Some good news about longevity testing of the concentrate:
It's been six months in the refrigerator, half full and opened once a month, and still no evidence of yellowing.
I have some D316 and Mocon in the refrigerator and freezer. The bottles of D316 are half full, and I've been opening them once a month for a few minutes to simulate typical hobbyist usage. It's been six months, and there's still no evidence of yellowing (oxidation) visible in a glass eyedropper. For comparison, when stored at room temperature and *not* opened, these glycol-based ascorbate concentrates start yellowing after 2-3 months. Evidently, they like to be kept cold.