Cooking Chemicals and Equilibria

In practice, the terms total, combined and free SO2 are used to characterize sulfite

cooking liquors. The content of stoichiometrically base bound hydrogen sulfite

and sulfite ions is referred to as combined SO2. The difference between the total

SO2 and the combined SO2 is then free SO2. According to this definition, half of

the pure hydrogen sulfite solution is free and the other is bound SO2. The following

definition is rather misleading from a chemistry point of view as no free sulfur

dioxide is present in a pure hydrogen sulfite solution:

2 HSO _3 __ SO 2_ 3 SO 2 H 2 O _157_

In the central European acid sulfite industry the technical sulfite solutions are

characterized more closely to the chemical state of the constituents. There, the

combined SO2 is equal to the pure hydrogen sulfite, whereas the (excess or true)

free SO2 accounts solely for the sulfur dioxide in its hydrated form (SO2.H2O) and

4.3 Sulfite Chemical Pulping 395

can be calculated as the difference between the total and the combined SO2. The

combined SO2 is calculated from the active base content, usually expressed as oxide

(e.g., CaO, MgO or Na2O). The following equation demonstrates the definition

of combined SO2 using magnesium hydrogen sulfite as an example:

Mg _ HSO 3_2__ MgO 2 SO 2 H 2 O _158_

According to Eq. (158), 1 mol MgO accounts for 2 mol combined SO2. On a weight

basis, one part of MgO corresponds to 3.179 parts of combined SO2. A typical acid

sulfite cooking liquor contains 17.5% total SO2 and 2.20% MgO, equal to 7.0%

combined SO2 on o.d. wood, respectively. The amount of free SO2 calculates to

10.5% (range 7.0–17.5), representing 60% of the total SO2 as free SO2.

For characterization of the composition of the sulfite cooking liquor, the latter

definition will be used.

To better exemplify the differences between the two ways of cooking acid specification

a comparison is provided on the basis of a typical acid sulfite cooking

liquor composition, as depicted in Tab. 4.52.

Tab. 4.52 Specification of a typical acid sulfite cooking liquor

expressed in two different terms (actual definition related the

more actual species concentrations as used in this book vs. the

Palmrose definition according to TAPPI Standard T604 pm-79).