The Chemistry Section: Dissolved Oxygen

The LaMotte Dissolved Oxygen kit

Equipment List Chemicals
1) Manganous Sulfate Solution
2) Alkaline Potassium Iodide Azide
3) Sulfamic Acid Powder
4) Sodium Thiosulfate
5) Starch Indicator solution
Glassware
1) Direct Reading Titrator (0 - 10 ppm)
2) Titration Tube, 20 mL
3) Water Sampling Bottle

The Chemistry

Step 1.

Rinse the sampling bottle and replace the cap. This is to clean out the bottle and make the sampling as uncontaminated as possible. Then submerge the bottle, remove the cap, allow the bottle to fill, and replace the cap. The cap has a cone-shaped plastic piece inside it which is designed to prevent air from being included in the sampling by forcing all of the bubbles out of the container. Air bubbles in the container will throw off the test. If you have bubbles, redo the sampling procedure until your bottle is bubble-free.

This means... When you take a sample from your water source and cap it up, you begin the process of measuring the oxygen in your water by closing your sample off from the atmosphere. Screwing the top on forces air bubbles containing oxygen out. If these bubbles remain in the bottle, they may cause the test kit to indicate more oxygen is present than may actually be there.

Step 2.

Add 8 drops of Manganous Sulfate Solution. Add 8 drops of Alkaline Potassium Iodide Azide, and allow the floc time to settle to the bottom of the bottle.

What is going on in this step:

The first chemical allows the addition of manganese (Mn(II) in a form which can easily react later under the right pH conditions to form Mn(IV). For the time however, no reaction occurs in the bottle.
The Alkaline Potassium Iodide Azide raises the pH of the solution to high enough levels that the manganese in the MnSO₄ (in the form Mn²⁺ in solution) reduced the O₂ molecules present in the solution, forming a solid manganese oxide. The manganese oxide precipitates out, and settles to the bottom of the container forming the brownish floc you see. For the time being, the Iodine and Azide components of the chemical will remain unreactive. The potassium is relatively unimportant to the reaction.
Allowing the floc time to settle in the bottle ensures that the chemical reaction occuring in the bottle has time to reach completion. Starting the next step before settling completes may lead to an inaccurate measurement of the amount of dissolved oxygen actually present. This process is sometimes called fixing the oxygen. In order for this fixation process to be completed, however, another reaction must take place. We need another reagent to make this occur...

Step 3.

Add 1 g. of Sulfamic Acid Powder

What does this do?
In appearance, the solid in the bottom of the bottle has dissolved, leaving a brownish solution. By creating a lower pH environment, the Sulfamic acid has caused the MnO₂ to oxidize the Iodide (I^-) molecules leaving 2 I₂ molecules and an Mn²⁺ molecule. For every four molecules of sulfamic acid added, one manganese oxide molecule is converted to an Mn²⁺ molecule, and 2 I^- molecules are converted to I₂.

MnO₂ + 4H⁺ + 2I^- = Mn²⁺ + I₂ + 2H₂O

We say at this point, that the oxygen is fixed. This means that all of the oxygen from the original sample which was in solution has now been chemically modified to a form which won't change when exposed to the air. It is now in a stable form, and can be transported back to a classroom for analysis if necessary. (For a more in-depth explanation, see the Winkler method titration page.)

Step 4.

Fill titration tube to 20 mL line with "fixed" sample. Cap.
Fill titrator with Sodium Thiosulfate. Add one drop at a time to sample, swirl between each addition until color is a very faint yellow.

What does this do?

As you add Sodium Thiosulfate (NaS₂O₃^2-), the Sodium (Na) dissociates (separates) from the thiosulfate (S₂O₃^2-) molecule. The thiosulfate molecule then reacts with any I₂ present.

2S₂O₃^2- + I₂ = 2S₄O₆^2- + 2 I^-

When the iodine (I₂) molecules react, they break up into I^- ions which are colorless.

What does this tell us about the amount of oxygen in the water? Stoichiometry (a fancy word meaning the chemical book keeping of the amount and concentration of chemicals in a reaction) tells us that 4 molecules of the Sodium Thiosulfate are required to change the color resulting from one molecule of O₂ in the original water. This clear definition allows us to get a very accurate estimate of the number of O₂ molecules in the original solution.

I hope that this brief report has stimulated some questions. Feel free to send email to us!
(roger@hwr.arizona.edu),
(martha@hwr.arizona.edu,
or to our assistant, Chris Gutmann (cgutmann@hwr.arizona.edu).

Sincerely,

Roger Bales & Martha Conklin

University of Arizona