Introduction: Measure Nitrate Concentration in Water With EOS 1

By Erie Open Systems
About: Erie Open Systems is the maker of EOS 1 (Erie Open Spec v1.0), an open-source spectrometer for measuring nutrient concentrations in water

This is a brief step-by-step instruction on how to use the EOS1 for measuring nitrate concentration in water. Similar steps can be used for measuring phosphate (a different test kid is required).

Step 1: Preparation: Check You Have Everything Ready

What you will need for this measurement:

  • EOS 1 spectrometer
  • your smartphone
  • water sample to be measured (more than 2mL)
  • API freshwater nitrate test kit (available on Amazon and other online stores)
  • disposable pipette with volume marks (for example, you can get a bag on 100 on Amazon)
  • disposable test tube with lid (for example, you can get a bag of 100 on Amazon)
  • [not shown in the picture] optically clear standard cuvette (for example, you can get a bag of 100 on Amazon)

Step 2: Take 2.0 ML of Water Sample

  • Check and make sure the water sample is clear. If not, use a centrifuge to sediment suspended solids first, and take only the clear liquid as sample.

  • Use the volume markers on the pipette to transfer (as accurate as you can) 2.0 mL of water sample to the test tube.
  • Double-check the volume of the water sample with markings on the test tube.

Step 3: Add 4 Drops of Reagent #1 (from Nitrate Kit)

  • Hold the bottle such that it's pointing straight down to ensure consistency on the size of droplets.
  • Carefully add 4 drops. Only squeeze the bottle lightly to let the droplets come out slowly (again to ensure consistency on the size of droplets).
  • Reagent #1 as marked on the bottle. The reagent should be yellow in color.
  • Shaking the bottle for 30 seconds before adding the reagent may help improve the consistency of the test results.

Step 4: Shake for 1 Minute to Mix

  • Make sure reagent #1 is evenly mixed with the water sample.
  • After mixing with reagent #1, the sample should turn yellow and stay that color until reagent #2 is added. However, it has been observed that some water samples may turn back to clear after a few seconds. In those cases, the nitrate test does not work

Step 5: Shake Reagent #2 for 1 Minute

  • The active component of reagent #2 may sediment. Therefore, it is crucial to ensure the evenness of the reagent before adding to the sample.

Step 6: Add 4 Drops of Reagent #2 (from Nitrate Kit)

  • Hold the bottle such that it's pointing straight down to ensure consistency on the size of droplets.
  • Carefully add 4 drops. Only squeeze the bottle lightly to let the droplets come out slowly (again to ensure consistency on the size of droplets).
  • Reagent #2 as marked on the bottle. The reagent should be clear.

Step 7: Shake for 1 Minute to Mix

  • After mixing with reagent #2, the sample may stay yellow the nitrate concentration is low, or it may turn orange or red (the higher the nitrate concentration, the more red it will turn).

Step 8: Wait 5 Minutes

  • Timing should start right after adding reagent #2 (i.e., the 5 minutes include 1 minute of shaking as mentioned in the previous step).

Step 9: Transfer Sample to a Cuvette

  • This step may take up to 1 minute.

Step 10: Put Cuvettes of Sample and Clear Water Into EOS1

  • Place the EOS1 such that the side with the LED faces towards you. Then place the reference cuvette (with clear water) into the left-hand-side slot, and the sample cuvette (with colored sample) into the right-hand-side slot. This is important for image analysis.

Step 11: Close the Cover and Turn on the LED

  • After this step, if you look closely into the hole on the top plate of EOS1, you should see two spectra (i.e., rainbows) such as shown in the picture.

Step 12: Take a Picture of the Spectra With Smartphone

  • Place your smartphone on top of EOS1. Align the phone camera with the hole on the top plate.
  • After the two spectra can be seen in the camera viewer, adjust the phone so that the two spectra align with the vertical axis of the picture (which should be a portrait image). Misalignment can cause measurement inaccuracy.

Step 13: Analyze the Picture With Our Python Code

  • Go to our Github page to find the Python code for image analysis, or use our Android app (coming soon).
  • If this is the first time you are doing this image analysis, please read this IPython (Jupyter) Notebook. It explains how the image analysis code works.
  • Assume you already have the "ImgAna_minimum.py" or "ImgAna_aligncheck.py" script (download from here: https://github.com/jianshengfeng/EOS1), you can either run it as a Python script (i.e., "python ImgAna_minimum.py"), or use it as a Python module (i.e., "import ImgAna_minimum") and get access to the EOS1_Img class.
  • If you run it as a Python script, you will first be asked to perform/update calibration (recommended). If you do a calibration, a calibration plot will be generated for your reference, also a calibration record "nitrate_calibration.csv" will be generated (overwritten if already exists).