Measuring levels of nitrates in water samples.
Background and nature of the task
The nitrate ion (NO3−), is an important naturally occurring plant nutrient found in soil. However, the widespread addition of nitrogen-containing fertilisers has, however, resulted in a significant increase in nitrogen-containing compounds in soil. High levels of nitrate in the soil can lead to high levels of nitrate in water sources, in particular as a result of run-off from agricultural land.
In ponds and rivers, the nitrate acts as a fertiliser for algae, leading to algal blooms. The algae can cover the surface of ponds, preventing light reaching plants and animals below the surface. This process is known as eutrophication and can lead to a considerable reduction in the biodiversity of a lake or pond with disruption of the entire aquatic food-web.
Nitrate can also cause other health issues. Nitrate in ground water can enter wells and other drinking water supplies where it can also be a problem. High levels of nitrate in drinking water can result in ‘blue-baby syndrome’ or methaemoglobinaemia, a condition that is particularly serious in infants under 6 months old. Thus, drinking water is routinely tested for nitrate levels – the current limit set by the EU, which is based on World Health Organization (WHO) guidelines, is 50 mg l−1 NO3− (milligrams of nitrate per litre of water).
In this online experiment you will determine the levels of total oxidisable nitrogen (TON) – namely nitrate and also nitrite (NO2−) – in ground water samples from springs that feed into a 30-mile stretch of the Suwannee River in Florida. You will use a spectrophotometric method to determine the level of TON in these samples.
The investigation consists of two specific tasks.
In Task 1 you will prepare a calibration curve to enable you to determine the concentration of nitrate and nitrite in the samples. You will:
- choose and prepare a range of standard solutions using appropriate volumetric glassware
- plot a calibration curve of absorbance versus concentration
- determine the linear working range for the calibration curve.
In Task 2 you will analyse a series of samples that were collected from springs in the Suwannee river basin. You will measure the absorbance of each sample, diluting your sample if required and use the calibration graph to calculate the nitrate concentration.
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