Anion Analysis 2 – Determination of Chloride and Bromide with
Ion-Selective Electrodes
Ion-selective
electrodes (ISEs) are specially designed electrodes that respond selectively to
a particular ionic species in a solution. Most ISEs are based on a measurement
of the potential (energy) difference that develops across a semi-permeable (to
the ion of interest) membrane that separates solutions with different
concentrations of the analyte ion. The most familiar example is the glass
electrode (the glass is semi-permeable by protons) in the pH meters that you’ve used extensively in the past.
In this experiment, we will use chloride and bromide ion-selective electrodes
that respond selectively to the chloride or bromide ions in a solution.
If the selectivity is high enough, chloride (or bromide) can be determined in
the presence of other ions such as bromide (chloride) and iodide that would
otherwise interfere if methods such as titration or gravimetric analysis by
AgNO3 were
used.
The
response of halide ion ISEs at 298 K can be described by the equation:
E = constant
- β (0.05916) log ax- (1)
where E is the measured voltage,
the constant takes different values for different types of electrodes,
β is the electromotive efficiency
which should be close to 1.00 for a good electrode (which we hope will be the
case here), and ax- is the activity of the ions being measured (for example aCl- for the chloride ion). Recall that the – log ax- term can also be stated as pX
as was done in the definition for pH. In a low ionic strength solution, the
activity can be approximated as the molar concentration, ax- ≈ [X-] and they always share the same
unit. The constant and β will be determined in calibration
procedures for the two ions. Because we usually experience non-linear
calibration curves (e.g., E vs. pCl) in this experiment, you will
use both direct (external)
calibration and standard addition
methods for quantitation of the unknowns and you will compare the results in
your report.
Apparatus
100 mL volumetric
flasks (9)
250 mL volumetric
flasks (3)
100 mL beakers (9)
500 μL
automatic pipettor (1)
10 mL pipet (1)
Instrumentation (See Appendix for Operating Instructions)
Accumet AB15
digital pH/ISE meter with Fisher Accumet bromide and chloride combination ISEs
(only one probe at a time can be connected to the meter)
(Note: these electrodes have integral
reference electrodes and junctions.)
Solutions available
(1) 2.5 M KNO3: measure out about 10 mL with a graduated cylinder.
(2)
1.0 M KNO3: measure out 50 mL
with a graduated cylinder.
Solutions to be prepared
NaCl stock solution (~ 0.1 M);
Accurately weigh about 0.5 g (to 0.001 g) of NaCl, quantitatively transfer into
a 100 mL volumetric flask, dissolve in deionized water, fill to the mark, and
mix thoroughly.
KBr stock solution (~ 0.1 M);
Accurately weigh about 1.2 g (to 0.001 g) of KBr, quantitatively transfer into
a 100 mL volumetric flask, dissolve in deionized water, fill to the mark, and
mix thoroughly.
Challenge “unknowns”:
Pipet 500 μL of the NaCl and KBr stock solutions into separate 250 mL
volumetric flasks and then add both to a third 250 mL volumetric flask, fill
all three to the marks with deionized water, and mix thoroughly.
Procedure
a) Dilute the 0.1 M NaCl stock solution as
follows (serial dilution) to give a set of standards with pCI ≈ 2.00, 3.00, 4.00, 5.00,
6.00, and 7.00. Dilute 10.00 mL of 0.1 M NaCl stock in a 100 mL volumetric
flask to produce a 0.01 M NaCl solution, of which 10.00 mL is transferred to
another 100 mL volumetric flask and diluted to make the 0.00100 M solution, and
so on. Be very careful during this series of dilutions, since the solutions
will be indistinguishable and thus easy to get confused.
b) To a series of
dry 100 mL beakers, add 50.0 mL of the standard solutions of pCI = 1 to
7 and the challenge “unknown” samples. Use a
graduated cylinder, but measure the volume as carefully as you can. Add 1.00 mL
of 2.5 M KNO3 solution to each
solution to control the ionic strength. Immerse the Accumet chloride ISE in
each solution, measuring the voltage after a stable reading is obtained on the
meter (i.e., no change in the reading for about one minute).
c) For the standard
solutions with pCI of 3 and 6, record the voltage reading on the
Accumet chloride ISE every half minute (after immersion of the electrode) for
three minutes and then every full minute for ten minutes or until a stable
reading is obtained.
d) Construct a
calibration curve with voltage vs. pCl for use in procedure (2)
described below. Retain the two chloride-containing-unknown solution beakers
for the next procedure.
(2) Standard Addition Method
for Chloride
Measure the voltage
of the chloride-only unknown using the Accumet chloride ISE and use it to
approximate the concentration of the unknowns from the direct calibration.
Measure the voltage of the mixed unknown using the Accumet chloride ISE. Add
500 μL spikes of an appropriate stock sodium chloride standard solution (a
good choice is the one that is ~100x more concentrated than the unknown) to
both chloride-containing unknowns,
stir well, and measure the voltages once they stabilize. {You don’t need to add
any additional 2.5 M KNO3 because the spikes
are small relative to the total solution volume.} Add a second 500 μL spike to both
solutions, stir, and measure the voltage. Repeat with a third, fourth, and
fifth spike.
(3) Bromide Measurements
Repeat Procedures
(1) and (2) above using the bromide stock solution and the three unknowns with
the Accumet bromide ISE attached to the meter. Parts (b) and (c) will produce
one direct calibration plot and one set of time responses (for pBr = 3 and 6)
using the Accumet bromide ISE. Use only the two bromide-containing unknowns in
the standard addition procedure.
Report: In preparing the
Final version of this report, you should consider/complete /discuss the
following, in addition to including the subjects you covered in the Partial
Report filed the first week:
(1) Response curve: Plot voltage vs. time for the standard
solutions with pCI and pBr of 3.00 and 6.00 and estimate a
response time for each electrode/activity combination. Discuss any differences
in response time for different activities or electrodes. What do these findings
imply about the measurements that you made during the other parts of the
experiment (and potentiometric methods in general)?
(2) Direct calibration : Tabulate and plot the voltage vs. pCI
and pBr for the direct calibration
experiments. Discuss the nonlinearity, if any is observed. Find the
concentration range of linear response, obtain the linear equations, and
calculate the concentration of chloride and bromide in the challenge “unknowns”. Is there any evidence
of cross sensitivity between chloride and bromide in this method (i.e., does
the chloride ISE respond to changes in bromide concentration and vice versa)?
(3)
Standard addition: Tabulate the voltage vs. volume of standard
solution added for the chloride and bromide experiments. Calculate the
concentration of chloride and bromide in the samples using the method described
below. Be sure to include the Y vs. Vs
plots in your report. Compare the concentrations of chloride and bromide
obtained by standard addition with the ones calculated using the direct calibration
curve and discuss the "matrix effect".
(4)
Compare the potentiometric method to the fluorescence quenching method. Discuss
analytical factors such as precision, accuracy, linear dynamic range, etc. as
well as more practical factors such as speed, cost, and ease of use and
analysis.
Appendix:
Calculations for standard addition experiment with an ion-selective electrode
The calculation for
the standard addition method with an ion-selective electrode is not
straightforward and is described below. Two equations are used: (1) the
equation for potentiometric determination of the ion, and (2) the dilution
equation, as applied to your system of spikes.
(1) The equation for the potentiometric
determination of CI- as plotted in the direct or external
calibration method is (from Equation 1):
E (mV) = a + b log aCl-
= a + b log([CI-]γCl-)
= a + b log [CI-] + b log γCl-
= a' + b log [CI-] (at constant ionic strength, maintained by adding KNO3)
where
a'= a + b log γCl- and γCl-
= activity coefficient of Cl-
Please note a few points:
1. The
potentiometric method measures the activity, not the concentration, of Cl-.
2. The slope b is a property of an ion-selective electrode; it is determined
from the direct calibration measurement and will not change with matrix in the
standard addition measurement. Note that
the sign of b here does not match up with that from a plot with pCl (as in your
direct calibration). You will need to
change this for the indirect method.
3. The constant a’ changes with the
matrix because of the effect of ionic strength on γCl-. Although a' is
also determined from the direct calibration, it will take on a different value
in the standard addition measurement (in which the matrix is changed).
(2) The dilution equation
of your system, as derived for the determination of [CI-] in the
unknown sample prepared in 1.0 M KNO3.
Assume: Cx
= the concentration of Cl- in the unknown, [CI-]x
Vx = 50.0 mL is the volume of the unknown used in the measurement (the
51.0 below is from when you added the 1.0 mL of 2.5 M KNO3 to the unknown.)
Cs = concentration of Cl- in the standard solution used
for spikes, [CI-]s
Vs = total volume of the spikes added
The two equations can be related by substituting the concentration
of Cl- in the standard solution, Cs,
into the potentiometric relationship as the concentration of Cl-.
Before
the addition of spikes (containing the initial concentration of Cl-):
E0
= a' + b log [(50.0/51.0)Cx]
After
the addition of spikes: E = a' + b log [(50.0Cx +
VsCs) / (51.0 + Vs)]
In order to find the concentration of Cl- in the
sample, the unknown constant a' can be removed by
subtracting the above two equations.
ΔE = E – E0 = b log[ { (50.0 Cx + VsCs)
/ (51.0 + Vs) } / { (50.0/51.0) Cx } ]
The constants in this equation can then be lumped into one
side with ΔE, and graphed as the dependent
variable in a linear plot:
10ΔE
/ b = { (50.0 Cx + VsCs) / (51.0 + Vs) } / { (50.0/51.0) Cx }
Y
= (50.0/51.0)(51.0 + Vs) 10ΔE /
b = 50.0 + ( Cs / Cx
) Vs
Remember that b has
been determined in the direct calibration part of the experiment. Now you can
calculate ΔE and thus Y for the measurement made after each addition. By
plotting Y vs. Vs, a straight line should
be obtained where the intercept is equal to 50.0 and the slope is equal to Cs
/ Cx.
So, the final concentration of chloride in the unknown is given by Cx
= Cs / slope.
Revised 2014-1-21 - DBA