Alkalinity of water, best practices

If one wants to make sure their water is alkaline, am I correct in understanding that any out of baking soda, lemon juice, or apple-cider vinigar will suffice in making drinking water alkaline? Is one better than the other? How much to use?

Next question :

Archus had a post about how during fasting, there should be no additions to the drinking water, even to make it alkaline.

I wanted to understand the Science of that better, and why that is. What if the water being drunk is acidic without the faster realizing it? Then alkalizing is needed just to neutralize the pH.

It appears that one can drink heavily Alkalized Water right before the fast, but not once it has begun.