Inorganic Reactions

As can be deduced from Scheme 4.5, nucleophilic attack of hydrogen sulfide

results in the formation of elemental sulfur, which in turn results in a temporary

loss of active hydrogen sulfide. Elemental sulfur can further react with hydrosulfide

ions to polysulfide (Scheme 4.25). This polysulfide may disproportionate to

thiosulfate and hydrosulfide. During a kraft cook, the amounts of polysulfide and

sulfide varies [12]. Gellerstedt et al. [12] demonstrated a rather constant concentration

of sulfide ions and a decrease in polysulfide towards the end of a conventional

kraft cook. Teder [134]showed that the stoichiometric composition of a polysulfide

184 4 Chemical Pulping Processes

solutions depends on the alkalinity, while the average size of the polysulfide

anions (SnS2–) in equilibrium with elemental sulfur is independent on the prevailing

alkalinity. However, the concentration of hydrosulfide ions decreases with

increasing alkalinity.

HSH

O 2 SxS

2- xS+ 0 +

S HS- m+nS

2-

SnS

2-

SmS

2-

OH-

+ + +

(a)

(b)

Scheme 4.25 Formation of polysulfide ions (a) and reaction

of polysulfide ions under cooking conditions.

The analysis of inorganic compounds in cooking liquors can be performed with

either ion-exchange chromatography or capillary electrophoresis [135,136]. Quinone-

type compounds are thought to be responsible for the rapid oxidation of sulfide

ions to sulfite and thiosulfate [134].

4.2.5