International standards for optimal lab water: pH, dissolved oxygen and conductivity measurements in aqueous solutions are most important to ensure top-quality lab water.
The International Prototype Kilogram (IPK) for the physical magnitude of mass is a body made from platinum and iridium stored as a reference object for mass determinations. Its principle role is to promote worldwide compatibility in mass measurements by providing calibrations traceable to the international prototype. The IPK has been held at the Bureau International des Piods et Mesures in Sevres near Paris since 1889 with official copies made for international meter convention members. The copies must not exceed a defined uncertainty in exact comparison weightings. Linked with these so-called country-specific measurement standards are further standards that also must not exceed certain tolerances. They are used as reference blocks for providers of measurement and calibration services and ensure that on a calibrated and adjusted scale, 1.00 kg of apples, for example, will weigh exactly 1.00 kg.
Measurement of pH value
According to Nernst, pH measurement is based on the electrochemical determination of the activity of hydrogen ions in aqueous solutions. This is the most common laboratory measurement performed in these solutions. For pH value, there is no "international prototype pH" to reference against. Instead, the most widely accepted practice is to use buffer solutions containing 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.
Institutes such as the National Institute of Standards in Gaithersburg, Md and the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, adopt complex measurement procedures against hydrogen standard electrodes to examine these solutions and deter mine their uncertainties. As a result of these examinations, a secondary reference material or standard has been created on which further standards can be built.
This system is important in order to comply with the DIN ISO 9000 ff standard. For companies that have achieved this certification, their testing equipment is monitored within the standard's quality assurance framework and must be traceable to international standards. This traceability must be documented and pro vided to customers by the manufacturers. The system also enables compliance with the inter nationally composed standards for DIN/NIST buffers, which most commercial instruments operate in accordance with.
Technical buffer solutions are also used by many for calibration. The unique feature of these buffers is that they have whole number pH values at certain reference temperatures including 20 and 25 C, such as 2.00; 4.00; 7.00. This eliminates previous issues with electrochemical pH measurement that made it difficult to obtain exact readings using analog instruments with dial displays.
The accuracy range of commercial buffer solutions is between [+ or -]0.01 and [+ or -]0.04 pH. This is used as a reference to check sensors and adjust the readings of the measuring transmitter according to the determined deviations. Ideally, the calibration temperature and measurement temperature should be the same. In 95% of cases, measurements are not critical and are performed at room temperature. However, critical measurements need to be taken above or below room temperature with buffer solutions and electrodes that have been tempered accordingly.
Users are not advised to store buffer solutions in a refrigerator as this increases settling time of the electrode and waiting time during calibration. Like all chemical products, buffer solutions also age and should be discarded after each use. Buffer in open containers should be used immediately; this especially applies to alkaline buffers which absorb more [CO.sub.2], consequently reducing their pH value.
Calibration frequency depends on the required accuracy and media that influence the sensor (drift). The frequency can range from several times per day in critical applications, such as those in pharmaceutical laboratories, to once every two weeks for relatively uncritical applications. When calibrating a pH combination electrode, the focus is on two parameters: zero point deviation and slope. According to the DIN 19266 standard, the zero point deviation must be within [+ or -]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 trace able to the primary standard arc established.
The Clark oxygen electrode, invented by Leland Clark in the 1950s, is a polarographic measurement method. Today, it is frequently used in water analysis to measure dissolved oxygen (DO) as an amperometric signal. Alternative techniques include optical methods and titration. To measure DO, ambient air is used as the standard since it contains approximately 21% oxygen by volume. This percentage is virtually constant in the biosphere and, as a result, can easily be used. Depending on the geo graphic location, the air pressure can be taken into account using a barometer. However, the measurement of dissolved oxygen 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, such as nitrogen and carbon dioxide.
Water vapor pressure is another important factor that is highly variable and must be taken into account for determination. However, determining the relative humidity and including it as a correction requires significant time and effort, not to mention 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 sensor creates a water vapor-saturated atmosphere above the sensor surface. As a result, the proportion of the water vapor at a known temperature can easily be compensated for by calculation.
The Winkler titration method is also frequently used in the U.S. for the calibration of dissolved oxygen sensors. The method is used as a comparison against external standard procedures and was developed for quantitative oxygen determination in the 19th century. As a procedure, it is highly accurate, but requires significant time and effort making it unsuitable for field applications. It is, however, used by manufacturers of dissolved oxygen measuring systems to validate their products.
A further method is calibration in air saturated water, an option that provides a very uncertain standard because of numerous sources of error during preparation. Oxygen calibration is typically performed as a single-point calibration at maximum signal. This requires a sensor that does nor 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 electronically suppressed.
The WTW OxiCal dissolved oxygen beaker from ITT Analytics, offers a viable solution for efficient dissolved oxygen measurements. To facilitate error-free calibration, the sponge in the beaker should be moist, but never wet. For accuracy reasons, users must place the sponge in the beaker, remove it again and check the membrane surface. If there are any droplets on the membrane, it is essential to squeeze out the sponge prior to calibrating; otherwise, there is the danger of the results being too high. In addition, the sensor must be calibrated regularly. As galvanic sensors are always in operation, their slope is subject to change even when not in use.
Conductivity in aqueous solutions 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 [micro]S/cm at 25 C. The H+ and OH- ions work as conductors according to the equation [H.sub.2]O= 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 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 because of 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 feature of conductivity is that, contrary to pH and dissolved oxygen measurement, it is not possible to measure the entire range of aqueous solutions with one single sensor, that is 0.055 [micro]S/cm to approximately 800 mS/cm. Suitable sensors are therefore used for the different ranges. Normally, conductivity sensors are not subject to much wear and the calibration interval according to the DIN EN 27888 standard is half a year. The same applies to standard solutions, which are available with different test values. More recently, certified products trace able to international standards have become available enabling the operator to check the sensor while taking the solution temperature dependency into account. As mentioned earlier, conductivity sensors are very sensitive to contamination, meaning calibration must be performed carefully to avoid incorrect measurements. Conductivity standards may also only be used once and the sensor must be cleaned first and moistened with the standard solution to avoid the displacement of adhering residues of sample or distilled water.
pH, dissolved oxygen and conductivity measurements in aqueous solutions are of utmost importance in order to ensure optimal water quality. The latest calibration methods facilitate accurate electrochemical determination of the values of these parameters, enabling meaningful and relevant documentation of research results, process steps and material parameters in compliance with official requirements.
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AT A GLANCE
* Testing against buffer solutions has made a new standard for quality of lab water.
* Factors like air and water vapor pressure are important to consider when measuring dissolved oxygen.
* Products adhering to international standards have improved calibration of conductivity measuring systems.
by Dr. Klaus Reithmayer, Product Manager. WTW part of ITT Analytics