Potable water chemical preservation I intend on making caffeinated water for personal consumption, in batches large enough that it will take several months to consume them, using containers which are not hermetically sealed, and for which it is not practicable to refrigerate. What would my options be as far as preservatives? 

Commercial caffeinated water products i've bought use sodium benzoate, but I can't seem to find information on how much to use. I am also interested in the possibility of using citric acid, but again i'm not sure what pH level I would be looking for. If it matters I do have a lab grade balance (Sartorius Entris 64-1S) which would be capable of measuring sub-milligram amounts and a dispenser to match.
It seems to me that the risk of adding soduim benzoate, while small, is higher than adding no preservative at all. I doubt you really need any type of preservative.  If you begin with impeccably clean bottles...perhaps sanitized the way a home brewer might sanitize beer bottles (using iodine or other sanitizing product), and work cleanly, I don't see a safety risk here. Use a known clean water source.  You could also decrease your worries further by using distilled water, but the taste might not be agreeable.  It seems like the biggest potential problem is that you might grow some algae.

Now, I don't quite understand your comment above where you state that a 16oz container would contain a hundred or so servings.  If you intend on taking a swig, and re-capping the bottle...well, every time you do that you introduce bacteria into the equation. Additional potential problems arise if you were sharing that bottle with others.  This, of course, could be remedied by pouring your dose into a cup.