Download 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


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
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
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Abstract
Introduction
Material and methods
Results
Discussion
Conclusion