|
|
Achieve meaningful and relevant results:
Calibrating standard electrochemical parameters in water analysis - pH value, dissolved oxygen (DO) and conductivity
|
|
By Dr. Klaus Reithmayer
|
"With enough measurements, you can prove anything, even the opposite."
Generations of laboratory staff have had to put up with this saying. Recording accurate measured values is one of the most
important elements for meaningful and relevant documentation of research results, process steps, material parameters, official
requirements and many more things.
What, however, is the correct method to achieve a universally valid and comparable basis?
The goal of this article is to provide insight on the concept of calibration and a comparable link with the measured values that are
electrochemically determined in water analysis most frequently: the parameters, pH, dissolved oxygen and conductivity.
The process of making an electrochemical measured value traceable and comparable is explained using a wellknown sample. For
the physical magnitude of mass, there is the International Prototype Kilogram (IPK). The IPK is a body made of platinum and
iridium, which has been stored as the reference object for mass determinations at the Bureau International des Poids et Mesures in
Sevres near Paris since 1889. Official copies of the IPK were made available to all nations that are members of the meter
convention. These copies must not exceed a defined uncertainty in exact comparison weightings'. Linked with these
so-called country-specifi c measurement standards are further standards that also must not exceed certain tolerances. They are,
for example, used as calibration blocks for measurement offices and calibration services and ultimately have the effect that, on a
calibrated and adjusted scale, 1.00 kg of apples weighs exactly 1.00 kg and neither 0.98 kg nor 1.20 kg is displayed.
Calibration of pH value
According to Nernst, pH measurement is based on the electrochemical determination of the activity of hydrogen ions in aqueous solutions. It
is the most common laboratory measurement performed in aqueous solutions. In Nernst's equation, electrical potentials are measured and
converted accordingly. For the pH value, there is no "International Prototype pH" that is used as the reference for all other "pHs".
Instead, the most broadly accepted practice is the use of buffer solutions, which contain salts that create determined hydrogen ion activity in
aqueous solutions. The term buffer solution is used because minor changes of the hydrogen ion concentration will remain virtually unchanged in
these solutions.
By means of very complex procedures of measurement against hydrogen standard electrode, these solutions are examined and their uncertainties
determined at institutes such as the National Institute of Standards in Gaithersburg, Maryland USA or at the Physikalisch-Technische Bundesanstalt
(PTB) in Braunschweig, Germany. Thus a secondary reference material or standard is created on which further standards can be built.
For example: Manufacturers can have their material (described as suitable) certified as reference material by one of the above mentioned institutions
thereby, using this as a baseline to create more reference material by comparing it in their own laboratories.
This system is especially important for certification according to the standard: DIN ISO 9000ff. The test equipment of companies that have
achieved certification is subject to monitoring within the framework of quality assurance and, for their own benefit, is traceable to international
standards. The traceability must be documented and provided by the manufacturers. Similarly are the internationally composed standards for DIN/NIST
buffers which most commercial instruments operate. Many users, however, also use the commonly called Technical buffer solutions for
calibration. The unique feature of these buffers is that they have whole number pH values at certain reference temperatures (20 or 25 °C), such
as 2.00; 4.00; 7.00. This originates from the early times of electrochemical pH measurement when it was difficult to obtain exact readings using
analog instruments with dial displays. This limitation no longer applies. However, after the introduction of digital pH meters, the
preference for Technical buffer solutions resurfaced.
The accuracy range of commercial buffer solutions is between ±0.01 and ±0.04 pH. Using this as reference, the customer checks their sensors and
adjusts the readings of the measuring transmitter (meter) according to the determined deviations similar to the scale described before. When
calibrating a pH combination electrode, the main focus is on two parameters: Zero point deviation, and slope.
According to DIN 19266 standard, the zero point deviation must be within +0.5 pH units. The ideal slope of a pH combination electrode is
59.2 mV/pH at 25 °C. By determining the deviations and electronically adjusting the combination electrode to the meter used, the
prerequisites for correct and comparable measurements, traceable to the primary standard in an uninterrupted chain are established. Further
points that contribute to a substantive documentation of the pH value are meters tested with traceable standards and sensors with individual series
numbers. With these, successful operation is guaranteed and clear allocations can be made that help to avoid mix-ups and errors.
Practical notes on pH calibration:
- Buffer solutions are chemical products and undergo an aging process. They cannot be used after the expiration date.
- Open containers should be used immediately, only minutes after opening. This applies especially to alkaline buffers that
tend to absorb more CO2, which reduces their pH value.
- Buffer solutions must be discarded after each use.
- Ideally, the calibration temperature should be the same as the measurement temperature. In 95% of all cases, measurements are
not critical and are performed at room temperature. If they are critical, measurements are taken at above or below room temperature
with buffer solutions and electrodes that have been tempered accordingly.
- As a general rule, do not store buffer solutions in the refrigerator. This will cause longer settling in time of the electrode and
longer waiting periods during calibration.
- Calibration frequency depends on the required accuracy and the media that influence the sensor (drift). The range is between several
times per day in critical applications (pharmaceutical laboratories) and once every two weeks for relatively uncritical applications.
For information on different pH electrode calibration methods and the difference between pH solutions please click here.
Calibration of Dissolved Oxygen
What happens when the oxygen dissolved in water is measured? The procedure carried out most frequently is the polarographic measuring method
by means of the Clark cell that measures dissolved oxygen as an amperometric signal. Additionally, there are optical methods and titration. The
ambient air is used as the standard. It contains approximately 21 per cent oxygen by volume. This percentage is virtually constant in
the biosphere thus, can easily be used. The air pressure, depending on the geographic location, can be taken into account by using a barometer
(modern instruments have an integrated barometer). There is, however, one important remark: The measurement of dissolved oxygen is a partial
pressure measurement. This means the oxygen applies a pressure in the ambient air that is in equilibrium with the pressure in the liquid to
be measured. This pressure is complemented by the pressure of other gases present such as nitrogen, rare gases and carbon dioxide. Another
important factor is the water vapor pressure in the atmosphere. It is highly variable and must be taken into account for the determination. It
is possible to spend much time and effort in determining the relative humidity and including it as a correction. This requires an additional
measuring instrument and calculative effort. Fortunately there is a defined dependency between the water vapor saturation and temperature. The
trick is quite simple: A moist sponge in a calibration beaker that is placed on the sensor creates a water vapor-saturated atmosphere above the sensor
surface. Thus, the proportion of the water vapor at a known temperature can easily be compensated for by calculation.
In the United States, a method frequently required for the calibration of DO sensors is the method of comparison against an external standard
procedure. This is the Winkler titration, developed for quantitative oxygen determination in the late 19th century. This procedure is
very accurate, but requires substantial time and effort and is unsuitable for field applications. It is, however, used by manufacturers of DO
measuring systems to validate their products. In addition, there is calibration in airsaturated water, a calibration method which provides
a very uncertain standard due to numerous sources of error during preparation. Dissolved oxygen calibration is typically performed as a
single-point calibration at maximum signal. This requires a sensor that does not create any signal in the absence of oxygen. Otherwise,
an existing signal has to be determined in an oxygen free environment (such as a nitrogen stream) and then suppressed electronically.
Notes on error-free DO calibration:
- Sponges used for DO Calibration should be moist but never wet.
- For safety reasons, place the sensor on the sponge, remove it again and check the membrane surface. If there are any droplets
on the membrane, it is essential to dry it and squeeze out the sponge prior to calibrating. Otherwise there is the danger of the
results being too high.
- Calibrate the DO sensor regularly. As galvanic DO sensors are always in operation, their slope is subject to change even when not being
used.
Calibration for Conductivity Measurement
The conductivity in aqueous solutions is measured as a composite parameter. The ions dissolved in water coming from covalent or ionic
compounds contributing to its conductivity. The lower conductivity limit is determined by the self-dissociation of water, it is approximately
0.055 uS/cm at 25 °C. The H+ and OH- ions work as conductors according to the equation:
H2O <-> H+(aq) + OH-(aq)
Conductivity measuring systems are calibrated with commercially produced standard solutions. The sensors consist of parallel electrode systems
working with alternating currents of different frequencies. These mechanical cells are actually characterized by their distance and surface
("cell constant, K = d/A") and have to be determined regarding their characteristics in aqueous systems. Standard solutions are used to
determine the effective cell constant, which may deviate from the merely geometrical cell constant due to surface roughness and electrical fields. The
standard solutions are diluted with ultrapure saline solutions that are very sensitive to contamination, especially in the lower conductivity range.
Another special feature of conductivity is that contrary to pH and DO measurement, it is not possible to measure the entire measuring range of aqueous
solutions, that is 0.055 uS/cm to approximately 800 mS/cm, with one single sensor. Suitable sensors are used for the different ranges. Normally,
conductivity sensors are not subject to much wear, the calibration intervals according to standard DIN EN 27888 is half a year. The same applies to
standard solutions, which are available with different test values. Nowadays, certified products traceable to international standards are
available. This enables the operator to check the conductivity sensor while taking the solution temperature dependency into account. Conductivity
sensors are, as already mentioned, very contamination sensitive. Therefore, calibration has to be performed very carefully to avoid incorrect
measurements. Conductivity standards may only be used once; the sensor has to be cleaned first and moistened with standard to avoid the displacement
of any possible adhering residues of sample or distilled water.
According to the correlation,
k = G * K
(k: electrolyte conductivity S/m, G: conductance (Siemens (S)), K: cell constant), the current cell constant is determined with a given conductance
and, if necessary, electronically adjusted until the actual and the measured conductance agree with one another.
For more information on water quality sensors click here.
For information on different pH electrode calibration methods and the difference between pH solutions please click here.
For more information on DO calibration and maintenance please click here.
For information on pH measurements please click here.
For specific information on pH glass electrode types please click here.
|
|
|
