Amperometry

Amperometry
Doug Glosup
CHE 4213.002
03/18/13

Introduction:
Amperometry in chemistry is the detection of ions in a solution based on electric current or changes in electric current. Another words, the measurement of current resulting from the electrochemical reaction at the electrode, is a simple means of determining the concentration of electro-active species. The type of electrochemical cell used in this experiment is a three-electrode cell. The cell consists of working, reference and counter electrodes. The working electrode is the glassy carbon electrode, the reference electrode is that Ag/AgCl reference electrode, and the counter electrode is a platinum wire. The potential is applied between the working in the reference electrodes, and the current measure between the working and counter electrodes. The working electrodes provide the surface for electron transfer to occur for the system under investigation.
Experimental:
The working electrode was first polished using 0.3 and 0.5 alumina. A 0.05 M solution with a pH of 7.3 Phosphate buffer solution (PBS), and a 50mM stock solution of Ru(NH3)6Cl3 was made. The PBS buffer was made using NaH2PO4 and NaH2PO4H2O in dilution with 200mL of water. Five different concentrations of Ru(NH3)6Cl3 and one unknown concentration was prepared. The amount needed for each concentration was calculated; 50 µL for 0.25 mM, 100 µL 0.5 mM, 200 µL for one mM, 300 µL for 1.5 mM, 400 µL for for 2mM. The unknown concentration required 80 µL. The electrochemical electrode was set up with three electrodes; the glassy Carbon electrode was the working electrode, Ag/AgCl reference electrode was the reference electrode, and the platinum wire was a counter electrode. Chi832 program with zero voltage, 750 seconds, and 1 e-5 Amp parameters were used to plot an ampoteric IT curve as shown below.

Results:

Conclusion:
One error in prep process of our solutions that did not directly affect our result data was measuring the pH. The pH meter was not first calibrated, so the resulting pH, 7.33(±0.01), came out lower than 7.40. According to the current vs. time graph, current exponetially grew proportional to time. Such that, current increased as time passed. According to the current vs. concentration graph, they are directly proportional to one another. The R2 value skewed from 0.9999 value, meaning the point values were not directly linear to one another, thus not a “perfect experiment”. Three injections of each of the six Ru solutions (this includes the unknown) into PBS in a three electrode experiment were made. The average of each currents were taken and plotted against concentration in the above graph. The LINEST function was used in excel to find the x value (concentration) of the unknown with 80μL injection. That value came out to be 0.44 mM. Amperometry Questions: 1. This amperometry lab utilized electrolytic cell. Reactions in galvanic cells are spontaneous. They supply energy in which is used to perform work. Reactions in electrolytic cells are non-spontaneous. Electrical energy is used to induce electrolysis. 2. The reference electrode provides a constant potential regardless of the composition of the solution it is placed in. Its provide a stable, reproducible voltage to which the working electrode potential may be referenced. The voltage difference between the reference and measuring electrodes is a function of the pH value of a solution. Common reference systems in use are (Ag/AgCl) and (Hg2Cl2). 3. The counter electrode, also referred to as the auxiliary electrode, acts to source electrons in the electrochemical circuit formed with the working electrode. With this electrode, the potential voltage is able to pass current through the analyte solution without passing current into or out of the reference electrode. It has little or no effect on the electro analytical measurement.
4. An example is electrorefining and electrowinning of purification of nonferrous metals. This uses electrical current to separate and break down metals from their accompanying impurities. Electrorefining works by running a current through an electrolytic cell and electrowinning is used for the collection of the purity.