Theoretical background#

Principle of the Winkler method#

Manganous chloride (MnCl2) solution is added to a known quantity of seawater and is immediately followed by the addition of an alkaline sodium hydroxide-sodium iodide solution (NaOH/NaI). Manganous hydroxide (Mn(OH)2) precipitates and reacts with the dissolved oxygen in the water with the formation of a hydrated tetravalent oxide of manganese (MnO(OH)2).

Mn2+ + 2OH– → Mn(OH)2
2Mn(OH)2 + O2 → 2MnO(OH)2

Upon acidification, the manganese hydroxides dissolve. In the acid solution, the tetravalent manganese in MnO(OH)2 acts as an oxidizing agent and liberates iodine (I2) from the iodide ions (I2).

2MnO(OH)2 + 8H+ + 4I- → 2Mn2+ + 2I2 + 6H2O

Two moles of I2 are formed for each mole of O2 present in the sample. The amount of I2 in the solution is determined by titration with a standardized sodium thiosulfate (Na2S2O3) solution.

I2 + 2S2O32- → 2I- + S4O62-

Two moles of thiosulfate are required to titrate each mole of I2. Since two moles of I2 were formed for each mole of O2 the final stoichiometry is four moles of thiosulfate equals one mole of O2. By knowing the concentration of the thiosulfate solution and the volume required to titrate the liberated I2 the amount of the oxygen dissolved in the seawater sample can be easily computed.

Volumetric Karl Fischer Titration with a double Pt-wire electrode with a bivoltametric end point determination#

  1. What is Karl Fischer Titration?

    The Karl Fischer Titration is a titration method for measuring water content in basically all types of substances. It was invented in 1935 by the German chemist Karl Fischer. The Karl Fischer Titration is based on an iodine/iodide reaction: The water reacts with iodine. The endpoint of the titration is reached when all the water is consumed. In the case of the dissolved oxygen Winkler titration, the analyte is iodine (I2) in acid condition (H2SO4) and will react with a standardized sodium thiosulfate solution (Na2S2O3).

  2. Bivoltametric (volumetric) Karl Fisher titration

    A small current (1μA) is applied between the two electrodes “Ipol” (between the 2 Pt wires) and the voltage required to maintain this current is measured. In an acidified solution, the iodine is reduced at the cathode to iodide (I2 + 2e- → 2I-) and the reverse reaction occurs at the anode (2I- → I2 + 2e-). During the titration, thiosulfate (Na2S2O3) is added and reacts with the I2 to form 2I-. When the endpoint of the titration is reached, free thiosulfates are available in the solution and the voltage drops to 500mV or more to maintain the Ipol current.

Quality control and measurement uncertainty#

To calculate the precision of the oxygen measurements 10 replicates from the same Niskin bottle were analyzed. Average value = 6.042 St. deviation = 0.0054 were: coeff% = 0.0887. Generally, four Thiosulfate standards calibration are run, and the endpoints are averaged. The endpoints should be within ±0.3% of each other.

Calculation of oxygen concentration#

Chemical reactions and stoichiometry:
Mn2+ + 2OH– → Mn(OH)2
2Mn(OH)2 + ½O2 + H2O → 2Mn(OH)3
2Mn(OH)3 + 2I– + 6H+ → 2Mn2+ + I2 + 6H2O
I2 + I– ⇌ I3–

The amount of I3– formed is chemically equivalent to the amount of oxygen:
I3– + 2S2O32– → 3I– + S4O62–

This means that:
2S2O32– = I2 = ½O2 and that under ideal conditions, 1 mol O2 will be equivalent to 4 mol thiosulfate.

These factors are used further in the equation for determining the amount of oxygen: 1 mmol / L O2 occupying 22.4 mL STP (standard temperature pressure). This means:
1 mL O2 = (1 mmol / L O2 (31.9988 mg = molar mass O2)) / 22.4 mL = 1.42857 mg O2
1 mmol / L = 0.25 thiosulfate mmol / L O2 = 7.9997 mg O2 = 5.598 mL O2

5598 mL is used further in the equation for determining the amount of oxygen.

Determination of thiosulfate normality (Nthio):
Nthio = (VIO3 × NIO3) / (Vthio – Vblk)

where V = volume IO3– iodate standard used (mL) NIO3 = normality of iodate standard Vthio = volume thiosulfate used for titration of standard (mL) Vblk = volume thiosulfate used in the blank (mL)

Determination of oxygen in the sample: O2 (mL·L–1) = [(Vthio – Vblk) × Nthio × 1000 × 5598 / (Vflask – Vreag)] – [(0.0017 / (Vflask – Vreag)) × 1000]

where Vthio = volume thiosulfate used for standard titration (mL) Vblk = volume thiosulfate used in blanks (mL) Nthio = normality of thiosulfate Vflask = volume of the sample bottle (mL) Vreag = volume of added Winkler reagent A and B (usually 2 mL) 0.0017 = amount of oxygen added with the reagents 5598 = mL O2 (STP) per mol thiosulfate titrated

By simplifying the equation: O2 (mL·L–1) = ([(Vthio – Vblk) × Nthio × 5598] – 1.7) / (Vflask – Vreag)

Conversion to weight basis: It is increasingly common to express oxygen concentration in seawater on a weight basis (µmol/kg). To convert mL·L–1 to µmol·kg–1, seawater specific gravity is required. This can be calculated from sigma theta based on potential temperature.

The equation becomes: O2 (µmol·kg seawater–1) = 44.660 × O2 (mL·L–1) / specific gravity of seawater

Seawater specific gravity = 1 + (sigma-theta / 1000) 44.660 = (1000 / molar volume of oxygen gas at STP) (Weiss, 1981)

References#