Method for producing organic solvent solution of quaternary ammonium hydroxide

A method for producing an organic solvent solution of a quaternary ammonium hydroxide,

• the solution having a water content of no more than 1.0 mass % on the basis of the total mass of the solution,
• contents of Na, Mg, Al, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Zn in the solution each being no more than 100 mass ppb on the basis of the total mass of the solution,
• the solution having a Cl content of no more than 100 mass ppb on the basis of the total mass of the solution,
• the method comprising:
• (a) subjecting a raw material mixture liquid to a thin film evaporation by means of a thin film evaporation apparatus, to remove water from the raw material mixture liquid,
• the raw material mixture liquid comprising:
• a quaternary ammonium hydroxide;
• water;
• and
• a first organic solvent dissolving the quaternary ammonium hydroxide, the first organic solvent being a water-soluble organic solvent having a plurality of hydroxy groups,
• the thin film evaporation apparatus comprising:
• an evaporation vessel;
• a raw material reservoir storing the raw material mixture liquid;
• and
• a raw material conduit transferring the raw material mixture liquid from the raw material reservoir to the evaporation vessel,
• wherein liquid-contacting portions of inner surfaces of the raw material reservoir and the raw material conduit are each made of resin.