Bellingen Riverwatch Online Water Quality Monitoring Manual
Part 2. Can be done at the river or at home / school

2.1.1. Dissolved Oxygen - using Winkler Titration Method

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Equipment:

  • Bottle of tap water

  • Distilled water

  • Glass Dissolved Oxygen bottle (with fixed sample)

  • Glass vial with plastic lid with hole

  • Latex gloves

  • Laminated yellow card on white paper

  • Liquid waste container

  • Paper towel

  • Pipette

  • Plastic chopping board

  • Safety glasses

  • Sodium Thiosulfate

  • Spirit level

  • Starch Indicator Solution

  • Titrator (pink & green syringe)

Important - The titrator MUST have a pink tip. If it doesn't, replace it! The test will not work properly!

1. Ensure you are working on a level surface.

Place the plastic chopping board down on the table's surface and use the spirit level to adjust it until it is a level surface.

2. Rinse 20ml vial twice with a small amount of the fixed DO solution.

3. Fill the small glass vial to the 20mL white line with yellow/golden solution.

Use the pipette to finely adjust and remove/add liquid until the bottom of the meniscus is sitting on top of the white line.

4. Depress the plunger of the titrator.

5. Insert the adapter tip into the special plastic plug in the Sodium Thiosulfate bottle.

6. Invert the Sodium Thiosulfate bottle. Hold the bottle and titrator firmly together. Slowly pull out the plunger until the bottom of the large ring on the plunger lines up with the bottom of the zero (0) line on the scale.

Tip: If an air bubble appears in the titrator barrel or the adapter tip, partially fill the barrel and pump the titration solution back into the inverted reagent bottle to expel the bubble. Repeat as necessary until you have 1ml of liquid in the titrator with NO air bubbles.

8. Turn the bottle right side up and remove the titrator.

9. Insert the adapter tip into the opening in the glass vial cap. Slowly depress the plunger to add one drop at a time. Swirl solution gently after each drop.

10. Stop when the solution turns pale yellow.

Use the Laminated yellow colour card on white paper to match the shade of yellow (approximately).

11. Important - Carefully remove the titrator syringe making sure the plunger doesn't move.
DO NOT DISCARD THE CONTENTS, YOU WILL NEED THIS AGAIN.

Lay the syringe on its side, being careful not to touch the plunger and lose any contents. At the end of the test your final result will depend on the total volume left in the syringe.

The yellow solution will turn deep blue when the starch is added.

12. Add 8 drops of Starch Indicator Solution.

13. Continue adding one drop at a time of Sodium Thiosulfate and swirling each time. Slow down and go carefully from the halfway mark. Stop when the solution becomes clear.

Use the Laminated yellow card on white paper create a white background. to see when the liquid has gone clear.

14. Record mg/L. Read off the TOTAL amount of Sodium Thiosulfate USED.

This is equivalent to the mg/L of Dissolved Oxygen in the water.

Tip: How to read the pink & green syringe - each graduation is 0.2mg/L.

Tip: If you are unsure about if it is clear or not :

  1. Read off your syringe at this point.

  2. Then add another drop and see if it made a difference and looks clearer.

  3. If yes, change your record to the new reading. if no, leave the reading as is.

Reference & Illustrations : LaMotte, Company, 2021.

If you’ve use a full syringe of Thiosulfate Solution…
(skip past this for packup)

As the weather cools down over winter and there is some rain and good flow in the river, we'll see some higher oxygen levels occurring.

If you've used up all the Thiosulfate solution in the 1ml syringe, follow these steps to complete the test (make the solution clear) and to get an accurate result.

1 . If the solution is still bluish after a full 1ml syringe of Sodium Thiosulfate has been used, simply draw up another half syringe (to 5 units say) from the Thiosulfate bottle.

2. Continue to 'drop and swirl' steps in the DO glass tube with the Thiosulfate syringe until it goes clear.

3. Read off the syringe how many units of the 5 units you've used.

4. Add the full syringe used (10 units) plus the amount you've used in the additional half syringe (e.g. 2 units). This gives you 12mg/L of dissolved oxygen.

This often means that the water is supersaturated with oxygen, but can be a function of time of day, water temperature, amount of plants photosynthesing etc.

Pack Up

1. Empty all chemicals into liquid waste container.

2. Put some distilled water in a beaker. Draw up distilled water into the pink & green syringe and swirl the tip in the distilled water. Squirt into the liquid waste container. Repeat twice.

3. Wash the D.O. glass bottle three times - two rinses with tap water and then the last rinse with distilled water. If you don't have tap , do three rinses with distilled water).

4. Drain all equipment until COMPLETELY dry. Leave to dry overnight if possible. If you completed your D.O. test at the river, make sure you take the equipment back out of your kit when you get home to allow it to dry completely.

5. Return all equipment to the kit in their correct locations.

  • A concave meniscus occurs when the particles of the liquid are more strongly attracted to the container (adhesion) than to each other (cohesion), causing the liquid to climb the walls of the container.

    Source: Wikipedia, 2021