The specific action of bicarbonate (as in AlkaSeltzer or TriSalts), does two things:

AlkaSeltzer original formula has aspirin in it...this works very very fast when in the effervescent form...if the nerve reactions are based on inflammatory causes the aspirin knocks them out quickly.

The fast absorption of the bicarbonate changes the pH of the blood for a short time, and since pain transmission is thought to be acidic, this is blocked for a while. Also the bicarb blocks histamine receptors which also are acidic. The blood will normalize very quickly, but by then the bicarb has done its work on the receptors.

One has to be careful with aspirin containing things and not overdo it. I find that AlkaSeltzer works for me once a day or even less when I am in a pain flare. I used it one day this past weekend, but none the whole week before. I save it for when I am not doing well.

If you cannot do aspirin, there are TriSalts which are available online, that some environmental doctors use, esp in children who cannot take aspirin.

Some of the gout internet pages recommend sodium bicarbonate too, to reduce gout attacks for the same reason...blocks acid formation.

This is a temporary intervention and not a cure.. for me one dose lasts a long time, usually a whole 24 hrs or more.

Ordinary aspirin is not absorbed nearly as fast as effervescent forms.
I think the blast into the bloodstream allows for the more efficient actions.
Regular aspirin sits around the stomach, and may cause more bleeding issues there as well.

The aspirin in the AlkaSeltzer WILL also act to reduce platelet clumping, and so people on any blood thinning drugs or with bleeding problems, cannot take this form...the original formula with aspirin.