Subject: 26. Electrochemical Techniques
     
26.1  What is pH?

The pH scale determines the degree of acidity or alkalinity of a solution,
but as it involves a single ion activity it can not be measured directly.

pH = - log10 ( gamma H  x  m H )

where gamma H = hydrogen ion single ion activity coefficient
          m H = molality of the hydrogen ion.

As pH can not be directly measured, it is defined operationally according to
the method used to determine it. IUPAC recommend several standardised methods
for the determination of pH in solution in aqueous solutions. There are 
seven primary reference standards that can be used, including 0.05 mol/kg
potassium hydrogen phthalate as the Reference Value Standard. There is an
ongoing debate concerning the relative merits of having a multiple primary
standard scale ( that defines pH using several primary standards, and their
values are determined using a cell without a liquid junction ) or a single
primary standard ( that requires a cell with a liquid junction ). Interested
readers can obtain further information on the debate in [1]. Bates [2], is a 
popular text covering both theory and practise of pH measurement. 
   
26.2  How do pH electrodes work?     

Contributed by Paul Willems <Paul.Willems@rug.ac.be>, and slightly modified 
by Bruce Hamilton.

The most common type of pH electrodes are the "glass" electrodes. They
consist of a special glass membrane that is sensitive to variations in pH, 
as pH variation also changes the electrical potential across the glass. In 
order to be able to measure this potential, a second electrode, the 
"reference" electrode, is required. Both electrodes can be present in a 
"combined" pH electrode, or two physically-separate electrodes can be used.

The glass electrode consists of a glass shaft on which a bulb of a special
glass is mounted. The inner is usually filled with 3 Mol/Litre aqueous KCl
and sealed. Electrical contact is provided by a silver wire immersed in the 
KCl.

For "combined" electrodes, the glass electrode is surrounded by a concentric
reference electrode. The reference electrode consists of a silver wire in 
contact with the almost-insoluble AgCl. The electrical contact with the meter
is through the silver wire. Contact with the solution being measured is via
a KCl filling solution. To minimise mixing of the solution to be measured and
the filling solution, a porous seal, the diaphragm, is used. This is usually
a small glass sinter, however other methods which allow a slow mixing contact
can also be used, especially for samples with low ionic strength. Besides the
"normal" KCl solutions, often solutions with an increased viscosity, and 
hence lower mixing rate are used. A gel filling can also be used, which 
eliminates the necessity for slow mixing devices. 

In contact with different pH solutions a typical glass electrode gives, when 
compared to the reference electrode, a voltage of about 0 mV at pH 7, 
increasing by 59 mV per pH unit above 7, or decreasing by 59 mV per pH unit 
below 7. Both the slope, and the intercept of the curve between pH and 
generated potential, are temperature dependent. The potential of the 
electrode is approximated by the Nernst equation :

E = E0 - RT log [H+] = E0 + RT pH
Where E is the generated potential, E0 is a constant, R is universal
gas constant and T is the temperature in degrees Kelvin.

All pH-sensitive glasses are also susceptible to other ions, such as Na or K. 
This requires a correction in the above equation, so the relationship between 
pH and generated voltage becomes nonlinear at high pH values. The slope tends
to diminish both as the electrode ages, and at high pH. As the electrode has
a very high impedance, typically 250 Megohms to 1 Gigohm, it is necessary to
use a very high impedance measuring instrument.

The reference electrode has a fairly constant potential, but it is 
temperature dependent, and also varies with activity of the silver ions in 
the reference electrode. This occurs if a contaminant enters the reference
electrode.

Calibration

From the preceding, it is obvious that frequent calibration and adjustment of
pH meters are necessary. To check the pH meter, at least two standard buffer 
solutions are used to cover the range of interest. The pH meter should be 
on for at least 30 minutes prior to calibration to ensure that all components
are at thermal equilibrium, and calibration solutions should be immersed for
at least a minute to ensure equilibrium. 

First use the buffer at pH 7, and adjust the zero (or the intercept). 
Then, after thorough rinsing with water, use the other buffer to adjust the 
slope. This cycle in repeated at least once, or until no further adjustments 
are necessary. Many modern pH meters have an automatic calibration feature,
which requires each buffer only once.

Errors

People assume pH measurements are accurate, however many potential errors 
exist. There can be errors caused by the pH-sensitive glass, reference 
electrode, electrical components, as well as externally generated errors.

Glass Electrode Errors

The pH-sensitive glass can be damaged. Major cracks are obvious, but minor
damage can be difficult to detect. If the internal liquid of the pH-measuring
electrode and the external environment are connected, a pH value close to 7 
will be obtained. It will not change when the electrode is immersed in a 
known solution of different pH. The electrical resistance of the glass 
membrane will also be low, often below 1 megohm, and it must be replaced.

Similar results occur if the glass wall between the inner and outer part of 
a combined electrode breaks. This may occur if the outer part is plastic.
The inner part can crack without any external signs. The electrical 
resistivity over the glass electrode is intact, but actual measuring between
both electrodes reveals a low resistivity. The electrode must be replaced.

The glass can wear out. This gives slow response times, as well as a lower 
slope for the mV versus pH curve. To rejuvenate, immerse the electrode in a 
3 Molar KCl solution at 55 degrees Celsius for 5 hours. If this does not 
solve the problem, try removing a thin layer of the glass by immersion for
two minutes in a mixture of HCl and KF (be careful, do not breathe the fumes,
and wear gloves). The electrode is then immersed for two more minutes in HCl,
and rinsed thoroughly. As an outer layer of glass has been removed, the new 
surface will be like a new electrode, however the thinner glass will result
in a shorter electrode life. Frequent recalibration will be required for
several days.

The glass can be dirty. A deposit on the glass will slow the response time,
make the response sensitive to agitation and ionic strength, and also give 
the pH of the film, not the sample solution. If the deposit is known, use a
appropriate solvent to remove it, and rehydrate the electrode in 3M KCl.
If the deposit is not known, first immerse the electrode for a few minutes 
in a strongly alkaline solution, rinse thoroughly, and immerse it in a 
strong acid (HCl) solution for several minutes. If this does not help, try 
using pepsin in HCl. If still unsuccessful, use the above HCl/KF method. 

Reference Electrode Errors

The diaphragm of the reference can become blocked. This is seen as unstable
or wrong pH measurements. If the electrical resistivity of the diaphragm is
measured, high values are reported (Most multimeters will give an over-range 
error). The most common reason is that AgS formed a precipitate in the 
diaphragm. The diaphragm will be black in this case. The electrode should be 
immersed in a solution of acidic thiourea until the diaphragm is white, and
then replace the internal filling liquid of the reference electrode 

There is no contact across the diaphragm, due to air bubbles. This appears
as if the diaphragm were blocked, except that the diaphragm is white. Ensure 
that the filling solution level in the reference electrode is always well
above the sample, so that liquid is always slowly flowing from the reference 
electrode towards the sample.

The electrode filling solution is contaminated. This appears as unstable or 
wrong pH measurements. Often the 0mV pH differs considerably from pH 7. The 
diaphragm has its normal colour and the electrical resistivity is normal. 
However, the solution often becomes contaminated due to low filling solution 
levels, and air bubbles may also appear in the diaphragm, which obviously
affects electrical resistivity. Replacing the reference filling solution 
several times should solve the problem, but the electrode may have been 
permanently damaged. The problem can be avoided by choosing gel-filled 
reference electrodes, double-junction electrodes, or ensuring there is an 
outflow of reference filling solution towards the sample.

The electrode was filled with the wrong reference solution. This appears as
as displaced pH measurements. Flush and replace the reference liquid.

Electrical errors 

Condensation or sample contamination of the electrode connecting cable. This 
appears as an almost-constant measurement of about pH 7, even when the pH 
electrode is disconnected from the cable, or as a pH which changes less than 
it should, when tested with two standard solutions. If the cable is 
disconnected from the meter, the pH will start to drift.

There is a short circuit in the cable. The symptoms are similar to the above
case, except that bending the cable may create sharp, spurious readings. In 
most pH cables, between the two copper conductors there are two layers which 
appear to be insulators. The inner layer is an insulator, whereas the outer 
layer is a conductor to avoid trace electrical effects. If this outer layer 
does contact the inner conductor, there will be a short circuit. Replace 
suspect cables.

The input stage of the meter is contaminated with conducting liquid. The 
symptoms are the same as above, except that removing the cable has no
effect. Closely examine the input stage of the meter for liquid or deposits. 
If present, rinse with distilled water, then ethanol, and dry thoroughly.

The input stage of the meter is faulty. This gives random measurements.
Shorting both input wires does not make any difference. Repair the meter.

The input stage appears faulty. Shorting both input wires gives a stable
pH measurement of about 7. The meter may be faulty, but probably the problem 
is elsewhere in the electrical circuit.
 
Externally-generated Errors

If a significant flow of liquid passes the electrode, then there can
be a minor electrical effect. This generates a potential on the glass
membrane, which is superimposed on the actual pH measurement. This effect
becomes negligible for highly-conducting liquids. It is seldom observed.
If the trace electric effect does influence pH measurements, the addition 
of a little salt to increase the conductivity, or changing the flux of 
liquid around the electrode, should solve the problem.

Ground loops and spurious electrical currents may generate unexpected 
electrical signals. Such signals can strongly influence pH measurements. 
A pH reading in the range of -15 to +20 is possible, even if the pH is 7. 
Ground loops can be eliminated by grounding the system according to the
manufacturer's instructions, and ensuring insulation is in good condition. 
Often these problems can be extremely difficult to detect and remedy.

Low ionic strength samples can be affected by electrolyte from the electrode, 
and special electrodes are available.

26.3  What are ion-selective electrodes? 

Ion selective electrodes are electrochemical sensors whose potential varies
with the logarithm of the activity of an ion in solution. Available types:
1. The membrane is a single compound, or a homogeneous mixture of compounds.
2. The membrane is a thin glass whose chemical composition determines the 
   response to specific ions.
3. The support, containing an ionic species, or uncharged species, forms the
   membrane. The support can be solid or porous.  
Popular texts on applications of ion-selective electrodes include 
"Ion-Selective Electrodes in Analytical Chemistry" [3], and "Ion-selective
Electrode Methodology" [4].

26.4  Who supplies pH and ion-selective electrodes?

The best known manufacturer of ion-selective electrodes is Orion Research. 
There are several pH electrode manufacturers, including Radiometer and
Metrohm.