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Potentiometric
Instruments
270
• Potentiometry is the field of electroanalytical
chemistry in which potential is measured. • The measured potential may then be used to determine:
– The analytical quantity of interest
– Generally the concentration of some component of the analyte solution. • The potential that develops is the result of the free energy change that occur
free energy change that occur
– if a chemical phenomena were to proceed until the equilibrium condition has been satisfied. 271
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1. Potentiometric Endpoint Detection
• Titrations can be carried out using a suitable electrode to measure the potential of the solution as the titration progresses. • The advantage of making potentiometric
measurements:
– The measurements can be made in solutions which are coloured
– Used in titration which give unambiguous endpoints where indicator colour changes are not clear or
where indicator colour changes are not clear or sudden. 272
• Electrode is usually used to make the measurements in potentiometric titrations.
• The electrode monitors the variation in the potential difference • The interaction of hydrogen ions with the outer surface of the pH sensitive glass membrane cause this potential difference.
• This electrode consists of:
1. pH sensitive glass membrane bulb which encloses:
• Phosphate buffer solution containing potassium chloride solution and saturated with silver chloride. • The solution is in contact with an internal reference electrode 273
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2. The internal reference:
– Which consists of a silver wire – It is sensitive to the hydrogen ion
– It develops a potential (voltage) directly related to
It develops a potential (voltage) directly related to
the hydrogen ion concentration of the solution. 3. The circuit is completed by a second external reference electrode
– Which is a second silver/silver chloride electrode – The reference electrode provides a stable potential The reference electrode provides a stable potential
against which the measuring electrode can be compared.
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• With decreasing of [H+] the potential becomes increasingly negative with respect to inner surface with increasing pH.
• The Nernst equation can be simplified and written in the following form for the glass electrode when the temperature is 20°C:
E = Ek ‐ 0.0591pH.
• Where E is then measured potential in volts • Ek is a constant composed of the sum various potential differences within the electrode which do not vary appreciably. • The electrode is constructed so that its potential is ca 0 V at pH 7 0
pH 7.0 • It can be seen from the equation above that E changes by 59.1 mV for each pH unit.
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• When potentiometric titration is carried out the volume of titrant added is plotted against the measured potential. The titrant is added in 0.1 ml amounts until the is added in 0.1 ml amounts until the
• The titrant
steep inflection in the titration curve is passed.
• The endpoint of the titration is the point where the slope of the titration curve is at its maximum. • Thus if dE/dV is plotted for the titration the maximum of the plot gives the endpoint. p g
p
• The endpoint can also be determined from the mid point of the inflection in the titration curve or from the tabulated data. 277
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• The figure shows a curve for the titration of 2 mM
of aspirin with 0.1 M NaOH. • The endpoint corresponds to the midpoint of the The endpoint corresponds to the midpoint of the
inflection
• It can be taken to be the mid point between the two volumes where dE/dV is greatest
• At 20.05 ml between 20 and 20.1 ml. 278
ml of 0.1M
NaOH added
14 16
18
19 19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.8
19.9 20
20.1
20.2
20.3
20.4
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potential
(mV)
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172
151
132
129
126
122
119
113
107
100
89
71
‐35
‐177
‐195
‐206
‐212
‐236
‐266
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The Use of Potentiometric Titration to Determine pKa Values
• Potentiometric titration provides the principal method for determining pKa values,
• It is best applied to substances with pKa values < 11. • For example the pKa of benzoic acid can determined as follows: – A 0.01 M solution of benzoic acid (50 ml) is titrated with 0.1 M KOH. – The KOH is added in 0.5 ml increments; – It would be expected that 5 ml of 0.1 M KOH would be required p
q
to neutralise the benzoic acid. – The pH of the titration is monitored with and glass electrode and the pH of the mixture after 2.5 ml of 0.1 M KOH has been added will equal the pKa value of benzoic acid since.
[C6H5COOK]
pKa = pH - log
[C6H5COOH]
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Automatic Titration
• Titrations can be automated and controlled by a microprocessor. • The titrant is delivered via an automatic burette and the endpoint is detected potentiometrically
and the endpoint is detected potentiometrically
with a glass combination electrode.
• If ions other than hydrogen are being measured another ion selective electrode may be used. • The microprocessor control also enables the instrument to be set to calculate a pKa values di tl f
directly from the pH profile it obtains by titration th H
fil it bt i b tit ti
of a sample.
• A sample changer can be incorporated so that several samples can be automatically titrated.
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• The apparatus is microprocessor controlled and can be programmed to run in various modes:
– The rate of delivery of the titrant can be controlled according to rate of change of potential • So it is added more slowly as the rate of change in potential increases i.e as the endpoint is approached. – For titrations which take time to equilibrate the instrument can be programmed to delay after
instrument can be programmed to delay after each incremental addition until the potential becomes stable. 282
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2. Karl Fischer Titration for water detection
• Coulometry is the name given to a group of techniques in analytical chemistry that determine
techniques in analytical chemistry that determine the amount of matter by measuring the amount of electricity (in coulombs) consumed or produced.
• The Karl Fischer titration is a type of coulometric
titration.
• Coulometry measures the amount of charge which has been passed through a solution of analyte undergoing reduction or oxidation. 284
• The amount of charge can be translated to the number of moles of analyte present in solution.
• According to Faraday's law one molecule reacts with one electron
– 1 mole of analyte will react with 96485 coulombs of electricity where coulombs = amps x s. – Current = coulombs/time
– Faraday = coulombs per mole (coul/mol)= 96,485
– Moles electrons = current x time / F
• In the case of the Karl Fischer titration the end‐
point detection is based on the following reaction
I2 + 2e-
2I285
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• The end‐point is detected by a pair of platinum electrodes
– They are set so that they supply a constant current to the titration cell. • When excess iodine is produced at the endpoint the resistance of the cell falls.
• Since the instrument is set to apply a constant current the endpoint is detected electronically as a steep fall in the applied voltage .
f ll i h
li d l
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• The Karl Fischer reagent consists of :
– A mixture of anhydrous methanol
– An anhydrous base • Like pyridine but bases such as imidazole or diethanolamine and more commonly used now),
– Iodine and sulphur dioxide.
– Various other inert co‐solvents may be used in the preparation of the reagent.
• The reaction that occurs in the presence of p
water involves the reduction of iodine to iodide by sulphur dioxide
H2O + I2 + SO2 + 3C5H5N + CH3OH
2C5H5N.HI + C5H5NH.SO4CH3
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• A variety of automated Karl Fischer systems are available. – The apparatus used basically consists of a titration vessel of about 60 ml capacity – Fitted with two platinum electrodes,
– A nitrogen inlet tube
– Burette
– A vent tube protected by a suitable desiccant. A vent tube protected by a suitable desiccant.
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• The substance being examined is introduced through an inlet tube
• Sample is stirred during the titration with a magnetic stirrer. i
• The potential is adjusted so a current of 10 µA passes between the platinum electrodes. • At the end of the reaction a steep fall in potential indicates the presence of excess iodine in solution. Karl Fischer volumetric titration can be used to
• Karl Fischer volumetric titration can be used to determine water in between ca 10 µg and several hundred mg.
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An alternative coulometric titration: • The instrument used generates iodine from iodide at a platinum anode
• Any water present in the sample solution immediately coverts the iodine back to iodide until the endpoint is reached. • The endpoint is detected as in the volumetric titration using a pair of platinum electrodes in addition to the anode used to generate the iodine in situ. • At the endpoint a steep drop in potential . • In this case the amount of water present is determined f
from the number of coulombs required to generate th
b
f
l b
i dt
t
iodine • The coulometric apparatus is best used for low levels of water between 10 µg and 200 mg per sample.
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The Karl‐Fischer titration is used to quantify
water in pharmacopoeial assays of:
• Ampicillin trihydrate, • Benzylpenicillin sodium,
• Gentamycin sulphate, • Tobramycin.
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