Information Sheet Dissolved Oxygen Background Dissolved oxygen (DO) is the small amount of oxygen gas dissolved in the water. It is essential for the respiration of fish, aquatic animals, micro-organisms and plants. To maintain a healthy and diverse aquatic ecosystem, the dissolved oxygen must be maintained at high levels. If DO falls, there will be reductions or losses in the more sensitive species. At low DO levels, only a very few hardy species may be present. So DO levels are a useful indicator of general water quality. DO levels in natural waters depend on three factors:1. Speed of uptake of oxygen into the water from air. This depends on agitation at the water's surface. Shallow flowing streams usually have high oxygen levels while stagnant pools have reduced oxygen. 2. Speed that oxygen is used up in the water. Bacteria will use up oxygen in water. If large amounts of organic matter enter the water course from point sources such as sewage plants and animal feedlots, there will be a high bacterial population and so greater oxygen usage. 3. Photosynthesis of plants and algae.

Aquatic plants and algae

release large amounts of oxygen into the water in daylight hours as a bi-product of photosynthesis.

Peak DO levels occur in early

afternoon and minimum levels before sunrise. Since plants produce oxygen during the day through photosynthesis and use it up at night with respiration, the time of the day when the sample is taken should be kept fairly constant and noted on the record sheet. Oxygen levels are also affected by the temperature of the water. Scientific studies suggest that 4.5mg/L DO is the minimum amount that will support a large and diverse fish population. The DO level in good fishing waters generally averages about 9mg/L. When DO levels drop below about 3mg/L, even the hardy fish die. Note: A dissolved oxygen test tells us precisely how much oxygen is dissolved in water, but it does not indicate how much DO the water is capable of holding at

the temperature of the test. The percentage saturation is a better measure of the availability of oxygen to aquatic organisms.

Sampling Procedure In slow moving streams, make sure to sample away from the shore and midway between the surface and the bottom. In free flowing streams, any sample taken below the surface will be representative.

Instructions - Merck DO Kit Equipment Distilled or deionized water Safety gloves and safety glasses Merck dissolved oxygen kit

Method 1.

Rinse the oxygen reaction bottle with the water to be tested.

2.

Fill the bottle to overflowing by submerging the stoppered bottle in the water body and remove the stopper under water.

3.

Add 5 drops of Reagent 1 and 5 drops of reagent 2. Close without trapping air bubbles with the slanted stopper and shake for approximately 30 seconds.

4.

Add 10 drops of reagent 3. Reseal and shake well again.

5.

Rinse the test tube with a small amount of the resultant reaction solution and fill to the 5 ml mark.

6.

Place the test tube on the colour card and match the colour to the closest standard colour on the card. You should attempt to estimate the concentration if the colour match appears between the colour standards provided.

7.

Report the concentration of dissolved oxygen as mg/L on your result sheet.

8.

To convert the mg/L result to % saturation using the conversion graph, mark the temperature of the test sample on the temperature

scale.

Mark

the

DO

level

in

mg/L

on

the

concentration scale. Connect the two points and note the value where the line crosses the % saturation line. 9.

Report the percentage saturation of dissolved oxygen on your result sheet.

Instructions - Visocolor ECO DO Kit Equipment Test kit Safety glasses Safety gloves. Stoppered glass bottle (rinsed with water to be tested).

Method 1. Pour a 1ml water sample into on of the measuring glasses and place it on position A in the comparator. 2. Rinse the oxygen reaction bottle several times with the water to be tested and fill until it overflows without air bubbles. 3. Add 5 drops of O 2 -1 4. Add 5 drops of O 2 -2, close the bottle with the stopper (avoid air bubbles) and mix by shaking. 5. After 1 min add 12 drops of O 2 -3, close the bottle and shake well until the deposit is dissolved. 6. Pour 1ml of the resultant reaction solution into the second measuring glass and place it on position B in the comparator 7. Slide the comparator until the colours match in the inspection hole on top. Check the measurement reading in the recess on the comparator reed. Mid-values can be estimated. 8. After use, rinse out the oxygen reaction bottle and both measuring glasses thoroughly and seal them. 9. Record the reading on the Physical and Chemical Test Record Sheet.

Instructions - Cyberscan DO300/310 Meter Equipment Distilled or deionized water Safety gloves and safety glasses Cyberscan DO 300 dissolved oxygen meter

Method 1. Switch on the Cyberscan meter. 2. Calibrate meter to 100% saturation in air. See below. 3. Rinse the probe with distilled rinse water. 4. Select the desired measurement mode by pressing the MODE key to move between modes: a. % Saturation b. mg/L O 2 c. date/time clock 5. Dip the probe into the sample. When the reading is stable, the READY indicator will appear. 6. Report the concentration of dissolved oxygen as mg/L or % saturation on the result sheet.

Calibration Air Calibration Note: The exact calibration value depends on barometric pressure. The meter is set to a factory default of 760 mm Hg. See instrument manual to change the pressure setting.

To calibrate to 100% Saturation: 1. Switch on the Cyberscan 300. 2. Select the desired measurement mode by using the MODE key to toggle between modes. 3. Plug the Dissolved Oxygen sensor into the sensor socket. 4. Rinse the DO sensor in distilled water and blot dry. Do not touch the membrane. 5. Press the MODE key to select the % saturation mode. 6. Hang the DO sensor pointing downwards in air. 7. Press the “CAL” button to do an air calibration. 8. Allow the reading to stabilize. The READY indicator will appear when the reading is stable. 9. Press the ENTER key. Note: Since the probe consumes oxygen from the sample, the sample must constantly flow past the membrane for accurate and stable readings.

Probe Care and Maintenance The probe consists of two parts:

 An upper part consisting of an anode, a cathode and cable  A lower part made up of a membrane cap, membrane and electrolyte solution. Oxygen diffuses through the membrane onto the cathode. This produces an electrical current that flows to the meter. The current is proportional to the oxygen that passes through the membrane and the layer of electrolyte.

Maintenance of Sensor Under typical operating conditions, the probe should last for several years. After using the probe, rinse it in clean water and wipe it with a soft cloth or paper to avoid any deposits. The membrane does not require replacement so long as it remains intact. If the membrane is punctured, or is leaking, it must be replaced. Oils, fats and other greasy substances clinging to the cell will reduce the sensitivity of the sensor. Replace the membrane in this situation rather than cleaning.

Storage of Sensor There are no special storage requirements for the probe.

Replacing the Membrane Replace the membrane only when the probe cannot be calibrated. Typically membrane problems involve puncture or wrinkles caused during measurements or cleaning. 1. Pull off the probe guard. 2. Unscrew the membrane cap from the probe. 3. Hold the probe under running water and brush away the white oxide on the cylindrical anode with a stiff plastic brush. 4. Using the installation tool, unscrew and remove the membrane lock from the membrane cap. 5. Remove the membrane and O-ring. Discard. 6. Rinse the membrane cap and lock in tap water 7. Install a new O-ring inside the membrane cap. 8. Install a new membrane, making sure that it covers the O-ring all around its circumference. 9. Using the installation tool, screw the membrane lock back into the cap. 10. Tighten the lock firmly over the membrane and O-ring. Do not overtighten. 11. Inspect the membrane for wrinkles. 12. Fill the cap with water to test for leaks. 13. Fill the membrane cap with electrolyte to the brim. 14. Screw the cap onto the probe and replace the probe guard. 15. Calibrate the probe. Note: The membrane can only be used once as it is stretched by the cathode.

Checklist •

During measurement, the probe can be fully or partially immersed in the solution.

•

Do not allow the probe’s membrane surface to touch anything. Leave the probe guard attached to the probe at all times.

•

Since the probe consumes oxygen from the sample, the sample must constantly flow passed the membrane to achieve accurate readings. A stirrer may be used to achieve this.

•

Calibrate the instrument to 100% saturation in air before taking any measurements. Re-check that the instrument is measuring 100% saturation in air after the first sample.

•

A zero calibration should be performed monthly. (See Manual for zero calibration instructions)