This document discusses calibration services provided by Dr. Elena Kardash at the National Physical Laboratory of Israel (INPL). INPL has developed national standards for pH and electrolytic conductivity that allow for traceable calibration of pH meters, glass electrodes, conductometers, and other equipment. Dr. Kardash offers on-site calibration of these devices using INPL's standards to ensure measurements can be tied directly to Israel's national measurement standards. Calibration procedures involve comparing the readings from a device to INPL's primary standards under controlled conditions.

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.: +972-2-6536-534
fax: +972-2-6520-797
e-mail: lena.ka@moital.gov.il
Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL

2. 2
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

3. 3
The following cell is required for the pH measurements:
glass electrode│test solution║bridge solution│ref. 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

4. 4
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:
1
,
2
+
2 1
pH pH
S S E E1
pH pH (E E ) pH
S 1 S
E E k'
1 1

 
  

where k' is the practical slope of the glass electrode.
Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL
(2)

5. 5
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 J·mol-1·K-1, T is the
Kelvin temperature, F is the Faraday constant, 96485.309
Coulombs·mol-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

6. 6
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
meter–electrodes assembly.
Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL

7. 7
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

8. 8
The national primary standard of pH value developed
at INPL consists of a precise pH/ion-meter and the
following electrochemical cell:
Ag/AgCl│3M KCl║3M KCl║Test sol-n│H2,Pt/Pt. (3)
The emf of cell (3) depends on the activity of hydrogen
ions according to the Nernst equation:
E = EAg/AgCl - k·lg aH (4)
Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL

9. 9
Calibration of cell (3) is carried out using six primary
pH buffers at 25°C. 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

10. 10
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

11. 11
Electrolytic conductivity () is the measure of a solution’s
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

12. 12
The standard is developed using the direct current
method and includes conductivity cell. This standard
allows to determine  values from 5ּ10-4 up to 10 S·m-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

13. 13
Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL
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 S·m-1. The results
obtained were in good agreement with the assigned
values.

14. 14
Calibration of a conductometer.
Calibration of a conductivity cell.
These calibrations assure the shortest unbroken
traceability chain to the national measurement
standards of Israel.

15. 15
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.

16. 16
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

17. 17
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