Lecture3_Module_19
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Transcript Lecture3_Module_19
Challenges with
simultaneous equilibrium
Speciation programs
(MINEQL)
Compilation, calculations and
QC of data
After the analysis the
data must be:
compiled,
reckoned in terms of
equivalent charge and
quality controlled by
ion balance and
agreement between
measured and
calculated conductivity
Speciation with different ligands present
In aqueous solution, containing both metal and ligand,
the distribution of species will depend on factors such
as concentrations, stoichiometry, pH and ionic strength
The calculations become very complex where a
metal cation have the opportunity to bind to more than
one type of ligands
Multiple iterations of the calculations are necessary
In natural water systems
Basic: CO32-, OH-, Org-, ClAcid: F-, SO42-, Org-, Cl-
• For such calculations we apply computer programs as MINEQL and ALCHEMI
Scheme for chemical equilibrium calculations
Relevant balanced equilibrium reactions
Definition of unknown
Equilibrium expressions
1
Mass balance
Charge balance
2 and 3
No
No of unknown
Approximations
are commonly done
by assuming the
concentration of
specific species are
0 Molar
Only mass
balance and
charge balance
equations can
be simplified
Stop
<
No of equations
Yes
Approximations
Solve equations
No
Are assumptions
justified?
Yes
Set of expressions
1. Equilibrium expressions
– KW, KSP, KA, KB, bn, KREDOX, Kd
2. Mass (read: concentration) balance
–
Set the equilibrium molarities (MX)
up against each other (MX vs. MY) and against
the analytical molarity (MX vs. cX)
–
Analytical concentration is the concentration of a substance dumped
into a solution. It includes all the forms of that substance in the solution.
3. Charge balance
– eqv./L positive charge = eqv./L negative charge
1.
Equilibrium expressions
• KW, KSP, KA, KB, bn, KREDOX, Kd
K W [ H 3O ][OH ]
KSP [Ba ][SO4 ]
2
2
[OH ][CH 3COOH ]
KB
[CH 3COO ]
2 n
[ Ni( CN )n ]
bn
2
n
[ Ni ][CN ]
2
[ H3O ][CH3COO ]
KA
[CH3COOH ]
3 5
[ Mn ][Fe ]
KRedOks
2 5
8
[ MnO4 ][Fe ] [H ]
[ I2 ]org
Kd
[ I2 ]aq
2. Mass balance
• Ex.1: BaSO4 in HCl solution
–
We see from the molecular formula
that:
So that:
The hydroniumion (H+) has two sources:
HCl (=cHCL) and the auto-proteolysis of water (=[OH¯]):
• Ex.2: Ag2CrO4 solution
–
We see from the molecular formula that :
So that :
3. Charge balance
• The law of physics demand that
–
Number of positive charge is equal to
number of negative charge
Charge contribution of a specie
= Valens · Molar concentration
n [ X ] m [Y ]
n
m
Ex. 1:
In neutral pH solutions one can
disregard the H+ and OH- ions
–
Ex. 2:
– No new information
Metal hydrolysis
The hydrolysis is described by a set of equilibrium reactions
pb1 pK1 3.05
Fe3 aq H 2O Fe(OH) 2 aq H
Fe3 aq 2H 2O Fe(OH) 2
Fe3 aq 3H 2O Fe(OH)3
aq
2H
pb 2 pK1 pK 2 6.31
aq
3H
pb 3 pK1 pK 2 pK 3 13.8
aq
4H
pb 4 pK1 pK 2 pK 3 pK 4 22.7
0
Fe3 aq 4H 2O Fe(OH) 4
C {Fe 3 } {Fe(OH) 2 } {Fe(OH) 2 } {Fe(OH) 3 } {Fe(OH) 4 }
0
{Fe3+} is determined by replacing each of the other parts of the mass
equation with their equilibrium expression:
Fe3 aq 2H 2 O Fe(OH) 2
aq
2H
{Fe(OH) 2 } {H }2
b2
{Fe 3 }
{Fe(OH) 2 }
b 2 {Fe 3 }
b3
b1
b2
b4
C {Fe 3 }
1
{H } {H }2
{H }3
{H }4
{H }2
Then the other species can be determined from the {Fe3+} and b
E.g.;
3
Fe(OH )2
b 2{Fe }
{H }2
Speciation; Shortcomings
The equilibrium model is based on a choice of
complexes and their stability constants, which
makes the results questionable
Speciation programmes
MINEQL+ is a chemical
equilibrium model
capable of calculating
aqueous speciation
solid phase saturation
precipitation-dissolution
adsorption.
An extensive
thermodynamic database
is included in the model
Tutorial
Start out by choosing
components that
define your system
Find thermodynamic
constants in database
in ”Scan Thermo”
Tutorial
The Calculation
Wizards Tool is a
collection of 5 input
options to describe
the chemistry of the
system
Running the
calculation
Tutorial
Multirun manager
Titration
2 way analysis
Field data
Output manager
Types of Output
•
•
•
•
•
The Header
The Log
The MultiRun Table
Component Groups
Special Reports
Tutorial
Graphics manager
Bar and X-Y plots
Run through the 4
problems
Assignment
Run
a complete speciation analysis on the
samples and describe the chemical
composition of the samples
Proposed issues
Soil – soil water interactions
Describe the changes in water
chemistry through the catchment
Why does the chemistry change is such
a manner? What are the main
processes controlling the water
chemistry through the watershed?
Temporal variation
Compare the generated data,
representing a snap shot in time, with
the volume weighted average
concentrations found in the literature
(http://www.nilu.no/projects/ccc/onlineda
ta/intro.html and
http://folk.uio.no/rvogt/KJM_MEF_4010/
Data%20from%20Skogforsk/
Why are the data different?
Proposed issues
Heavy metal mobility and toxicity
Make a risk assessment of
heavy metal contaminants at
the site. Base the discussion
on your data of heavy metal
content and speciation results
Are there any significant
amount of heavy metals in the
water samples? Is the
environmental conditions
favourable for heavy metal
mobility? Are the heavy metals
in a toxic form?
Report
The
report (~ 4p) should include the
following paragraphs
Abstract
Introduction
Material and methods
Results
Discussion
Conclusion