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Calibration of pH meters and glass electrodes at the National Physical Laboratory of Israel

Elena Kardash and Ilya Kuselman

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

    pH is one of the most frequently measured physico-chemical properties in fields of health care, safety, pharmaceutical industry, environmental control and others [1]. The pH value is defined as the negative decimal logarithm of the hydrogen ion activity aH:

                                                           pH = -lg aH                                                          (1)

Only in highly dilute (so-called “ideal”) solutions, the activity is equal to the hydrogen ion concentration. Single ion activity is not measurable by any thermodynamically valid method and requires a convention for its evaluation. For this reason, a pH scale is based on the pH values of a series of standard buffer solutions determined by the potentiometric technique [2]. Routine potentiometric measurements of pH are carried out by means of a pH meter, accompanied by a temperature sensor, a pH-indicator glass electrode and a reference electrode. The following cell is required for the pH measurements:

          glass electrode│test solution║salt bridge solution│reference electrode,            (2)

where the single vertical bar is the electrode-solution interface, and the double vertical bar is the liquid junction denoting an interface between the test solution and the salt bridge solution. Glass electrode may be designed in combination with an external

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