Question

b) Electrochemistry is the branch of physical chemistry that studies the relationship between electricity, as a measurable and quantitative phenomenon, and identifiable chemical change, with either electricity considered an outcome of a particular chemical change or vice versa. These reactions involve electric charges moving between electrodes and an electrolyte (or ionic species in a solution). Thus electrochemistry deals with the interaction between electrical energy and chemical change. When a chemical reaction is caused by an externally supplied current, as in electrolysis, or if an electric current is produced by a spontaneous chemical reaction, as in a battery, it is called an electrochemical reaction. Chemical reactions where electrons are transferred directly between molecules and/or atoms are called oxidation-reduction or redox reactions. In general, electrochemistry describes the overall reactions when individual redox reactions are separate but connected by an external electric circuit and an intervening electrolyte. Figure 4.1 shows the analogue electronic circuit that controls and measures the electrochemical experiment. In brief, the circle in the middle indicates the liquid cell containing the electrolyte in which the reaction under the influence of an electric potential takes place. The whole system can be seen as a three electrodes system. The working electrode (WE), which makes contact with the liquid cell, must apply the desired potential in a controlled way and facilitate the transfer of charge to and from the liquid. The reaction on this electrode characterised by measuring the current between electrode and electrolyte at a given potential A second electrode, the counter electrode (CE), acts as the other half of the cell and supplies the current for the reaction happening at the working electrode.

The Reference electrode (RE) measures the potential inside the electrolyte. Its only role is to act as reference in measuring the potential inside the electrolyte and controlling the working electrode's potential in order to archive the correct potential inside the electrolyte. At no point does it pass any current. In other words, the reference electrode ensures that the measurement is corrected for the resistance of the leads and working electrode as well as compensates for any reactions that might take place there. In the circuit are three operational amplifiers labelled I, II and III each with their own function.

+ II R СЕ R R REXWE out + + III R

The various components have the following values, R1 = 100 kΩ, R2 = 10 kΩ, R3 = 1 kΩ, R4 = 5.1 kΩ, C = 10 µF. The current from the working electrode varies between ±1.5 mA.

i) Assuming that all components are ideal, what is the transfer function of amplifier I and what is the ideal gain?

ii) The electrochemical cell has a noise level of ±100 mV on the output voltage and the resistors used in the circuit have a tolerance of 5%. To connect the output to a computer you have an ADC available with a maximum and minimum input of +10 V and -10 V respectively. Assuming the noise sources are independent determine the number of bits that carry relevant information.

iii) Determine the transfer function for the electrochemical cell assuming all components are ideal. What is the function of amplifier II?

iv) What is the function of amplifier III?