Dissolved Oxygen

Dissolved oxygen is important as all aquatic animals need it to breathe underwater. Furthermore, oxygen concentrations affect the behaviors of other chemicals in the water.

The State of Utah has set minimum dissolved oxygen concentrations to protect fish and other aquatic animals. In addition, different species and different activities require different concentrations of oxygen. You can find the water quality standards as part of the Utah Administrative Code  R 317-2. For more information on dissolved oxygen read Understanding Your Watershed: Dissolved Oxygen.

For instructions on how to use the dissolved oxygen kit, watch the video below or follow these steps. You will also need to record barometric pressure right before or after monitoring your site.

Steps

Illustration

1. Collect Water Sample

  1. Rinse and discard the Water Sampling Bottle 3 times with the lake or stream water you're sampling.


  1. Put the lid on tightly and submerge the bottle to the desired depth. Try to sample at least 15 cm (6 in.) below the surface.

  1. Remove the lid and let the bottle fill.

  1. Tap the sides of the bottle to free any trapped air bubbles.  Place the cap on the bottle while it is underwater.

  1. Bring the bottle out of the water and make sure there are no air bubbles. If air bubbles are present repeat the collection steps.

  

2. Add the Reagents

NOTE: Make sure to wear gloves

  1. Remove the lid from the bottle and immediately add 8 drops of Manganous Sulfate Solution and 8 drops of Alkaline Potassium Iodide Azide. It is important to hold the bottles vertically to ensure the drops are a consistent size. Do not let the reagent bottle tips come in contact with the sample.

  1. Put the lid on the bottle and mix by inverting several times. Make sure there are no air bubbles. A yellow-brown color and precipitate should form. This indicates the presence of oxygen.

  1. Wait for the precipitate to settle below the shoulder of the bottle. If it does not settle after three minutes, shake 10 times and wait 2 minutes, then proceed, settled or not.

  1. Take the lid off the bottle and add 8 drops of Sulfuric Acid to the solution. The reagents will sink to the bottom, so don't worry about spilling over as this will not affect the test.

  1. A rust colored precipitate or flecks should form.

  1. Put the lid on and mix by gradually inverting the bottle until the precipitate and reagent have completely dissolved (this may take up to 10 minutes).

  1. If there are any flecks remaining that are not brown or rust colored it's possible that suspended sediments or organic matter (e.g., leaves or pollen) are present in your sample. When pouring sample out of the bottle into the titration vial, avoid pouring out the organic material. The solution will be clear yellow to orange if dissolved oxygen is present.

  1. If the flecks are still brown or rust colored after 10 minutes of inverting add 2 additional drops of sulfuric acid to the sample. Put the lid on and continue inversions for approximately 2 more minutes. If the precipitate remains, you may add additional sulfuric acid drops, one at a time, inverting the sample for approximately 2 minutes between drops. Do not add over 8 additional drops.

NOTE: Now the sample has been "fixed" and contact between the sample and the atmosphere will not affect the results. If desired, samples may be held at this point and titrated up to 4 hours later. In this case, keep the sample cool (cooler with wet ice) until you are ready to continue.

3. Titration

NOTE:  You need to repeat these steps twice in order to meet our Quality Control standards by repeating a duplicate sample.
  1. Rinse the titration tube with 5 mL of the fixed sample and discard into a waste container.

  1. Carefully fill the titration tube with the fixed sample to the 20 mL line and cap the tube.

  1. Push down the plunger of the titrator and insert the titrator into the plug on the top of the Sodium Thiosulfate, 0.025N (4169) titrating solution.

  1. Invert the bottle and carefully withdraw the plunger until the large ring on the plunger is opposite the zero on the scale.

NOTE: If air bubbles appear in the titrator barrel, free them by partly filling the barrel, then pump the titration solution back into the reagent container. Repeat until there are no more bubbles.

  1. Place bottle in the upright position and remove the titrator. If the sample is a very pale yellow, skip the next three steps.

  1. Insert the tip of the titrator into the opening of the titration tube lid.

  1. Gradually depress the plunger to add the titrating solution a drop at a time until the color changes from yellow-brown to a very pale yellow color. Carefully swirl the tube between each drop for the titration to mix.

 
  1. Carefully remove the Titrator and lid. Do not disturb the Titrator plunger.

  1. Add 8 drops of Starch Indicator Solution. The sample should turn blue.  Gently swirl to mix evenly.

  1. Cap the titration tube and insert the tip of the Titrator into the opening of the titration tube lid.

  1. Keep titrating until the solution becomes colorless. If the plunger ring reaches the lowest line on the scale 1.0 mL (10 ppm) before the solution becomes colorless, refill the Titrator and continue titration. Record the value of the original amount of reagent dispensed.

 
  1. Read and record the results from the scale where the large ring on the Titrator meets the Titration barrel (in ppm Dissolved Oxygen). Each minor division on the Titrator scale equals 0.2 ppm.

    Empty any remaining Sodium Thiosulfate into the waste container.
  1. Repeat these steps for the duplicate sample and then dump the contents of the waste container down a sink drain with running water.