In a reply from Jon Herskovits of Five Star Chemicals (I had asked him about long term storage in a carboy):Yes, storage in a glass container or plastic (HDPE) would be suitable.
"Star San will kill below a pH of 3.5, when the pH drifts above 3.5, it means
that it is no longer effective. You could also Titrate to determine the
exact ppm of acid and DDBSA. Most people use the pH method.