Can I substitute baking soda for kansui powder?
Harold McGee tackled alkaline noodles a while back. He found that baking baking soda actually changes it from sodium bicarbonate to sodium carbonate. This is a reasonable substitute for the kansui called for in alkaline noodles and can be substituted 1:1 in recipes. The noodles may not get AS yellow as they would with both alkalines present in kansui but it's a small price to pay for not having to hunt down that ingredient. 

http://www.nytimes.com/2010/09/15/dining/15curious.html?_r=0

The key part:


  Just spread a layer of soda on a foil-covered baking sheet and bake it at 250 to 300 degrees for an hour. You’ll lose about a third of the soda’s weight in water and carbon dioxide, but you gain a stronger alkali. Keep baked soda in a tightly sealed jar to prevent it from absorbing moisture from the air. And avoid touching or spilling it. It’s not lye, but it’s strong enough to irritate.