Question: We purchase 4 ounce bottles of dyne solution at 72 dynes/cm. Can you tell me how you determine the expiration date? We don’t always use up the container before its shelf life is over.
Answer: Given that your purchase is of 72 dyne/cm surface tension test fluid, containing only reagent-grade water and Methyl Violet dye, this is an interesting question. The typical dyne solutions containing 2-ethoxyethanol and formamide (mixed per ASTM Std. D2578) definitely have a finite shelf life, as the constituents will react with one another over time, eventually changing their wettability regardless of whether they have been used or not. I do not believe it has ever been determined whether the dye acts as a catalyst, whether it is the cause, or whether the change happens with dye or without. At any rate, historically, suppliers of these test fluids have assigned a shelf life of approximately 3 to 4 months from date of manufacture. We are confident enough with our reagent grade materials to offer a shelf life of 5 months.
No data is available on the aging of water/Methyl Violet mixes. My best guess is that any degradation would be slower than with the binary mix dyne levels, but I am not willing to bet on that and change the expiration dates for this single constituent dyne level (the same can be said for 30 dyne/cm test fluid, which contains only 2-ethoxyethanol and Methyl violet dye, or 57 dyne/cm test fluid, which contains only formamide and Methyl violet dye).
A final concern pertaining to the stability of the 72 dyne/cm test fluid regards potential leaching of compounds from the packaging bottle (HDPE for narrow and wide mouth bottles; LDPE for dropper bottles) into the test fluid. A good deal of information on this phenomenon is available from the literature, but most of it pertains specifically to health effects, and most of the studies are not limited to additive-free polymer formulations. One review(1) presents a litany of mostly solvents and short-chain molecules that have been identified leaching from HDPE water pipes – if these do, in fact, leach from unmodified HDPE, they would generally reduce the surface tension of water over time. Another study(2) demonstrates changes in LDPE over time when exposed to pure water. It seems reasonable to assume that eventually there will be some effect on the test fluid due to polymer leaching.
You could run a study comparing results from fresh test fluids vs. those that are aged, though this could be a challenge, as any number of variables may affect the aging process (average storage temperature and RH, degree of temperature and humidity cycling, exposure to light, etc.). Realistically, the best strategy would be to purchase one or two ounce bottles, which will offer full usage before there are any problems related to aging.
1) no author cited, https://plasticpipesleach.org/wp-content/uploads/2018/01/White-Paper_A-Review-of-Chemical-Substances-Shown-to-Leach-from-Common-Drinking-Water-Piping-Materials.
2) S. Massey, A. Adnot, A. Rjeb, and D. Roy, “Action of water in the degradation of low-density polyethylene studied by X-ray photoelectron spectroscopy,” eXPRESS Polymer Letters, 1, No.8 (2007) 506–511.