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7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 1
Accurate Measure-ment of Dissolved O2
The current status of WP4
Ivo Leito, Lauri Jalukse, Irja Helm, Anna Aas UT
Michal Mariassy SMU
Teemu Näykki SYKE
ivo.leito@ut.ee
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 2
Importance of O2 concentration
O2 besides CO2 is a big player in
shaping our climate and
influencing biodiversity
Climate studies are among the
most important research
directions for the humankind
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 3
Importance of O2 concentration
• Oxygen depletion in oceans
• Oxygen depletion in the atmosphere
– Currently 20.95% and it is decreasing
• O2 dynamics is important for climate
– Closely related to CO2
G Shaffer et al, Nature Geosci. 2009, 2, 105
Keeling et al, PNAS 2002, 99, 7848
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 4
Problem
These atmospheris/oceanologic
trends are slow and complex
It is necessary to measure small
differences of large quantities and
compare results from different
locations
High-accuracy measurements and
calibrations are needed
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 5
Needs for fostering metrology of
dissolved O2
High-accuracy measurement/calibration
methods needed
1. Winkler titration
2. Coulometry
Jalukse et al, ACQUAL 2006, 10, 562
Fisicaro, P. MetChem 2007
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 6
ENV 05, WP 4
Metrology of dissolved oxygen
• Task 4.1: Gravimetric Winkler titration procedure
• Task 4.2: Coulometric measurement procedure of
dissolved oxygen
• Task 4.3: Good Practice Guide and an ILC
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 7
4Mn2+ + 8OH- + O2 +2H2O 4Mn(OH)3(s)
O2
Mn2+
Mn(OH)3
4Mn(OH)3(s) + 12H+ + 4I- 2I2 + 4Mn2+ + 12H2O
4S2O32- + 2I2 2S4O6
2- + 4I-
1 Winkler iodometric titration
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 8
Uncertainty of the classical
Winkler method
• Review of literature indicates that in the case of
normal laboratory conditions by a highly skilled
operator:
U = 0.1 .. 0.2 mg/l *
– Pure water, saturation conditions
Jalukse et al, ACQUAL 2008, 13, 575
1
* All uncertainties are given as U at k = 2 level
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 9
How to achieve
lower uncertainty?
• Advancements to be incorporated:
– Weighing instead of volumetry
• Done
– Reagents with well-defined oxygen content used
• Almost done
– Incoming oxygen amount minimized and estimated and results corrected
• In process
– The extent of Iodine volatilization estimated
• In process
1
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 10
How low uncertainty?
– The current results indicate that uncertainty
U = 0.03 .. 0.05 mg/l
(at saturation, in pure water)
seems realistic
1
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 11
M_KIO3
0.0% m(KIO3I_transf)
P_KIO3 0.0% m(KIO3II_transf)
m(KIO3)s 0.0% 0.0%
0.0% m(KIO3II)
m(KIO3I) 0.0% F_m_KIO3
0% m(KIO3III) 0.0%
C_KIO3 0.0% 0.9% F_m_Na2S2O3_KIO3
0.0% 0.0%
F_m_KIO3_endp
1.3%
F_I2
C_Na2S2O3 1.4%
3.7%
V_reag C_O2_Korr_II M_O2
0.0% 14.9% 0.0%
F_m_sample
57.6% 0.0% F_m_Na2S2O3_sample
0.0%
F_m_sample_endp
15.5%
F_I2
ref C_O2 [mg/dm3] C_O2_sample 8.4%
9.01 9.030 mg/dm3
0.043 U , k =2
3
3_322
KIO
KIOOSNa
m
m
_sample3O2S2Na
sample
m
m
8.920
8.940
8.960
8.980
9.000
9.020
9.040
9.060
9.080
C(O
2)
mg
/dm
3
C_O2_sample
ref C_O2 [mg/dm3]
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 12
Coulometry
• Different chemistry types possible
– Oxygen is “converted” into iodine via NO:
• 2NO + O2 → 2NO2
• NO2 + 2I- + 2H+ → NO + I2 + H2O
• Iodine can be determined coulometrically
– Oxygen reacts with methyl viologen radical cation
• 2MV*++ ½O2+ H2O → 2MV2+ + 2OH-
• Determination may be possible as coulometric titration with amperometric end-point
2
Karlsson et al, Talanta. 1974, 21, 957
J. Electroanal. Chem., 43, 1973, 251–255
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 13
Current favorite: methyl viologen
• It is a reversible reaction with E0 (on GC electrode) ca -0.65 V (against Ag/AgCl)
2
Applied Spectroscopy, Vol. 40, Issue 2, pp. 251-258 (1986)
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 14
Current status
– Preliminary cyclic voltammetry experiments have been carried out
– Some potential problems have been identified
• Influx of O2 into the cell – KF titrator is able to monitor
this
• O2 is produced in anode compartment
2
Applied Spectroscopy, Vol. 40, Issue 2, pp. 251-258 (1986)
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 15
How low uncertainty?
• Coulometry
Uncertainty expected:
U = 0.04 .. 0.06 mg/l
(at saturation, in pure water)
• At the moment we are not yet able to assess
whether we will reach this
2
7.10.2011 ENV05 Ocean Kickoff, Berlin, Sept 20-21, 2011 16
Use in the field
• The Good Practice Guide
• In-situ intercomparison will be arranged for assessing
proficiency of DO sensors (on-line/field analyzers)
• High accuracy Winkler and/or Coulometric titration will
be used as a reference method
© Jere Riikonen © Ilkka Lastumäki
3
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