chem2601 aquatic tutorial 2013
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The University of the West Indies
Department of Chemistry
CHEM2601 – Aquatic Chemistry Tutorial 3 -2013
1) EDTA and NTA are often used as model compounds to approximate the behavior of organic
ligands in natural water. Suggest advantages and disadvantages of using these ligands to model
the behavior of organics in water systems.
2) a) Water in a gully has a pH of 12.3, total lead (Pb2+
) concentration of 2.0 x 10-5
M and a total
NTA concentration of 2.0 x 10-5
M. What will be the ratio of complexed to uncomplexed
lead under these conditions. (Use the relevant equations and their associated equilibrium
constants from your notes).
b) Water at pH 12.3 contains lead (Pb2+
) at a concentration of 4.0 x 10-15
M.
If the solubility product of Pb(OH)2 is 1.62 x 10-20
, determine whether any of the Pb2+
contained in the water will precipitate out.
3) a. Derive the equilibrium line relevant to a log-concentration diagram, for the reaction of
Fe(OH)3 under basic conditions to form Fe(OH)4-.
b. Using the diagram overleaf, discuss the changes in iron speciation that would occur if
natural water remained at pH 4 while the total iron concentration gradually increased from
1 x 10-10
to 1 x 10-2
M.
c. Industrial effluent containing 10-5
M total dissolved Fe leaves a mining company at
pH 1 via a small canal. Subsequent changes in the pH and total dissolved Fe content of the
waste are shown in the table below.
Sample Points A B C D E
pH Value 1.0 3.0 3.6 5.0 8.0
-log [Fe]Total 4.9 6.0 6.2 7.0 9.5
i) Plot the data on the Log-concentration diagram for iron.
ii) Discuss the changes in iron speciation throughout the journey of the waste. Use chemical
equations to illustrate the reactions that occur and rationalize any increase or decrease
observed in the total dissolved iron content.
4) Explain the meaning of the following:
a. Acid neutralizing capacity b. Bicarbonate alkalinity
c. Clay hardpan. d. pE/pH diagram
5. Derive equations to represent the:
(i) upper pE limit for the oxidation reactions of iron in natural water.
(ii) equilibrium line between Fe(OH)3 and Fe(OH)2 on a pE-pH diagram.
Calculate points for plotting the lines on a pE-pH diagram. Refer to the equations:
Fe3+
+ e- ⇋ Fe
2+ pE
o = +13.2 volts
Fe(OH)2 + 2H3O+ ⇋ Fe
2+ + 2H2O K = 8 x 10
12
Fe(OH)3 + 3H3O+ ⇋ Fe
3+ + 3H2O K = 9.1 x 10
3
d. You have been invited to address a group of environmental professionals on the subject:
‘From polyphosphates to EDTA and NTA: in search of ideal detergent additives’.
Carefully summarize the information you would present. In your discussion, include the
role of such additives, potential hazards associated with their use and a description of the
characteristics detergent additives shoulsd possess for them to be considered
environmentally friendly.
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Log-Concentration Diagram of Fe
pE/pH Diagram the Fe-C-O-H System