I need to do work with muriatic acid and was wondering if storing the waste (12 Oz of acid per gallon of water) is okay to do in a regular plastic 5 gal bucket. My concern is whether the acid could melt the plastic then spill out. Should I get a metal bucket just in case?
A metal bucket is much MORE prone to be attacked by acids in general than most plastics, especially those commonly used for buckets.
As it's "waste acid" throw in a box or two of baking soda and you'll neutralize it right then and there. Preferably outside due to the release of carbon dioxide. Or use marble/limestone chips or dust.
Do not use metal! That goes for any acid. Generally its pretty easy to find good resources on plastic manufacturers websites. Try here. (muriatic acid = Hydrochloric acid)
This chart gives data for 0.4M and 4M. If this is waste acid, you are probably more in the range of 0.4 but you might as well not take any chances.
Home depot buckets are most likely HDPE which is pretty good according to this chart, but you should verify. It should have a #2 recycling symbol if it's HDPE.
You may also find PVC (#3), or polypropylene(#5) containers which also do well.
Pretty much everything else on the list is either not suitable for acids, not generally used for containers or high weight polymers used in more specialized applications and you aren't likely to find them at a local hardware store.
NUTRALIZATION (read me, keep your eyes)
I also wanted to add that if you are dealing with a reasonably concentrated acid, neutralization may not be as safe and straightforward as you might think. If you must neutralize, do it slowly, with googles and gloves.
Plastic vessel. Add a base, easiest is barn lime (white is better), garden lime. Both have fine particles. Dilute your liquid with water, at least to 2/3 of the vessel. SLOWLY add lime. This process releases CO CO2? and H use good ventilation! Add a bit more lime after percolation stops. Now you have Lime water. Add to your garden!