B sc_I_General chemistry U-III(B)Molecular formula and empirical formula
Module 3 in Methodology course
1.
Methods
in
Water
Science
and
Technology
Scientific
research
Cross
validation
of
two
different
COD
test
kits
(Kit
with
Hg
and
kit
without
Hg)
Written by: Eric Clayderman CAZOLI
December 2014
University
of
Stavanger
Department
of
Mathematics
and
Natural
Sciences
4036
Stavanger
2. Abstract:
COD
measurements
on
COD
standard
solutions
and
wastewater
samples
were
done
by
using
two
different
COD
test
kits
(kit
with
Hg
and
kit
without
Hg).
Data
obtained
from
both
kits
were
analyzed
by
looking
essentially
at
statistical
parameter
mean
X
and
standard
deviation
σ.
The
objective
of
this
work
was
to
compare
the
analyzed
data
in
order
to
check
the
cross
validation
of
the
two
kits.
Results
of
this
work
showed
that
the
two
kits
are
the
same
when
measuring
COD
standard
solutions,
while
they
do
not
really
overlap
when
measuring
wastewater
samples.
Key
words:
COD,
wastewater,
test
kits,
mean,
cross
validation
1.
Introduction:
Chemical
Oxygen
Demand
(COD)
is
a
term
used
in
both
water
and
wastewater
treatment
to
measure
the
amount
of
a
specified
oxidant
reacting
with
a
given
sample
under
controlled
conditions
(Al-‐Momani,
2003).
The
diochromate
ion
(Cr2O72-‐)
is
the
specified
oxidant
in
colorimetric
method
and
its
amount
is
expressed
in
terms
of
its
oxygen
equivalence.
Under
the
presence
of
catalysts
(sulphuric
acid
H2SO4,
mercuric
sulphate
AgSO4
and
sulfamic
acid
H3NSO3),
the
dichromate
(Cr2O72-‐)
oxidizes
organic
material
in
a
sample
after
incubation
of
2h
at
150°C.
This
oxidation
reduces
Cr2O72-‐
(hexavalent)
into
Cr3+
(trivalent).
Each
of
these
chromium
species
has
a
direct
relationship
with
oxygen
consumed
(Association,
Association,
Federation,
&
Federation,
1915).
Colorimetric
method
for
COD
analysis
is
a
time
consuming
since
you
have
to
prepare
both
digestion
and
catalyst
solutions.
Nowadays,
COD
test
kits
are
available
for
an
easy
and
quick
COD
analysis.
A
COD
test
kit
is
like
a
small
glass
tube
(vial)
on
which
there
is
a
unique
barcode
label
that
is
automatically
read
by
a
spectrophotometer
to
identify
the
appropriate
method
and
take
the
COD
measurement.
COD
test
kits
also
contain
both
digestion
and
catalyst
solutions
(like
in
the
colorimetric
method)
that
react
with
samples
to
be
measured.
A
COD
test
kit
may
present
a
risk
for
the
environment
when
it
contains
harmful
chemicals.
A
kit
3. containing
Hg
is
harmful
for
the
environment
compared
to
the
one
that
has
Hg
free.
Regarding
the
environmental
aspect
and
the
price
of
kits,
it
would
be
recommended
to
use
the
one
with
Hg
free
and
the
one
that
is
less
costly.
For
this
reason,
I
did
some
COD
analysis
on
COD
standard
solutions
and
wastewater
samples
by
using
two
types
of
COD
test
kits
(kit
with
Hg
and
kit
without
Hg).
For
this
COD
analysis,
three
known
COD
concentrations
of
standard
solution
were
prepared
from
Potassium
hydrogen
phthalate
(C8H5KO4).
The
objective
of
this
work
was
to
check
the
cross
validation
of
the
two
kits
by
analyzing
the
statistical
parameter
mean
and
standard
deviation
of
obtained
data.
Data
obtained
from
this
work
and
the
results
of
statistical
analysis
are
presented
and
explained
in
the
result
section
of
this
paper.
2.Theory
A
COD
test
kit
contains
all
necessary
chemicals
that
digest
organic
matters
in
a
given
sample
and
catalyse
reactions
that
happen
inside.
The
following
stoichiometry
shows
these
reactions
and
the
relationship
as
well
as
the
theoretical
ratios
between
chromium
species
and
O2.
Oxidation:
C6H12O6
+
6
H2O
===
>
6
CO2
+
24
e-‐
+
24
H+
Reduction:
24
e-‐
+
24H+
+
32
H+
+
4
Cr2O72-‐
===
>
8
Cr3+
+
28
H2O
Redox
reaction:
C6H12O6
+
+
32
H+
+
4
Cr2O72-‐
===
>
8
Cr3+
+
6
CO2
+
22
H2O
In
reality,
O2
is
the
electron
acceptor:
C6H12O6
+
6
H2O
===
>
6
CO2
+
24
e-‐
+
24
H+
24
e-‐
+
24
H+
+
6
O2
===
>
12
H2O
From
the
reactions
above,
we
can
see
that
1
mole
of
O2
takes
up
4
e-‐
and
1
mole
of
Cr2O72-‐
takes
up
6
e-‐
.
=
>
4
e-‐
/6
e-‐
*
mole
Cr2O72-‐
/
mole
O2
=
4
mole
Cr2O72-‐
/
6
mole
O2
=
>
Δ
Cr2O72-‐
=
3/2*ΔO2
=
COD
4. =
>
8
Cr3+
/4
Cr2O72-‐
*
4
Cr2O72-‐
/6
mole
O2
=
>
4
mole
Cr3+
/3
mole
O2
=
>
Δ
Cr3+
=
¾*
ΔO2
=
COD
Practically,
we
measure
Cr3+
for
the
high
range
COD
(100
and
900
mg/L).
The
relationship
between
theoretical
COD
and
ΔCr3+
is
obtained
by
a
standard
curve
calibration.
Reagents:
According
to
(Association
et
al.,
(1915),
different
reagents
are
needed
during
the
set
up
of
analysis
in
order
to
have
a
complete
oxidation
reaction
and
also
to
remove
any
possible
interferences.
Specifically,
these
reagents
are
mercuric
sulphate,
sulfuric
acid
and
sulfamic
acid.
Mercuric
sulphate
is
added
to
remove
complex
chloride
ions
present
in
the
sample.
Without
the
mercuric
sulphate,
the
chloride
ions
would
form
chlorine
compounds
in
strong
acid
media
used
in
the
procedure.
These
chlorine
compounds
would
oxidize
the
organic
matter
in
the
sample,
resulting
in
a
COD
value
lower
than
the
actual
value.
Sulfamic
acid
is
added
to
remove
interferences
caused
by
nitrite
ions.
Without
sulfamic
acid,
the
COD
of
the
sample
would
measure
higher
than
the
actual
value.
Potassium
dichromate
is
used
as
the
oxygen
source
with
concentrated
sulfuric
acid
added
to
yield
a
strong
acid
medium.
5.
3.
MATERIALS
AND
METHODS
3.1.
Materials
Different
materials
(devices,
chemical
and
instruments)
were
used
during
the
work
in
a
laboratory.
These
materials
are
listed
in
the
following
table
(tab.
1).
Table
1:
list
of
materials
that
were
used
during
the
experiments,
and
their
functions
Material
Function
Potassium
hydrogen
phthalate
(C8H5KO4)
Chemical
used
to
prepare
COD
standard
solutions
Distilled
water
Used
for
dilution
Analytical
balance
Used
to
weight
the
amount
of
C8H5KO4
to
be
used
2
types
of
COD
test
kits:
kit
with
Hg
and
Kit
without
Hg
Contain
chemicals
needed
to
react
with
samples
Pipettes
Used
to
take
a
precise
volume
of
sample
Flask
Erlenmeyer
Used
for
mixing
chemicals
and
solutions
Gloves
Hands
protection
Glass
Yes
protection
Incubator
Used
to
cook
COD
kits
containing
samples
Spectrophotometer
Measures
COD
concentration
of
sample
as
a
function
of
the
color
intensity.
Stop
watch
Record
time
3.2.
Methods
First
of
all,
I
prepared
three
known
concentrations
of
COD
standard
solution
from
Potassium
hydrogen
phthalate
(KHP=
C8H5KO4).
This
preparation
was
done
according
the
American
standard
for
COD
analysis
(Association
et
al.,
1915).
KHP
has
a
COD
of
1.176
gO2/g
KHP.
This
value
is
obtained
by
the
following
reaction
and
calculations:
6.
C8H5KO4
+
aO2
+
H+
=
>
8
CO2
+
K+
+
3
H20
===
>
a=
15/2
*
mole
O2/mole
KHP
=
>
7.5
mole
O2/mole
KHP
=
>
γO2/KHP
=
7.5
mole
O2/mole
KHP
*
(32gO2/moleO2)
/
(204
gKHP/mole
KHP)
=
>
1.176
gO2/gKHP
=
1.176
gCOD/gKHP
(1
gCOD/L)/
(1.176
gCOD/gKHP)
=
0,85
gKHP/L.
Preparation
of
the
three
known
COD
concentrations:
C1=
stock=
1500
mg/L
(add
1275mg
KHP
into
1000mL
distilled
water)
C2=
525
mg/L
(add
35
mL
stock
into
100
mL
volumetric
flask
containing
distilled
water)
C3=
150
mg/L
(add
10
mL
stock
into
100
mL
volumetric
flask
containing
distilled
water)
The
second
part
of
this
work
was
to
measure
the
COD
of
these
three
known
COD
concentrations
and
measure
the
COD
of
wastewater
samples.
The
procedures
for
COD
measurement
are
explained
in
the
appendix
i.
The
third
part
of
this
work
was
to
analyse
the
data
obtained
from
the
COD
measurement.
Mean
and
standard
deviation
were
the
two
main
statistical
parameters
analysed
to
compare
the
data
obtained
from
both
Kits.
This
analysis
was
done
under
Excel
software.
7.
4.
Results
and
discussion
4.1.
Raw
data
The
raw
data
obtained
from
all
measurement
during
this
work
is
presented
in
table
2.
For
simplification,
let´s
use
the
terms
“Hg”
and
“Hg
free”
respectively
for
kit
with
Hg
and
kit
without
hg.
Table
2:
Raw
COD
data
obtained
from
the
two
kits.
Expected
concentrations
of
the
three
known
COD
standard
solutions
(mg
COD/L)
C1
C2
C3
1500
525
150
COD
concentrations
of
blanks
(CODmg/L)
Blank
1
Blank
2
Hg
83
65
Hg
free
49
0
COD
concentrations
of
COD
standard
solution
after
the
measurement
(mgCOD/L)
C1
C1
C1
Hg
1556
1569
1551
Hg
free
1528
1561
1515
C2
C2
C2
C2
C2
C2
Hg
569
571
573
571
577
570
Hg
free
530
533
537
531
534
530
C3
C3
C3
Hg
205
202
200
Hg
free
153
158
157
COD
concentrations
of
wastewater
samples
(mgCOD/L)
sample
1
sample
2
sample
3
sample
4
sample
5
sample
6
Hg
1304
409
388
400
354
392
Hg
free
441
422
637
465
354
8.
4.2.
Analysed
data
4.2.1.
Mean
(X)
and
standard
deviation
(σx)
During
the
statistical
analysis,
parameter
mean
X
and
standard
deviation
σ
were
calculated.
Table
3
and
4
show
respectively
the
mean
and
the
standard
deviation
of
COD
values
obtained
from
the
two
kits.
The
average
COD
concentrations
of
standard
solutions
and
wastewater
samples
were
subtracted
by
COD
content
of
blanks.
Table
3:
Average
COD
concentrations
of
standard
solutions
(C1,
C2,
C3)
and
wastewater
samples
COD
concentrations
(
mgCOD/L)
Blank
C1
C2
C3
Wastewater
Hg
74
1485
498
128
467
Hg
free
26
1510
508
131
439
Table
4:
Standard
deviation
of
COD
values
(σx)
C1
C2
C3
Wastewater
Hg
9
3
3
374
Hg
free
24
3
3
105
4.2.2.
Rejection
of
data
By
looking
at
the
table
2,
we
can
see
that
some
of
the
COD
concentrations
of
wastewater
samples
look
specious.
The
value
1303
mgCOD/L
(from
kit
with
Hg)
and
the
value
637
mgCOD/L
(from
kit
without
Hg)
seem
anomalously
large.
By
applying
the
Chauvenet´s
criterion,
we
can
decide
the
rejection
of
these
two
values.
Assuming
provisionally
all
COD
measurement
of
wastewater
samples
is
legitimate.
a.
N=6
(1303,
409,
388,
400,
354,
392);
the
mean
X
is
here
467
and
the
standard
deviation
σx
is
374.
The
difference
between
the
suspect
Xsus=1303
and
the
mean
X=
467
is
836,
or
2.2
standard
deviations;
that
is,
Tsus=
(xsus-‐x)/
σx
=
(1303-‐467)/374
=
2.2
9. Referring
to
the
table
in
Appendix
ii,
the
probability
that
a
measurement
will
differ
from
X
by
2.2σx
or
more
is:
Prob(outside
2.2σx)
=
1-‐
Prob(inside
2.2σx)
=
1-‐
0.972
=
0.028
In
6
measurements,
I
would
expect
to
find
0.168
of
one
measurement
as
deviant
as
the
suspect
result.
Since
0.168
is
less
than
the
0.5
set
by
Chauvenet´s
criterion,
I
should
reject
the
suspect
Xsus=
1303
mgCOD/L.
So,
the
new
mean
and
standard
deviation
for
COD
of
wastewater,
which
was
measured
by
kit
with
Hg,
would
be
respectively
315
mgCOD/L
and
21.
b.
N=5
(441,
422,
637,
456,
354);
the
mean
X
here
is
439
and
standard
deviation
σx
is
105.
The
difference
between
the
suspect
Xsus=637
and
the
mean
X=
439
is
198,
or
1.88
standard
deviations;
that
is,
Tsus=
(xsus-‐x)/
σx
=
(637-‐439)/105
=
1.88
Referring
to
the
table
in
Appendix
ii,
the
probability
that
a
measurement
will
differ
from
X
by
1.88σx
or
more
is
Prob(outside
1.88σx)
=
1-‐
Prob(inside
1.88σx)
=
1-‐
0.939
=
0.061
In
5
measurements,
I
would
expect
to
find
0.305
of
one
measurement
as
deviant
as
the
suspect
result.
Since
0.305
is
less
than
the
0.5
set
by
Chauvenet´s
criterion,
I
should
reject
the
suspect
Xsus=
637
mgCOD/L.
So,
the
new
mean
and
standard
deviation
for
COD
of
wastewater,
which
was
measured
by
kit
without
Hg,
would
be
respectively
421
mgCOD/L
and
48.
4.2.3.
Correct
estimator
for
all
measurements
If
we
assume
95
%
confidence
for
our
measurements,
the
average
COD
for
our
samples
would
be:
COD=
X
±
(σx/√n)*tα/2,n-‐1
where:
x:
mean
σx
:
standard
deviation
n:
number
of
measurements
α:
accepted
error=
5%=
0.05
10.
For
95%
confidence,
the
real
mean
of
COD
(mg/L)
for
the
standard
solutions
and
the
wastewater
sample
are
the
following:
Conclusion:
By
looking
at
the
real
mean
of
COD
for
standard
solutions,
we
can
see
that
the
two
test
kits
overlap.
Therefore,
we
can
say
that
the
two
tests
are
the
same.
For
the
wastewater
samples,
we
can
see
that
the
two
kits
almost
overlap.
The
reason,
why
they
do
not
exactly
overlap
for
wastewater
samples,
could
be
from
the
subtraction
of
the
COD
concentrations
of
samples
by
the
mean
of
the
COD
of
blanks
that
seem
incorrect.
Reference:
Al-‐Momani,
F.
(2003).
Combination
of
photo-‐oxidation
processes
with
biological
treatment:
Universitat
de
Barcelona.
Association,
A.
P.
H.,
Association,
A.
W.
W.,
Federation,
W.
P.
C.,
&
Federation,
W.
E.
(1915).
Standard
methods
for
the
examination
of
water
and
wastewater
(Vol.
2):
American
Public
Health
Association.
Taylor,
J.
(1997).
Introduction
to
error
analysis,
the
study
of
uncertainties
in
physical
measurements
(Vol.
1).
C1
C2
C3
wastewater
Hg
1485±
22
498±
3
128±
7
315±
26
Hg
free
1510±
60
508±
3
131±
7
421±
77
11.
Appendix
i
a.
Procedures
for
COD
analysis
using
Kit
with
Hg
