Hans Wenking is an engineer-physicist who devoted his lifetime to the development of equipment for chemistry and biocybernetics. Dipl.-Phys. Hans Wenking developed the first electronical potentiostat on the European continent. He was the first who described the basic principles of potentiostats. In 1952 Hans Wenking designed a bulb amplifier which was used to control a mechanically recording amplitude-controlled oscilloscopeoperating in both polarities.
A mirror-galvanometer was used to record fast oscillations up to 100 Hz. The light beam was recorded on photographic paper. Later this amplifier was extended with a power supplier to drive amplitude-controlled oscillating boards for biological applications.
This oscilloscope-amplifier may be called the ancestor of the modern semiconductor-based operational amplifiers. It was showing all the features of them, e.g. it demonstrated some similarity with the well known TL 071 amplifier (Texas Instruments), or comparable devices.
The features of the Wenking’s amplifier included: differential input with an inverting and non-inverting inputs, difference-forming in the next stage, high amplification (up to 105) using constant current circuits, dynamic feed-back. This amplifier was later used as a core of the novel potentiostat, only a power supplier had to be added.
However, Wenking made his the most important contribution to electrochemistry – construction of a potentiostat – during his work at the Max Planck Institute in Goettingen, Germany. He joint an excellent group of physico-chemists working in Goettingen headed by Professor Karl Friedrich Bonhoeffer and his assistant Weil. Wenking was assigned to develop a potentiostat that was needed for electrochemical experiments, particularly, for studying corrosion.
A potentiostat is a control amplifier which, besides its own phase shift, has to control the current passing a cell. The electrode system within the cell forms a low pass filter, which itself introduces an unknown phase shift – any angle between nearly zero and 90o – into the control loop. Therefore, the allowed maximum phase shift of a potentiostat should be 90o.
This problem had to be solved. Early potentiostat manufacturers neglected this problem. The consequence was “singularities” during the experiment where the potentiostat started to oscillate. Sets of filters were used to damp the oscillations: filters which had to be switched during the experiments when critical points were reached.
The solution for the potentiostat instability was quite simple, in principle. The method is called “over-all-compensation” of the phase shift. By this method, the phase angle remains below 90o until the frequency-gain product is unity or less, i.e. the point where the amplifier damps and does not longer amplify. To follow electrochemical reactions, this critical frequency shall be higher than typical changes within the cell. Otherwise, the cell itself may start to operate in an undesired manner. These features were realized in the first potentiostat designed by Wenking.
Some other novel principles were used in his instrument. Former potentiostats (e.g. a potentiostat designed by Staubach) fixed the counter electrode to ground, and the signals of both working electrodeand reference electrode floated with respect to the ground. Wenking fixed the working electrode to ground, and the potentiostat was operated like an operational amplifier today.
His first potentiostat showed practically the principles of a presentoperational amplifier: Differential input as the first stage, difference forming in the next stage, followed by a high-gain stage and a power stage. The features were high amplification, high common-mode rejection and internal dynamic feedback. The potentiostat performance was discussed at the CITCE meeting in Stuttgart in 1955, however, the respective paper was not written.
Wenking’s potentiostat electronic circuit.
Until 1957 Wenking’s potentiostat was manufactured only for the internal use of the Max Planck Institute in Goettingen. Later Hans Wenking together with Gerhard Bank established “Elektronische Werkstatt Goettingen” to commercialize the potentiostats.
From 1959 the company operated under the name “Gerhard Bank Electronik”. Wenking designed the instruments as free lance but the brand “Wenking potentiostat” soon became a famous trade mark. The consequence of the potentiostat development was a rush in the development of electrochemical science. The phenomena of metal passivity could be better explained, mechanisms of oxide layer formation, and far beyond the materials science. The potentiostat became a standard instrument for most electrochemical investigations.
Independent of Wenking’s work, similar instruments were designed by other companies. Tacussel was one of those companies who came to similar conclusions as Wenking (probably slightly later), and started potentiostats manufacture in France. In the U.S.A. however, Wenking’s potentiostats were leading on the market. This and Wenking’s habit of using English as language in his manuals led to the misunderstanding that both the company Bank and Wenking himself were American.
Wenking has never published a line in scientific papers. Rare hints on his theoretical works are found as citations “unpublished”, e.g. by Vetter. On the other hand, Wenking never concealed the technical details of his instruments.
His circuits and layouts were included in the operation manuals for the instruments, and even in some manuals a detailed theoretical treatise was given (pages which probably never fascinated a chemist or material scientist). The timidity to publish his work may be one of the reasons why until today publications on potentiostats can be found which do not meet the criterion of absolute stability.
For Wenking the potentiostats were not a primary source of income, but a welcome add-on. He felt as a physicist, his foremost aim had always been to solve measurement and control problems. He solved different great problems, amongst others the realization of an automatic operating ellipsometer. He did not like to be captured by institutions or companies, he liked to remain free lance, to be free to choose his tasks.
This love for freedom is documented by his biography: in 1963 he joined the Max Planck Institute for Biocybernetics in Tuebingen, not as an employee, but as a consultant. There, he constructed instruments to investigate the optosensorics of insects. At the same time, he was working part-time with Zeiss in Goettingen. His optical instruments were never commercialized, however, the US Navy bought his patent for aninstrument to measure the precise speed relative to the ground of an aircraft by analogue correlation of the signals of two photosensors.
Nevertheless, Wenking was further interested in potentiostats. In 1961 he received a revolutionary part from an American scientist – a field effect transistor (FET). This FET was the first solid state transistor which reached the qualities of a bulb with respect to low input current and high input impedance.
These FETs were instantly used to design the next generation ofoperational amplifiers. In Wenking’s circuit diagrams these FETs were referred to as “BOP” or “FOV”. These were discretely soldered amplifiers which were superior to commercial products at that time. The first integrated operational amplifier which reached the quality of those discretely soldered ones was the CA 081. It was introduced in Wenking’s instruments in the late seventies.
Wenking’s advanced potentiostat circuit.
Further work on potentiostats was undertaken by Wenking to make the instruments faster: higher slew rates, higher bandwidth to obtain most precise square wave response. Furthermore, the method of current measurement was improved.
Wenking’s idea was to separate the internal ground of the power supplier from the “signal” ground which was the grounding point of the workingelectrode. Doing so, and by introducing the current-to-voltage converting resistor between these two grounds, the current through the workingelectrode is forced to pass this resistor. As a result, he obtained the current – proportional voltage ground – referred without lack of bandwidth. Especially for high-currents and high-voltage power potentiostats, this principle is still superior.
Wenking tried nearly all concepts, and the best were introduced for commercial models. His critical view to the works of others as well as to his own designs was sometimes extreme. Sometimes this led to some mistrust in the capabilities of potentiostats. However, Wenking’s instruments were always state-of-the-art and electrochemists of 50s-70s used them for many different applications.