Chemistry of Freshwaters - Principles of Environmental Chemistry - page 117

30

20

O 2 /H 2 O

10

Cr 3+

CrO 4 2-

0

Cr(OH) 3 0

Cr(OH) 4 -

H 2 O/H 2

-10

0 2 4 6 8 10 12 14

pH

Example 3.12: Construction of a pe-pH diagram for As-H 2 O. Assume
that no solid phases are formed.
Acid-base behaviour of As V :

H 3 AsO 4 Ð H 2 AsO 4 þ H þ K 1 ¼ 10 2 : 24

H 2 AsO 4 Ð HAsO 2 4 þ H þ K 2 ¼ 10 6 : 76

HAsO 2 4 Ð AsO 3 þ H þ

K 3 ¼ 10 11 : 60

4

Acid-base behaviour of As III :

H 3 AsO 3 Ð H 2 AsO 3 þ H þ K 4 ¼ 10 9 : 23

H 2 AsO 3 Ð HAsO 2 3 þ H þ K 5 ¼ 10 12 : 10

Reduction of As V to As III :

H 3 AsO 4 þ 2H þ þ 2e Ð H 3 AsO 3 þ H 2 O K 6 ¼ 10 19 : 46

H 2 AsO 4 þ 3H þ þ 2e Ð H 3 AsO 3 þ H 2 O

K 7 ¼ 10 21 : 70

HAsO 2
4

þ 4H þ þ 2e Ð H 3 AsO 3 þ H 2 O K 8 ¼ 10 28 : 46

HAsO 2 þ 3H þ þ 2e Ð H 2 AsO 3 þ H 2 O K 9 ¼ 10 19 : 23

4

AsO 3 þ 4H þ þ 2e Ð H 2 AsO 3 þ H 2 O K 10 ¼ 10 30 : 83

4

AsO 3 þ 3H þ þ 2e Ð HAsO 2 K 11 ¼ 10 18 : 73 þ H 2 O

4 3

Construction of vertical lines:
e.g. K 1 ¼ {H 1 }{H 2 AsO 4 }/{H 3 AsO 4 }
On the line, the concentrations of the As V species are equal,
so pH ¼ pK 1 ¼ 2.24.
Construction of sloped lines:
e.g. K 6 ¼ {H 3 AsO 3 }/{H 3 AsO 4 }{H 1 } 2 {e } 2