Transcript Field Methods of Monitoring Atmospheric Systems

Field Methods of Monitoring
Aquatic Systems
Unit 10 – Ion-Selective Electrodes
Copyright © 2006 by DBS
ISE Theory
•
Uses potentiometry to measure concentration
– Membrane responds selectively to a given ion
– mV reading between sensing and reference
electrode (stable mV)
•
Samples must be aqueous to avoid damage to
membrane
•
•
Pros: Much cheaper than IC or AA instruments
Cons: Limited life-span
ISE Theory
Variety of ISE’s
•
Polymer membrane
– Ca2+, BF4-, NO3-, ClO4-, K+, Mg2+
•
Gas-sensing
– NH3, CO2, HCN, HF, H2S, SO2, NO2
•
Crystalline membrane (solid state)
– Br-, Cd2+, Cl-, Cu2+, F-, I-, Pb2+, Ag/S2-, SCN-
Theory
• Ion Selective Electrodes (including the most common pH
electrode) work on the basic pricipal of the galvanic cell
• By measuring the electric potential generated across a
membrane by "selected" ions, and comparing it to a reference
electrode, a net charge is determined
• The strength of this charge is directly proportional to the
concentration of the selected ion
Theory
•
Solid-state electrodes follow the Nernst equation:
Ecell = E0 + 2.303 RT log [M]
nF
Where E = voltage (V), E0 = constant, 59.2 = slope, n = charge, and
[M] = concentration of ion
2.303 x RT / F = 0.0592
mol-1
Where R = 8.316 J
T = temperature (K) and
F = 96487 C eq-1
K-1,
E
log[M]
Question
Write down the Ecell expression for a fluoride ISE in mV
Ecell = E0 + 0.0592 log [F-]
-1
1 V = 1000 mV,
For Ecell (mV)
Ecell = K - 59.2 log [F-]
Calcium ISE
•
•
ISE, reference electrode and voltmeter (mV)
PVC membrane coated with organic molecule that
selectiveley absorbs Ca2+
•
Ca2+ concentration gradient generates potential
Ecell = K + 29.6 log [Ca2+]
Where E = voltage (mV), K = constant,
29.6 = slope
Ionic Strength Adjuster
• Ca ISA = 4 M KCl
• It is likely that samples will be higher ionic strength than
standards
• Variable ionic strength of solutions varies electrode response,
also calibration curve is affected
• ISA adds ionic strength (does not add ion under study)
• Swamps ionic effects of host solutions and gives uniform ionic
strength in all solutions
• Ca ISA does not buffer solution or complex interfering ions like
F- ISAB
Procedure
Requires ISA, electrode and standards
1.
2.
3.
4.
5.
Set up electrode and allow to equilibriate in high
concentration solution (30 mins)
Make up standards and analyze with ISA from low to high
0.1, 1, 10, 100, 1000 ppm Ca2+
Make a graph of Ecell (mV) vs. log[Ca2+], ensure slope is at or
near 29.6
Analyze unknowns
Calculate concentration of samples
Question
Why is it important that a refrigerated sample or standard be
removed from storage 30 minutes prior to measurement?
Fluoride ISE
• Occurs in minerals somay be naturally present
• Low incidence of cavities with high levels of F– Decay of hydroxyapatite Ca10(PO4)6(OH)2
– F- replaces OH- with F- to form more resistant fluorapatite
Ca10(PO4)6F2
• F- > 3 ppm causes decay and poisoning
– In SW US F- > 4ppm requiring removal
Fluoride ISE
•
•
LaF3 crystal doped with EuF2
F- passes through holes in crystal lattice produced
by EuF2 impurity
•
F- concentration gradient across the crystal
generates potential
Ecell = K - 59.2 log [F-]
Where E = voltage (mV), K = constant,
-59.2 = slope
Ionic Strength Adjuster
•
•
•
•
ISA Buffer maintains pH 5.2 – 5.7 reducing interference from OH- and formation
of dangerous HF
ISA masks Al3+ and M2+ (complexes them with citrate) that would otherwise bind
F- strongly
F- measurement is best carried out under conditions of high ionic strength
Minimizes change in electrode response as a function of the concentration of
other ions in solution
Measurement Considerations
1.
2.
3.
4.
5.
6.
Read operating instructions
Agitation – supplies constant flow of ions to sensor
Response time – at least 15 mins for equilibrium
Calibration – at least 3 standards
Rinsing – do not use a cloth! Shake off excess water
Conditioning – ISE needs to remain most even when not in
use
Measurement Considerations
1.
2.
3.
4.
5.
6.
Read operating instructions
Agitation – supplies constant flow of ions to sensor
Response time – at least 15 mins for equilibrium
Calibration – at least 3 standards
Rinsing – do not use a cloth! Shake off excess water
Conditioning – ISE needs to remain most even when not in
use
General Considerations
1.
2.
3.
4.
Electrodes with a polymer membrane must not come in
contact with organic solvents
Do not store in water for extended periods—dry before storing
Store Combined Ion Selective Electrodes in dilute ISA (ionic
strength adjuster) solution—for long term storage, remove
reference solution and store dry.
Clean crystal membranes with a mild abrasive, then rinse with
water. Toothpaste is an excellent cleaning agent, for fluoride
electrodes use a fluoride toothpaste
Latest Technology
New NextSens ISE with USB
(no meter required)
Ex-Stick
Source: http://www.nextsens.com
Journal Articles
• Frank and Ross (1966) Science, Vol. 154, pp.1553.
• Frant, MS. (1994) History of the Early Commercialization of IonSelective Electrodes. Analyst, Vol. 199, pp. 2293-2301.
• Meyerhoff, ME and Opdycke, W.N. (1986) Ion Selective
Electrodes. Advances in Clinical Chemistry, Vol. 25, pp. 1-47.
• http://www.chemistry.nmsu.edu/Instrumentation/IS_El
ectrod.html
Text Books
•
•
•
•
•
•
•
•
Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil.
Wiley-VCH.
Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker.
Keith, L.H. and Keith, K.H. (1996) Compilation of Epa's Sampling and Analysis Methods.
CRC Press.
Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis
Publishers.
Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books.
Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative
methods. Taylor & Francis.
Reeve, R.N. (2002) Introduction to environmental analysis. Wiley.
Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the
Examination of Water and Wastewater, 20th Edition. Published by American Public Health
Association, American Water Works Association and Water Environment Federation.