Muriatic acid in big box retail stores is already diluted, usually somewhere around 30% - 35% of full strength. The ideal dilution to etch a floor depends on several factors, clearly the strength in the package available to you, the condition of the concrete, etc., so it's a good idea to run a quick test.
The first task is to be sure all safety precautions are followed. Use safety gear, no kids around, proper mixing containers, good ventilation, read the instructions and warnings, follow them, etc. etc. Wear safety eye protection! Always have a large supply of clean water easily available to quickly dilute and wash off spills.
I first try a 1:2 ratio of acid to water, that is 1 part acid to 2 parts water. Note that you should always slowly add the acid to the water, do not pour water into the acid. Then apply a bit of that solution to an inconspicuous spot of concrete. What you want to see and hear is a small amount of interaction - a little fizzing, an bit of bubbling.
That would indicate that the acid is reacting with components of the concrete, primarily the calcium, however it does not react with all the elements that make up the concrete. So where the calcium is dissolved by the acid small pockets are formed, that leaves a rough texture which provides more anchor points for the subsequent coating to mechanically grab onto. The acid also reduces the alkalinity of the concrete, which allows for a better chemical bound with the epoxy, assuming the epoxy has that ability - not all do.
If the test batch of diluted acid has visible "smoke" fumes, then the concentration is way too high - not good for the concrete or your health. Whip up another small batch with a lower ratio of acid.
If there's no visible or audible reaction, then that ratio isn't strong enough for that floor, so try another test with a slightly higher concentration of acid.
Eventually you'll find the right ratio.
I'd pick up a gallon for every 200 ft2, it's unlikely you'll need that much, but I hate having to go back to the store, so get more and hang onto that return receipt.
When you apply it to a larger area there may be some spots where it still looks like nothing is reacting, use a broom with stiff bristles and scrub that area directly to promote the chemical reaction (first be sure to confirm that the bristles aren't affected by the acid).
Do not allow the etching solution to dry on the concrete, that leaves behind that white residue (calcium carbonate) that can effect the adhesion of the coatings. If you do end up with a white powder residue, sometimes you can scrub it off, but often it will require another acid etching to remove. To help avoid that don't acid etch during the high heat of the day, etch smaller areas, and use enough of the mix to so you don't spread it too thin.
The acid will do most of its work in the first 10 minutes or so. It will be largely neutralized by its reactions with the concrete, but it's still a good idea to use an additional neutralizing agent, like baking soda.Just add some baking soda (or garden lime) to water and spread it around. I use a 1/2 cup per gallon of water, and figure one gallon for every 300 ft2. Again, use that broom to work the neutralizer into the wet floor.
Rinse real well with clear water, pressure rinse if possible. It should be weak enough to not pose a problem to driveways or greenery, but I'd try channel the runoff away from the landscaping just in case. Also check beforehand to see if there are any local runoff restrictions.
It's probably pretty clear by this point that muriatic acid is strong stuff. It's very corrosive to metal, can damage landscaping, and can result in long term injuries and health problems if used incorrectly.
There are variations on muriatic acid, and even some other chemicals, that offer the same results, but have far fewer hazards and shortfalls. We supply an alternate etching chemical to facilities that can't tolerate the dust and mess that comes from grinding or shot blasting, but also can't tolerate the fumes or corrosive effects of muriatic acid.
Environments that fall on that list include zoos and other animal housing (kennels, research labs); hospitals, assisted living homes; electronic equipment assembly facilities, equipment rooms (power distribution, hvac etc.), or areas where the residents are sensitive/allergic to chemical fumes. A limiting factor to using those substitutes is they're more expensive.
Muriatic acid is probably the most nasty stuff sold in the typical hardware store - be careful.
BTW - Interesting that muriatic acid - hydrochloric acid - is one of the main ingredients in our stomach acid. Although we like to blame spicy foods, the hydrochloric acid is actually secreted by the stomach. When you have heartburn or have one of those nasty belches - that burning sensation is caused by the same chemical that we're using here to etch concrete! Sometimes life is just too amazing for me to comprehend.
