Reviews of hard boiled egg recipes are always mixed, with most rating them either excellent or terrible and very few in between. Assuming everybody is following the recipe carefully, I think the reason for this is that the recipes never control the water acidity well enough. Vinegar works by making your water acidic. Acid dissolves calcium carbonate from the eggshell, which makes it softer and more flexible. That means you don't need to use as much force when pulling the shell off, so it's less likely you'll tear the white before the membrane lets go. The more acidic the water is, the faster it softens the shell. Municipal tap water generally has a pH from 6.5 (a tiny bit acidic) to 8.5 (slightly alkaline), and varies from city to city or even seasonally. Well water is usually quite alkaline. When you add vinegar (acid) to alkaline water, it goes toward neutralizing the water first; only if there's more acid than alkaline will it tip the pH into the acid range where shell softening can occur. So depending on how alkaline your water is, you may need more vinegar before it has any effect at all. And of course, the more water you use, the more vinegar you need. If you don't want to stink up the kitchen by adding more vinegar, you can use different water instead. Distilled water should be very nearly neutral, meaning all your vinegar will go toward softening the shell. Unfortunately, water from a home filtration system is unpredictable; it depends on the adsorbent used and how long since it was cleaned or changed. Most commercially bottled water is acidic to start with, which means you can use LESS vinegar (but also means it's LESS healthy than municipal water if you drink it frequently). It's been claimed that baking soda can also make eggs easier to peel because it weakens the bond between the membrane and the white. I haven't tried that, but it might work. Still, don't add both vinegar and baking soda because you think their effects will combine to make super-easy peeling: Baking soda makes the water more alkaline, so the effects will mostly cancel each other out.
