2. What is a pH Meter?
• An instrument that measures the
H+ ion concentration (pH) of a
solution using an ion sensitive
electrode which will ideally
respond to one specific ion, in
this case H+
• The pH of a solution is the
negative logarithm of the H+ ion
concentration
• A typical modern pH meter has a
glass and reference electrode in
one tube
3. How Does a pH Meter Measure
H+ Concentration?
The pH meter has two electrodes in one tube, glass
and reference electrode. A saturated KCL and
HCL solution is contained in a tube which is
inside of an outer tube that will have contact with
the solution to be measured. This outer tube has a
double glass bulb with NA+ which makes an ion
specific electrode. When measuring the pH of a
solution, a salt bridge forms. The NA+ ion, not H+,
crosses the glass membrane of the pH electrode
and allows for a change in free energy which is
measured by the pH meter as the concentration of
H+.
4. Calibrating a pH Meter
• Make sure the meter is in pH mode
• For a 3 point calibration, use high pH (--),
pH 7.0 and low (-.0) solution
• Before calibration, rinse probe thoroughly
with de-ionized water or a rinse solution
• Immerse the end of the probe completely in
the calibration solution
• Stir the probe gently to create
a homogenous sample
5. Measuring the pH of a Solution
• Always rinse electrodes with
de-ionized water prior to
placing in a solution for pH
measurement
• Allow meter to stabilize for 30
seconds or a minute, then read
• Remove electrodes and rinse
with de-ionized solution
• The pH bulb should always be
stored wet preferably in pH 4.0
buffer with 1/100 KCl
• Other buffers or tap water can
be used for storage
6. Maintenance of pH Meter
• Wipe off exterior of pH meter with a damp
cloth after use
• The outside of the meter is made of polyester
and is not affected by most solvents but is
affected by some organic solvents
• pH the “power of hydrogen”
8. Litmus Paper
• Litmus paper is useful
to determine the pH of a
solution when you do
not have a pH meter
available
• Proper storage of the
litmus paper is essential
to maintain quality
control
9. The pH Scale
• The pH measures the
concentration of H+ in a
solution
• The lower the number,
the more acidic the
solution
• The higher the number,
the more basic the
solution
The pH meter measure the H+ concentration (the pH ) of a solution using an ion sensitive electrode. An ion selective electrode will ideally respond to only one specific ion. A typical modern pH probe is a combination electrode, which has the glass and reference electrode in one tube. The bottom of a pH electrode balloons out into a round glass bulb. The electrode contains a tube within a tube with the inside electrode having the KCL and HCL solution as well as the cathode terminus of the reference probe. The outer tube wraps itself around the inner tube and also ends with some type of reference tube like the inner tube. The outer tube only has contact with the solution and ions cross through a porous plug that serves as a salt bridge. The measuring part of the pH meter, the glass bulb on the bottom is coated both inside and out with a 10nm layer of hydrated gel. These two layers are separated by a 0.1mm layer of dry glass. The metal cations(NA+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel, which makes the pH electrode an ion selective electrode. H+ does not cross the glass membrane of the pH electrode, it is the NA+ which crosses allows for a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by NA+ transport and thus the concentration of H+ on the outside of the membrane is ‘relayed” to the inside of the membrane by Na+.
Some machines will indicate of what type of mode is presently in use with symbols. With the machine is in pH mode rinse the electrodes with de-ionized water before you calibrate. I want to emphasize the importance of rinsing the electrodes off before each calibration and before they are placed in any solution that needs to be pH. The pH bulb should always be stored wet, preferably in pH 4.00 buffer with 1/100 part of KCL added. Other pH buffers or tap water are acceptable storage media. This step needs to done especially at the end of the day when the machine will no longer be used. After you have rinsed the electrodes dip the probe in the calibration buffer. It is suggested that you standardize (calibrate) with at least three points before the use of the machine. Some machines have calibration dials (cal ) dials used do standardization. If you need 2 calibrations to standardize the machine the knobs may say ca1 1,2 or 3. If your machine has an “auto read” and it is turned on , it will automatically endpoint when the reading is stable 30 seconds to a minute). For a 1 point calibration place the tip of the electrode in the first buffer and press “cal”. Some machine have 5 calibration buffers are shown on the display. Make sure you rinse the electrode between each calibrator.
Have different beakers of clear liquids and have students dip litmus paper in them to determine acid or base. Examples, bleach, clear soda, vinegar. Have different beakers of clear liquids and have students dip litmus paper in them to determine acid or base. Examples, bleach, clear soda, vinegar.
