The perfect thing to do is yes collect test samples in a suitable container preferably one with a child safety cap, make sure you label it with the contents or you could be fined by law, an old excel bottle with the excel label isn't going to cut it here people, make sure they are marked with proper corrosive labeling and keep those inserts as they have the MSDS information you need to have on hand just in case of a potential environmental spill, or little Johnny next door decides it's cool aid and drinks it.
Next lets take our 500ml bottles to the local waste water treatment facility when they are full and turn them over for proper disposal. When the staff looks at you like you lost your mind explain to them EXACTLY what you have, and why your storing it, and why your bringing it to them. Don't forget to tell them just how long it took you to collect that 500ml as well, also don't forget to have a pretty close idea of the amount of caustics/acids that are contained in your container by % so they know just how responsible your being...
Coming from the manufacturing side of chemicals (both caustics and acids) as a manager having to know the EPA compliance regulations and having reported spills that hit the sewer drains and in much greater excess than even the whole bottle of a testing solution contains, only to be told that the threshold was of no concern.
There is not one single EPA rep here in the states that is going to bat an eyebrow over the very minimal amount of potentially toxic chemicals going into the drain when we check our water quality. The amount is so negligible it's pathetic, I understand the mindset of accumulative effect and being an avid outdoors man myself I have as much respect for the environment as any other person here, but really, some people here can't be serious. The accumulative affect of house hold drain cleaners people use daily isn't an issue and those chemicals are MUCH more potent than our test kits.
Granted the chemicals we use to test our tank water quality are toxic, you certainly don't want to drink them, if you get them on you wash them off with cool tap water. But one would need MUCH more than the bottles we use in an API test kit to create even a blip on the water quality radar, pour it down the sink, flush it with tap water diluting it even further. Not one single waste water treatment operator here in the states would find cause for comment or concern, I'm sure there are a few operators in the hobby that do just that, pour it down the sink without a care or worry in the world