怎樣才算是好的天然指示劑？ what means by a good acid-alkali indicator? (?

1. The change is significant when reacting to acids and bases
2. The result should be different in colours when reacting to acids and alkalis respectively. i.e. not the same colour for both acids and alkalis
3. do not react to other materials except acids and alkalis, otherwise the results will be disturbed by extra substances in your samples
4. must be water soluble
5. (optional) non-toxic and do not cause your allergy