Sub-Product

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^[1].

Spent caustic concentration is typically: 

• Low concentration: 1-5% w/w
• Medium concentration: 5-15% w/w
• High concentration: 15-25% w/w

Density values and molarity of NaOH solutions at 20°C are presented in the following table^[2]:

References

1. Wikipedia, Spent caustic.Brave Search Summarizer, 30^th Jul 2024, Potassium Hydroxide Solution.
2. Japan Soda Industry Association, 20th Nov 2006, Safe Handling of Caustic Soda.