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

2 pH pH is defined as the negative decimal logarithm of the hydrogen ion activity a H. pH scale is based on the pH values of a series of standard buffer solutions determined by the potentiometric technique. pH meterpH-indicator glass electrode,reference electrodeautomatic temperature compensator 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 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 adjustment 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 pH value If emf corresponding to two buffer solutions S 1 and S 2 with pH S1 and pH S2 are E 1 and E 2, 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)

5 The practical slope k' usually differs from the theoretical slope - Nernst coefficient - k = RT/F (R is the universal gas constant J·mol -1 ·K -1, T is the Kelvin temperature, F is the Faraday constant, Coulombs·mol -1 ). between 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 systematic errors Some factors lead to systematic errors in pH measurement results, which can not be compensated by the adjustment procedure: asymmetry potential asymmetry potential of glass electrode, liquid junction potential liquid junction potential of cell (1), clogging ofdiaphragm clogging of glass electrode diaphragm etc. a calibration of the pH meter–electrodes assembly 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 traceability chain from the national metrological institution 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 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/AgCl│3M KCl║3M KCl║Test sol-n│H 2, Pt/Pt. (3) The emf of cell (3) depends on the activity of hydrogen ions according to the Nernst equation: E = E Ag/AgCl - k·lg a H (4) Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL

9 six primary pH buffers 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 ± pH. interlaboratory international key comparisons 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 pH-meter Calibration of a pH-meter (as a voltmeter). automatic temperature compensator Calibration of an automatic temperature compensator (as a thermometer). glass electrode Calibration of a glass electrode. buffers for pH- measurements. Certification of buffers for pH- measurements. shortest unbroken traceability chain 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  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 direct current  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. five standard conductivity solutions 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 Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL Conductometer calibrationcomparison Conductometer calibration is based on the comparison of the measuring results obtained by the tested conductometer and by INPL electrolytic conductivity standard. interlaboratory key comparison good agreement In the year 2003 INPL took part in the interlaboratory key comparison organized by the CCQM for solutions with low conductivity levels S·m -1. The results obtained were in good agreement with the assigned values.

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

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 Calibration of a 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. 53 mV/pH 59.5 mV/pH The experimental slope Sl HE of the linear calibration curve “E-pH”, measured with hydrogen electrode, and the similar slope Sl GE of the tested glass electrode should be not less than 53 mV/pH and not greater than 59.5 mV/pH at 25 o C. The ratio (Sl GE / Sl HE )  100, % should be not less than 90%. Calibration of a 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. 53 mV/pH 59.5 mV/pH The experimental slope Sl HE of the linear calibration curve “E-pH”, measured with hydrogen electrode, and the similar slope Sl GE of the tested glass electrode should be not less than 53 mV/pH and not greater than 59.5 mV/pH at 25 o C. The ratio (Sl GE / Sl HE )  100, % should be not less than 90%. Dr. Elena Kardash - Calibration of pH meters, glass electrodes and conductometers at INPL

17 pH meter calibration 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 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