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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
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
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