The document summarizes an investigation into using transition metal ions like copper (II) to remove chloride interference from nitrate ion selective electrodes. Nitrate measurements were made using ion selective electrodes in flow injection analysis. The addition of copper (II) ions was found to complex with chloride ions, reducing chloride interference and allowing detection of nitrate down to 5 ppm. Selectivity coefficients improved from around 0.01 without copper (II) to around 0.001 when copper (II) was added, representing a tenfold reduction in chloride interference. Further optimization of copper (II) concentrations could provide an inexpensive way to measure nitrate in chloride-containing waters like seawater.

1. Abstract
Introduction
Experimental
Nitrate measurements were made with a Cole Parmer (K-27502-31) Nitrate Ion
Selective Electrode and a Thermo Fisher Scientific Combination Nitrate
Electrode (9797BNWP) Epoxy Body in Chemflow FIP-4 wall-jet configuration
flow cell and a Global FIA cell and a MicroLab FS-522 in conjunction with a
Dell laptop. 60 µL injection volumes and a flow rate of 4.5 mL per minute were
employed via a Gilson MP3 peristaltic pump and a Reodyne injection valve.
Ultra pure water was used for all prepared solutions: Barnstead, E-pure; 17.9
megaohms.
The following reagents were employed: Ammonium Sulfate, EMD ACS, Cupric
Sulfate as CuSO4.5H2O, EM Science USP, Potassium Nitrate, Sigma-Aldrich
ACS, Ammonium Chloride, Sigma-Aldrich ACS.
.
Results and Discussion
Acknowledgements
SLCC Chemistry Department Staff, and Sara Ranquist
References
1. Flow injection analysis of environmental samples for nitrate using an
ion-selective electrode. E. H. Hansen, Animes,h K. Ghose and J. Růžička
Analyst, 1977, 102, Pages 705-713
2. Potentiometric selectivity coefficients of ion-selective electrodes. Part
II. Inorganic anions (IUPAC Technical Report), Yoshio Umezawa,
Kayoko Umezawa, Philippe Bühlmann, Naoko Hamada, Hiroshi Aoki
Jun Nakanishi, Moritoshi Sato, Kang Ping Xiao, and Yukiko Nishimura.
Pure and Applied Chemistry 1995 Volume 74, Issue 6, Pages 923–994
3. Analysis of plants, soils and waters for Nitrate by using an ion-selective
electrode. P. J. Milham, A. S. Awad, R. E. Paull and J. H. Bull
Analyst, 1970, 95, Pages 751-757
4. Frant, M. S. and Ross, J. W. Jr, Use of a Total Ionic Strength
Adjustment Buffer for Electrode Determination of Fluoride in Water
Supplies, Anal. Chem. 1968, 40 (7), Pages 1169-1171
5. Corn Leaf Nitrate Reductase, A nontoxic alternative to Cadmium for
photometric determination of Nitrate. Patton, C.J., Fischer, Campbell
W.H. and Campbell, E,R, Environ Sci Tech 2002, 36, 729-735
Conclusion
Selectivity Coefficients plots showing reduction in Chloride interference in
the presence of Copper (II) sulfate
Investigation of the use of Transition Metal Ions to remove
Chloride interference from Nitrate ISEs
Peter Iles, Ashley Rihm, Sarah Moore, Alan Abbinanti, Lourdes Peralta, Tyler Fullmer, William Ballard, Kenny Robbins, Kent Jones,
Tallon Nielson, Laura Dalby, Ryan Holcomb, Neil Bastian, Luther Giddings, Mary Alvarez and Ron Valcarce
Salt Lake Community College 4600 South Redwood Rd. Salt Lake City Utah, 84130
Nitrate is traditionally measured via colorimetric techniques employing a
Copper-Cadmium reduction column. This technique though sensitive is
cumbersome and tedious. Analysis by Nitrate Ion Selective Electrode (ISE)
coupled with flow injection analysis coupled can provide a rapid and
automatable procedure for Nitrate determination. Unfortunately Chloride ions
interfere significantly with the Nitrate ISE prohibiting its use in seawater and
salt-water aquaria. Precipitation of Chloride as insoluble Silver Chloride has
been used to remove Chloride ions from solution enabling the use of the
Nitrate electrode in brackish water samples. This approach is very expensive.
This work reports the investigation of transition metal ions capable of
complexing Chloride as a means to eliminate Chloride interference when
using the Nitrate ISE in Flow Injection Analysis (FIA) systems. Sample
treatment with Copper (II) has shown to provide a 10 fold improvement in
selectivity.
The Living PlanetAquarium and the Biotechnology Department at SLCC have
expressed interest in developing a rapid and sensitive method for the measurement of
Nitrate in various Chloride media. Ion Selective Electrodes (ISEs) with Flow Injection
Analysis (FIA) provide these capabilities1. Unfortunately Chloride interferes
significantly with Nitrate ISEs in particular in sea water2. Methods using Nitrate ISEs
have been reported for brackish waters, wastes and plant materials3. These methods
employ Silver Sulfate to precipitate Chloride to remove the interference before
measurement with the ISE. This is expensive and limited to removal of Chloride only
up to 0.015 M due to the poor solubility of Silver Sulfate.
Fluoride ISE measurements use TISAB II which contains CDTAto release Fluoride
bound to transition metals such as Iron (III) andAluminum (III) so that all Fluoride
exists in solution as free Fluoride ions, which can respond at the ISE4. In this work we
have attempted to use this process in reverse where the interfering free Chloride ions are
bound to transition metal ions. Copper (II) ions are known to form Chloro complexes
and have been investigated as a means to remove interfering free Chloride ions
effectively from the solution.The literature reports selectivity coefficients KPot
i,j
generally in the order of 1 x 10-2 . In this work the selectivity coefficients were
determined in FIAmode by the constant interference method using 0.1 M Cl- and
0.015 M Cl-.
It can be seen from Figures 1 and 2 that the presence of a large excess of Copper (II) as
background electrolyte greatly reduces the Chloride interference. These plots were used to
determine the potentiometric selectivity coefficients in Flow Injection Analysis mode.
The literature reports selectivity coefficients KPot
i,j generallyin the order of 1 x 10-2 . In this work the
selectivity coefficients were determinedin FIAmode by the constant interference method using 0.1 M
Cl- and containingvaryingNitrate levels . Both brands of electrode were evaluatedand the KPot
i,j was
also determinedin solutions containing varyingNitrate levels, 0.1 M Chloride and a background of 0.15
M Copper(II). No difference in the values was observedbetweenthe brands of NitrateISE.
KPot
i,j ≈ 1 x 10-2 i = NO3
-, j = Cl- measured
KPot
i,j ≈ 1 x 10-3 i = NO3
-, j = Cl- measured when a background of 0.1 to 0.15 M
Copper (II) is present.
The data show a 10 fold increase in the removal of Chloride interference which
allows detection down to 5 ppm. Preliminary qualitative experiments with
Aluminum (III), Iron (II) and (III),Cadmium(II) and Nickel (II) aqueous ions were
shown to provide no improvement in selectivity and have been removed from
further investigation.
Using ISEs in FIA mode provides rapid and inexpensive measurements without a
lack of sensitivity and minimum exposure of the sensing membranes which may
prolong sensor lifetime. Future work will include a more detailed study of the
effects of Copper (II) but it is considered that an enzyme5 nitrate reductase
colorimetric FIA method may provide the best solution to sea and brackish waters.
NO3
-only NO3
- + Cl- NO3
- + Cl- + Cu2+
Figure 2. Calibration plots of Nitrate with various electrolyte backgrounds.
Figure 1. Triplicate injections of 5 ppm Nitrate