How much Lactic Acid is too much?

So, a few months back I got my first pH meter. Imagine my surprise when I learned during my first all-grain brew with it that my mash pH was about 6.4.

Well, since then I did some reading and starting playing with my malt bill to try to add some acid. No luck.

On my last batch I added a table spoon of 5.2 buffer to the mash and it only dropped the pH by a tenth. Luckily, on my last trip the my LHBS I picked up some Lactic Acid. Everyone cautioned me to use it very sparingly and that drops are my friend.

Well, I used a ml dropper and added about 4 ml in .5 ml increments and was able to get the pH to drop from 6.4 to 6.0. I stopped at this point for fear of adding too much acid to the mash.

My question - how much Lactic Acid should it take to drop my mash from 6.4 into an acceptable range?

Here is the malt bill:

10+ gallon batch - 5.8 gallon in mash, 9 gallon sparge.

Ingredient Amount:
US 2-Row Malt 13lb 0oz
US Vienna Malt 1lb 14oz
US Caramel 80L Malt 1lb 14oz

I know my city water is rather hard, I do not have a water report at my tap, but here are the numbers the city provided for the water they put into the system (with the caution that the water on my end will be different):

FINISHED WATER - MARCH 2011 (they send me monthly data for the last year):
Total Alkalinity (CaCO3) - 60
Noncarbonate Hardness (CaCO3) 63
Total Hardness (CaCO3) - 122
Phenol Alkalinity
Calcium (Ca) - 36
Magnesium (Mg) - 8
Sodium (Na) - 76.1
Potassium (K) - 3.9
Sulfate (SO4) - 133.3
Chloride (Cl) - 55
Fluoride (F) - 1.02
Nitrate (N) - 4.1
Total Phosphate (PO4) - 0.51
Total Dissolved Solids (TDS) - 392
Turbidity (NTU) - 0.06
Color (Pt-Co Units) - 0.6
Conductivity (uS/cm) - 620
pH Value (pH Units) - 7.8
Chlorine-Free - 1.43
Chlorine-Total
Total THM's (ug/L) - 13.6
HAA5 (ug/L)
Total Organic Carbon (TOC) - 2.45

Oh, and I run the tap water through an activate carbon RV water filter prior to use.

Something is off. With a total alkalinity of 60, and 4ml of lactic acid, and that grain bill, you should be pretty close to the 5.4 range (probably even lower). Is your pH meter calibrated? What meter is it? Your water report is pretty close to balanced.

so, i am an idiot... that last time i used it the meter was bang on. I just re-tested it and the calibration was off by .7. That would be the issue.

So glad I stopped adding acid when i did.

I have the yellow Milwaukee tester from Northern Brewer.

It's a bit of a pain, but I like to calibrate mine before I take the pH reading for my mash. It doesn't take more than about 5 minutes or so.

One of those things to add to your brew day check list: Calibrate ph meter.
I'm a huge fan of checklists, they make it harder to screw something up, even if I'm drinking while brewing

do i need to keep buying test solution or can i calibrate it with distilled water?

You need calibration solution. This will last you quite a while: http://www.northernbrewer.com/shop/buffer-solution-ph-7-01-220-ml.html. You just need enough to cover the probe. Also, if you're not storing the probe in storage solution, you should be. Also, it wouldn't hurt to read AJ's guide to pH meter calibration in the stickies of the Brew Science section.

will do. thanks. i only had the single serving sized test solutions. the large bottle is a good suggestion. thanks