Title Potentiometry: Titration of a Halide Ion Mixture Name Manraj Gill (Lab partner: Tanner Adams) Abstract Potentiometric titrations of a sample using a system of a electrolytic cell can be used to analyze the concentration and identify species of halides present in any sample. We identified I - and Cl - halides in Unknown #21 and accurately determined their concentrations in this sample. And we additionally determined that around 0.5M Cl - is present in seawater and it is the most prominent halide present in seawater. Purpose In this series of experiments, we use potentiometric titrations (gradual addition of titrant to a solution while measuring the potential of a electric cell) to measure the concentrations of halides in solutions. This is achieved by titrating the sample containing the halides with a standardized silver ion solution (in this case, silver nitrate, AgNO3). This titration leads to the halides precipitating out of the solution (described in detail in Theory and Methods) and thereby allows us to potentiometrically determine the concentrations of the halides that were initially present. We use this approach to determine the identity of an unknown solution in terms of its specific constituents. And we then extend this approach, in Part II, to the analysis of seawater to determine the most prominent halide in the seawater sample and the total salinity of this sample! Theory and Methods The approach used towards understanding the composition of the samples analyzed relies on measuring the potential by observing a voltmeter. The half-cells created for this purpose consist of an indicator electrode and a reference electrode. The indicator electrode is this case is the unknown (or seawater in Part II). We create the reference electrode is immersed in a solution of 1M KNO3 and we use this reference electrode (or reference half-cell) because of it’s ability to maintain a reproducibly high and constant potential (1). Additionally, the used of a salt-bridge allows us to separate the two electrodes (reference and test/indicator) from each other and allows for the reference electrode potential to be constant! This approach, described above, allows us to correlate any chemical alterations reflected in the measured potential of the entire cell to be reflective of the test electrode! Additionally, we use the reduction-oxidation (redox) couple of silver metal (submerged in the test electrode) and silver ion (the titrant or silver nitrate (AgNO3 mentioned in the purpose section) as an “internal standard” (2). This allows us to derive the concentrations of the halides present in the indicator electrode because of the different potentials of the redox reactions between silver and the halides! The titrant added dissociates into silver ions and these silver ions interact with the halides in solution. This interaction leads to the concentrations of the halide ions decreasing as
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Title Potentiometry: Titration of a Halide Ion Mixture
Name Manraj Gill (Lab partner: Tanner Adams)
Abstract
Potentiometric titrations of a sample using a system of a electrolytic cell can be used to
analyze the concentration and identify species of halides present in any sample. We
identified I- and Cl- halides in Unknown #21 and accurately determined their
concentrations in this sample. And we additionally determined that around 0.5M Cl- is
present in seawater and it is the most prominent halide present in seawater.
Purpose
In this series of experiments, we use potentiometric titrations (gradual addition of titrant
to a solution while measuring the potential of a electric cell) to measure the
concentrations of halides in solutions. This is achieved by titrating the sample containing
the halides with a standardized silver ion solution (in this case, silver nitrate, AgNO3).
This titration leads to the halides precipitating out of the solution (described in detail in
Theory and Methods) and thereby allows us to potentiometrically determine the
concentrations of the halides that were initially present. We use this approach to
determine the identity of an unknown solution in terms of its specific constituents. And
we then extend this approach, in Part II, to the analysis of seawater to determine the most
prominent halide in the seawater sample and the total salinity of this sample!
Theory and Methods
The approach used towards understanding the composition of the samples analyzed relies
on measuring the potential by observing a voltmeter. The half-cells created for this
purpose consist of an indicator electrode and a reference electrode.
The indicator electrode is this case is the unknown (or seawater in Part II). We create the
reference electrode is immersed in a solution of 1M KNO3 and we use this reference
electrode (or reference half-cell) because of it’s ability to maintain a reproducibly high
and constant potential (1). Additionally, the used of a salt-bridge allows us to separate the
two electrodes (reference and test/indicator) from each other and allows for the reference
electrode potential to be constant!
This approach, described above, allows us to correlate any chemical alterations reflected
in the measured potential of the entire cell to be reflective of the test electrode!
Additionally, we use the reduction-oxidation (redox) couple of silver metal (submerged
in the test electrode) and silver ion (the titrant or silver nitrate (AgNO3 mentioned in the
purpose section) as an “internal standard” (2). This allows us to derive the concentrations
of the halides present in the indicator electrode because of the different potentials of the
redox reactions between silver and the halides!
The titrant added dissociates into silver ions and these silver ions interact with the halides
in solution. This interaction leads to the concentrations of the halide ions decreasing as
the Ag-X (X denoting a halide species) compound precipitates out of the solution. The
potential of the cell starts off at a negative value as the flow of electrons is from the silver
wire to the reference electrode. This potential remains constant (for an experimentally
relative time) and becomes progressively positive as more titrant is added. It is due to this
reversal in the flow of electrons that we observe plating on the silver wire used in the test
electrode because once the potential is positive, the silver wire is serving as the cathode!
The important measurement is of the volume of the titrant added at the equivalence points
because it from these endpoints that we can calculate the amount of each halide present.
The equivalence points can be determined using the first-derivative method of analysis as
the equivalence point is the small amount of volume that leads to the drastic increase in
measurement (pH or in this case, voltage). Therefore, the volume at the maximum of the
first-derivative corresponds to the volume at the equivalence point.
Results and Application of Theory
In Part I of this experiment, we determine the concentration of each halide ion in the
unknown sample. Unknown sample number: #21
The halides present (of varying concentrations) in each unknown are Chlorine (Cl-),
Bromine (Br-) and Iodine (I-). The following page shows a preliminary titration
performed to gain a rough idea of where the equivalence points are found because this
then allows us to be more precise in the three consequent “accurate” measurements of the
titrations that can be used for statistical analysis.
As evident by the performed first derivative analysis of this preliminary titration, the
equivalence points lie around silver nitrate volumes of ~14.75ml and ~42.75ml (the two
maxima of the 1st derivative plot).
Based on this preliminary observation, three similar titrations were performed to obtain
accurate values (with small standard deviations) for the equivalence points. The three
repeat measurements give us the following values for the equivalence points (the graphs
are over-laid to represent consistency in measurements and to avoid redundancy in the
represented data)
1st Equivalence Point (ml) 2nd Equivalence Point (ml)
1st Measurement 15.15 42.82
2nd Measurement 15.03 42.65
3rd Measurement 15.35 42.05
Mean Value 15.17 42.51
Standard Deviation 0.16 0.40
95% Confidence Interval +/- 0.27 +/- 0.68
Therefore,
1st equivalence point at 15.17 +/- 0.27ml and
2nd equivalence point at 42.51 +/- 0.68ml
Analysis of the potentiometric data and determination of the halide concentrations is
continued after the Voltage to Volume titration plots and 1st derivative graphs below.
-0.4
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Vo
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Me
asu
red
Silver Nitrate (ml)
Titration Plot of Actual Voltage Measured:
Preliminary Titration of Unknown
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0
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0 10 20 30 40 50 60
1st
De
riva
tive
(d
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/ d
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lum
e)
Silver Nitrate (milliliters)
1st Derivitive Plot:
Preliminary Titration of Unknown
-0.4
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0 10 20 30 40 50 60
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d
Silver Nitrate (ml)
Titration Plot of Actual Voltage Measured:
Accurate Titrations of Unknown
1st
2nd
3rd
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0
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0 10 20 30 40 50 60
1st
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e)
Silver Nitrate (ml)
1st Derivite Plot:
Accurate Titrations of Unknown
1st
2nd
3rd
These potentiometric titration curves plot the voltage of the entire cell as milliliters silver
nitrate is added. Upon addition of this silver nitrate titrant, the reaction of silver ions with
the halide ions results in the formation of the solid precipitate that we observe (yellowish
in color, see Observations column in the excel data sheet for more details).
Therefore, in measuring the potential of the test electrode, we need to keep in mind [Ag+]
(i.e. the concentration of the Ag+ ions). We focus on the Ag+ ions and not only on the
concentrations of the halide ions because the indicator electrode responds to [Ag+]!
The potential for the test electrode is given by the Nernst Equation (3) as follows:
E(test) = E°(Ag/AgX) – (RT/F) ln[X-]
Which, if we consider the variation in the different solubilities of the AgX compounds
and take into account the solubility products (Ksp), turns to: