What does one do with 3 quarts unused from a gallon container (bought at Sears)?

How long can the 3 quarts be stored safely in the plastic container?

Give it to someone with a swimming pool to reduce the PH of the water, or give it to someone for etching their garage concrete floor prior to coating with epoxy. If it is diluted with substantial amounts of water, I suppoe that it could be put down the drain. Great care must be taken when mixing the acid with water. Never add water to acid; add the acid to water. I don't know how long the plastic container will last, but I have seen gallon bottles of muriatic acid more than a couple or three years old without any deterioration of the container.

Or you could give it to me or anyone who owns an aluminum boat. We use muratic to clean boat hulls. It should store in the original container for 6 months or longer. However, make sure the cap is tight and it is not near any metal shelf or container. Last but not least you could fill your laundry tub and add the 3 quarts to the water. Winfields sewer pipes will love it.

It can be neutralized with lye or baking soda and then disposed of down the drain. The question is how much. You can always get some PH test paper, I suppose. A PH of 7 is neutral. If you know someone with an aquarium, they could show you how to test.

Household Hazardous waste disposal.

There are a number of uses for muriatic acid, just a few mentioned here already. Do you live in a brick home? If so you can use it to clean around the base of the home to remove algea, and stains. Be sure to follow all safety precautions.

The acid could be diluted and used to reduce the alkalinity of your flower beds. There are a number of plants that love low PH soil.

I sometimes use Muriatic Acid when I am refinishing brass. Please give it to me! I am not sure of the shelf life of Muriatic Acid.

I have two one-gallon bottles leftover from my pool construction. One is full and the other is about half. They are both over a year old. The labels have fallen off but I've saved the labels and have relabeled the bottles with a black marker.
Shelf-life? I haven't tested the effectiveness of the contents, but a slight and very judicious whiff indicates that it is in fact still good stuff.
Come summer, I will be repainting the garage floor and plan to use some to etch the concrete. I will keep you informed.

Hey Cap, that stuff will be good for your purposes for quite some time. I don't keep it over 6 months because it can leak fumes through that little paper gasket even with the lid closed. I saw rust forming inside my metal cabinet where I store gas and other stuff the grandkids need to stay away from. The rust was just right at the bottle and a chemistry teacher friend of mine explained about the fumes.

I believe that Jeffrey Dahmer used acid to dispose of unwanted parts. The uses seem unlimited for this compound.

Hey Cap, that stuff will be good for your purposes for quite some time. I don't keep it over 6 months because it can leak fumes through that little paper gasket even with the lid closed. I saw rust forming inside my metal cabinet where I store gas and other stuff the grandkids need to stay away from. The rust was just right at the bottle and a chemistry teacher friend of mine explained about the fumes.
I keep it stored outside, away from the house, in a $10 plastic locker. I've heard plenty of horror stories. Thank you for your words of warning.