Citizen Science - Total Ammoniacal Nitrogen (TAN) Measurement, Part 1

 Although not part of the standard suite of tests performed by the Wye Alliance citizen science project, the measurement of 'ammonia' is important due to its toxicity towards aquatic life and as an indicator of pollution from sewage, agricultural run-off (fertilizers) and waste discharge from various sources (residential, commercial and industrial). Some groups within the Wye Alliance (e.g.Wye Salmon Association) have been monitoring ammoniacal nitrogen for a number of years; however, this analysis may be deployed more widely within the River Wye Catchment in the future.

Ammonia, NH₃, and its conjugate acid - the ammonium ion, [NH₄]⁴ - co-exist in water with the latter predominating in most natural water systems where the pH is typically between 6.5 and 8.5 (Figure 1). Ammonia is toxic to aquatic life even at concentrations below 1 ppm whereas the ammonium ion is less harmful. Ammonium ions are converted to toxic ammonia by increases in both pH (i.e. more alkaline) and temperature.

Figure 1: Effect of pH on Ammonia/Ammonium Equilibrium

The Nessler reaction is used to quantify the total ammoniacal nitrogen (TAN) in water. The colour-forming reagent (potassium tetraiodomercurate II) reacts with ammonia to form a yellow (Photo 2) to reddish brown (Photo 3) complex. The intensity of the colour is proportional to the concentration of the ammonia. The reaction is carried out under strongly alkaline conditions (pH >11) to convert all the ammonium ions to ammonia (see Figure 1). Values are reported as TAN ppmN; i.e. the nitrogen concentration in the water that is present as either ammonia or ammonium. To quantify the toxic free ammonia concentration in the original water sample, you will need to measure its temperature and pH and use an on-line calculator.

Test kits for ammonia come in various forms:

• Test strips (e.g. SimplexHealth) which are cheap and easy to use (just dip into the water) but less accurate as they rely on visual colour matching against a colour card
• Liquid reagent kits (e.g. API Ammonia Test Kit) where reagents are added to a water sample in a test-tube and the resulting colour is matched against a colour card. Possibly a little more accurate than the test strips but still subjective.
• Colorimeters such as the Hanna Checker Ammonia range where reagents (liquids or tablets) are added to the water sample in a cuvette and the colour intensity measured optically. Good accuracy at a reasonable price though more expensive than the first two options.

About a year ago, I bought a Hanna Low Range Ammonia Checker for use within our small group (the Belmont Testers) who regularly test the Newton Brook, a small tributary to the River Wye. The kit (Photo 1) comes with two glass cuvettes and two small bottles of reagent (Reagent A is a sodium hydroxide solution and Reagent B is a potassium tetraiodomercurate II solution) sufficient for 25 tests. I now buy the reagents in larger bottles sufficient for 100 tests as this is more economical.

Photo 1: Hanna Checker for Ammoniacal Nitrogen (0.0 to 3.0 ppmN)

The procedure is very simple:

• rinse the cuvette and cap three times with the sample
• fill cuvette to the 10 ml mark with sample
• switch on the Checker unit so that 'C1' is displayed, insert the filled cuvette, close the lid and  press the button (this is the sample blank)
• when 'C2' is displayed, remove the cuvette and add 4 drops of Reagent A, cap and mix for a couple of seconds
• add 4 drops of Reagent B, cap and mix for a couple of seconds
• put the cuvette back in the Checker unit, close the lid and perform a long press until 3:30 appears on the display
• 3 minutes and 30 seconds later, the result appears on the display
• finally, empty the contents of the cuvette down the toilet and wash with warm water followed by 3 rinses with distilled or de-ionised water.
• the cuvettes can be left to dry although I prefer to store them filled with distilled water

Below I've included a couple of photos showing how the intensity of colour changes with increasing TAN content.

 

Photo 2: Unreacted and Reacted Samples containing 0.3 ppm TAN

Photo 3: Reacted Sample (>3 ppm TAN)

In Part 2 we will look at some typical results obtained during our citizen science work.