pH meters are electronic devices which measure the pH of an aqueous solution. We will cover the theory behind pH meters when we study electrochemical cells. Let it suffice for now that the pH probe is a combination electrode containing two half cells, the pH half cell and a reference half cell. The potential across these cells is logarithmically related to the hydronium ion concentration (see Nernst eq.) and thus a measure of the pH. For this reason the pH meter has two scales, a pH scale and a voltage scale.

Before making any measurements you need to calibrate the pH meter. This is done by successively placing the electrodes into different buffer solutions of increasing pH while the instrument is operating in calibration mode. When you accept each calibration point the meter locks that value in. If you run the meter for several hours you should redo a calibration to offset any electrode potential drift.

Calibration Techniques:

We will use 3 buffers of pH

4.00

7.00

10.00

Pour about an inch of each buffer

into a separate beaker.

Make sure the solutions are

deep enough to cover the bottom of the

electrode. Do not waste buffer solutions

by using excess buffer.

Fill a beaker with distilled water and

wash. Immerse the electrode in the

water and then wash again with a

more distilled water in a squeeze bottle.

In order to prevent a dilution effect

as you make your measurements

you should dry the outside of the

electrode with a Kim wipe prior to

immersing in any solutions.

Never dry the electrode itself,

just the outside covering.

Place the electrode

into the most acidic buffer.

If the pH meter does not read

in pH, press mode until it does

-the arrow on the bottom of the

display points to pH

and not mV or Rel mV

-Press "2nd Calibrate"

The meter now displays calibrate

above the reading and states

P1 below (indicating this is the

first calibration value.)

When it reads "ready", hit "yes" if you want to

automatically accept the value and "no"

if you want to manually set the value. If you

hit no, you now have to hit the up or down

button twice and the left most digit will change.

Once it is correct, hit "yes"

and you will be toggled to the next digits position.

Repeat until finished.

Now the meter will change to P2 and you

are ready to take the second reading.

Don't forget to wash the probe

between successive measurements

When you accept the second value

the meter will prompt you for a third

value (pH 10) if it uses a three point

calibration or it will display the slope

if it is uses a 2 point calibration.

The buffers need to bracket the pH range

you desire to test. (The above values would

be inappropriate to measure the pH of a

basic solution.)

The value of the slope should be

between 95 and 105.

The meter is now calibrated and you are

ready to run your experiments.

Note how it reads "measure" and

not "calibrate"

"ATC" means automatic temperature

compensation. This allows you to measure

samples at different temperatures than the

standards used in the calibration.

A final note, never leave the probes outside or let them dry. They come in a test tube containing a saturated potassium chloride solution. It is OK if some of the solution evaporates and KCl crystals form on the outside of the test tube as long as the electrode is immersed in solution. Notify your instructor if your electrodes are not immersed in solution during the beginning of your experiment.