Winkler and Vernier methods

In the Winkler method, we used titration to learn how much dissolved oxygen was in the sample.  We filled the container to the top with water, making sure that there weren’t any air bubbles.  (There always seemed to be a bubble that didn’t want to leave the container, so this part was harder than it sounds.)  We then added alkaline iodide and manganous sulfate to the water sample.  This produces manganous hydroxide.  Upon acidification, the oxygen in the sample converts this to a manganese compound.  This reacts with the iodide and colors the water yellow.  Then, using titration we can tell the amount of iodine.  We did this by adding sodium thiosulfate until the sample is once again clear.  This method is .1 to .6 percent precise.

Then we performed the Vernier method.  Using a handheld measuring device we measured the carbon dioxide, dissolved oxygen, pH level, temperature, turbidity, and nitrates of water.  This method is easier and more accurate.  Using a computer to measure the data cuts down on human error.  The two methods are similar in the fact that we were able to accomplish the same experiment of measuring the dissolved oxygen plus much more.  They are different because of the ease of use and the ability to do more things with the Vernier.  Also, the Vernier method makes it much easier to do the experiment right at the ecosystem, while the Winkler method is much more inconvenient to do on scene.  Doing the experiment right away cuts down on sources of error.  We encountered one of these sources of error.  The water sat for three days in the classroom.  That will definitely change the data.  If we were on scene with the Vernier we would have taken data quickly and never have ran into this source of error. 

While on the topic of sources of error, there was also a source of error if we stirred up the water while trying to get the air bubbles out of the container.  Also when titrating if it didn’t turn yellow that means that the data was 0.  Since we didn’t know this in the beginning of our experiment, when it didn’t turn yellow we just added more of the solutions, which can definitely distort the data.