From a Reddit post:
I keep Star-San in a 5gal bucket for re-use. Is there a definitive way to know that it is still good? Is there a safe time-frame or will I need to get a pH meter/paper and check acidity? Answer: The most definitive way to tell if Star San is still good is by titrating for Phosphoric acid. However, you can check the solution by the ph, it should be 3.5ph or below.
What is the maximum effective pH of it, according to them? Answer: 3.5
I have hard water that doesn't seem to work well for mixing Starsan. Is there anything I can do, so I don't have to buy distilled water from the store, other than setting up my own water still or RO system? Answer: Some people have found that putting the water through a carbon filter works. The real question is “How hard is your water?” Star San should be fine, but it will start to deactivate sooner. This means make up a fresh solution every time. Don’t keep for more than one hour.
How long can I reuse water with PBW in it? How do I know when I should make a fresh solution (for example, when I'm washing bottles in a bucket with PBW)? Answer: PBW is best effective if used within 12-24hours of making your solution. However, you can reuse the PBW until it doesn’t get the bottles clean any more.
Are there metals that I should not leave in contact with PBW/StarSan for a long time? Answer: Don’t use on Aluminum. Copper is okay but not more than a few hours. Stainless and most plastics are fine.
Re:Starsan, can I use a little less per gallon, but just let it soak for longer? Whats the limit of this? Answer: NO, Star San must be used at a ratio between 1oz/ 5 gallons and 1oz/ 3gallons.
Finally, if something is wet with Starsan, is it assumed its sanitized (for example, if I wet my thermometer and then put it in the wort)? Or does it need to be wet for a short period of time to kill the bacteria? Answer: It should have a contact time of at least 2min to fully do the job.
Can you please address the "is the foam ok for my wort" question that's constantly asked by newbs so we can put that one to bed? Answer: Don’t Fear the Foam, the foam will not hurt or dis-flavor your product.
I use PBW for CIP in my hard plumbed homebrewery. One of the suggestions that I generally ignore for the sake of time is rinsing with the same temperature water as the PBW. I usually use 120F PBW and then rinse with cold tap water. How important is rinsing with the same temperature water and what are the ill effects if I don't?
Answer: I wouldn’t worry about that. The main reason is so that you don’t break glass or plastic carboys with a temperature shock. The other reason is to ensure a proper rinse. Some chemicals can take more water at colder temperatures to completely rinse off.
What does star-san break down into? Answer: Star San will break down into Phosphates and other biodegradable products.
Are they hiring? Seriously I would like to be one of their biologists that I'm sure they must need (right? Everywhere needs a staff biologist) Answer: Not at this time, but keep your ears open. It never hurts to send in a resume so we have it on file.