Don't use baking soda. Pulverize some eggshells (that have been rinsed\dried) in a coffee grinder for several seconds. Or if you can, get some micronized dolomitic limestone\garden lime. Start by mixing a tablespoon per gallon of container into the top few inches of soil, water thoroughly and allow for sufficient run-off. After roughly a few cups of water have drained through your container take a run-off sample and check the pH of that. If you need to repeat with a bit more lime until it tests around 7. Once you get there the pH should remain balanced for a month or more.
Egg shell is primarily calcium carbonate, which is the same thing as some brands of garden lime, and oyster shell flour. Dolomitic limestone contains magnesium as well and is preferred for that reason. The lime particles are dissolved slowly (depending on their size) and neutralize acidity over a longer period of time. Baking soda contains sodium, and too much is bad for plants. It also isn't likely to work over the period of time lime would.