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Opens external link in new windowpH of sea water, PTB News, 3, 2018, p. 3

Opens external link in new windowIs the Baltic Sea acidifying? IOW researcher adapt optical pH measurement method for brackish waters, Press release Leibniz-Institute for Baltic Sea Research Warnemünde (IOW), 19.09.2018


Traceability chain for measuring the acidity in the Baltic Sea realized

In a joint project between PTB and Leibniz-Institut für Ostseeforschung Warnemünde (IOW), it has been possible for the first time to measure the acidity constant pK of the indicator dye m-cresol purple (mCP) as a function of temperature and within a salinity range that is typical of the Baltic Sea. Knowing this pk value is an essential precondition for traceable pHT measurement results whose definition differs from that typically used in metrology and in industry.

Measuring the anthropogenic acidification of the world oceans is of essential importance because of the associated negative effects on biological and biochemical processes. In oceanography, the pHT value measured in situ (e.g. on ships) is typically used as a measure of acidification. This optical measurement method uses an absorption spectrum to measure the ratio between the percentage of the dye mCP to which a hydrogen ion has bound and the percentage to which no hydrogen ion has bound. This ratio depends on the concentration of hydrogen ions in the seawater and the value of the acidity constant pK, which – in turn – is a function of salinity and temperature. In contrast to the pHT value, which is sensitive to the absolute hydrogen ion concentration, the metrological definition of the pH value is based on the activity of the hydrogen ions. The activity describes the effective concentration of the hydrogen ions, which is significantly influenced by the medium in which they are found.

The pHT values were measured in artificial seawater samples with different salinities ranging between 5 and 20 g/kg by means of PTB's primary pH measuring device, the so-called Opens external link in new windowHarned cell apparatus. The IOW has added mCP to the same samples and measured them by means of spectrophotometry. From these measurement data and from the pHT values measured at PTB, the pk value of mCP was determined as a function of salinity and temperature. This made it possible to close the traceability chain for the optical measurement method.

An open issue that still needs to be worked on in a future project is the characterization of the purity of the dye which impacts the measurement uncertainty of the spectrophotometric measurement of the pHT value.

These procedures can also be extended to other spectrophotometrical indicator dyes, e. g. for thymol blue.


PTB develops an empirical method to quantify the state of health of high-energy lithium-ion battery cells by means of electrochemical impedance spectroscopy (EIS)

PTB has developed a method for measuring the state of health of high-energy lithium-ion battery cells in a time-efficient way. For this purpose, an electrochemical impedance spectrum of the battery is recorded. Based on an empirically obtained specific reference curve, the battery's state of health can be quickly measured.

The quick and reliable measurement of the state of health of lithium-ion batteries is vital when they are used in electric vehicles. In contrast to the capacity of fuel tanks, a battery's capacity deteriorates over time. In the case of smartphones or laptops, it is usually sufficient to get just a rough indication of the remaining lifetime of the battery. The information on the cruising range of an electric vehicle should, however, be exact. A promising method for measuring a battery's state of health is electrochemical impedance spectroscopy. The AC resistance (the so-called impedance) measured in this method depends on the electrochemical processes in the battery cell. If these change due to battery aging, then the impedances measured at different frequencies (the so-called impedance spectrum) change as well. As a result, these changes can in principle be used as a measure of a battery cell's state of health.

So far, this method has suffered from the fact that the results were not clear. Changes in the impedance spectrum could not be clearly attributed to the electrochemical processes, and varying operating conditions seemed to lead to varying changes in the spectrum of one and the same battery type. Measurements conducted by PTB with regard to the aging of batteries as well as a meticulous impedance analysis and the application of appropriate statistic models for data analysis suggest that it is possible to gain the SoH from impedance spectra if the measurement conditions are carefully defined. The method is now being validated by additional measurement sequences under modified operating conditions. For this validation, the measurement results are supported by model calculations where the impedances are computed on the basis of fundamental charge and mass transport equations. PTB's medium-term goal is to implement a reference measuring set-up that enables measuring the specific aging curve of a specific battery type. This curve is to be used as a reference for respective measurements in the field of quality assurance, in workshops, in research as well as in the further development of batteries.