I've read a lot of posts online about using baking soda to neutralize ph in the soil and kill and prevent weeds. Many say to use it around flower garden edges to keep the grass from invading. What I am wondering is how close to the wanted grass and flowers can you get with the baking soda? What is the radius of effected soil, I guess is a better way to put it, maybe. Help.

Sodium bicarbonate does not belong in a garden to my way of thinking.
Sodium is toxic to plants and critters!

Find a weed somewhere with no nice plant too close and put some baking soda on it. (crack in a sidewalk maybe?) See what happens.
I suspect it isn't strong enough to be effective as a weed killer.

It isn't going to travel sideways much, but the sodium will go down and eventually dilute with rain.

This is the main post I was reading. It says 6-8 inches away from plants. I was just wondering what a general opinion was. It seemed to work in this article.

https://wife2jason.wordpress.c...

I think it would take a lot of it to kill anything. It will only neutralize the soil if you have acid soil.

I can't find fault with that article. (other than the unspecific use of 'neutralize"). Sodium is soluble mineral and will eventually dissolve away/ dilute. It is initially burning the weeds' roots, but the weeds should be able to grow back. We had similar results with a small experiment with vinegar on weeds.
Might be worth a try, just begin cautiously until you see how it works. And like any weed killer, use as little as needed for the control you want.