1. pH is a
number which represents conventionally the hydrogen ion concentration of an
aqueous solution.
2. The pH of
a solution can be measured by potentiometric instrument (pH meter) capable of
reproducing pH value to 0.02 pH
units.
3. The
potentiometric determination of pH is made by measuring the potential
difference between two appropriate electrodes immersed in the solution to be
examined.
4. One of
these electrode is sensitive to hydrogen ions (usually glass electrode) and
other is reference electrode (for example a saturated calomel electrode)
5. The pH of
a solution to be examined is related to that of a reference solution (pHs) by
following Nernst equation.
E - Es
pH= pHs - ---------------
K
Where
E = Potential in volts of
cell containing the solution to be examined.
Es= Potential in volts of
cell containing the known pH (pHs)
K = Change in potential per
unit change in pH expressed in volts.
Theoretically K=
(0.05916+.000198(t-25) volts at any temperature t.
Measurement of pH:
pH is defined as -Log [H+].
It is important to be able to measure pH accurately. And also it is important
to know how the instrument measure pH because several factors can cause the
observed value to differ from the
actual pH.
A pH meter measures the voltage between
two electrodes placed in the solution. The
important part of the system is an electrode whose potential is pH
dependent. The most commonly used pH dependent unit is glass electrode. The
action of this electrode is based upon the fact that the certain types of
borosilicate glass are permeable to H+ ions but not other cat ions
or anions. Therefore, if thin glass layer of such glass is interposed between
two solutions of different H+ ion concentration H+ ions
will moves across the glass from the solution of high to that of low H+
concentration. Because passage of H+ ions through the glass adds a
positive ion to the solution of low H+ concentration and leaves
behind (high H+ concentration) a negative ion an electric potential
develops across the glass.
|
If inner H+ moves to outer, a negative
ion at inner wall and a positive ion at outer wall developed.
If outer H+ moves to inner, a negative
ion at outer wall and a negative ion at outer wall developed
|
The magnitude of this potential is
given by the equation
Potential=E= {2.303 RT/F}{Log [H+]1/
[H+]2}
R=Gas constant
T=Absolute temperature
F=Faraday constant
[H+]1 & [H+]2
= The H+ concentration of inside and outside of the glass
respectively.
Clearly if H+ concentration
of one of the solution is fixed, the potential will be proportional to the pH
of the other solution. So the second part of the pH meter is a reference
electrode whose concentration is fixed. (most commonly contains Hg-HgCl2 paste
in saturated KCl. If high temperature is required Ag-Agcl2 is required instead
of Hg-HgCl2). This is called a calomel electrode. KCl serves to make contact
between Hg-HgCl2 or Ag-Agcl2 unit and the solution being measured. The calomel
electrode tube is made of glass that is impermeable to H+ ions (so
that potential is pH independent
So the pH measured by such system is
primarily the difference between the potentials of that of the glass and
reference electrodes. And then the potential is converted to pH.
1 pH=59.12 mv
However there are
three other potentials present in the circuit.
1.The liquid junction potential of the
reference electrode resulting from the fact that K+ & Cl- do not diffuse at
the same rate so that a charge is generated at the interface between the KCl
solution in the reference cell and the sample.
2. Asymmetry potential which develop
across glass even when the pH on both sides is the same.
3. Potential of Ag-Agcl in glass
electrode unit which is itself an electrode because of its contact with the CL-
of the HCl.
These three potentials and that
the reference electrodes itself are relatively independent of pH and of ionic
strength in the range normally encountered. Hence voltage V, measured with the
total system may be expressed as the difference between the fixed potentials
and that of the glass electrode. Therefore the voltage generated is linearly
related to the ph of the solution.
pH electrode
standardization – Calibration:
Before measuring the pH you have to
calibrate (standardize) electrode. To calibrate the electrode you need at least
two solutions of known pH. Most commonly used commercially available
calibration buffers have pH of 4.01, 7.00 and 10.00.
Details of calibration procedure depend
on the pH meter model.
First step is usually related to temperature
correction. Some models will measure temperature by itself, others need
external temperature probe, or you will have to enter temperature measured by
others means using dials or buttons. Buffer pH changes with temperature.
Check the pH of buffer 4.01, 7.00 and
10.00 at 25. If they are not in 0.02 ranges adjust with up and down arrows. (If
desirable check the buffers of pH 1.00 and 12.45 at 20°)
Then observe the slope value which displays automatically whether is in specified limit
of 80-120 or not.
pH electrode test procedure or Slope
calculation:
Mathematical
difference between two buffer mill volts (Electrode span)
Slope = --------------------------------------------------------------------
X100
Theoretical span (Theoretical
span)
Electrode
span= 7 buffer millivolts reading – 4 buffer millivolts reading (at 25°)
Theoretical span= 176.9
Cleaning of pH
electrode:
·
General
·
Soak in 0.1M HCl for
half an hour.
·
Drain and refill the
reference solution.
·
Soak the electrode in
filling solution for one hour.
·
Inorganic
Contaminants
·
Soak in 0.1M tetra
sodium EDTA solution for 15 min.
·
Drain and refill the
reference solution.
·
Soak the electrode in
filling solution for one hour.
·
Protein Contaminants
·
Soak in 1%
pepsin/0.1M HCl for 15 min.
·
Drain and refill the
reference solution.
·
Soak the electrode in
filling solution for one hour.
·
Grease and Oil
·
Rinse with detergent
or ethanol solution.
·
Drain and refill the
reference solution.
· Soak the electrode in
filling solution for one hour. Electrode response may be enhanced by
substituting a mixture of 1:1 pH 4 buffer and filling solution for the soaking solution.
Cleaning of clogged
junction
·
Pollution by sulfides
· Use a solution of 8%
thiocarbamide in 1mol/L HCl.
· Keep the electrode in
the above solution till junction color turns pale.
·
Pollution by Silver
chloride
·
Use concentrated
ammonia solution.
·
Keep the electrode
above solution for about 12 hours.
·
Rinse and put into pH
4 buffer for at least 1 hour.
Other contaminants have to be removed by cleaning with
distilled water, alcohol or mixture of acids .If nothing else help you may
consider use of ultrasonic cleaners as last resort.
Regenerating pH electrode:
Following procedure is the last resort. They may work, they
may won’t .You may try them before throwing electrode away.
First of all clean
the electrode as described in electrode cleaning section then
·
Soak the electrode
for 4-8 hours in 1M HCl solution.
·
Rinse it and move to
pH 7 buffer for an hour.
If electrode is still
not working
·
Fill the electrode
with filling solution.
·
Move to the fume hood
·
Place the electrode
in the 10% nitric acid solution on a hotplate. Heat to boiling and keep it in
the solution for 10 min.
·
Place 50ml of filling
solution in a second clean beaker. Heat although boiling is not necessary.
·
While the electrode
is still hot, transfer it to the beaker of heated filling solution, set aside
to cool.
·
Some manufacturers
suggest the electrode may be reactivated by treating with diluted solution of
hydrofluoric acid followed by subsequent conditioning in filling solution.
(Before going to HF, remember it is highly dangerous and dissolves glass).
pH electrode maintenance:
·
Handle electrode with
care-it is fragile (easily broken or damaged).
· Keep electrode always
immersed. Use the solution recommended by manufacturer or neutral solution of
KCl (3M-4M).
· Remember to always
keep internal level of filling solution above the level of measured solution.
· Don’t put electrode
in a solution that can dissolve glass-HF (or acidified fluoride solution),
concentrated alkalies.
· Don’t put electrode
into dehydrating solution such as ethanol, sulphuric acid, etc.
· Don’t rub or wipe
electrode bulb, to reduce chance of error due to polarization.
·
Fill the electrode
with correct filling solution to not let it dry internally.
· If you are using
electrode in a solution containing substances able to clog the junction or
stick to the glass bubble clean the electrode as soon as possible after use.
· If electrode will not
be used for long time, you may store it dry to prevent aging (aging take place
only when electrode is wet).
· If dried
incidentally, or after storing-soak for at least 24 hrs before using.
pH electrode storing
· A wet stored
electrode allows an immediate use and a short response time, which is not true
for dry storage ones. Unfortunately, the wet stored electrode is aging faster,
because the process of aging (changing the structure in the membrane) proceeds
also in the case of non-use. Keeping the electrodes wet should preferably be
made in KCl solution. Most electrodes have preventive cap that can be filled
with storage solution before placing.
· To store electrode
dry you must first remove internal solution, rinse the electrode in DI/RO
water, and let it dry. (in fact electrodes that can be stored dry are getting
more and more rare)
· Always check
electrode owner manual for details, as these may depend on the electrode make.
USP Criteria for pH measurement:
·
Measure at Temperature
25 +2o.
·
Reproducibility
0.02.
·
To standardize the
p H meter, select two buffers whose difference does not exceed 4 units and such
that the expected pH of the test solution should falls in them.
·
Check the first
buffer and set the value to std and check for second buffer, if more than +0.07
units examine the electrode.
·
If electrode is
good, adjust slope or electrolyte changing etc.
·
Repeat the buffers
checking until the pH values within+0.02
