Photometer and reagent ammonia tests

If you are using a Photometer or Reagent test that measures NH3-N or TAN then care needs to be taken to understand exactly what is being measured. There are a number of test methods that measure "Ammonia" & NH3 however, it is not always as simple as reading the value.

Is an Ammonia or NH3 test kit reporting the same NH3 value as seneye?

In short, No:

Why?

Many tests use the “indophenol method” for measuring the ammonia by drastically increasing the pH of the sample. The movement in pH means that all of the ammonium (NH4) in the sample will be liberated to the NH3 form. The NH3 then reacts with the chemistry in the test to create the colour change that is measurable by the photometer or eye. The value that is outputted is either TAN or NH3-N.

This method of testing means that the NH3 level that is being measured is true of the sample after the pH increase. However, the value tells you nothing of the actual NH3 in the aquarium because the pH of the sample is not the same as the aquarium water. The actual amount of NH3 in the aquarium will be dependent on the factors explained on this page. 

Often these tests allow you to use multiplication values to find NH3 or NH4 from the NH3-N value reported. This is misleading as the multipliers only allow you to put a numeric value on if all of the TAN was NH3 or NH4, not the ratio between them that is actually present in the aquarium water.

Can we do a comparison between the output of the seneye and these tests?

Yes, but caution should be taken. As a guide you should be able to take the TAN value and use a free ammonia calculator which takes into account pH, temperature, salinity and the TAN value. These calculations are based on exact chemisty and as such any minor error in the measurements of the input data will have a drastic effect on the calculated NH3 value. 

This is why seneye measures NH3 directly.