What pH range is suitable for cooking on teflon?
We shouldn't cook acids, alkali and concentrated salts on teflon.


This is incorrect.  Teflon (PTFE) itself is one of the most non-reactive substances you can use on cookware, in some ways better than ceramic.  To quote Wikipedia:


  It is nonreactive, partly because of the strength of carbon–fluorine
  bonds, and so it is often used in containers and pipework for reactive
  and corrosive chemicals.


So why is there so much advice on not cooking high-acid foods in teflon pans?  As far as I can tell, it's because teflon pans scratch easily, and acids can get into scratches.  This has two potential effects:


On pans with poor bonding between teflon and the backing metal, the acid can dissolve some of the adhesion between PTFE and pan, causing it to flake off.
Many teflon pans have the teflon backed by aluminum, and aluminum gives off flavors to acidic foods.  So even the small amount of aluminum exposed by the scratches can cause the high-acid food to taste "off", especially if it is slow-cooking.


Given the above, however, I think you can see that there is no specific pH that's bad for teflon pans.  The lower the pH the worse the two above effects can potentially be, but a lot more depends on the manufacture of the pan, how scratched or abraded the teflon is, and even how long and how hot the food is in the pan.  And if your teflon pan is brand-new and high-quality, it's one of the best vessels for making high-acid foods you have.