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

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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

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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

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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

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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

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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

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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

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How low uncertainty?

– The current results indicate that uncertainty

U = 0.03 .. 0.05 mg/l

(at saturation, in pure water)

seems realistic

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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]

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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

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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)

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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)

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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

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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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Thank you for your attention!

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