Determining acid strength in sour mash

I do sour mashes 2-3 times a year. I take about 20-30% of my grist and do a saccharification infusion in a small cooler. After it cools down to 120, I inoculate it with a small handful of unmilled grain and cover it with plastic wrap. Every 12 hours or so I infuse it with enough boiling water to bring it back up to 120. I keep this up for about 3 days. Last year I did back-to-back brews with sour mashes. The Kentucky Common (basically a Cream Ale with a sour addition) turned out great, but the Wit turned out more like a Berliner Weiss. Is there a way to measure the strength of the lactic acid in the small mash so I can have more consistent results? I'm planning to brew a stout a la Guinness in addition to the Wit and the Common and would really like to be able to dial in the sourness.

Yes, there is. It is conceptually very simple and there may even be an easy path to being able to do it conveniently. What you want to determine is the "titratable acidity". This is done by measuring out a known amount of the mash and then incrementally adding base of known strength to it until a specified pH is reached. The challenges are:

1) measuring out the proper amount of mash,
2) obtaining standard strength base
3) measuring out the base in increments accurately
4) determining when the target pH is reached.

Obviously, a balance is the best way to handle 1) but if all you are interested in is relative strengths and you intend to determine how much to add to a main mash based on volume a volume measurement might do. You'd mix up the mash thoroughly and draw off, say, 50 mL into a small beaker.

WRT 2) - vintners like to measure the titratable acidity of their musts and so kits are sold for measuring it. These contain sodium hydroxide solutions of known strength. The only problem I see with this is that once the stuff is exposed to the air it starts taking up CO2 which reduces its alkalinity so that over time its strength declines.

For 3) - the wine maker's kit must contain some means of measurement perhaps as simple as an eyedropper with the measured acidity expressed in terms of the number of drops it takes to reach the end point pH. The calibration may be something like "as citric acid per drop" but again, if you are only interested in relative values that may be sufficient.

WRT 4) - A pH meter is the obvious solution but again the kits must contain something that allows the end point to be detected. I don't remember what this is but I can't fathom it being an indicator dye as I would think the color of the must would mask the color of the dye and I can't imaging that it would be a test strip as they are so inaccurate.

At this point my suggestion would be to go to your LHBS (assume they also serve vintners) and ask if they carry acidity kits.

I thought it had something to do with titration, but it's been 25 years since I was in a chemistry lab, and to tell you the truth, I didn't exactly pay the closest attention then. Thanks for pointing me in the right direction. I am looking for relative strengths that I can plug into a water calculator spreadsheet.