Testing for nitrate and nitrite in natural waters.

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Testing for nitrate and nitrite in natural waters.

Inorganic nitrates and nitrites.

Nitrates in natural waters can come from the breakdown of organic matter or from external sources eg fertilizers. Normally in well aerated waters, organic matter is slowly broken down in a process called nitrification first forming ammonia then nitrites and finally nitrates. Nitrification in natural waters is a process that requires oxygen. In poorly aerated water or water overloaded with organic contamination, low oxygen levels can inhibit this process leaving most of the nitrogen as ammonia or nitrite. Both of these are relatively toxic to organisms.

Measuring ammonia, nitrites and nitrates can reveal a lot about the 'health' of water. Nitrates themselves are only hazardous in higher concentrations. However nitrates with phosphates are implicated in 'eutrophication' which can seriously degrade water through exceesive plant growth and subsequent decay.

There are 2 main methods for testing nitrates that can be called cadmium reduction and azo dye methods. Both rely on converting nitrate to nitrite and then measuring nitrite. Therefore nitrate is actually measured indirectly. This can be confusing because it means that if nitrite is already present then that will be included in the nitrate estimate.

If a test specifically for nitrite shows that there is nitrite present then the next or 'same time' test shows the nitrates present then the real nitrate value is the measured nitrate take away the nitrite value.

Test strips usually use the Azo dye method. The technical description is: "Nitrate is reduced to nitrite by a reducing agent. In the presence of an acidic buffer, the nitrite is converted to nitrous acid which diazotizes an aromatic amine, this coupled with N-(1-naphthyl)-ethylenediamine to form a red-violet azo dye".

Hach Aquachek nitrate / nitrite strips have two pads on each test strip and therefore two colour scales. The nitrite pad on the strips can be read in about 30 seconds. The nitrate pad contains a reducing agent that converts the nitrate to nitrite before creating the pink-red colouration of the azo dye. This reaction takes a little longer. So the nitrite is read first then the nitrate to get two values. If there is nitrite present then that value must be subtracted from the nitrate reading to get an actual nitrate value.

See them at Hach Aquachek nitrate / nitrite test strips.

Visual methods like test strips usually can't give the resolution needed for accurate measurements at all levels. However some test kits like the Macherey Nagel Visocolor ECO nitrate kit can improve accuracy and resolution. This kit can be used for seawater as it includes a conversion table. Also importantly this kit contains no hazardous materials.

See them at Macherey Nagel Visocolor ECO nitrate kit.

Some nitrate kits contain an inhibitor sometimes sulfamic acid that reacts with and removes nitrite. The resulting nitrate measurement is more accurate.

The cadmium reduction method is used in some kits and photometer methods. Usually the reacted sample needs to be shaken a number of times and then left to stand before any measurements are taken. Read the instructions and make sure you are consistent so that results are more reliable. Technically the cadmium reduction method is: "Cadmium metal reduces nitrates in the sample to nitrite. The nitrite ion reacts in an acidic medium with sulfanilic acid to form an intermediate diazonium salt. The salt couples with gentisic acid to form an amber colored solution".

Test kits for nitrate like the Hanna Instruments HI3874 use the cadmium reduction method.

Care should be taken in choosing test strips and kits for the required range. Many strips and kits show results as Nitrate - N (or Nitrite - N). This is the nitrogen part of the molecule. To convert Nitrate - N to nitrate multiply by 4.34.

Most nitrate kits work best in surface waters eg drinking waters and in some waste waters. However high ion concentrations particularly chloride can interfere with nitrate determinations. Either special calibration solutions can be used, a conversion table can be used or a different method chosen. Chemetrics make a nitrate kit that uses Zinc reduction. This is more suitable for seawater.

To measure organic nitrogen a nitrate test kit can be used but the sample has to be pre-digested to release the nitrogen. Organic nitrogen can then be calculated as the 'after digestion nitrogen' minus the 'pre digestion nitrogen'. At Apps Labs we have found that digestion using hot potassium persulphate is acceptably safe and reliable.

 

 

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