Five drops from one bottle, five drops from another, and within minutes, a colour is generated revealing a previously unknown property of your aquarium or pond’s water. The secret chemistry that happens within a test tube appears mysterious and prone to misconceptions. Let’s explore some of these ideas and see what’s going on:
Nitrite tests from different manufacturers measure nitrite differently.
FALSE: The underlying chemistry used to generate a colour that intensifies as the compound of interest increases is largely the same, whether it’s an NT Labs test or any other. There can be subtle tweaks in formulations to slightly alter performance, but the fundamental chemical reaction is the same.
NT Labs Nitrate Test measures nitrogen whilst some others measure nitrate ion.
FALSE: Like the first answer, the chemistry is the same. Where there is a difference is how the concentration is reported. It’s similar to measuring length; you can report the physical length of an object in centimetres, inches, metres or yards, but the methodology of measurement is still the same.
When you use an Ammonia/Nitrite/Nitrate NT Labs Test Kit, you need to multiply the result to convert it.
FALSE: You only need to perform any numeric conversion on a result if you need to know the concentration of the ion of interest, but this is rarely required. In the case of nitrate for example, this is the difference between NO3-N and NO3-. The former reports the mass of nitrogen as a concentration (as if the ‘O3’ bit does not exist), while the latter considers the mass of the entirety of the compound (technically, only half of the compound as these negative ions cannot exist in isolation). The former is preferred for three reasons. Reporting as NO3-N is entirely unambiguous in what is reported, whereas NO3 does not share the same clarity. When comparing a nitrate concentration to known toxicity values, these toxicity values are usually in terms of NO3-N; like-for-like comparisons are needed to know if your water is safe. Finally, reporting in terms of nitrogen makes working with these numbers much easier. For example, 14 mg of NH4-N will be converted by your filter to 14 mg of NO2-N, then to 14 mg NO3-N; the actual amount of nitrogen doesn’t change, irrespective of its form. If you use the actual molecular weight of the ions, it turns out that 18 mg of ammonia will be converted to 46 mg of nitrate ion, then converted to 62 mg of nitrate ion! Both examples are correct, but reporting in terms of nitrogen is much more comprehensible.
All liquid-based ammonia tests measure total ammonia
TRUE: This is certainly the case where the test uses the “indophenol blue” method or its variants. Ammonia (NH3) in water coexists with its ionised form, ammonium (NH4+), and one can readily change into the other and back again: a phenomenon known as ‘chemical equilibrium’. Like an old-fashioned set of scales, the position of this equilibrium is determined by the pH and temperature of the water. Alkaline pH and higher temperatures tip the balance in favour of free ammonia, while acidic conditions and lower temperatures favours ionised ammonium. However, when running the test, which requires strongly alkaline conditions to work, all of the ammonia is converted to the NH3 form. Consequently, this type of test will measure total ammonia. In conjunction with the pH and temperature, it is possible to calculate the concentration of the harmful free ammonia in your pond or aquarium. However, in practice, any positive total ammonia concentration reading indicates a problem and will most likely result in an elevated nitrite concentration shortly after.
Phosphate in a reef aquarium should be 0.03 mg/L PO4-.
FALSE: But the devil is in the detail. Ideal phosphate concentration in a reef aquarium should be 0.03 mg/L as phosphorus (or using the convention as above, PO4-P), which means that phosphate ion concentration should be about 0.1 mg/L as phosphate ion. However, phosphate ions are a little more nuanced; phosphate ions that can lead to algae growth or affect coral health can take numerous forms, with varying quantities of phosphorus and oxygen bound together. It would therefore be wrong to assume that all phosphorus in a water sample is in the form of PO4, but we can all agree on the total amount of phosphorus present when we report it as phosphorus (PO4-P). It would be like trying to work out the number of vehicles on a road; you could count all of the wheels belonging to cars and divide by 4, count all of the wheels belonging to motorbikes and divide by 2 and add the two together
A KH test that reports dKH can be converted to mg/L.
TRUE: each dKH (or degrees carbonate hardness) is equivalent to 17.85 mg/L, but here’s the important bit: as calcium carbonate (CaCO3). Again, this isn’t to imply that 1 litre of water with 1 dKH contains 17.85 mg/L calcium carbonate, but it is equivalent to having this amount dissolved in water. Many minerals contribute to the effect we call carbonate hardness, such as carbonates, bicarbonates, even hydroxides. Only sophisticated high-tech testing would be able to accurately measure each of these components separately, and it is lucky we don’t require that information. Consequently, we can simplify and effectively pretend that it all originates from calcium carbonate. For the fans of chemistry, CaCO3 also handily has the nice, round atomic weight of 100 g/mole!
Conclusion: The hobby of fishkeeping is unique amongst pet ownership in that it allows a fishkeeper, if they so wish, to explore a lot of the science that governs the habitat of our fish. That’s not to say it’s a must – our test kits are easy to use with simple step-by-step instructions, complete with colour charts showing what’s acceptable or what requires action. If you’re new to fishkeeping or maybe intimidated by the chemistry, it’s usually sufficient to know that ammonia and nitrite should be zero, nitrate should be as low as your source water allows, the pH is correct and stabilised by a good level of KH. Equally, if you want to enjoy the science and get stuck into the numbers, understanding these measurement conventions can really help put these concentrations into a meaningful context.
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