Hi notapillow,

Baking soda (aka sodium bicarbonate) is a very weak base and will give you a pH at around 8.5, which basically means that more than half of spice will be salted and therefore washed away. DMT has a pKa of 8.68, which in plain terms means that at pH of 8.68 50% of DMT will be salted  (and thus water soluble and 50% will be freebased. Washing soda on the other hand (sodium carbonate) is a moderately weak base which will give you a pH at around 12, which is strong enough to keep the spice freebased while you wash any NaOH remains.

You can also easily dissolve 10-50 grams of sodium carbonate per litre of water to get a pH of ~12

Hope that helps!