Chemistry Lab and Field Work Demo Day Marine Sciences (01/11:628:200)
Liz Sikes, 26 October 2004
Today's class is a demonstration of chemical oceanographic techniques that should review some useful material which will appear on the second exam. We will also show some pictures which illustrate some of the joys and trials of doing oceanographic field work. Please ask questions during the demonstration and make sure to move to a better seat if you can't see. There is a list of thought questions at the end (back side) of this handout. Look them over as the demonstration proceeds. We will try to work through the answers at the end.
A. Water Sampling and Sensing Technology
Objectives:
1) To demonstrate the use of sensors to characterize properties of the water column without taking water samples back to the laboratory.
2) To demonstrate how water samples are collected at sea at various depths in the water column.
One obvious way to characterize the oceans is to collect water samples. It is important to do this in a way that does not modify the properties of seawater. A device that is commonly used to sample seawater is a Niskin bottle. The open Niskin bottle is lowered to the particular depth of interest on a cable ("hydrowire") and once there, the bottle is closed by triggering it from the surface with a weight ("messenger") which slides down the wire. When the messenger reaches the first bottle it triggers it to close, releasing a second messenger to slide down and trigger the next bottle, and so on. Niskin bottles can also be deployed in a “rosette”; a circular array of 6-24 bottles around a CTD that is linked to a ship-board computer, through a long, conducting cable. The scientist sitting at the controls can hit a button to trigger each bottle in order as the rosette is lowered/raised to various depths.
CTDs (Conductivity, Temperature, Depth) are instruments that measure water properties continuously while being lowered through the water (or towed laterally), using a variety of sensors. The device that you will see today measures conductivity (can be equated to salinity), temperature, and pressure (can be equated to depth). In addition it measures fluorescence (a measure of phytoplankton abundance) and turbidity (cloudiness). The CTD gives a continuous profile, filling in large gaps in the data between Niskin bottle samples.
B. Determination of Dissolved Oxygen in Water Samples
Objectives:
1. To demonstrate the measurement of dissolved oxygen using an oxygen electrode.
2. To explain the concept of equilibration between a water sample and the atmosphere, and the term saturation.
3. To demonstrate differences in saturation values for oxygen as a function of salinity and temperature.
With the exception of salinity determinations, dissolved oxygen is probably the most commonly measured chemical in the ocean. Dissolved oxygen concentrations are generally expressed in micromoles per liter (mmol/L). Values from 0 to 400 mmol/L are encountered in natural waters. The saturation values for dissolved oxygen increase with decreasing temperature, and salinity. They range from 178-400 mmol/L throughout most of the ocean. Generally, biological processes are responsible for the wide range of values that are encountered. Phytoplankton photosynthesis, for example, can cause oxygen to be supersaturated, while respiratory processes can remove all of the oxygen originally present in some regions.
Oxygen can be measured with electrodes or by chemical methods. The chemical titration techniques are more accurate and precise but take much more time. The principle behind the oxygen electrode depends on redox sensitivity (transfer of electrons). The electrodes are polaragraphic consisting of a platinum electrode in a KCl solution, covered with a membrane which is permeable to gases. If an electrical voltage is applied, O2 reacts with water and generates electrons. This occurs at the electrode surface, causes an electrical current, which is proportional to the oxygen concentration in the water sample.
In this demonstration we will analyze four samples: One seawater sample at room temperature, a seawater sample at 4°C, a fresh water sample at room temperature, and a fresh water at 4°C. Each water sample has been allowed to equilibrate with the atmosphere. Consider whether salinity or temperature had the greatest effect on oxygen saturation values.
C. Salinity Measurement
Objectives:
1. To understand the basic principles behind three methods of determining salinity of seawater.
2. To appreciate the uncertainties associated with salinity values derived using each method.
Recall the simple definition of salinity: total mass of dry solid material per mass of seawater. The average salinity of the ocean is about 35 (the modern convention is to use NO units, but you can think of salinity in parts per thousand or ‰). Oceanographers need to measure salinity very accurately in order to calculate density (temperature is also needed, of course). Salinity determinations at different locations can be used to estimate mixing ratios of different water masses and to understand circulation in the ocean. However, salinity is not necessarily straightforward to measure accurately. There have been several techniques used to determine salinity:
1) Gravimetric: weigh the dry salts from a known mass of seawater. This is a direct measure of the concentration of all dissolved solids in the seawater sample. However, it is not very accurate and no longer used. Can you guess why (see questions below)?
2) Inductive electrical: The conductivity sensor on many CTDs is this type. Very fast and easy technique, and pretty precise, but not as precise or accurate as a salinometer.
3) Electrical: measure conductivity of seawater using a salinometer. The conductivity (inverse of resistance, i.e., the ease with which electrical current passes through a volume of water) is dependent on the total concentration of ions as long as temperature is held constant. The measured conductivity of a sample is compared to that of a standard of known salinity. This is routinely done to “calibrate” the measurements taken from a CTD.
As you watch these demonstrations try to get a feeling for the relative accuracy (=how close to the correct true value) and precision (=how reproducible) of these methods, and understand why great accuracy and precision is important to understanding the oceans.
Questions:
1. As salinity increases, conductivity [increases or decreases].
2. As depth increases, pressure [increases or decreases].
3. All four samples should have been at equilibrium with air (i.e. at saturation). Based on the results did temperature or salinity have the greater effect on saturation values?
6. Where in the ocean would you expect to find the lowest oxygen concentrations?
7. Why might the gravimetric salinity measurement give an inaccurate answer?
8. What is the difference between accuracy and precision?