Sodastream sells a device that will charge a 1 liter bottle of water with CO2 gas so you can drink fizzy beverages. I always use 40F cold water from the fridge to fill the bottle, and seal it tightly, but I find that after only 1-2 days 90% of the fizz is gone, a much shorter life than fizz from 2 liter factory soda bottles that have been opened, the cap retightened, and kept in the fridge. What I initially thought is that there must be a leak, but I submerged the bottle in the sink, shook it (with some CO2 still forming visible bubbles inside), and there were no bubbles at all from the cap or anywhere else, after 30 minutes. So the only idea I have left has to due with the city water, which has a pH of about 9.3 (limestone surface rock). Factory soda is presumably made with initially neutral water, which becomes acidic from the CO2, but in this case, it's starting out with solutes present (bicarbonate I'm guessing, calcium, etc.) and the higher pH. So, is it possible that during that 1-2 days when the fizz goes away, the pH of the water is dropping and the CO2 is being bound into some solute particle? I take it a system with carbonic acid and bicarbonate is a "buffer system" but I'm not sure I fully understand what would happen in my sodastream bottle.