pH Adaptor & Electrode
11
Oil, Grease
: - Carefully wash the electrode under warm tap water using a non-filming dish washing
detergent or stain removing pre wash pre-treatment. Do not use automatic or electric dish washing
detergents. On overnight soak may be needed if build-up is heavy. Rinse thoroughly with fresh tap
water followed by a three rinses of distilled water. Soak the electrode in its storage solution for at
least 30 minutes before use.
Protein & Fatty Materials
: - Either gently wipe the bulb with a tissue soaked in propanol or soak in
1% pepsin in 0.1 mol dm
-3
hydrochloric acid (HCl) for at least 10 minutes or soak the pH electrode in
contact lens enzymatic cleaner solution overnight. Rinse thoroughly with distilled.
Highly resistant deposits
: - Clean with H
2
O
2
or sodium hyperchlorite.
Bacterial cultures
: - Chemically sterilize with ethylene oxide, soak a cloth to wipe the entire body.
CAUTION - Do not use strong solvents such as halogenated hydrocarbons, petroleum ether, etc. for
cleaning.
Theory of pH measurement
pH is a unit of measure which describes the degree of acidity or alkalinity of a solution and is usually
written as:
pH = -log [H
+
]
Where ‘p’ is the mathematical symbol of the negative logarithm and [H
+
] is the concentration of
Hydrogen ions.
pH levels generally range from 0 to 14. A pH value of 7 is described as neutral - the point at which the
activities of hydrogen and hydroxide in solution are equal. When the pH value is less than 7, the
activity of hydrogen ion is greater than that of the hydroxide ion and the solution is described as
acidic. Conversely, as the hydroxide ion activity is increased the solution becomes alkaline (or basic)
and the pH value is greater than 7.
The pH electrode is actually a combination of a two half-cells (electrodes) within a single body
Internal pH Half Cell, the measuring electrode, whose voltage varies proportionately to the
hydrogen activity of the solution, and a
Reference Cell, the reference electrode, which provides a stable and constant reference voltage
and completes the electrical circuit.
The pH Half Cell consists of a thin membrane of hydrogen ion sensitive glass blown on the end of a
high resistance glass tube. Within this tube is an internal reference system, which remains constant.
The Reference cell uses a similar system, but without using a hydrogen sensitive glass. It is housed
concentrically between the outer body of the electrode and the glass half-cell. It is comprised of a
reference element (silver/silver chloride) and an electrolyte solution that seeps through a porous
liquid junction (a small filter) to make the necessary electrical connection with the sample (the
external liquid).
The pH adaptor measures the difference between the pH Half-cell and the Reference cell in millivolts
DC. This millivolt reading is displayed in pH units.
The electrode signal varies with the pH according to the Nernst Equation:
E = Eº + 2.303 RT · log [H
+
]
nF
Where:
E = Measured electrode potential
Eº = Standard potential of the system (constant)
R = gas law constant (8.314 J mol
-1
K
-1
)
