Calibrating standard electrochemical parameters in water analysis
The calibration process of water analysis instrumentation allows for both reliable and relevant results. Therefore, calibration with reliable standards is of the utmost importance if accurate measurements and adequate operation procedures are sought.
In water analysis, pH, dissolved oxygen (DO) and conductivity are the three most commonly measured values. Recording precise measured values for these properties is vital to ensuring optimal water quality and accurate documentation of research results, process steps and material parameters in order to comply with official requirements.
The latest multiparameter measurement technology has helped to overcome industry challenges by offering efficient measurement of all three values. There has been significant discussion, however, regarding the correct method to achieve a comparable and universally recognized basis for water analysis.
This article will address the concept of calibration and its comparable link with the aforementioned values, which are routinely determined electrochemically in water analysis.
pH is a measure of the activity of free hydrogen ions in a particular aqueous solution. Put simply, it is a measure of how acidic or basic a water sample is. Although it is the most common laboratory measurement performed in aqueous solutions worldwide, there is no current international pH reference for calibration purposes.
As an alternative, the most widely used practice is to buffer solutions containing salts that create determined hydrogen ion activity in aqueous solutions. The term buffer is used because small changes of the hydrogen ion concentration will remain virtually unchanged in these solutions.
In order to ensure the reliability of results, many institutions—including the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, and the National Institute of Standards in Gaithersburg, Md.—have implemented rigorous measurement protocols against hydrogen standard electrodes to develop a greater understanding of these solutions and determine their uncertainties. These assessments have led to the creation of a secondary reference material or standard on which further standards can be developed. In addition, this system is key to ensuring compliance with the DIN ISO 9000ff standard.
The achievement of the DIN ISO 9000ff standard certification dictates that a company’s testing equipment is monitored within the standards quality-assurance framework and be traceable to international standards. Manufacturers must fully document and provide customers with a commercial instrument’s traceability. The system ensures compliance with global standards for DIN/NIST buffers, with which most instruments operate.
The unique features of technical buffer solutions mean that they have whole-number pH values at certain reference temperatures, including 20°C and 25°C, such as 2.0, 4.0 and 7.0, making them particularly suitable for calibration.
The accuracy range of commercial buffer solutions is between ±0.01 and ±0.04 pH. This is used as a reference to check sensors and adjust the readings of the meter according to the determined deviations. The temperature and measurement temperature ideally should be the same.
In as many as 95% of cases, measurements are not critical and are carried out at room temperature. It is essential that those critical measurements are taken with buffer solutions and electrodes that have been equilibrated accordingly. This is very important when the sample to be measured is above or below room temperature. It also is advisable that buffer solutions are not refrigerated, as this not only increases settling time of the electrode but also waiting time during calibration. As with all chemical products, buffer solutions are impacted by age and should be discarded after use.
The number of times that calibration must be carried out depends on the required accuracy and media that influence a sensor (drift). Frequency can vary from numerous times per day for critical applications (i.e., those in pharmaceutical laboratories) to as little as once every two weeks for less critical applications.
The calibration of a pH electrode focuses on two different parameters: zero point deviation and slope. The DIN 1926 standard states that zero point deviation must be within ±0.5 pH units with the ideal slope of a pH combination electrode being 59.2 mV/pH at 25°C. By determining the deviations and electronically adjusting the combination electrode to the meter used, the prerequisites for accurate measurements traceable to the primary standard are established.
The Clark oxygen electrode polarographic measurement method commonly is used in water analysis to measure DO as an amperometric signal. Alternative techniques include optical methods and titration. DO measurement is vital for the health of water bodies because if it drops below normal levels, the water quality is harmed and creatures and organisms begin to die. Higher DO concentrations correlate with a thriving population and little pollution.
To measure DO, ambient air is used as the standard because it contains approximately 21% oxygen by volume. This percentage is virtually constant in the biosphere and, as a result, can be used easily. DO measurement is a partial pressure measurement, meaning that 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 (e.g., nitrogen and carbon dioxide).
Another factor that can be variable and therefore must be taken into consideration is water vapor pressure. Determining the relative humidity and including it as a correction requires significant time and effort, as well as additional measuring equipment. Fortunately, there is a simple, defined dependency between the water vapor saturation and temperature. A moist sponge in a calibration beaker placed on the relevant sensor creates a water vapor-saturated atmosphere above the sensor surface. Consequentially, the proportion of the water vapor at a specified temperature can be compensated for by calculation.
The Winkler method is a technique that uses titration to determine DO levels in a water sample. The method is used as a comparison against external standard procedures and was developed for quantitative oxygen determination. Although the technique is highly accurate, it requires significant time and effort, rendering it unsuitable for field applications. It is, however, widely used by manufacturers of DO measuring systems to validate their products.
Another method is calibration in air-saturated water. This option provides a highly uncertain standard, namely due to the user not allowing enough time for the air to become saturated during preparation. Oxygen calibration typically is performed as a single-point calibration at maximum signal and 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 (i.e., in a nitrogen stream) and then electronically suppressed.
Specific conductance, or conductivity, is a measurement of the ability of a material to conduct an electrical current. For this discussion, we refer to aqueous solutions.
Conductivity is dependent on the amount of dissolved solids (e.g., salt) in the water. Pure water will have a very low specific conductance, while seawater will have a high specific conductance. Conductivity is an important water quality measurement because it provides an understanding of the amount of dissolved ions in the water. In aqueous solutions, conductivity is measured as a composite parameter. The ions dissolved in water from covalent or ionic compounds contribute to its conductivity. The lower conductivity limit is determined by the self-dissociation of water and is approximately 0.055 μS/cm at 25°C, while the H+ and OH- ions work as conductors, according to the equation H2O ←→ H+ (aq) + OH– (aq).
Calibrated with commercial standard solutions, conductivity measuring systems consist of a parallel electrode system working with alternating currents of different frequencies. These mechanical cells are characterized by their distance and surface (cell constant, K = d/A) and have to be determined by 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, which are very sensitive to contamination, especially in the lower conductivity range.
Unlike pH and DO, it is not possible to measure conductivity across all aqueous solutions using a single sensor. Different sensors must be used depending on the nature of a specific application. The DIN EN 27888 standard specifies that calibration of conductivity sensors occur every six months. Standard solutions for calibration also are available with different test values. More recently, certified products traceable to international standards have become available, enabling the operator to check the sensor while taking the solution temperature dependency into account.
Conductivity sensors are highly sensitive to contamination, meaning calibration must be performed carefully to avoid incorrect measurements. In addition, standards must be used only once; the sensor must be cleaned first and moistened with the standard solution. This will overcome any issues associated with the displacement of adhering residues of sample or distilled water.
The latest multiparameter technologies, including the WTW MultiLine from Xylem Analytics, offer measurement of pH, DO and conductivity in water analysis. The ability to measure all three parameters in a single system provides reliable determination of the quality and health of a specific water body, while enabling measurement of both samples in the laboratory and in the field.
In conclusion, pH, DO and conductivity must be monitored in aqueous solutions to ensure that water is of a high quality and fit for its purpose.