How to test for dissolved oxygen in freshwater with the Chemetrics testkit

What is it? This is the amount of gaseous oxygen dissolved in the water. Oxygen gets into the water in various ways, including absorbing it from the atmosphere, by rapid movement of the water, or as a product of photosynthesis.

Why is it important? Oxygen is what gives the water life! Fish and plants alike depend on certain levels of oxygen to survive!

What does it mean? The concentration refers to the amount of oxygen (milligrams) dissolved in every litre of water. Dissolved Oxygen changes over the seasons. Both temperature and the depth of your sample may change the levels. For the best results, trends throughout the year should be compared to previous years.

Understanding basic levels

0-2 mg/L: not enough oxygen to support life
2-4 mg/L: only a few fish and aquatic insects can survive
4-7 mg/L: good for many aquatic animals, low for cold water fish
7-11 mg/L: very good for most stream fish

Learn more

How to test

We use the Chemetrics dissolved oxygen testkit. It is the most user-friendly (no measuring chemicals) and is easier to interpret than other tests we’ve tried! It measures dissolved oxygen in milligrams/litre as a concentration. While a probe will give you more fine-grain results, we haven’t found inexpensive ones that we thought were as good as this test (plus, they need calibration every time you use them).

Take out an ampoule and your sample cup. Notice the ridges in the bottom. Notice the white tip on the ampoule. This is a weak point in the glass.
Take a water sample as far below the surface as you can reach. Do not shake. It is more important to get the sample quickly than to get exactly 25mL.
IMMEDIATELY put the ampoule, tip down, into the sample. Break the ampoule’s tip off using one of the ridges at the bottom of the sample container by pressing against the ridge and pushing inward. It takes less pressure than you think!
The ampoule will fill with water. Once mostly filled, remove from the sample and invert two times (watch the bubble travel up/down two times).
Hold the tip facing upwards for 2 minutes (sand timer/stopwatch).
Take out your comparison chart. Place the ampoule flat between the comparison chart until you and your testing companion agree on the reading. If it’s between two values, take the halfway point.
Record your results. Carefully pour water out, leaving the glass tip in your sample cup. Remove the tip and used ampoule and put them in a disposal cup. When you return home, you can dispose of the waste in regular garbage.
Pack up your kit. Do not expose the comparison chart or ampoules to light except while conducting the test since the dye is affected by sunlight.