100
Exercise 7
Both anions and cations can be measured with IC [e.g., Gjerde and Fritz (1987) and
Small et al. (1975)], but IC is particularly valuable for the analysis of ionic sulfur and
nitrogen compounds.
High-Performance Liquid Chromatography (HPLC)
In high-performance liquid chromatography (HPLC) liquid samples are forced under
high pressure through a column packed with an appropriate stationary phase.
Components separated from the liquid by interaction with the stationary phase emerge
from the column in the order of their reaction, i.e., the least retarded component elutes
first. Several different detectors can be used to quantify elutants, including fluorescence
and electrochemical.
HPLC is used commonly to measure NH 4 , N0 2 , N0 3 , S04' plant pigments, and
high molecular weight organic compounds [e.g., Lodge (1989) and Snyder and
Kirkland (1986)].
Continuous Flow Spectrophotometric Procedures
Automated systems such as an Auto-Analyzer (AA) or Flow Injection Analysis (FIA)
bring sample and reagents together under carefully controlled, automated conditions.
In AA, the chemical reactions occur in continuously flowing, air-segmented streams of
liquid (sample plus reagents). In FIA, the sample and reagents are mixed by laminar
flow in narrow-bore tubing without air segmentation. Color formation resulting from
the chemical reactions is measured with a spectrophotometer.
These methods are used commonly for the measurement of N0 2 -, N0 3 -, NH4 +,
Cl-, P0 4 3 -, dissolved Si, and total dissolved N [e.g., Shugar et al. (1981) and Willard
et al. (1988)].
ION SELECTIVE ELECTRODES
Much attention has been given in recent years to the use of ion selective electrodes for
measuring the concentrations or activities (or apparent concentrations) of ions in
solutions. The principle of ion selective electrodes is identical to that of pH electrodes:
In a glass pH electrode the thin glass bulb acts as a membrane specific for hydrogen
ions. Other ion specific electrodes function similarly as electrochemical cells, where two
solutions containing the same ion in differing concentrations are separated by a
membrane sensitive to this ion. A voltmeter or a potentiometer is used to measure an
electrical potential generated across the active surface of the membrane. The difference
in potential between the internal electrode in the sample solution and a reference
electrode is related exponentially to the activity of the selected ion in the sample
solution. Detailed discussions of construction and membrane properties of ion selective
electrodes are given in Durst (1969), Carson and Keeney (1971), and especially
Whitfield (1971).
Electrode response is a function of ion activity, not of ion concentration. The
membrane potential is logarithmically related to the measure of ionic activity by the
Nernst equation. The Nernst equation modified for membrane electrodes is
,
As
E = E + Slog-A;
Exercise 7
Both anions and cations can be measured with IC [e.g., Gjerde and Fritz (1987) and
Small et al. (1975)], but IC is particularly valuable for the analysis of ionic sulfur and
nitrogen compounds.
High-Performance Liquid Chromatography (HPLC)
In high-performance liquid chromatography (HPLC) liquid samples are forced under
high pressure through a column packed with an appropriate stationary phase.
Components separated from the liquid by interaction with the stationary phase emerge
from the column in the order of their reaction, i.e., the least retarded component elutes
first. Several different detectors can be used to quantify elutants, including fluorescence
and electrochemical.
HPLC is used commonly to measure NH 4 , N0 2 , N0 3 , S04' plant pigments, and
high molecular weight organic compounds [e.g., Lodge (1989) and Snyder and
Kirkland (1986)].
Continuous Flow Spectrophotometric Procedures
Automated systems such as an Auto-Analyzer (AA) or Flow Injection Analysis (FIA)
bring sample and reagents together under carefully controlled, automated conditions.
In AA, the chemical reactions occur in continuously flowing, air-segmented streams of
liquid (sample plus reagents). In FIA, the sample and reagents are mixed by laminar
flow in narrow-bore tubing without air segmentation. Color formation resulting from
the chemical reactions is measured with a spectrophotometer.
These methods are used commonly for the measurement of N0 2 -, N0 3 -, NH4 +,
Cl-, P0 4 3 -, dissolved Si, and total dissolved N [e.g., Shugar et al. (1981) and Willard
et al. (1988)].
ION SELECTIVE ELECTRODES
Much attention has been given in recent years to the use of ion selective electrodes for
measuring the concentrations or activities (or apparent concentrations) of ions in
solutions. The principle of ion selective electrodes is identical to that of pH electrodes:
In a glass pH electrode the thin glass bulb acts as a membrane specific for hydrogen
ions. Other ion specific electrodes function similarly as electrochemical cells, where two
solutions containing the same ion in differing concentrations are separated by a
membrane sensitive to this ion. A voltmeter or a potentiometer is used to measure an
electrical potential generated across the active surface of the membrane. The difference
in potential between the internal electrode in the sample solution and a reference
electrode is related exponentially to the activity of the selected ion in the sample
solution. Detailed discussions of construction and membrane properties of ion selective
electrodes are given in Durst (1969), Carson and Keeney (1971), and especially
Whitfield (1971).
Electrode response is a function of ion activity, not of ion concentration. The
membrane potential is logarithmically related to the measure of ionic activity by the
Nernst equation. The Nernst equation modified for membrane electrodes is
,
As
E = E + Slog-A;