PASCO PS-2108 User Manual
Instruction Manual
Manual No. 012-07688E
PASPort Dissolved Oxygen
Sensor
Model No.
PS-2108
Instruction Manual
Manual No. 012-07688E
PASPort Dissolved Oxygen
Sensor
Model No.
PS-2108
®
Model No. PS-2108 Dissolved Oxygen Sensor
Table of Contents
Equipment List .......................................................... 1
Introduction ............................................................. 1
DO Sensor Setup and Calibration...................................... 3
Prepare the Sensor for First Use....................................................................................................3
Set up the Dissolved Oxygen Sensor with DataStudio..................................................................3
Equilibrate the Probe in 100% Humidified Air.............................................................................4
Perform a Single-point Calibration (mg/l Dissolved O2)..............................................................4
Setup to Calibrate in Percent (%) Saturation.................................................................................4
General Sampling Procedure ............................................ 6
Measurements in a Controlled Laboratory Setting........................................................................ 6
Using the Dissolved Oxygen Sensor with other PASCO Sensors:................................................6
Maintenance ............................................................. 7
Changing the Electrolyte solution..................................................................................................7
Replacing the Membrane............................................................ .................................. ...... ...........7
Replacing the O-ring........................................................ ..............................................................8
Storage .................................................................. 9
Short-Term Storage (two weeks or less)........................................................................................9
Long-Term Storage (more than two weeks)..................................................................................9
Troubleshooting ......................................................... 9
Theory of Dissolved Oxygen: ......................................... 10
What is the chemical mechanism by which diatomic oxygen dissolves in water?......................10
Citations:......................................................................................................................................14
Experiment 1:
Introduction to the Operation of the Dissolved O2 Sensor ........ 15
Purpose.........................................................................................................................................15
Materials and Equipment Needed................................................................................................15
Optional Equipment Suggested ...................................................................................................15
Procedure .....................................................................................................................................15
Data Analysis/Questions..............................................................................................................16
Experiment 2:
Photosynthesis and Oxygen Generation with Aquatic Plants ........ 17
Purpose.........................................................................................................................................17
Materials and Equipment Needed................................................................................................17
Procedure .....................................................................................................................................18
During the lab time......................................................................................................................18
Collecting Data ............................................................................................................................19
Sample Data.................................................... ...... .......................................................................19
Analysis .......................................................................................................................................19
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®
Model No. PS-2108 Dissolved Oxygen Sensor
Experiment 3:
Effect of Sodium Sulfite on Dissolved O2 Concentrations ......... 20
Materials and Equipment Needed................................................................................................20
Optional Materials .......................................................................................................................20
Purpose.........................................................................................................................................20
Procedure .....................................................................................................................................20
Questions .....................................................................................................................................21
Experiment 4: Biochemical Oxygen Demand * ....................... 22
Background..................................................................................................................................22
Procedure .....................................................................................................................................22
Experiment 5:
The Effect Of Respiration On Dissolved O2 Concentration ........ 23
Purpose.........................................................................................................................................23
Materials required........................................................................................................................23
Optional materials ........................................................................................................................23
Procedure .....................................................................................................................................23
Questions .....................................................................................................................................24
Appendix A: Tables.................................................... 27
Copyright Notice....................................... ...... ...... .................................. ..... ...... ..........................25
Limited Warranty....................................................... ..................................................................25
Equipment Return........................................................................................................................25
Appendix B: Tables .................................................... 27
Specifications ...............................................................................................................................26
Replacement Parts..................................... ...... .............................................................................26
Technical Support........................................................................................................................26
Appendix C: Tables .................................................... 27
Table 1: Concentration of dissolved oxygen (mg/L) in water at
various temperatures and pressures .............................................................................................27
Table 2: Salinity correction factors for dissolved oxygen in water (based on conductivity)......32
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Model No. PS-2108 Dissolved Oxygen Sensor
®
PASPort Dissolved Oxygen
Sensor
Model No. PS-2108
Equipment List
Included Equipment Replacement
Model Number*
1. Dissolved Oxygen Probe (1)
2. PASPORT Dissolved Oxygen Sensor box (1)
3. Membrane Replacement Kit (1)
4. Soaker Bottle (1)
5. Electrolyte Solution (1)
6. Syringe for filling cartridge housing (1)
699-06320
CI-6541
R001068
*Use Replacement Model Numbers to expedite replacement orders.
Introduction
The PASCO PS-2108 Dissolved Oxygen Sensor can be used to monitor and explore factors
that affect the concentrations of dissolved oxygen molecules (O2) in aqueous solutions,
particularly in applications related to ecological studies of water environments. The
Dissolved Oxygen Sensor is specifically designed for use with a PASPort interface or logger,
®
plus DataStudio
of temperature, water movement, inorganic chemicals, organic matter, and living organisms
on levels of dissolved O
ecological surveys of aqueous habitats, including BOD (Biological Oxygen Demand) studies.
The Dissolved Oxygen Sensor is designed for use in aqueous media at temperatures ranging
from 10 °C to 40 °C. For greatest accuracy, the following requirements should be met:
although the unit is temperature compensated, it should be calibrated at approximately the
same temperature as the test solution; the sensor must equilibrate for a short period when the
temperature of the test solution changes-the greater the temperature change, the longer the
period of equilibration required; the test solution should constantly flow past the membrane
of the probe.
data acquisition software. In the laboratory, students can explore the effects
. They can also monitor dissolved O2 levels in the field as a part of
2
The Dissolved Oxygen Sensor has a polarographic probe composed of a platinum cathode
and a silver (Ag) anode surrounded by a potassium chloride (KCl
*PASPORT sensors require a PASPORT computer interface.
) electrolyte solution.
(aq)
1
PASPort Dissolved Oxygen Sensor Model No. PS-2108
®
Figure 1
Oxygen molecules pass through the
semipermeable membrane into the
electrolyte surrounding the electrodes
Figure 2
O
2
molecules diffuse across the membrane and react with water
molecules in the presence of electrons from the cathode to form
hydrox ide ions (OH
-
). Hydroxide ions diffuse to the anode and react with
silver (Ag) atoms, forming silver oxide (Ag
2
O), water, and free electrons.
O22H2O4e-4OH
-
Reduction potential = 0.40V
++
The sensor functions by measuring the electric current
produced by a chemical reaction in the probe. The
chemical reaction involves the reduction of oxygen (O2)
molecules and the oxidation of the silver (Ag) atoms of
the anode. A voltage of 0.7 volts is applied across the
electrodes, causing the desired redox reaction (see
below) to be favored. When the dissolved O2 probe is
placed in an aqueous medium, such as deionized water
that contains dissolved O2, the dissolved O2 molecules
diffuse across a thin silicon membrane into the
electrolyte that surrounds the electrodes of the probe
(Figure 1). The membrane is semipermeable, allowing
the dissolved O2 to pass through it, but preventing
passage by most other molecules that might interfere
with the chemical reactions at the elect rodes. The
chemical reactions produce electrons that cause electric
current to flow through the sensor's electric circuit. Since
the rate of diffusion is dependent on the concentration of the dissolved O2, the number of
diffused O2 molecules will vary approximately in direct proportion to the concentration of
dissolved O2 in the test solution. Accordingly, the number of electrons produced by the redox
chemical reactions of the dissolved O2 will be almost directly proportional to the
concentration of dissolved O2 in the test solution.
The following is an overview of the
chemical and electrical processes at
each of the electrodes that are
involved in measuring dissolved O2
with the Dissolved Oxygen Sensor .
As soon as the dissolved O2
molecules pass through the silicon
membrane into the electrolyte
solution, they come into close
proximity to the platinum cathode
(Figure 2). The negative charge
(excess electrons) of the cathode
induces the reduction of the
dissolved O2, forming hydroxide
-
ions (OH
):
1
2
Model No. PS-2108 Dissolved Oxygen Sensor
®
4Ag 4OH
-
2Ag2O2H2O4e
-
Reduction potential = 0.343V
+++
Figure 3
Plug the DO
2
probe into the DO2 sensor box and connect the sensor box to a PASPort USB interface that is
connected to a computer’s USB port or to a USB hub that is connected to the computer.
The negatively charged hydroxide ions diffuse to the silver anode. There they combine with
silver (Ag) atoms from the silver anode, forming silver oxide and releasing electrons that join
the current in the electrode in the following chemical reaction:
The released electrons produce a current that passes from the electrode and is amplified. The
current due to the chemical reactions of the O2 molecules must be corre cted for temperature
variations, since the rate of reaction varies directly with the temperature.
The correction is accomplished through the use of a temperature-sensing thermistor that is
built into the probe. With the temperature sensing thermistor, the temperature of the probe is
monitored, and the gain of the amplifier is automatically adjusted to compensate for the
temperature dependence of the chemical reactions in the probe. A signal representing the
temperature-compensated dissolved O2 concentration of the solution is fed to the computer
interface and displayed by DataStudio in concentration (mg/l) or saturation (%).
DO Sensor Setup and Calibration
Prepare the Sensor for First Use
Prior to the first use of the sensor, you will need to fill the Dissolved Oxygen Sensor electrode
membrane cartridge and cartridge housing with an electrolyte solution. See the maintenance
section for instructions.
Set up the Dissolved Oxygen Sensor with DataStudio.
1. Attach the probe to the sensor box (Figure 3).
2. Plug the sensor box into a PASPort interface or logger connected to a computer.
3. The PASPortal window should open, allowing a choice between DataStudio or EZScreen.
Select DataStudio.
4. The Digits display opens automatically. Open additional or alternate displays as desired.
1
Find a more detailed discussion in Micha el L. Hit ch m an, M e asurement of Dissolved Oxygen, John Wiley and Sons,
New York, 1978, pp. 59-123.
3
PASPort Dissolved Oxygen Sensor Model No. PS-2108
®
Water level
Shake
vigorously
Soaker
Bottle
Figure 4
To equilibrate the probe, insert the
end of the probe into a clean soaker
bottle containing ~5 mL of deionized
water. Shake the probe vigorously for
about 10 Sec.
Equilibrate the Probe in 100% Humidified Air
1. Place about 5 mL (to a height of about 1 cm) of deionized
water into a clean soaker bottle. Slip the cap and O-ring of the
soaker bottle over the end of the probe.
2. Insert the probe into the soaker bottle and screw on the lid.
Adjust the height of the end of the probe to about 2 cm above
the top of the water (Figure 4).
3. Shake the soaker bottle vigorously for about 10 seconds.
Shake off any large water drops from the membrane.
Perform a Single-point Calibration (mg/l Dissolved O2)
1. Obtain current barometric pressure and temperature readings
for your location. You can use the PAPSPORT Barometer (PS-
2113) and Temperature Sensor (PS-2131), or the PASPORT Weathe r Sensor (PS-2154) to
take the necessary measurements.
2. Refer to the Solubility table 1 in Appendix C and find the appropriate dissolved oxygen
value for the temperature and barometric pressure at your location.
Example: At a temperature of 25ºC and a barometric pressure of 760 MM. Hg, 8.2 mg of oxygen will
dissolve in one liter of water at 10 0% saturation.
3. In DataStudio, click the Setup button on the toolbar. Make sure mg/L is selected next to
the Calibrate… button in the Dissolved Oxygen Sensor dialog.
4. With the DO2 sensor equilibrated as described above, click the Calibrate… button.
5. Refer to the tables for the value of mg/l dissolved O
in saturated water at the temperature
2
and uncorrected barometric pressure at which you are measuring. (If you do not know the
barometric pressure, assume that it is 760 mm Hg.) Enter the value in the mg/l box. Click
OK.
Setup to Calibrate in Percent (%) Saturation
1. (DataStudio only) Click the Setup button on the DataStudio toolbar. Click the mg/L button
next to the Calibrate… button and select % from the drop-down menu
Note: Always calibrate the probe before measuring absolute (rather than relative) concentratio ns. Calibrate at or
close to the temperature and barometric pressure at which the test solution is to be measured.-
Note: Percent (%) saturation output is valid only at the temperature and barometric pressure at which the probe is
calibrated.
Note: Before taking measurements of dissolved oxygen (DO) concentration (rather than % saturation), change
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Model No. PS-2108 Dissolved Oxygen Sensor
®
Cg=k Pg
your software settings back to mg/L.
DataStudio: Click the Setup button on the DataStudio toolbar. Click the % button next to the Calibrate button and
select mg/L from the drop-down menu.
Note: Calibration is not required for measurements of relative DO content, such as before/during/after
experiment relative changes.
2. Click the Calibrate… button to open the Calibrate dialog.
3. Place the probe in 100% humidified air. When the readings stabilize around a value, click
the Set button. Then click OK. It may take a few minutes for the sensor to equilibrate for
calibration. The readings may not completely stabilize at one specific point.
Calibration in 100% humidified air is equivalent to calibrating in 100% air-saturated
water. This is because the oxygen must first pass through the electrolyte solution in the
Dissolved Oxygen Sensor to get to the cathode. The concentration of oxygen in both the
100% air-saturated water and the electrolyte solution will be equivalently proportional to
the partial pressure of oxygen in air, following Henry's law where
Cg = the solubility of the gas (oxygen)
Pg = the pressure of the gas over the solution
This relationship holds true whether the end of the sensor is immersed in the water or
suspended in air.
4. Before taking measurements of dissolved oxygen concentration, change your unit settings
back to mg/L.
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PASPort Dissolved Oxygen Sensor Model No. PS-2108
®
General Sampling Procedure
1. Immerse the Dissolved Oxygen probe into the test solution until the silver temperature
compensation band is submerged.
2. Click the Start button on the DataStudio tool bar or select Monitor Data from the
Experiment menu.
3. Gently, swirl the probe for a minute or two until the dissolved oxygen readings stabilize
around a value. As an alternative, use a magnetic stirrer to slowly stir the test solution (do
not stir fast enough to entrain air bubbles or form a large vortex.
Measurements in a Controlled Laboratory Setting
In a laboratory setting, you can make long-term measurements of dissolved oxygen levels by
gently and continuously stirring the solution with a magnetic stir plate and stir bar. Minimize
the surface area of the atmosphere/liquid interface to retard gas exchange with the
atmosphere. A vessel such as an Erlenmeyer flask or a large test tube works well.
Note: DO NOT USE MINERAL OIL, as it is difficult to clean from the membrane.
Using the Dissolved Oxygen Sensor with other PASCO Sensors:
Some PASCO sensors, including the Dissolved Oxygen Sensor, emit electrical signals into
the test solution and may interfere with other sensors taking simultaneous measurements. If
you want to take simultaneous measurements with the Dissolved Oxygen Sensor and another
PASCO sensor (such as the Conductivity Sensor or pH Sensor), conduct controlled
experiments and ensure that no intra-sensor interference occurs under your experimental
conditions.
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Model No. PS-2108 Dissolved Oxygen Sensor
®
Figure 5
Removing the cartridge
housing and filling with
electrolyte.
Figure 6
Replacing the cartridge
housing.
Maintenance
Changing the Electrolyte solution
The electrolyte solution (probe filling solution) should be
periodically replaced and the silver electrode cleaned to maintain
optimal performance of the probe. If the probe is not performing
optimally, replace the electrolyte solution as follows:
1. Unscrew the end of the probe by turning it to the left, and
remove the cartridge housing (Figure 5).
Always hold the probe below the stainless steel band when
unscrewing the cartridge housing or otherwise applying torque
to the end of the probe.
2. Rinse the electrode with tap water (or deionized water in areas
with hard tap water) and rub it dry with a paper towel, removing
loose silver oxide (Ag2O) from the anode.
3. Rinse and air dry the cartridge housing.
4. Pull about 10 ml of the Polarographic solution into the syringe,
being careful not to introduce air bubbles.
5. Place the tip of the syringe very close to, but not touching the
membrane, and slowly fill the membrane cartridge and cartridge
housing to approximately 5 mm from the top of the housing.
Note: Tap the cartridge housing while filling to avoid introducing air bubbles.
6. Holding the probe in a vertical position, slip the cartridge housing
over the electrode and turn to the right to tighten (Figure 6).
7. Dry with a paper towel or tissue.
Replacing the Membrane
If the silicon membrane becomes torn or otherwise damaged, replace it
as follows:
1. Follow steps 1 through 3 under "Changing the Electrolyte Solution"
above.
7
PASPort Dissolved Oxygen Sensor Model No. PS-2108
®
Figure 7
Removing and replacing the
membrane cartridge.
A
B
Figure 8
Examine the O-ring for
damage.
2. Use the supplied plunger to push the membrane cartridge
out of the cartridge housing (Figure 7A).
3. Examine the O-ring (Figure 8) and replace it if it is
damaged (See "Replacing the O-ring" below.).
4. Insert a replacement membrane cartridge and use the
plunger to push it down until it is seated at the end of the
housing that has the O-ring (Figure 7B).
5. Fill with ele ctrolyte and reas semble the pr obe as directed
in steps 4 through 7 of "Changing the Electrolyte
Solution."
Replacing the O-ring
The O-ring should rarely if ever require replacing. However, if it develops nicks or splits and
begins to allow leakage of the electrolyte solution from the probe, replace it as follows:
1. Follow steps 1and 2 of the procedure for replacing the membrane
cartridge (above).
2. After removing the membrane cartridge, remove the O-ring with a pair
of fine-tipped tweezers and insert a new O-ring.
3. Insert the membrane cartridge as directed in step 4 of “Replacing the
Membrane”.
4. Fill with electrolyte and reassemble the probe as directed in steps 4
through 7 of "Changing the Electrolyte Solution."
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