Silica-based sols

The invention relates to an aqueous silica-based sol comprising a nitrogen-containing organic compound and silica-based particles with a specific surface area of at least 300 square meters per gram of silica. The invention further relates to a process for the production of an aqueous silica-based sol comprising a nitrogen-containing organic compound which comprises incorporating a nitrogen-containing organic compound into a silica-based sol containing silica-based particles with a specific surface area of at least 300 square meters per gram of silica. The invention also relates to the use of an aqueous silica-based sol comprising a nitrogen-containing organic compound and silica-based particles with a specific surface area of at least 300 square meters per gram of silica as a flocculating agent in the production of pulp and paper and in water purification. The invention further relates to a process for the production of paper from an aqueous suspension containing cellulosic fibers, and optional fillers, which comprises adding to the suspension (i) an aqueous silica-based sol comprising an organic nitrogen-containing compound and (ii) at least one charged organic polymer, forming and draining the suspension on a wire.