A coulometer is an instrument that can be used to measure the amount of an electroactive substance present based on the amount of electricity passing through the solution.
weight in grams of substance =
= (molecular weight in grams) * (coulombs for the reaction) / ((number of electrons involved in electrochemical process) * 96494)
where:
• 96494 is 1 Faraday and is the amount of electricity required to oxidize or reduce 1 equivalent of an electroactive species (such as silver). 96485.3 is given in the CRC Handbook as the 1986 CODATA recommended value while 96487 is given in Tietz.
• Coulombs are in ampere•seconds.
coulombs for the reaction =
= (current in amperes at time 0) / (factor for experimental conditions)
The plot of the current over time is an exponential curve which drops from the maximum (current in amperes at time 0) to 0 at the end of the reaction.
current flow through coulometer in amperes =
= (current in amperes at time 0) * EXP((-1) * (factor for experimental conditions) * (time in seconds))
To calculate the current at time 0, it is necessary to measure the current at different times until the reaction is complete.
Steps:
(1) Take the logarithms of the current readings.
(2) Find the slope of the line with time in seconds as the X axis and log(current).
(3) The slope of the line is the negative (-1) of the factor for experimental conditions.
(4) The current at time 0 is the exponential for the intercept = EXP(intercept).
(5) Once the current at time 0 is known, then the coulombs for the reaction can be calculated.
(6) All of the factors are available to complete the initial equation.
