Chemistry 425

Instrumental methods of analysis

Amperometric titration for the determination of ferrous ion

End-point indication using amperometric titration is inherently more accurate than titration with visual indicators. The accuracy of the end-point determination is primarily given by the accuracy of titrant delivery. There are other suitable electrochemical methods for end-point indication, but amperometric titrations possess greater sensitivity than conductometric and potentiometric titrations. In addition, equipment needs for amperometric titrations are much simpler than typically required for other electrochemical methods. Typical analyses are in the range of 0.1 mol/l -- 0.0001 mol/l, with analyte not needing to be an electroactive species. The titration can be conducted by utilizing redox system present either before or after the end point.

In amperometry, the potential of the working electrode (vs. the reference electrode) is held at a constant value and the resulting current is measured. The electrodes are usually in a stirred or flowing solution. Alternatively, the electrode may be realized in the form of an ultramicroelectrode, where stirring is not essential. Analytical determinations are made from the current, which is proportional to the concentration of the electroactive species. Usually, the potential is held at the limiting current region of the electroactive species of interest. Amperometry is now commonly used as a means of detection for flow injection analysis (FIA) and liquid chromatography. In these techniques, the sample is injected into a flowing stream of electrolyte, and a current peak is recorded as the sample passes through the electrochemical cell.

In its conventional and/or most common form, the arrangement for the amperometric titration consists of a polarizable microelectrode, e.g., a platinum flag, in combination with a large surface non-polarizable reference electrode. A constant potential is impressed across the indicating system such that it is on the diffusion current plateau for either the titrant, reactant or both. During the titration experiment the current flowing through the system is recorded as a function of the volume of titrant.

With its three electrode-configuration, the CV-27 voltammograph (Bioanalytical Systems, West Lafayette, IN) is a suitable controller for this application. The three-electrode system employs additional, third electrode, the auxiliary electrode, which is the actual current carrier, rather than the reference electrode in a two-electrode system. The use of this instrument will be illustrated with the titration of ferrous ion using the ceric ion as titrant.

        Fe²⁺  +  Ce⁴⁺    -->  Fe³⁺  +  Ce³⁺

Standard and sample solutions

0.0100 mol/l ceric ammonium sulfate in 1.0 mol/l sulfuric acid
0.0100 mol/l ferrous ammonium sulfate
0.00100 mol/l ferrous ammonium sulfate
unknown solution from the TA

Procedure

In a clean beaker, dispense a known volume of the 0.0100 mol/l Fe (II) standard solution. Place a platinum flag working electrode and the Ag/AgCl reference electrode in the beaker, using appropriate stand. Place the counter electrode in a tube with a fritted disk and put the tube in the beaker. Deliver enough deionized water into the beaker to cover all the electrodes (the solution should flow slowly through the fritted disk into the counter electrode compartment). Add 1 ml of phenanthroline indicator to the solution to guide you visually in the process of the titration.

Connect all three electrodes to the CV-27 (RED-auxiliary, BLACK-working and WHITE-reference). Fill a clean burette with the Ce(IV) titrant solution. Record the initial volume reading. Turn the CELL MODE to STANDBY. Turn the POWER switch to the ON position and adjust E1 to +0.8 V. Switch the DISPLAY to I out position. Turn on the magnetic stirrer. Turn the CELL MODE switch to CELL. Read and record the cell current (I out) at zero volume of titrant.

Add 0.5 ml of titrant to the sample solution and record the current reading from the CV-27. Continue to add the titrant and record the current at each volume increment. Note the volume of titrant that produces an indicator end-point (from orange to light blue or clear). Continue adding 0.5 ml of titrant and recording the current until 10 ml past the indicator end-point.

Repeat the titration for the 0.00100 mol/l Fe(II) solution, and for the unknown solution. Smaller volume increments may be needed.

Results

Plot the current versus titrant volume added for all titrations. Determine the end point for the known solutions and compare how the end-point volume agrees with the volume expected for titration with the known cerium standard. Also, how does the amperometric end-point agrees with the visual end point as determined by your observation.

Calculate and report the Fe(II) concentration in your unknown.

Discuss the possible sources of error in your result. Is there any discrepancy between the indicator endpoint and the amperometric endpoint? If so, explain why.

Why is the counter electrode placed in the fritted tube?

PVTJQ

Copied from previous handout: 24 August 1998 
Last revised: 08 November 2002 10:20
© Petr Vanýsek
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