EurOCEAN 2000 - Vlaams Instituut voor de Zee

In order to keep the pH constant a small amount of Sodium Hydrogen carbonate (NaHCO3) (~
0.2 gr/L) was added to each solution. Initially tests were carried out in ultrapure water and in
solution containing 0.2 g/l Sodium Hydrogen carbonate solutions to measure the combined
influence of pH and carbonate. The resulting pH (of approximately 8.1 to 8.6) and carbonate
concentration corresponds to the conditions expected in marine waters. Different Nitrate salts
{such as Potassium Nitrate (KNO3) and Sodium Nitrate (NaNO3)} were added to determine the
influence of the cation (the Potassium and Sodium ions). Furthermore the interference of
chloride, bromide, sulphate and nitrite ions were tested in the Sodium Hydrogen carbonate
(NaHCO3) solution. Concentration ranges similar to those expected in seawater were chosen.
The influence of chloride was examined at two different molalities (0.25 M and 0.5 M, the
latter to be expected in seawater) in order to demonstrate the growing impact of interference.
The interference of pH, bicarbonate, chloride, nitrite, bromide, fluoride and sulphate ions in
nitrate/bicarbonate/ water mixtures at constant temperatures was assesed in order to determine
the selectivity coefficients as a function of the concentrations for the various sensors. All
measurements were carried out at constant Nitrate concentrations (up to 0.1 M). With the
exception of the pH interference the pH of solution is fixed by the carbonate buffer system to
approximately 8.4.
To measure the pH interference at a constant, i.e. zero bicarbonate concentration the pH was
varied by adding Nitric Acid (HNO3) or Sodium Hydroxide (NaOH) to bicarbonate/ free water/
Nitrate solutions. To prevent the absorption of Carbon Dioxide (CO2) from the air the
solutions were sparged with Nitrogen. When the bicarbonate concentration was varied by
adding Sodium Hydrogen Carbonate (NaHCO3) the pH was fixed by adding Nitric acid
(HNO3). At pH 8.2 to 8.4 the fraction of dissolved Carbon Dioxide (CO2) is negligible and the
fraction of carbonate (CO3 - ) only about 2 percent of the total carbon so that the bicarbonate
concentration is equivalent to the amount of Sodium Hydrogen Carbonate (NaHCO3).
Measurements of Nitrate were then carried out in an artificial seawater matrix, varying
systematically the Nitrate level and the concentrations of all interfering ions found.
THE MEASURING PROGRAM FOR THE NITRITE ION SELECTIVE
ELECTRODES (ISE) IS AS FOLLOWS:
Measurements have been performed on Nitrite electrodes from REAGECON and Orion. In all
experiments the Nitrite (NaNO2) concentration was varied from 0.0001 M up to 0.01 M at
temperatures of 20 o C and 10 o C. The interference of chloride, bromide, sulphate and nitrate
ions were tested. For these ions concentration ranges that may be expected in seawater have
chosen. Since a pH range of 4.0 – 5.0 is recommended for the Orion electrode, most of the
measurements have been carried out in dilute aqueous HCl solutions at pH 4.5. According to
the technical information supplied by Orion long term exposure to pH greater than 8.0 may
damage the nitrite electrode membrane. That being the case the Orion ISE is not suitable for
Marine applications.
THE MEASURING PROGRAM FOR THE AMMONIUM ION
SELECTIVE ELECTRODES (ISE) IS AS FOLLOWS:
Testing was carried out on Ammonium sensors obtained from Orion and Mettler as well as
those manufactured by Reagecon. In all experiments the Ammonium (NH4Cl) concentration
was varied from 0.00005 M up to 0.1 M and measurements performed at temperatures of 20 o C
and 10 o C. From an evaluation of the toxological data compiled by the consortium it has been
established that an expected maximum concentration of approximately 0.0003 mol NH4/l in
seawater.
704