In the sulfite process, a mixture of sulfurous acid and the bisulfite ion are used to attack and solubilize in the cooking liquor, and the chemical structure of the lignin is left largely intact. The chemical base for the bisulfite can be ionic calcium, magnesium, sodium, or ammonium. Sulfite pulping can be carried out over a wide range of pH. "Acid sulfite" denotes pulping with an excess of free sulfurous acid (pH 1-2), while "bisulfite" cooks are carried out under less acidic conditions (pH 3-5).
Sulfite pulps are lighter in color than Kraft pulps and can be bleached more easily, but the paper sheets are weaker than equivalent Kraft sheets. The sulfite process works well for such softwoods as spruce, fir, and hemlock, and for such hardwoods as poplar and eucalyptus. Resinous softwoods and tannin-containing hardwoods, however, are more difficult to handle. This sensitivity to wood species, along with the weaker strength and the greater difficulty in chemical recovery, are the major reasons for the decline of sulfite pulping relative to Kraft. The trend toward whole-tree chipping puts sulfite at a further disadvantage because of its intolerance to bark.
