A pH-meter is an electronic apparatus often allowing the measurement of pH of a aqueous Solution.

Operation

The pH-meter generally consists of a black box allowing the posting of the numerical Value of the pH and of a electrode of Verre allowing measurement. Its operation is based on the report/ratio which exists between the concentration in ions $H\_3O^+$ (definition of the pH) and the difference of Electrochemical potential which is established in the electrode of glass. Actually the latter consists of two electrodes, one standard whose potential constant and is known, the other with variable potential (function of the pH). With pH 7 (Acid neutral neither nor Basic) the Potentiel between the two electrodes is null. Once the apparatus standard born using solutions Plug (often of pH 4,7 and 10), one can determine the value of the pH by simple correlation, the potential difference proportionally evolving/moving with the value of the pH according to this formula:

$\backslash \; Delta\; E=a\; (pH\_a-pH\_b)\; +b\; \backslash ,$

• $\backslash \; Delta\; E$ is the potential difference between the two electrodes.

• $pH\_a$ is the pH of the solution to be measured.
• $pH\_b$ is the pH of the reference solution.
• has and B depend on the apparatus, they are revealed during the calibration of the pH-meter.

Calibration

One calibrates a pH-meter with buffer solutions. According to measurements which one will take, one calibrates either by a solution of pH=7 then by a solution of pH=4 to make measurements in acid medium, or by a solution of pH=7 then a solution of pH=10 for measurements in basic medium. (these values of pH are most commonly recorded at the buffer solutions.)

Diagram of the electrode

1. active Glass
2. Crystals of KCl (visible in certain cases)
3. Solution saturated with KCl
4. Calomel electrode (or Ag/AgCl)
5. Body with the electrode
6. Reference to calomel (or Ag/AgCl)
7. Junction with the solution to study

See too

• Arnold Orville Beckman, chemist, inventor of the pH-meter,

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