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Pickling - Part I Because I had never pickled anything before, I decided to perform a test on two of the cutting scraps before subjecting my billets to acid etching. I took two cutting scraps:  and kept them separate. One would be tested in sulfuric acid (H2SO4) and one is nitric acid (HNO3). These acids must be kept in a safe place, such as a proper corrosives cabinet:  The actual pickling should be done somewhere with good ventilation such as (ideally) in a fume hood, outside, or at least by an open window with a fan blowing out. Luckily I had a fume hood at lab:  A fume hood is a work space with a constant suction, and a glass door (called a "sash") that can be raised or lowered to protect you from chemicals and to ensure a good flow rate into the hood. That way, if something generates toxic vapors they are sucked up the chimney and stay out of the room.  I decided to test each acid at a 9:1 ratio, or 1 part acid to 9 parts water. NOTE: ALWAYS ADD ACID TO WATER, not the other way around!!! If you add water to acid, it can splash back at you violently! Add the acid a little bit at a time  while stirring constantly:  I then set the timer for half an hour, lowered the sash (while leaving it a crack open for air flow) and walked away:   "ALWAYS ADD ACID TO WATER." But
why?When a solute dissolves in a solvent (in this case acid dissolves in water), there is a heat of dissolution associated with the reaction. Many chemicals, such as most acids, get very hot when they are diluted with water. If you add a few drops of acid to water and stir, the temperature only changes by a tiny bit at once. However, if you add water to acid, the water can heat up so quickly that it flashes to steam! This steam expands rapidly, and can force the acid out of its container and an to you!!!
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