Captan is very pH sensitive, At pH of 8 half the Captan is destroyed in 10 minutes. Citric acid is probably the best for pH adjustment since it’s a powder and only a small amount is required per gallon of water. Locally you can usually find it in the food canning supplies area since citric acid is used in canning. Here is a link to an older post talking about adjusting pH for spraying Captan.