Sodium hydroxide (Sodium hydroxide, NaOH), sodium hydroxide to adjust the pH in the distribution process to a pulp. In alkaline conditions, which is acidic - pH of about 9.5 to 11.0 to make the fibers absorb water faster and more flexible, but the addition of alkali into pulp, mechanical pulp, paper may turn yellow and darker skin called the macula (. Alkaline darkening)