PBW batch lifetime
I typically accumulate a lot of commercial bottles because I have a hard time tossing out a non-twist-off beer bottle (it is an obsession and I've promised my wife I'll seek help). I will clean them by filling a 5-gallon bucket with a PBW solution and soaking the bottles in batches of about a dozen (it cleans them up and takes the labels right off). Obviously the PBW solution gets dirtier with each batch.
One can monitor the efficacy of Star-San just by measuring the pH; is there a simple way to monitor the efficacy of PBW or similar cleaners, or is one stuck making subjective judgements by how murky the solution looks?