'll probably have to wait until it rains and then do it 5 to 10 more times before the soil pH drops into the 4.5 to 5 range deeper down. 

We also mixed in 40 lbs of elemental sulfur granules, but that was earlier in the spring and I don't think they've had time to do their thing yet.

Thank you!