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Methods of measuring
 

Methods for measuring phosphorus and nitrogen in natural waters differ largely. There are different reasons to measure the concentration of these elements and in most cases the methods are adapted to the objective of the person analyzing water samples. If for example the test is done to establish if water is safe as drinking water, the only information needed is if the maximum allowed content is exceeded or not. Even with this simple measurement, there might be some confusion. Where some countries require a value lower than 45 ppm nitrate (NO3), others require a value lower than 10 ppm nitrate-N (only the weight of the nitrogen in the NO3). These values lay only 2% apart, but the use of different “standards” make that this can not be readily seen. To compare different measurements, it is thus necessary to convert them all to the same standard (unit). On this web page, the unit chosen is ppm (or mg/l) for the nitrogen or phosphorus weight only – not the weight of the whole nitrate or phosphate molecule (this is sometimes expressed as nitrate-N and phosphate-P). Chemists prefer a unit that does not express weight, but a number of molecules – called molar units. You may thus also find some data where mol/liter or M is the unit.

The cheapest and simplest method to analyze water samples for phosphate and nitrate concentration is the use of chemical tests. Depending on the intended use (aquarium, drinking water quality monitoring, environmental purposes) they each have a specific range for detecting nitrate or phosphate. It is thus important to select a test that is able to detect the nutrient levels to be expected in local waters. The resolution of these tests is not high. The idea is that chemicals added to the water sample make it change color. The more phosphate or nitrate the sample contains, the more intense the color. A chart with various colors then allows comparison between the sample and the printed colors to find a value. Most water tests offer a range of 4 up to 10 different colors (concentration levels), but since the human eye is not a very sensitive instrument, the steps are rather big.

To allow precise reading of colored samples, photometers can be used. They replace the human eye with a sensor that measures the desired frequency (light color). Such meters can detect lower values (water that to the human eye would still appear not to be colored) and give a higher resolution – there is no more need for distinct steps between the different color levels. Photometers are first calibrated to the color of the water sample, by first inserting a sample without chemicals and setting the instrument to zero. Once the zero setting is done, the prepared samples can be measured.

Another method would be the use of an ion specific probe. These instruments look like a pen that is held in the sample. The probe is equipped with a membrane that allows only a certain molecule to pass (for example nitrate). On the other side of the membrane the “activity” or presence of these molecules is measured. A calibrated digital instrument then gives the content of the molecule (in this example nitrate) in ppm.

Chemists in a laboratory essentially make use of the same methods, but they correct for any variation in the probe, the photometer, or personal “mistakes” in measuring the sample, by first measuring a series of samples with a known concentration and creating a “standard”. This is a graph that plots the output of the instrument (mostly in mV) against the concentration in the probe. Software such as excel can then express the slope of the line created in the graph as a formula, which can be used to assign a concentration to any measurement in mV (for the range in which the “standard” produces a straight line – at some point the measurements “drift” away because of detection problems).

The range to be expected in fresh water is from 0.04 to 0.2 ppm for phosphorus, depending on the type of water, and from 1 to 4 ppm for nitrogen. The range relates to different waters. Rivers are mostly on the high end, while deep lakes are mostly measuring lower values (if the nutrient content is more or less natural). The values in salt water can be lower: 0.1 for phosphorus falls in the same range, but nitrogen can be as low as 0.04. This makes it harder to measure the nitrogen content in the sea than in freshwater.


Source: IT College

Source: IT College

Source: IT College

Source: IT College
 
 
 
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Converter Nitrate
 

ppm
Nitrate

ppm
Nitrate-N

µmol/l
 
 
 
Converter Phosphate
 

ppm
Phosphate

ppm
Phosphate-P

µmol/l
 
 
 
FAQ
 

What is the recommended method for the nutrient loading program?

Photometers provide readings on nutrient concentrations that are accurate enough for the purpose of this program. Color chart comparison is not very accurate and is for that reason not recommended (but allowed).

Are there special considerations for measuring nutrient levels in saltwater?

In saltwater you can expect the concentration of nitrate to be very low. Standard single parameter photometers may not be able to detect such low levels. Universal photometers can be used with different chemicals, including those that are meant to measure in the (extremely) low range. During testing we used a product available with Hach-Lange.

Can water be tested when it is turbid due to particles?

Yes, but it has to be filtered before analyzing. A “normal” coffee filter can be used for that purpose, just as (of course) paper filters meant for use in laboratories.

What can be done when the nutrient levels are too high to be analyzed with the available equipment?

The probe can be diluted. For this purpose you must use water that is free of nutrients.

What can be done when the nutrient levels are too low to be detected by the available equipment?

Techniques to concentrate the sample may proof to be too complex to perform with the available tools and with the available knowledge of laboratory techniques and procedures. It is thus important to have a general idea of the concentrations of nutrients to be expected in local waters, before deciding which equipment to use.

 
 
 
Photometric measurements
 
 
 
 
Laboratory techniques
 
 
 
 
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