Voltage, current and time
In an electrochemical reaction, electrons are pushed around the circuit by an external power supply - part of this circuit is the reaction mixture with two electrodes separated by some distance. The current is the rate of flow of these charged particles, and is measured in amperes. The current is directly related to the rate of electron flow around the circuit. For a given current and time you can calculate the moles of electrons that flow. This modeller shows the relationship between these. Later on we will show you how this can guide your experiments.
Current and time to moles of electrons
The voltage is a measure of the work done to push electrons around the circuit - some circuits are low resistance and you only require a small voltage to push the electrons, whilst a circuit with a high resistance means you have to work harder : the potential difference (measured in volts) is greater.
The resistance is dependent on the reactants and products, the choice of solvent and any electrolytes you add (to reduce the resistance), and the surface area and distance between electrodes (which is why controlling these with ElectroReact is important). It might also change over the course of the reaction. Voltage, current and resistance are linked. Since resistance is determined by the solution mixture and reactor, you can't independently control voltage and current for your circuit, they are proportional to each other - so double the current and you double the voltage. More generally `V=IR`, where `V` is the voltage (volts), `I` is the current (amperes) and `R` is the resistance (ohms). Generally you fix either the voltage or the current to be constant over the course of the reaction, that is the reaction can be run in one of two ways - constant current (galvanostatic), or constant potential (potentiostatic) conditions. Next we explore the differences.
Running a reaction under constant current (galvanostatic conditions) means you know exactly the rate of electron flow. And since the number of electrons are known, then you can relate this to the moles of required electrons in your chemical reaction. The modeller below uses the known relationship between current and the resulting flow of electrons to estimate the time for your reaction to complete. You need to specify the number of electrons required per mol of substrate to give the final product. This does assume 100% Faradaic efficiency (see Determining the efficiency of a reaction). So whilst a big advantage of constant current means you know the number of electrons flowing, a disadvantage is that you are not controlling the potential, as this varies automatically to maintain the rate of electrons flowing in the circuit. Too low a potential (as a result of a low resistance) creates very mild conditions under which the reaction might not run. Too high, and undesired reactions such as decomposition or side reactions may occur. This may be managed through modifying the resistance (e.g. through electrolyte type, concentration etc.) or the reactor setup (e.g. through modifying the distance between electrodes, charge density etc.).
Time estimator
When running under constant potential, you can ensure that the potential lies in a band to give you an effective reaction without causing side reactions (e.g. within the solvent). There are tables of standard reduction potentials that can help set the potential within your reaction, or it is possible to measure these using cyclic voltammetry. However, you now don't have a guide to how long your reaction might take since you are not controlling the current (flow of electrons) and unless you have logged the current as a function of time, can't determine efficiency.
Finally, however you run your reaction, efficiency can be established by calculating the conversion of the starting material (conversion) or formation of the product (yield) but with respect to the quantity of electrons supplied. We provide some guidance on our pages on measuring efficiency.