Most canning books and pickle recipes tell you never to use homemade vinegar to make pickles because it isn't safe." Actually, there is a way to safely use your homemade vinegar in pickling, but it does require one extra step. You need to test your vinegar to verify that it is acidic enough to kill off any harmful bacteria.
Vinegar pickles (as opposed to lacto-fermented pickles) rely primarily on the acidity of vinegar to kill off harmful bacteria and preserve the food. The logic behind saying "never use homemade vinegar for pickles" is that you have no way of knowing how acidic your homemade vinegar is. But there is a way, and it is very simple to do.
The number you need to remember is 4.5% acetic acid. That is the percentage at which a vinegar is acidic enough to use in pickling recipes. Most commercial vinegars have that high a percentage of acetic acid, or even higher (it will say on the label).
To find out if your homemade vinegar is in the 4.5% or higher range, you will need to order something called an acid titration kit from a home winemaking supplier. They are inexpensive and one kit will last you for many batches of vinegar.
The acid titration kit will include:
20 ml syringe
150 ml plastic testing cup
100 ml of standard base liquid (0.2 N sodium hydroxide)
15 ml dropper bottle of indicator solution (phenolphthalein)
Because the type of acid you are testing for in vinegar is different from that in wine, and usually at a much higher amount, you need to follow different instructions from the ones that come with the wine-testing kit.
To test your vinegar, first use the syringe to measure 2 ml of homemade vinegar and transfer it to the testing cup.
Add 20 ml of water and 3 drops of indicator solution and to the vinegar and stir (I use a chopstick).
Fill the syringe with 10 ml of standard base. Add the standard base to the mixture in the testing cup 1 ml at a time, stirring after each addition.
At first, the liquid will turn clear after each addition of standard base. Eventually, it will darken and turn pink. Stop adding standard base at this point.
Once the liquid has turned pink, make a note of how much standard base you added to get to that point. For example, since you started with 10 ml of standard base, if you have 2 left after the addition that made the testing sample change color, then you added 8 ml of standard base.
Now comes a little math. Multiply the number of milliliters of standard base you added by 0.6. The result is the percentage of acetic acid in your vinegar. If you added 8 ml of standard base, for example, multiply 8 x 0.6 and you get 4.8, or 4.8% acetic acid.