Calibration of pH meters, glass electrodes and conductometers at INPL

Dr. Elena Kardash, Supervisor of the ProjectAddress: The National PhysicalLaboratory of Israel (INPL), Givat Ram, Jerusalem 91904, Israel tel.: +972-2-6536-534 fax: +972-2-6520-797e-mail: lena.ka@moital.gov.ilDr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL

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

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

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:

where k' is the practical slope of the glass electrode.

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

(2)

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

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

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

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

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

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

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

The standard is developed using the direct current method and includes conductivity cell. This standard allows to determine values from 510-4 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

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

Calibration of a conductometer. Calibration of a conductivity cell.

These calibrations assure the shortest unbroken traceability chain to the national measurement standards of Israel.

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.

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

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

*Dr. Elena Kardash Calibration of pH meters and glass electrodesDr. Elena Kardash Calibration of pH meters and glass electrodesDr. Elena Kardash Calibration of pH meters and glass electrodes