Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
|» Home » Field & Laboratory Protocols » Chapter 5|
SUMMARY: Seawater is collected from known depths using CTD-rosette sampling protocols. Subsamples are drawn into precalibrated iodine flasks and dissolved oxygen is chemically bound by the formation of a manganese (III) hydroxide floc. The floc is subsequently dissolved under acidic conditions which stochiometrically converts the original dissolved oxygen (DO) oxidizing equivalents to triiodide. The latter is quantitatively titrated with sodium thiosulfate to a potentiometric end-point using a high-precision computer-controlled titration system.
The oxygen content of seawater is a fundamental measurement in oceanography providing information which can elucidate water mass movements, net primary productivity, atmosphere-ocean interactions and carbon remineralization processes. The oxygen content of a seawater sample is largely determined by a balance between: (a) the exchange of atmospheric oxygen with the upper mixed layer, (b) net increases due to photosynthetic processes and (c) net decreases due to respiratory demands and heterotrophic processes. From an oceanographic perspective, the measurement of dissolved oxygen is a parameter of fundamental importance.
In this procedure, a divalent manganese solution and a strong alkali are added to the water sample which results in the formation of a floc (1). In the presence of oxygen the manganese II is oxidized to manganese III (2). On subsequent acidification (1 < pH < 2.5), the manganese hydroxide floc dissolves, and I2 is produced in stoichiometric proportion to the original O2 concentration (3). The iodine, in the form of triiodide (4), is titrated with a standardized thiosulfate solution (5), and the oxygen content is calculated from the quantity of thiosulfate consumed.
It is extremely important to prevent contamination of the sample with atmospheric oxygen during sampling, fixation and storage. Drawing tubes and sample flasks must be free of bubbles (3.1 and 3.2.1), as must the autopipette system which dispenses the reagents (3.2.2). If the water samples cool significantly or the seal dries during storage, air can infiltrate the flask. The rims of the flasks are filled with seawater and the samples are stored in a location where temperature fluctuations are minimized (3.4).
The solubility of oxygen in seawater is temperature-dependent and deep samples may warm up as they are brought to the surface (i.e. through the thermocline). It is therefore necessary to measure the temperature in the Niskin bottle at the time the sample is drawn in order to calculate the oxygen concentration in situ (3.3). Oxygen samples are always the first samples drawn from the Niskin bottles.
3. Field Sampling
Calculate the DO concentration using the following formula:
(Vt-Vb) * Nt * E µmol O2 l-1 = ________________ - RDO (Vf - Vr)
This value is derived from Carpenter 1965 (i.e., DO content of fixing reagents when 1 ml of each reagent is used in 140 ml of sample equals 0.018 ml O2 l-1).
6. Precision and Accuracy
The method as outlined above is capable of a precision of 0.1% or less (as defined by the coefficient of variation for triplicate samples). The accuracy of the Winkler titration procedure, when the Carpenter modifications are employed, has been determined to be 0.1% (Carpenter, 1965a,b).
Currently, we are using a high quality commercially prepared potassium iodate certified reference standard to independently assess the accuracy of our potassium iodate primary standard normality. A batch of primary standard is prepared and the theoretical normality normalized to the certified primary standard value. Our primary standard is periodically checked against the certified standard until the batch or lot is exhausted.
Manganese chloride reagent (3 M)
Dissolve 600 g of manganese chloride tetrahydrate (MnCl2.4H2O) in 800 ml distilled water and make up to 1 liter in a volumetric flask.
Alkaline iodide reagent
Dissolve 320 g sodium hydroxide (NaOH) in 500 ml distilled water and, separately, dissolve 600 g sodium iodide in 500 ml distilled water. Mix the he two solutions 1:1, by volume.
Sulfuric acid reagent (10 N)
Mix 280 ml of concentrated sulfuric acid into distilled water using a 1 liter volumetric flask
Sodium thiosulfate reagent (0.05 N)
Dissolve 12.41 g of reagent grade sodium thiosulfate (Na2S2O3. 5H2O and make up to one liter with distilled water. Determine exact normality as described under 4.3 above.
Potassium iodate reagent (0.025 N)
Dissolve 0.8918 g of dry (100°C, 2 hours) KIO3 into 800 ml distilled water and bring up to 1 liter in a volumetric flask.
CSK 0.0100 N KIO3 1° standard
Wako Chemicals, Inc., 1600 Bellwood Rd., Richmond, VA 23237.