Xylem WQ pH SENSORS - Manual (Page 8)

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e. The pH sensor must be stored where the sensing element remains wet
or the sensing element will be damaged. It can be stored in a
container of clean water or a wet sponge can be placed inside the
protective cap shipped with the sensor. Place the cap over the
sensing element to keep it wet. The sensor can then be stored on a
shelf or hung on a wall.
f. The pH value of a substance is directly related to the ratio of hydrogen
ion (H+) and Hydroxyl ion (OH-) concentrations. If H+ is greater than
OH-, the solution is acidic, i.e., the pH value falls in the 0 to 7 range. If
the OH- is greater than the H+, the material is basic with a pH value in
the 7 to 14 range. If equal amounts of H+ and OH- ions are present,
the material is neutral with a pH value of 7.
The pH electrode can be thought of as a battery whose voltage
changes as the pH of the solution in which it is inserted changes. It
consists of two basic parts: 1) a Hydrogen ion-sensitive glass bulb,
and 2) a reference electrode. The special glass of the sensitive bulb
has the ability to pass H+ i.e., it is said to be H+ sensitive. This ability
allows the H+ inside the bulb to be compared to the H+ outside of the
bulb, and a voltage to be developed that is related to the difference.
The bulb then is a half-cell that needs a reference voltage in order to
function.
The voltage produced by the complete probe is a linear function of
pH, generally about 60 mV per pH unit. For example, at 7.00 pH the
probe produces zero volts while at 6.00 pH it produces +60 mV. If the
voltage had been negative it would indicate that the solution had a
value of 8.00 pH.
A buffer solution is a solution with a well-defined pH value and has
the ability to resist changes in pH. These characteristics are well
suited to the standardization of pH measuring systems. Buffers are
available in a wide range of pH values and come either in pre-mixed
liquid form or as convenient dry powder capsules. When selecting
buffers for use with your particular system, a value should be chosen
nearest to that of the sample being measured.