Dissolved Oxygen Testing Wonderments

Christian asks: Today, our environmental science class was in the field doing stream checks.

We have some questions about the test procedures and how they might affect the end result for mg/L Oxygen. For consistency, test groups kept the same kit from one stream station to another. Results varied between groups and this was expected, but the same group order of results occurred for all stations.

Group 1 was always higher than Group 2 which was higher than Group 3. Besides individual procedural differences, our questions about the results include these wonderments:

The precipitate - how soon before one should add the next chemical to dissolve the precipitate and the chemical crystals?
When titrating the first time, what is yellow? Is it just a turn from the reddish brown or a light yellow? Those who took the titration to a light yellow had the highest results of all the kits.
To test this, we performed a trial varying the intensity of the color of yellow before the final chemical was added turning the solution deep blue. We did seem to change the groups results in the order of all the groups.

We hope this is not too confusing and you might be able to tell us information that will help us in the performance of our field tests so that we may have the highest levels of accuracy and consistency.

From Charlie, Technical Service:

Dear Chris,

When the first two chemicals have been added and mixed in the sample, you MUST wait until the cloudiness falls BELOW the neck of the bottle, before you add the Sulfamic Acid powder (kit # 7414) OR the Sulfuric Acid (kit # 5860).
The actual shade of yellow is not important; I suggest that you titrate the sample until it turns the color of a Post-it note or legal pad. This way, the starch indicator color will be a distinct purple-blue.
Remember that after you add the starch indicator, you continue the titration using the partially filled titrator. Titration must continue where they left off (Do not refill the titrator after adding starch to the sample.)
The most common reasons for different results;

Sample collection. Bottles must be filled completely to the top while the bottle is underwater, and the cap should be screwed on while it is still underwater. Be sure that your caps have a v shaped plastic piece inside, and that they fit the bottle tightly. You should not see any air bubbles in the sample.
Not filling titrator correctly. Be sure there are no air bubbles in the titrator, and that the the plunger starts on the 0 mark. If you have the newer plastic titrators, make sure you review the procedure and determine the 0 mark which is different from the glass titrators with the black rubber tips. If you have glass titrators, make sure that the tips are not broken.
Hold bottles and titrators vertically over the sample so that the drops are the correct size.
The bottle WILL overflow when the stabilizing chemicals are added to the sample bottle during the "fixing" process. This keeps additional oxygen from entering the bottle.
Measure the "fixed" sample carefully when it is added to the titration tube. Remember to measure from the bottom of the meniscus.
During the titration, after starch is added, you should swirl after each drop.
Make sure that the endpoint occurs when the sample becomes completely clear. Use a piece of white paper to help see color change.

Dissolved Oxygen- Why starch it?

Steven from Texas says this is a common question from his monitors: "Is it OK to add starch at the beginning of the dissolved oxygen titration?"

From Tim, Technical Service Supervisor, Sodium thiosulfate is added to the dissolved oxygen sample until it becomes a faint or straw yellow color. A starch indicator is added at this point. Titration is continued until the endpoint, which occurs when the solution turns clear. The titration is actually titrating iodine from yellow to clear. Since the yellow to clear change is very hard to see, we add starch, which turns blue in the presence of iodine. Once all the iodine has been titrated out, the starch goes clear. The blue to clear is much easier to see than yellow to clear. The reason that we titrate some of the iodine out by titrating to faint yellow before adding starch is two-fold:

The starch stays dark blue right up until it goes clear, unlike most titrations where the color gradually moves toward the endpoint. Therefore, it is easy to become complacent during the titration and overshoot the endpoint, thus artificially increasing the DO value. You only want to add drops or half drops for a short period of time before the endpoint.
Starch can be partially decomposed by a large amount of iodine. Therefore, the starch should not be added until the bulk of the iodine has been reduced (titrated out).

P.S. Steven's monitors only average dissolved oxygen readings that are within 0.5 mg/L of each other. If the results are not within 0.5 mg/L, the test must be run again.

	Downloadable Instruction for 0377 in PDF format (Titrator)
	Downloadable Instruction for 5860 in PDF format (DO Liquid)
	Downloadable Instruction for 7414 in PDF format (DO Powder)
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