Some of the bottles are labelled "corrosive," "irritant" and "harmful." It's not a bad idea to wear a glove to avoid getting the leakage on your hands (I have boxes of latex gloves I picked up for another application. I just reuse the same one when testing till it gets worn out enough to replace). If you use enough paper towel or a clean rag, that would do the trick too.
Wow.. I'm glad I'm not the only one. I actually sent an email to API complaining about the leaky vials, but I haven't heard back from them yet. If I do, I'll let you all know what they say.
I seem to only have a problem with leaks when I'm testing for Nitrates, and I have to shake for a long time. I started using paper towels to wrap the vial up when I shake it, I was tired of getting splatter everywhere.
A test seems to be when adding reagent #1 and mixing, no leaks. When adding the second chemical and shaking it starts to leak a little. That suggests that there's a little gas released from the reaction.
No biggie IMHO. I think all of mine leak a little.