Hello, I'm reading a book called "Getting Pregnant" by Lauersen and Bouchez and I remembered he mentioned something about this - sorry it took me a couple of days to backtrack and find it.

In it, he says:
To reduce vaginal acid: Use a low-pressure douche of two tablespoons of baking soda in one quart of water just before intercourse.
The increase vaginal acid: Use a low-pressure douche of two tablespoons of vinegar in one quart of water just before intercourse.

He then goes on to caution that home doucing instruments can be prone to bacteria growth so be very careful to properly clean all the bits just before use with lots of soap and hot water.

HTH
:-)