Compounds Species Concentration pKa

Base Acid mol/l

Hydroxide OH– H2O 0.347 >14

Hydrogen sulfide HS– H2S 0.217 7.05

Carbonate CO3

2– HCO3

– 0.180 10.33

Hydrogen carbonate HCO3

– (H2CO3) 0.180 6.35

Formic acid HCOO– HCOOH 0.300 3.45

Acetic acid CH3COO– CH3COOH 0.200 4.75

Phenolate PhO– PhOH 0.350 10.18

Catecholate PhO2

2–

PhO2H–

PhO2H–

Ph(OH)2

0.005

0.005

12.82

8.42

In strongly alkaline solutions only catecholic structures play a role in the acid–

base reactions. Since the concentration of these compounds is very low in black

liquor (Tab. 4.6), their influence on determination of the OH– ion content is

almost negligible. The (calculated) neutralization curve reveals three equivalence

4.2 Kraft Pulping Processes

0,2 0,4 0,6 0,8 1,0 1,2 1,4

EP3

EP2

EP1

pH-value

[H+], mol/l

experimental calculated

Fig. 4.4 Neutralization curve of black liquor at room temperature

([OH– ]= 0.347 mol L–1, [HS– ]= 0.217 mol L–1, [CO3

2–] =

0.18 mol L–1) using 0.5 mol L–1 hydrochloric acid as titrator

acid at room temperature. Experimental curve compared to

theoretical curve using Eq. (20) (Mathematica 4.1.) [6].

points at about pH 11.7, 8.8 and 5.0 which can be ascribed to the presence of OH–

+ PhO2

2–, PhO– + CO3

2– and HSO3

– + HCO3

–+ OAc–. A fourth equivalence point at

about pH 2.2 can be detected; this is probably due to carbonic acid and formic

acid. Based on the shown equilibria, a neutralization curve can be calculated by

using the algorithm shown in Eq. (20). The correspondence between experimental

and theoretical neutralization curves is surprisingly good, although the experimental

curve contains many more inflection points which are less resolved due to

the many compounds present in black liquor (Fig. 4.4). Differentiation of the

experimental curve reveals eight inflections points only in the pH range 13 to 4.5

(pH of 12.6, 11.9, 10.6, 9.5, 8.3, 6.4, 5.8, and 5.2).

In most cases, the hydroxide ion concentration is determined from the first

inflection point or by direct titration with acid to a predetermined pH of 11.3

[15,18]. Temperature and dilution of the sample affect the acid–base properties of

the inorganic and organic compounds present in the liquor [16]. For a more precise

determination of the hydroxide content of black liquors, a potentiometric

titration of the sample using Gran’s method can be recommended [14,17].

4 Chemical Pulping Processes

4.2.3