Abstract: [Problem] Provided is a semipermeable membrane that achieves chemical resistance (chlorine resistance, acid resistance, alkali resistance), membrane performance, and membrane strength by sulfonating a polyphenylene-based semipermeable membrane base material under specific conditions. [Solution] A polyphenylene-based semipermeable membrane that is composed of a material containing a polyphenylene-based resin, wherein the semipermeable membrane has a first surface of which at least a portion is sulfonated, and wherein, when concentration distribution of elemental sulfur derived from a sulfonic acid group is measured by elemental analysis in a cross section in a thickness direction of the semipermeable membrane, a ratio of a sulfonated layer obtained by the following expression is 50% or less: Ratio of Sulfonated Layer (%)=Sulfonated Layer Thickness/(Sulfonated Layer Thickness+Non-Sulfonated Layer Thickness)×100.

Abstract: A composite membrane comprising a supporting membrane that includes polyphenylene oxide and a separation layer that is disposed on one main surface of the supporting membrane, wherein the poiyphenylene oxide is sulfonated on the one main surface of the supporting membrane and the separation layer includes polyvinyl alcohol having an ionic functional group.

Abstract: There is provided a simple method of producing in a good yield a tetraalkylammonium hydroxide aqueous solution in which the content of metal ions such as alkali metal ions is reduced to an extremely low level.High-purity trialkylamine and high-purity alkyl chloride are reacted with each other in ultrapure water to obtain an extremely high-purity tetraalkylammonium chloride aqueous solution substantially free from metal ions such as alkali metal ions, and then, the tetraalkylammonium chloride aqueous solution is subjected to electrolysis-dialysis in a solution state without drying it into a solid to produce a tetraalkylammonium hydroxide aqueous solution. Thereby, an extremely high-purity tetraalkylammonium hydroxide aqueous solution with little reduction in purity is obtained in a high yield.