right on the money------- 'Baking Soda' is 'Sodium BI-carbonate'----meaning more carbonates than Sodium Carbonate---Carbonate (such as C02) lowers the pH of solutions ----which means, when baking soda is dissolved it will lower the pH of a neutral solution......
when you bake 'Baking Soda' your cooking off the carbonate (into C02 which escapes in the air) and converting the sodium bicarb. to sodium monocarbonate--without the Bi----which would now raise neutral solutions to more alkaline pH------'baked Baking Soda' is also known as Soda Ash---which you can also buy cheaply ----so either bake your own baking soda---or buy soda ash as the dude above mentioned----both will work---both are cheap----one doesn't require an oven----you decide