Barker square-wave instrument

Because Barker in this period confined his work to studies at the dme, he focused on and attempted to solve problems peculiar to that electrode. One of those problems is capillary noise or capillary response . These terms refer to the current required to charge the interface between mercury and the film of solution which penetrates some distance into the capillary. Charge must be supplied when the potential is changed, but also the interface itself is unstable, so that the charging current varies from drop to drop. In order to understand the reasoning which led Barker to normal and differential pulse polarography, it is useful to describe in more detail the operation of his square-wave instrument. 

Thus the capillary response problem could not be solved by simply operating the square-wave instrument at a lower frequency. Faced with this difficulty. Barker decided to limit the experiment to one pulse per drop. Having made that decision, it became clear that one could employ either the normal pulse or differential pulse waveform as described above. This created an interesting technical problem, for now the instrument required two precisely timed intervals of different magnitude. Barker solved this by using a mechanical timer based on a slowly rotating cam to time the 2 s interval between the fall of a drop and the initiation of the pulse and measurement sequence, and electronic timing to establish the 20 ms intervals over which current was sampled and the 40 ms interval of the pulse. 

The Metrohm 646 VA Processor is another microprocessor based instrument. Used in tandem with the 647 VA Electrode Stand and 675 VA Sample Changer, this system is capable of performing automated data acquisition, including the use of the standard additions method. Data analysis features include smoothing and differentiation, and a peak shape analysis routine that performs independently of the base current. Pulse polarographic techniques that can be performed include dp, which can be optimized for reversible and irreversible systems staircase, with current measurement during the final 20 ms of each current step and Barker square wave, which employs a waveform composed of five square wave oscillations superimposed upon a staircase, with currents measured for 2 ms at the end of each half cycle of the second, third and fourth oscillations. The 1988 price of this instrument is 14,000. 

It is possible to exploit the difference in the time-dependencies of y(pro-portionalto /1 /6) and ic (proportional to f 1/3) to separate the two (J. N. Butler M. L. Meehan, J. Phys. Chem. 69 (1965) 4051), and thereby to push back the lower limit somewhat. However, instrumental refinements such as square wave polarography (G. C. Barker I. L. fenkins, Analyst 77 (1952) 685) and, pulse polarography (G. C. Barker A.W. Gardner, Z.Anal. Chem. 173 (1960) 79) are much more efficient in doing so, and are therefore the methods of choice for concentrations between 10-5 and 10 7 M. 

Pulse voltammetric techniques, most used in electrochemistry, are normal pulse voltammetry (NPV) and differential pulse voltammetry (DPV). In square wave voltammetry (SWV), there may be a non-faradaic contribution to the individual currents but the current sampling strategy essentially eliminates this through subtraction, as will be seen in Sect. 2.2.4.3. SWV was pioneered by Barker [1] in the 1950s, but due to instrumentation development only 40 years... 

These developments were all based on the dropping mercury electrode and in each case the central feature is the instrument which is the embodiment of the technique. The developments of Barker are particularly significant because his was the first electronic instrument and because it was soon commercialized by Mervyn Instruments as the Mervyn-Harwell Square Wave Polarograph. A photograph of this instrument is shown in Figure 2. This pattern was to prove increasingly important because the electronic implementation of pulse voltammetric techniques required expertise and time outside the reach of the average scientist. Thus the use of these techniques would depend on the availability at an acceptable price of reliable commercial instruments. 

Laser illumination of electrodes as a thermoelectrochemical method dates back to 1975 [50], when Barker and Gardner applied pulsed diode laser light to implement thermal modulation as a new method. That time, many authors experimented with different modulation techniques in order to separate useful signal from noise. A scheme of the instrument of Barker and Gardner is given in Fig. 4.7. When periodic heat pulses in the form of a square wave function are imposed at the electrode interface, thermal changes of the electrode processes result in periodic... 

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