Calibration of pH meters, glass electrodes and conductometers at INPL Dr. Elena Kardash, Supervisor of the Project

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Calibration of pH meters, glass electrodes and
conductometers at INPLDr. Elena Kardash,
Supervisor of the Project
Address The National Physical Laboratory of
Israel (INPL), Givat Ram, Jerusalem 91904,
Israeltel. 972-2-6536-534 fax
972-2-6520-797 e-mail lena.ka_at_moital.gov.il
Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Introduction to calibration of pH meters

• pH is defined as the negative decimal logarithm
  of the hydrogen ion activity aH.
• pH scale is based on the pH values of a series of
  standard buffer solutions determined by the
  potentiometric technique.
• Routine potentiometric measurements of pH are
  carried out by means of a pH meter equipped by a
  pH-indicator glass electrode, a reference
  electrode and an automatic temperature
  compensator.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Cell and pH meter for routine pH measurements

• The following cell is required for the pH
  measurements
•  
• glass electrodetest solutionbridge
  solutionref. electrode, (1)
•  
• where is the electrode-solution interface, and
  is the liquid junction denoting an interface
  between the test solution and the salt bridge
  solution.
• pH meter is a high impedance voltmeter. Direct pH
  readings are possible after adjustment of a pH
  meter.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Calculation of pH value

• If emf corresponding to two buffer solutions S1
  and S2 with pHS1 and pHS2 are E1 and E2,
  respectively, the pH value of the unknown (test)
  solution can be calculated from its measured emf
  value E by the following formula

(2)
where k' is the practical slope of the glass
electrode.
Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Nernst coefficient

• The practical slope k' usually differs from the
  theoretical slope - Nernst coefficient - k
  2.3026 RT/F (R is the universal gas constant
  8.31451 Jmol-1K-1, T is the Kelvin temperature,
  F is the Faraday constant, 96485.309
  Coulombsmol-1).
• The pH value to be determined should lie between
  the pH values of the two adjacent buffer
  solutions.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Why calibration is necessary

• Some factors lead to systematic errors in pH
  measurement results, which can not be compensated
  by the adjustment procedure
• asymmetry potential of glass electrode,
• liquid junction potential of cell (1),
• clogging of glass electrode diaphragm etc.
• This problem can be solved by a calibration of
  the pH meterelectrodes assembly.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Traceability

• The IUPAC recommendations of the year 2002
  require at establishing a traceability chain of
  the pH measurement results from the national
  metrological institution level down to field,
  laboratory and industry measurements.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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The national primary standard of pH value

• The national primary standard of pH value
  developed at INPL consists of a precise
  pH/ion-meter and the following electrochemical
  cell
• Ag/AgCl3M KCl3M KClTest sol-nH2,Pt/Pt.
  (3)
• The emf of cell (3) depends on the activity of
  hydrogen ions according to the Nernst equation
• E EAg/AgCl - klg aH
  (4)

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Key comparisons of pH measurement results

• Calibration of cell (3) is carried out using six
  primary pH buffers at 25C. These buffers are
  prepared from the NIST standard reference
  materials with expanded uncertainties of
  0.005 pH.
• The national pH value standard have participated
  successfully in two interlaboratory international
  key comparisons in the years 2002 and 2004
  organized by the CCQM.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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INPL services based on the pH national standard

• Calibration of a pH-meter (as a voltmeter).
• Calibration of an automatic temperature
  compensator (as a thermometer).
•   Calibration of a glass electrode.
•   Certification of buffers for pH- measurements.
• These calibrations assure the shortest unbroken
  traceability chain to the national measurement
  standards of Israel.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Development of the national standard of
electrolytic conductivity
Electrolytic conductivity (?) is the measure of
a solutions ability to conduct current. Reliable
conductivity measurements require traceability to
national or international measurement
standards. INPL has developed the such Israeli
standard.  
Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Scheme of the national standard of electrolytic
conductivity
The standard is developed using the direct
current method and includes conductivity cell.
This standard allows to determine ? values from
5?10-4 up to 10 Sm-1. Calibration of the
conductivity cell at fixed temperature is carried
out using five standard conductivity solutions
from NIST.
Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Calibration of conductometers and conductivity
cells
Conductometer calibration is based on the
comparison of the measuring results obtained by
the tested conductometer and by INPL electrolytic
conductivity standard. In the year 2003 INPL
took part in the interlaboratory key comparison
organized by the CCQM for solutions with low
conductivity levels. 0.005 Sm-1. The results
obtained were in good agreement with the
assigned values.
Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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INPL services based on the national standard of
electrolytic conductivity

• Calibration of a conductometer.
• Calibration of a conductivity cell.
• These calibrations assure the shortest unbroken
  traceability chain to the national measurement
  standards of Israel.

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Conclusions

• The national standards of pH and electrolytic
  conductivity values have been developed and
  passed successfully interlaboratory key
  comparisons.
• INPL proposes calibration of corresponding
  equipment based on these standards.

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Calibration of glass electrode

• Calibration of a glass electrode is performed by
  measuring of emf values with this electrode
  simultaneously with the reference hydrogen
  electrode in the same cell and buffer solutions.
• The experimental slope SlHE of the linear
  calibration curve E-pH, measured with hydrogen
  electrode, and the similar slope SlGE of the
  tested glass electrode should be not less
  than 53 mV/pH and not greater than 59.5 mV/pH
  at 25oC. The ratio (SlGE / SlHE)?100, should be
  not less than 90.

Dr. Elena Kardash - Calibration of pH meters,
glass electrodes and conductometers at INPL
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Calibration of pH meter and temperature
compensator

• In order to make a pH meter calibration
  independent 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