Here is a link to the material safety data sheet for Etch All cream. I couldn't find one for the liquid, but it would be very similar:
http://www.alpineglass.com/shop/imag...lCreamMSDS.pdf
The disposal section states that moderate amounts can be flushed down the sanitary sewer system with plenty of water - flush with water or neutralize with lime.
I would make a saturated solution of baking soda in water about twice the volume of the etch all (just keep dumping baking soda into the water until it doesn't dissolve anymore). Wear eye protection and gloves. Slowly add the etch-all to the baking soda solution a little at a time. It will fizz and bubble. When it's all neutralized you can pour in down the drain slowly with lots of water.