First off: Never add anything you don't test for!
Baking soda (bicarbonate of soda) may be employed to raise alk:
1 - Bake it in a flat (clean) cookie sheet at 300 degrees for 30 minutes (it will get much lighter and powdery)
2 - As soon as it cools a little, immediately store it in an air-tight container (big mason jar, mayo jar pickle jar, etc..),-glass container if possible so it doesn't absorb any moisture/impurities from the air or the jar (don't use plastic or metal containers if possible) - Baking Soda is probably the worlds greatest "odor eater" and this is why - it will absorb anything it comes in contact with, so it's important to store it air-tight in a totally inert container that can be cleaned to the level of sterility (like glass).
3 - Now - don’t guess at how much to add - use one of the many online reef calculators out there on the web where you input your tank water volume, and your current parameters, and desired parameters, and your type of additive, and it will tell you how much to add to reach your target level.
Dissolve it in RO/DI or distilled water before dosing and do not dose all at once. This should be slowly dosed into your aquarium. Monitor pH while dosing. Do not allow immediate impact of pH to be more than +/-0.20.
Yes - there are commercial buffers available, and then there is what I've been using (Arm & Hammer baking soda). I recommend employing both - until you decide which is best for your system.
Information gleaned from discussions with cmanning.