This is part of a series on Aquaponic Water Testing
In an Aquaponic system, we have to rely on some sort of chelate to keep iron available to our plants. Without a chelator, iron is only stable at pH levels below 4. Fe-EDTA is the most common form of chelated iron found, but EDTA is only stable under acidic conditions and it’s breakdown products are phytotoxic.
Suitable chelator choices include Fe-DTPA, Fe-EDDHA, and FE-HBED. Fe-DTPA is a great iron chelate product, but it exhibits rapid photodegredation. One study reported a half life of 30 minutes in full sunlight. When I used Fe-DTPA, I found my iron levels would drop from 2.5ppm to 0.0ppm within 3 days. Since iron chelates are expensive, having to re-dose iron on a regular basis can end up being a large cost. If you are able to keep your water dark, Fe-DTPA may work for you.
The alternatives are Fe-EDDHA, and Fe-HBED. Fe-EDDHA is stable up to a pH of 10, and Fe-HBED is stable up to a pH of 12! Both of these products will stain your water slightly red.
Above:No, that’s not red wine, or kool-aid. It’s iron chelate! From left to right Fe-HBED, Fe-EDDHA, Fe-DTPA. All at 20ppm.
Below: what Fe-EDDHA looks like at 0.5, 1.0, 1.5, 2.0, and 3.0ppm concentrations
If this color was “permanent”, colorimeters/spectrophotometers would just compensate for it when you zero out the meter. Unfortunately, certain circumstances cause the pink hue to disappear mid-reaction. The can be caused by the pH going high, and the Fe-EDDHA being degraded, or by the iron being reduced or complexed with another molecule. So far, the problem has been noted with:
- Nessler based Ammonia Tests
- Molybdenum blue based Silica and Phosphate tests
- Iron tests
Interference with Iron Tests:
Most iron tests are phenanthroline based. An Iron reducing agent is introduced, and then the iron reacts with phenanthroline to cause a red hue which the photometer reads. Most tests have the reducing agent and phenanthroline combined into one packet. If you were to use a YSI Mid-Range Iron test, you would notice it had two reagents. The first reagent is the iron reducer. After adding the iron reducer, the pink color disappears from the sample. If we zero the meter at this point, then continue the test, we end up with the correct result.
If the meter is zeroed before the iron reducer is added, the pink hue is counted against the further color development. This leads to a reported value that is lower than actual.
There are a few ways to deal with this interference.
- Treat your sample blank with Sodium Hydroxide to raise the pH and destroy the Fe-EDDHA. Unfortunately, phenanthroline is only stable up to a pH of 9.0, so you can’t treat the sample.
- Treat your sample blank with Sodium Dithionite, an Iron reducing agent. Sodium Metabisulfite is much easier to find and will also work, but will requires 2x as much compared to Sodium Dithionite.
- Treat your sample blank with RoVer, an Iron reducing agent.
- Use distilled water as your sample blank, accepting that this will cause errors.
- Apply a calculated correction to your result.
- Stop using Fe-EDDHA/Fe-HBED.
My recommendation is #3 – using RoVer. It’s concentrated, and works like a charm.
Interference with Ammonia Tests
During a Nessler based Ammonia test, the pink hue of Fe-EDDHA disappears after the photometer has been zeroed. This leads to “Inverted Cuvette” errors or bad results.
Interference with Molybdenum blue based Silica and Phosphate Tests
During a Molybdenum blue based Silica or Phosphate test, the pink hue of Fe-EDDHA disappears after the photometer has been zeroed. In testing, the Hach 8048 TestNTube was not susceptible to interference.
With Fe-EDDHA, it is possible to increase the pH and destroy the EDDHA molecule. 10 drops of Sodium Hydroxide (1N solution, which can be found in the API Calcium Test, bottle 1) will raise the pH enough.
Fe-EDDHA and Fe-HBED Spectral Analysis