Spent caustic
Spent caustic is a waste industrial caustic solution that has become exhausted and is no longer useful (or spent). Spent caustics are made of sodium hydroxide or potassium hydroxide, water, and contaminants. The contaminants have consumed the majority of the sodium (or potassium) hydroxide and thus the caustic liquor is spent, for example, in one common application H2S (gas) is scrubbed by the NaOH (aqueous) to form NaHS (aq) and H2O (l), thus consuming the caustic.