A sol of the present invention is a sol comprising zirconium oxalate compound as a dispersoid, wherein a mol ratio of oxalic acid to Zr (oxalic acid/Zr) is from 1.2 to 3 and a particle diameter D50 of the dispersoid is from 10 to 100 nm. This sol is produced by adding oxalic acid to a dispersion liquid of zirconium hydroxide, and the addition of oxalic acid is carried out in twice.
A sol of the present invention is a sol comprising zirconium oxalate compound as a dispersoid, wherein a mol ratio of oxalic acid to Zr (oxalic acid/Zr) is from 1.2 to 3 and a particle diameter D50 of the dispersoid is from 10 to 100 nm. This sol is produced by adding oxalic acid to a dispersion liquid of zirconium hydroxide, and the addition of oxalic acid is carried out in twice.