Daily Updates from
our Teacher at Sea
Sampling for Oxygen
| Today we learn about "Dissolved Oxygen"
or oxygen sampling. Ron Patrick and John Calderwood, from Scripps
Institute of Oceanography, are in charge of sampling for oxygen via running
titrations on the samples.
They work opposite each other in shifts (one works noon to midnight, the other works midnight to noon) in order to keep the analysis going around the clock.
|Why do we analyze water samples for oxygen?
Oceanographers use oxygen as a tracer of water masses.
Just like CFC's, oxygen gas is mixed into the water by wave breaking and gas exchange at the surface of the ocean. So, if we find water with a high oxygen concentration deep down, we know it has recently been at the surface.
As a water mass moves away from the surface, it carries its oxygen content with it, and the biological activity and oxidation processes will gradually decrease the oxygen content.
Typically, oceanographers know the oxygen content of different water masses and so can use this information to help recognize where a sample of water originated. If we find a water sample with a low oxygen concentration, then it could be very old, or have seen a lot of biological activity.
Ron removing water samples
from the CTD Rosette.
Ensuring no air bubbles
contaminate the samples.
After the rosette is brought on board, CFC's are usually sampled first, then oxygen. As the seawater is drained from the Niskin bottle, it allows more oxygen to enter into the bottle. The lower the water level is in the bottle, the greater the chance for contamination to occur from laboratory air.
While Ron is sampling, the seawater flows from the Niskin bottle through rubber tubing and then into a numbered flask (ie: 5688). That number is recorded in a sample log (which is what I am holding, because I am "SAMPLE COP") along with the Niskin bottle number.
As he is sampling, Ron makes sure that no air bubbles are in the tubing or in the flask as this will contaminate the reading.
He is filling the flask up and letting it overflow with seawater. While this is happening, he reads what the temperature of the seawater is by using an electronic temperature sensor. It measures to the nearest tenth of a degree. This temperature value is also recorded in the sample log.
| Ron then adds 1 milliliter (ml) of MnCl2
and 1 ml of NaOH / NaI. These reagents are added to the flask, making
sure that no bubbles are being added in the process.
These chemicals then react with the seawater samples and form a quantity of iodine equal to the quantity of dissolved oxygen in the sample.
Ron then puts the top back onto the flask and does the "Ron Twist". He has to shake the flask making sure that the solution and reagents mix together.
Adding reagents to the sample flask.
Adding acid to solution.
John monitoring titration of a sample.
John Calderwood adds 1 ml of an acid and a stirring bar to the flask.
The flask is then placed into a water bath, next to an ultraviolet (UV) light. The iodine compound has color to it. This compound stops the UV light.
The solution also has a buret tip pointing downward in the flask. As the bar is stirring the solution, thiosulfate is slowly added. Iodine reacts with thiosulfate, which converts the iodine to iodate, thereby making the solution colorless and transparent to UV light.
When all the iodine is converted to iodate the desired endpoint of the reaction has been reached.
At the endpoint, the amount of thiosulfate is known, which has been added to the solution.
This amount is used to calculate the concentration of oxygen in the sample.