I believe these are the instructions I used
•Creating and Maintaining a Holding Solution
Citric acid (Alternatively, you can use tartaric to avoid contributing residual citric to your wine)
pH meter and calibration solutions
Free SO2 testing equipment (or access to a wine lab)
Mix Instructions: Fill the barrel with de-chlorinated water. Acidify to 3.7 pH or below by dissolving Citric or Tartaric acid into water and then mixing into the water in the barrel. Add an initial addition of free SO2 of at least 100 ppm.
How much acid is needed to reach 3.7 pH?
This will depend on what is in your water. A water with very little to no ion content, such as distilled or reverse osmosis water, will take much less acid to reach 3.7 pH than a water with high ion content, particularly alkalinity. You might use our staff table-top experiment as a guideline as follows.
We treated two very different water sources with Citric Acid until they dropped below 3.7 pH. One was Santa Rosa city water with about 160-180 ppm of alkalinity, the other was Reverse Osmosis water with only about 5 ppm of total dissolved solids. Here were the results:
Santa Rosa Municipal Water (160-180 ppm alkalinity)
0.4 grams per liter of water to reach 3.61 pH