Baking Soda (Sodium Bicarbonate) adds alkalinity, and sodium ions.
It is useful in darker beers to raise the pH back to a reasonable level to prevent the roasted malt character tasting harsh or acrid. It is preferable to Calcium Carbonate as it is more soluble, and does not increase the calcium ion levels.
Bring the container back to refill and get $1 off.
To determine the salt additions for your brew, we recommend using the Bru'n Water online tool: https://sites.google.com/site/brunwater/