Engineering Chemistry Module 1 : Reference Electrodes

Students will be able to undersatnd the concept of Reference Electroede. Working of calomel , Ion Selective and glass electrode.

1. Dr. Swastika Das
Professor, Chemistry
BLDEACET, Vijayapura

2. Reference electrodes:
The electrodes of known potential are called Reference
electrodes
The electrode potential of ref electrode is stable and
reproducible.
Using ref electrodes, the potentials of other
electrodes can be measured.
Two types: i. Primary. ii. Secondary.

3. Primary Reference Electrode:
Standard Hydrogen Electrode :
TheStandardHydrogenElectrodeisoftenabbreviatedto
SHE,anditsstandardelectrodepotentialisdeclaredtobe
0atatemperatureof298K.Thisisbecauseitactsasa
referenceforcomparisonwithanyotherelectrode.
TheredoxhalfcelloftheSHEiswherethefollowing
reactiontakesplace:
2H
+
(aq)+2e
–
→H
2(g)
Thereactiongivenabovegenerallytakesplaceon
aplatinumelectrode.Thepressureofthehydrogengas
presentinthishalfcellequalsto1bar.

4. Standard Hydrogen Electrode:

5. Disadvantages of Primary electrode
ItisdifficulttomaintainH
+
ion
concentrationas1M.
Itisdifficulttomaintainaflowof1Atm
ofhydrogenattheinterfaceofPt
electrode.
Impuritiespresentinhydrogenflowmay
poisonthePtelectrodewhichaltersthe
potentialvalue.

6. Advantages of secondary
electrodes :
Theyhaveconstantandstablepotential
valuesataparticulartemperature.
TheyarecalibratedwithrespecttoSHE.
Theyhavetheleastpotentialgradientwith
temperature.
Thepotentialvaluesarereliableand
measurementscanbemadewithgreat
accuracy.

7. Calomel Electrode

8. Construction :
Calomelelectrodeisametal-sparinglysolublesaltelectrode.
Theelectrodeconsistsofanarrowglasstubeprovidedwithtwoside
tubes.
AlittleHgisplacedatthebottomoftheglasstube
followedbyalayerofHg
2Cl
2(mercurouschlorideor
calomel)andHg.
TheremainingpartoftheglasstubeisfilledwithKClsolutionof
definiteconcentration.
Athinglasstubewithaplatinumwireistheninserted,takingcarethat
theplatinumwiredripsintomercury.
Representation:
Hg|Hg
2Cl
2|KCl(Sat)
Hence,threetypesofCalomelelectrodesareavailable,viz.,
normal(1N),decinormal(0.1N)andsaturated.

9. Working :
The potential of the calomel electrode depends upon the concentration of the
chloride ions in solution. If the electrode is saturated calomel electrode (SCE),
some crystals ofKClare placed over theHg−Hg
2Cl
2paste so as to keep the
solution saturated.
Oxidation : If the electrode serves as anode, then half reaction that occurs on
it will be oxidation. Mercury is first oxidisedto mercuric ions.
2Hg(l)⟶Hg
2
2+
+ 2e−
The chloride ions supplied byKClsolution combine with mercuric
ions[Hg
2
2+
]to form insoluble mercurous chloride.
Thus,
Hg
2
2+
+2Cl
−
⟶ Hg
2Cl
2(s)
Overall reaction is,
2Hg(l) + 2Cl
-
(aq)−⟶Hg
2Cl
2(s)+2e−

10. Reduction : If the electrode is cathode(+)in the galvanic
cell, the half reaction that occurs on it will be reduction :

Hg
2Cl
2(s)⟶Hg
2
2+
+2Cl
−
 Hg
2
2+
+ 2e− ⟶2Hg(l)
Overall reaction:
Hg
2Cl
2(s)+2e
−
⟶2Hg(l)+ 2Cl(aq)
−
Equilibrium :
Hg
2Cl
2(s)+2e
−
⇌2Hg(l)+ 2Cl(aq)
−
Nernst equation :
 E = E
0
–0.0591 log [Cl-]
The potential of calomel electrode decreases with increase in the concentration of
chloride ions at a given temperature. Thus, electrode is reversible with respect to
concentration of chloride ions.
[KCl] = 0.1 N 1.0 N Saturated
E.P. = 0.3335 V 0.2810V 0.2422 V

11. Advantages & Disadvantages:
Advantages:
Easy construction
Easy maintenance
Reproducible and stable potential
Disadvantages:
Calomel electrode should not be used at above 50 0C
because the mercurous chloride breaks down yielding
unstable readings.
Since mercury is toxic this electrode is losing popularity.

12. Applications:
Used as a reference electrode in pH meter.
Used to measure potential of any other electrode.

13. Ion selective electrodes(ISE)
Ionselectiveelectrodes(ISE)–ThemembraneofISEissensitivetoa
particularkindofionsinasolution.
-Ionexchangetakesplacebetweenthemembraneelectrodeandthe
solutioncontainingthespecificionsanddevelopsapotential,which
dependsontheconcentrationoftheparticularionicspecies.
-Selectivelybindsandmeasurestheactivityofoneion.
Examples:•(i)pHelectrodeorglasselectrode
•(ii)Calcium(Ca
2+
)electrode
•Fluoride(F-)electrode
Anion-selectiveelectrode(ISE),alsoknownasaspecificionelectrode
(SIE),isatransducer(orsensor)thatconvertstheactivityofaspecific
iondissolvedinasolutionintoanelectricalpotential,whichcanbe
measuredbyavoltmeterorpHmeter.

14. AnidealI.S.E.consistsofathinmembraneacross
whichonlytheintendedioncanbetransported.
Thetransportofionsfromahighconc.toalow
onethroughaselectivebindingwithsomesites
withinthemembranecreatesapotential
difference.

15. How ISE work??
IonSelectiveElectrodes(ISE),aspecificionelectrode(SIE),isa
transducer(orsensor)thatconvertstheactivityofaspecificion
dissolvedinasolutionintoanelectricalpotential,whichcanbe
measuredbyavoltmeterorpHmeter.
Worksonthetheorythatanelectrodedevelopsapotentialdue
toion-exchangeoccurringbetweenthesampleandtheinorganic
membrane.
Thispotentialismeasuredagainstastablereferenceelectrodeof
constantpotential.
Thepotentialdifferencebetweenthetwoelectrodeswilldepend
upontheactivityofthespecificioninsolution.Thisactivityis
relatedtotheconcentrationofthatspecificion.

16. Components of ISE
1. Ion selective electrode with membrane at the end –
allows ions of interest to pass, but excludes the passage
of the other ions
2.Internal reference electrode –present within the ion
selective electrode which is made of silver wire coated
with solid silver chloride, embedded in concentrated
potassium chloride solution (filling solution) .
3.An internal reference solution depending on the
nature of the analytic ion.

17. The potential developed at the ion selective sensor is a
measure of the concentration of the ionic species of
interest.
The electrodes cannot only detect but also measure the
concentration of a specific chemical species directly in
a chemical solution are referred to as ion selective
electrode.
An ion selective membrane is the key component of all
potentiometric ion sensors. The voltage developed
between the sensing and the reference electrodes is a
measure of the concentration of the reactive ion being
measured. As the concentration of the reacting ion at
the sensing electrode varies, so does the voltage
measured between two electrodes.

18. TYPES OF ION SELECTIVE
ELECTRODE (ISE)
TYPES OF ION SELECTIVE ELECTRODE (ISE)
• Glass Membrane Electrode
• Solid State Electrode
• Liquid Membrane Electrode
• Gas Sensing Electrode
GLASS MEMBRANE ELECTRODE :
Glasselectrodeareresponsivetounivalentcations(H
+
,Na
+
).
GlassmembraneismanufacturedfromSiO2glassmatrixwith
variouschemicals.Insidetheglassbulb,adiluteHClsolutionis
usedasimternalreferencesolution.Asilver-silverchloride
electrodeisusedasinternalreferenceelectrode.
TheelectrodeisimmersedinthesolutionandpHtobe
measured.

19. Applications:
1. The most commonly used ISE is the pH probe.
2. CATIONS: Ammonium (NH4
+
), Barium (Ba
++
), Calcium
(Ca
++
), Cadmium (Cd
++
), Copper (Cu
++
), Lead (Pb
++
),
Mercury (Hg
++
), Potassium (K
+
), Sodium (Na
+
), Silver
(Ag
+
).
3. ANIONS: Bromide (Br-), Chloride (Cl-), Cyanide (CN-)
Fluoride (F-), Iodide (I-), Nitrate (NO3 -), Nitrite (NO2 -),
Perchlorate(ClO4 -) Sulphide(S-), Thiocyanate(SCN-).
4. Water hardness can be measured with Calcium ISE.
5. Nitrate monitoring in water sample can be done with
Nitrate ISE
6. Salinity and chloride monitoring in sea water
7.Monitoring steams for ammonium.
8. Determination of Calcium in milk.

20. Glass electrode:
ItisapHsensitiveelectrodeandismostwidelyused
forpHdetermination.
ItconsistsofaglassbulbmadeupofSiO2.
TheglassbulbisfilledwithasolutionofconstantpH
(0.1N)HCl.
OneAg-AgClelectrodeisusedasinternalreference
electrode.
Theglasselectrodeisdippedinasolutionwhich
containsH
+
ionsorthesolutionofunknownpH.

21. A glass electrode and Ag-AgClelectrode
combination
Glass electrode used in
laboratories.

23. Electrode potential of glass electrode:
The potential of glass electrode is given by the
following equation:
E
glass= E
0
glass+ 0.0591/n log [H
+
].
=>E
glass= E
0
glass–0.0591 pH.
Since, n=1 ; pH= -log [H
+
].

24. pH of unknown solution determination:
The glass electrode is connected to a saturated calomel
electrode.
Glass electrode is made anode and calomel electrode is made
cathode.
The complete cell representation:
Ag-AgCl/HCl(0.1N)/glass/analytesolution//KCl(sat),Hg2cl2,Hg(l)
(pH=?)
Whenathinglassmembraneisplacedbetweentwosolutions
ofdifferentpHvalues,apotentialarisesacrossthe
membrane.ThepotentialdifferencevariesasthepHofthese
twosolutionsvary.Inpractice,pHofinternalsolutioniskept
constantandthereforetheelectrodepotentialdependson
thepHoftheexperimentalsolutionsonly.

25. E
cell= E
cathode–E
Anode
E
cell= E
Calomel–E
glass
E
cell= 0.2422 –[E
0
glass–0.0591 pH]
E
cell-0.2422 + E
0
glass= 0.0591 pH.
=> pH = (E
cell-0.2422 + E
0
glass)/ 0.0591

26. Advantages :
1. It can be used in oxidizing and reducing environment
2. Electrode does not get poisoned.
3. Accurate results can be obtained for the pH range 1-14.
Disadvantages :
It doesn’t work properly in pure alcohol and in some organic
solvents.