NOTES ON NITROGEN TESTS (Dustin Laurence, laurence@alice.caltech.edu) Tests for nitrogen compounds (ammonia or NH3, nitrite or NH2--, and nitrate or NH3-) measure the substance in question on one of two scales; ion concentration or nitrogen-ion concentration. Concentration is generally expressed in ppm (parts per million). Ion concentration is then written as ppm NH3, ppm NO2--, or ppm NO3-, and nitrogen-ion concentration is written as ppm N-NH3, ppm N-NO2--, or ppm N-NO3-. (When these symbols are typeset in a book or instruction sheet, the number are written as subscripts below the letters and the minus signs which follow NO2 and NO3 are written as superscripts; the charges (minus signs) are often omitted entirely.) The difference between these measures is not difficult to understand, but it can cause a lot of confusion because it is often not obvious which measure that a particular author or test kit refers to. First, the bottom line for those who don't want to know the details: [N-NH3] (ammonia-nitrogen) = 0.8 [NH3] (ammonia ion) or [NH3] = 1.3 [N-NH3] [N-NO2--] (nitrite-nitrogen) = 0.3 [NO2--] (nitrite ion) or [NO2--] = 3.3[N-NO2--] [N-NO3-] (nitrate-nitrogen) = 0.23 [NO3-] (nitrate ion) or [NO3-] = 4.4 [N-NO3-] Where [] indicates concentration in parts per million (ppm). Ammonia and nitrite concentrations should be too low to measure with hobby equipment in a cycled aquarium, so the distinction between the two measures is rarely useful to the amateur hobbyist; 1.3 times zero is still zero! However, nitrate concentrations will in general not only be nonzero but increase over time, and it is important for the aquarist to monitor their levels as necessary and to know what levels are acceptable for her tank. Therefore, you can usually buy relatively cheap ammonia and nitrite kits and spend more on a good nitrate kit. One general rule of thumb is never buy a kit that asks you to compare the color of a clear liquid to a color on an opaque chart; comparisons of colors of vastly different optical depths are very hard to estimate. Good kits give you a colored transparency for the comparison. You can't trust the zero of many kits! Do a test on distilled, reverse osmosis, or de-ionized water; often you will get a false reading. Subtract this reading from each test of your aquarium water for a somewhat better estimate of what the true levels are. Most aquarium authors give recommendations for acceptable nitrate levels in terms of nitrate ion concentrations. Recommendations for marine tanks range from 10 to 40ppm nitrate ion for fish-only tanks and <5 to 10ppm nitrate ion for reef systems, though many fish-only tanks run at much higher levels (sometimes with no ill effects, and sometimes...). On the other hand, many tests read in terms of nitrogen-ion concentrations, since this measure is more commonly used by professionals. Trouble often occurs when the hobbyist measures her nitrate levels in nitrogen-ion concentrations and unwarily compares the resulting number with a recommendation in ion concentrations, possibly resulting in nitrate concentrations up to four times that intended. If a nitrate test does not mention which scale is being used, it may be possible to determine this by testing an established, well maintained tank. However, someone who has access to a well maintained tank usually already knows the scale of his favorite test, and most often it is the person who is trying to set up a tank for the first time that needs this information. A list of common tests and their measure is included at the end of this section as well. If you absolutely must use a test that does not mention the scale, it is best to be safe and assume nitrogen-ion concentration. However, one may legitimately question the likely quality of a test which does not provide this information, and a move to a better test is probably indicated. Now the details: the measure of concentration used on both scales is parts per million, abbreviated ppm, which means: units of mass of the measured substance per million units of mass of the final solution. (For aquarist's purposes, this is identical to milligrams per liter, or mg/l; one liter of fresh water is almost exactly one kilogram, and a liter of salt water is only a couple of percent more.) On the ion scale we measure the mass of the complete ion which makes up the substance, while in the nitrogen-ion scale we only measure the mass of the nitrogen atom in that ion. The first scale measures the concentration of nitrate ions, while the second measures the concentration of nitrogen in the form of nitrate. In fact, it could be called the "nitrogen as nitrate" scale. Which scale is more natural depends on one's intent. Most aquarists are interested in the toxicity of, say, nitrate ions in aquarium water, in which case the ion scale is the most natural; it directly measures the amount of the toxic ion present. On the other hand, a biologist may be studying the nitrogen cycle in a particular system, and would thus be interested in measuring the total amount of inorganic nitrogenous wastes present in a water sample. Then the nitrogen-ion scale is most natural, since the concentrations can be added directly. One could not add the ion concentrations for this purpose; this would be exactly like adding apples and oranges. The conversions between the scales is easily derived as follows: mass of the complete ion [ion] = ------------------------ [nitrogen-ion] mass of a nitrogen atom And clearly 1 over this conversion factor is used when converting ion concentration to nitrogen-ion concentration. The interested reader can reproduce the above numbers with this formula and the following information. Masses: To within a percent, we can use mass of a hydrogen atom (H) = 1, mass of a nitrogen atom (N) = 14, and mass of an oxygen atom (O) = 16. Chemical Species Formula Mass Nitrate ion NO3- 14+3*16=62 Nitrite ion NO2-- 14+2*16=46 Ammonium ion NH4+ 14+4*1=18 Ammonia molecule NH3 14+3*1=17 The calculation for the ammonia conversion factor is complicated by the fact that in solution ammonia exists in two forms. In a given sample of water, part of the ammonia will be in the form of ammonia (NH3) and part in the form of ammonium ions (NH4+). Further, the ratio of ammonia to ammonium is pH dependent. However, even if we assume that the ammonia present is all in one form or the other, the difference between the resulting numbers is not significant for our purposes (1.21 vs 1.29). The commonly quoted figure of 1.3 probably results from the fact that if the ratio of the two species was exactly 50%, the conversion factor would be exactly 1.25. Since at the pH of most aquaria more of the ions are ammonium than ammonia, the conversion factor must be more than 1.25 and so we round to 1.3. [This last sentence is a guess on my part, since I don't really know what the pH of the average freshwater aquarium is. It should be true for all marine aquaria.] The following is an (incomplete) list of common tests, the measure used by each, and comments be various people on the net. DryTab ion From Dustin Laurence: These kits are IMHO almost completely worthless. Buy something like the SeaTest kits instead for good, fairly inexpensive tests. Dupla ion? From Ken Koellner: English instructions are translated from the German with European conventions intact. American aquarists beware things like 0,2 for 0.2 . HACH Saltwatermaster kit nitrogen-ion From Mike Loughlin: Designed for aquaculture, sensitive to low concentrations. Excellent technical support. Cost: $35.75 in the 1992 catalog. HATCH low range nitrate nitrogen-ion From Greg Smith: Range: 0-1ppm N-NO3 in .02 increments. Smallest readable increment: .02ppm. Cost: ~55. Comments: repeatable, consistent, kit uses colorwheel. This is the best kit I have used, better than lamotte, seatest, kordon. Number of tests: (30, 50? not sure). Keith Rogers Adds: Note that you get erroneous results in salt water unless you get the proper SW version of NitraVer 6 (one of the reagents used). They will *not* substitute the SW reagent in place of the FW one. So, if you need the kit for SW you also need to buy the other reagent for an extra $18.50. Shipping cost is $8. The up side to this is you get a FW and SW nitrate kit which is better than all the rest bar none. Kordon low range nitrate nitrogen From Greg Smith: Range: 0-55ppm NO3. Smallest readable increment: 5ppm. Cost ~10. Number of tests: (10 or 20 not sure). Comments: good kit. Lamotte nitrogen-ion? SeaTest nitrogen-ion From Dustin Laurence: Good kits for average aquarists. Reef aquarists will want a better (and more expensive!) nitrate kit after their reef has matured. From Ray Wallace (on nitrate kit): Range: 0-12 mg/L (ppm) and 0-100 mg/L (ppm) Nitrate-Nitrogen (NO3-N) scales. Uses a slide with gradual color scales. Smallest numbered increment is 1.2. 25 tests using premeasured powder in packets.