Hydrochloric acid
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Hydrochloric acid (HCl) is a common acid both in the body and in the lab. It is, for example, a major component of gastric acid (pH 1-2, 0.5% HCl). In experiments, it is used among other things to set the pH in buffers (e.g. Tris) and to reveal antigens (e.g. BrdU).