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

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.
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.

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.
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)
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.
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.
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%.
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.
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