12.097 Environmental Chemistry of Boston Harbor – IAP 2006

Lab 1:
DETERMINATION OF DISSOLVED OXYGEN BY WINKLER TITRATION
1. Background
Knowledge of the dissolved oxygen (O2) concentration in seawater is often necessary in environmental and marine science. It may be used by physical oceanographers to study water masses in the ocean. It provides the marine biologist with a means of measuring primary production - particularly in laboratory cultures. For the marine chemist, it provides a measure of the redox potential of the water column.
The concentration of dissolved oxygen can be readily, and accurately, measured by the method originally developed by Winkler in 1888 (Ber. Deutsch Chem. Gos., 21, 2843).
Dissolved oxygen can also be determined with precision using oxygen sensitive electrodes; such electrodes require frequent standardization with waters containing known concentrations of oxygen. They are particularly useful in polluted waters where oxygen concentrations may be quite high. In addition, their sensitivity can be exploited in environments with rapidly-changing oxygen concentrations. However, electrodes are less reliable when oxygen concentrations are very low. For these reasons, the Winkler titration is often employed for accurate determination of oxygen concentrations in aqueous samples.
2. Scope and field of application
This procedure describes a method for the determination of dissolved oxygen in aqueous samples, expressed as mL O2 (L water) -1. The method is suitable for the assay of oceanic levels of oxygen in uncontaminated seawater and is based on the
Carpenter (1965) modification of the traditional Winkler titration.
3. Definition
The dissolved oxygen concentration of seawater is defined as the number of milliliters of dioxygen gas (O2 ) per liter of seawater (mL L -1 ).
4. Principle of Analysis
The chemical