DETERMINATION OF NITRITES IN THE WATER THROUGH RAPID DETERMINATION
Introduction
Nitrogen is one of the macro-elements of fertility, as it is part of the nucleic acids, proteins and other macromolecules necessary for life and is indispensable for the development of plants.
Plants extract nitrogen from the soil through the circulating solution (water + mineral salts) which is absorbed by the roots.
The main reservoir of nitrogen in soil is organic nitrogen (an integral part of soil organic matter), which accounts for about 95% of the total. The process of mineralization of organic matter, carried out by various microorganisms, leads to the release of nitrogen in various forms, ammoniacal (NH4+), nitrous (NO2-) and nitric (NO3-)
Plants absorb nitrogen from the soil predominantly as nitric NO3- and alternatively ammoniacal NH4+. Nitric nitrogen is free in the circulating solution and its assimilability is much greater than that of ammoniacal nitrogen which is adsorbed on the exchange complex (soil colloids, clays and organic matter) and is poorly washable.
Nitrate (NO3-) is naturally found in the environment and is the most mobile and susceptible to leaching. Leakage can be as high as 99% of the nitrates present, so it can become a source of groundwater and surface water pollution. The loss of nitrates by leaching depends on the NO3- concentration in the soil, the volume of water drained, the texture and structure of the soil and climatic factors.
The nitrite (NO2-) is generally not present at significant concentrations, except in a reducing environment, since nitrate is the most stable oxidation state. It can be formed by microbial reduction of nitrate by microorganisms in the soil.
The extent of the forms of mineral nitrogen present in the soil, represented by nitrites (NO2 -), nitrates (NO3-) and ammonium ion (NH4+), depend on the biochemical activities taking place in it.
Nitrate is mainly used in inorganic fertilizers; it is also used as an oxidizing agent, in the manufacture of explosives, in the manufacture of glass. Sodium nitrite is used as a preservative, especially for salted meat.
Nitrogenic substances, and in particular nitrates, can cause adverse environmental impacts on rivers, lakes and coastal waters because, together with phosphorus, they promote eutrophication, i. e. the uncontrolled proliferation of aquatic plants, algae in particular, because of the abundance of nutrients present and leading to a reduction in the quality of these environments. The large amount of biomass produced by decomposing causes sharp decreases in the amount of oxygen in the water, causing e. g. fish deaths
In 1991, the European Union issued a directive, 91/676/EEC, better known as the “nitrates directive”, which indicated to member states the actions to be taken to identify areas vulnerable to nitrates of agricultural origin on their territory and the limits on the amount of nitrogen that can be distributed by fertilisers in these areas.
The content of nitrates and nitrites in water is an important parameter for determining water quality and for identifying possible sources of pollution. These nitrogen compounds may result from industrial, civil and agricultural discharges, in particular due to the excessive or incorrect use of nitrogenous fertilizers. They may be released into the environment as such or may result from the degradation of other substances such as organic substances.
The maximum concentration of nitrates and nitrites in drinking water is defined by law. Directive 98/83CE and its transposition into Legislative Decree 31/2001, set a parameter value of 50 mg/L for nitrate (as ion) and 0. 5 mg/L for nitrite (as ion).
The risks from nitrates are primarily environmental. The danger to human health lies not so much in their relatively low toxicity as in their transformation into nitrites which, when combined with other substances in the body, can give rise to potentially carcinogenic compounds. Hemoglobin and its ability to carry oxygen into the blood (methaemoglobinaemia).
Objectives
Determine the presence of nitrites in water samples
Materials
Gloves and personal protective equipment
Kit for the determination of nitrite
2 Tubes per sample of water
Water samples
5 ml syringe or pipette
Reaction
Measurement of the intensity of the red-fuchsia colour formed by reaction of nitrous ion with sulphanilic acid and naphthylamine in an acidic environment.
Procedure
Respect security measures.
Shake the sample to homogenise.
Rinse the syringe (or pipette) and tubes with the water to be analysed.
Withdraw 5 ml of water to be analysed using the syringe (or pipette) and place it in each of the two tubes.
Place a test tube (blank test) in the left hole of the comparator.
Place the comparator on the colour scale with the coloured fields under the blank test piece.
In the second test tube add 8 drops of reagent A and shake well.
After 10 minutes, place the test tube with the reagents in the right hole of the comparator.
Observe from above and slide the comparator until you find the colour of the sample.
Read the corresponding value, indicated by the comparator arrow.
If the concentration is high dilute the sample and determine the value taking into account the dilutions
Expected results
The presence of nitrates determines the colouring of the sample
The colour scale is used to determine the value of nitrites present expressed in parts per million (ppm) or milligrams per litre (mg/l).
The more coloured the sample, the higher the nitrite content in the water.
Values above 0. 5 mg/l indicate waters with nitrogen pollution problems.
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