Calibration-pH-meter.ppt

Calibration of pH meter, use, principle and calibration

1. 1
Calibration of pH meters, glass electrodes
and conductometers at INPL
Dr. Elena Kardash, Supervisor of the Project
Address:The National Physical
Laboratory of Israel (INPL),
Givat Ram, Jerusalem 91904, Israel
tel.: [phone]
fax: [phone]
e-mail: [email protected]
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

2. 2
pHisdefinedasthenegativedecimallogarithmofthe
hydrogenionactivitya
H
.
pHscaleisbasedonthepHvaluesofaseriesofstandard
buffersolutionsdeterminedbythepotentiometric
technique.
RoutinepotentiometricmeasurementsofpHarecarried
outbymeansofapHmeterequippedbyapH-indicator
glasselectrode,areferenceelectrodeandanautomatic
temperaturecompensator.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

3. 3
ThefollowingcellisrequiredforthepHmeasurements:
glasselectrode│testsolution║bridgesolution│ref.electrode,(1)
where│istheelectrode-solutioninterface,and║isthe
liquidjunctiondenotinganinterfacebetweenthetest
solutionandthesaltbridgesolution.
pHmeterisahighimpedancevoltmeter.DirectpH
readingsarepossibleafteradjustmentofapHmeter.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

4. 4
IfemfcorrespondingtotwobuffersolutionsS
1
andS
2
withpH
S1
andpH
S2
areE
1
andE
2
,respectively,thepH
valueoftheunknown(test)solutioncanbecalculated
fromitsmeasuredemfvalueEbythefollowingformula:1
,
2
+
21
pH pH
SS EE
1
pH pH (E E ) pH
S 1 S
E E k'11



  

wherek'is the practical slope of the glass electrode.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL
(2)

5. 5
Thepracticalslopek'usuallydiffersfromthetheoretical
slope-Nernstcoefficient-k=2.3026RT/F(Risthe
universalgasconstant8.31451J·mol
-1
·K
-1
,Tisthe
Kelvintemperature,FistheFaradayconstant,96485.309
Coulombs·mol
-1
).
ThepHvaluetobedeterminedshouldliebetweenthe
pHvaluesofthetwoadjacentbuffersolutions.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

6. 6
Some factorslead to systematic errorsin pH
measurement results, which can not be compensated by
the adjustment procedure:
asymmetrypotentialofglasselectrode,
liquidjunctionpotentialofcell(1),
cloggingofglasselectrodediaphragmetc.
ThisproblemcanbesolvedbyacalibrationofthepH
meter–electrodesassembly.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

7. 7
TheIUPACrecommendationsoftheyear2002requireat
establishingatraceabilitychainofthepHmeasurement
resultsfromthenationalmetrologicalinstitutionlevel
downtofield,laboratoryandindustrymeasurements.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

8. 8
ThenationalprimarystandardofpHvaluedeveloped
atINPLconsistsofaprecisepH/ion-meterandthe
followingelectrochemicalcell:
Ag/AgCl│3MKCl║3MKCl║Testsol-n│H
2,
Pt/Pt.(3)
Theemfofcell(3)dependsontheactivityofhydrogen
ionsaccordingtotheNernstequation:
E=E
Ag/AgCl
-k·lga
H
(4)
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

9. 9
Calibrationofcell(3)iscarriedoutusingsixprimary
pHbuffersat25°C.Thesebuffersarepreparedfromthe
NISTstandardreferencematerialswithexpanded
uncertaintiesof±0.005pH.
ThenationalpHvaluestandardhaveparticipated
successfullyintwointerlaboratoryinternationalkey
comparisonsintheyears2002and2004organizedby
theCCQM.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

10. 10
CalibrationofapH-meter(asavoltmeter).
Calibrationofanautomatictemperature
compensator(asathermometer).
Calibrationofaglasselectrode.
CertificationofbuffersforpH-measurements.
Thesecalibrationsassuretheshortestunbroken
traceabilitychaintothenationalmeasurement
standardsofIsrael.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

11. 11
Electrolyticconductivity()isthemeasureofasolution’s
abilitytoconductcurrent.Reliableconductivity
measurementsrequiretraceabilitytonationalor
internationalmeasurementstandards.
INPLhasdevelopedthesuchIsraelistandard.
Dr.ElenaKardash-CalibrationofpHmeters,glasselectrodesandconductometersatINPL

12. 12
Thestandardisdevelopedusingthedirectcurrent
methodandincludesconductivitycell.Thisstandard
allowstodeterminevaluesfrom5ּ10
-4
upto10S·m
-1
.
Calibrationoftheconductivitycellatfixedtemperature
iscarriedoutusingfivestandardconductivity
solutionsfromNIST.
Dr.ElenaKardash-CalibrationofpHmeters,glasselectrodesandconductometersatINPL

13. 13
Dr.ElenaKardash-CalibrationofpHmeters,glasselectrodesandconductometersatINPL
Conductometercalibrationisbasedonthecomparison
ofthemeasuringresultsobtainedbythetested
conductometerandbyINPLelectrolyticconductivity
standard.
Intheyear2003INPLtookpartintheinterlaboratory
keycomparisonorganizedbytheCCQMforsolutions
withlowconductivitylevels.0.005S·m
-1
.Theresults
obtainedwereingoodagreementwiththeassigned
values.

14. 14
Calibrationofaconductometer.
Calibrationofaconductivitycell.
Thesecalibrationsassuretheshortestunbroken
traceabilitychaintothenationalmeasurement
standardsofIsrael.

15. 15
ThenationalstandardsofpHandelectrolytic
conductivityvalueshavebeendevelopedandpassed
successfullyinterlaboratorykeycomparisons.
INPLproposescalibrationofcorresponding
equipmentbasedonthesestandards.

16. 16
Calibrationofaglasselectrodeisperformedby
measuringofemfvalueswiththiselectrode
simultaneouslywiththereferencehydrogenelectrodein
thesamecellandbuffersolutions.
TheexperimentalslopeSl
HEofthelinearcalibration
curve“E-pH”,measuredwithhydrogenelectrode,and
thesimilarslopeSl
GEofthetestedglasselectrode
shouldbenotlessthan53mV/pHandnotgreater
than59.5mV/pHat25
o
C.Theratio(Sl
GE/Sl
HE)100,
%shouldbenotlessthan90%.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL

17. 17
In order to make a pH meter calibrationindependent of
the vagaries of electrode performance, the calibration is
conducted by the pH meter comparison with known d.c.
potentiometer traceable to the national standard of volt.
Calibration of a automatic temperature compensator
is done by comparison of temperature values measured
using this compensator with simultaneous readings of the
reference glass thermometer, traceable to the national
temperature standard.
Dr. Elena Kardash -Calibration of pH meters, glass electrodes and conductometers at INPL