Electrogravimetry and Coulometry Based on an analysis that is carried out by passing an electric current for a sufficient length of time to ensure complete oxidation or reduction of the analyte to a single product of known composition Moderately sensitive, more accurate, require no preliminary calibration against standards i. Absolute analysis is possible. Electrogravimetry The product is weighed as a deposit on one of the electrodes the working electrode. Coulometry The quantity of electrical charge needed to complete the electrolysis is measured. Electrogravimetric Methods Involve deposition of the desired metallic element upon a previously weighed cathode, followed by subsequent reweighing of the electrode plus deposit to obtain by difference the quantity of the metal Cd, Cu, Ni, Ag, Sn, Zn can be determined in this manner Few substances may be oxidized at a Pt anode to form an insoluble and adherent precipitate suitable for gravimetric measurement. Easily reducible metallic ions are deposited onto a mercury pool cathode Difficult-to-reduce cations remain in solution Al, V, Ti, W and the alkali and alkaline earth metals may be separated from Fe, Ag, Cu, Cd, Co, and Ni by deposition of the latter group of elements onto mercury.
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Dynamic techniques, in which current passes through the electrochemical cell and concentrations change, also are important electrochemical methods of analysis. In this section we consider coulometry. Voltammetry and amperometry are covered in Chapter Coulometry is based on an exhaustive electrolysis of the analyte.
By exhaustive we mean that the analyte is oxidized or reduced completely at the working electrode, or that it reacts completely with a reagent generated at the working electrode.
There are two forms of coulometry: controlled-potential coulometry , in which we apply a constant potential to the electrochemical cell, and controlled-current coulometry , in which we pass a constant current through the electrochemical cell. If the current varies with time, as it does in controlled-potential coulometry, then the total charge is. In this section we consider the experimental parameters and instrumentation needed to develop a controlled-potential coulometric method of analysis.
Here we use the Nernst equation to help us select an appropriate potential. If we define a quantitative electrolysis as one in which we reduce As a result, the rate of electrolysis—recall from Chapter Because time is an important consideration when designing an analytical method, we need to consider the factors that affect the analysis time.
For an exhaustive electrolysis in which we oxidize or reduce From this equation we see that a larger value for k reduces the analysis time. For this reason we usually carry out a controlled-potential coulometric analysis in a small volume electrochemical cell, using an electrode with a large surface area, and with a high stirring rate.
A quantitative electrolysis typically requires approximately 30—60 min, although shorter or longer times are possible. A three-electrode potentiostat is used to set the potential in controlled-potential coulometry see Figure For example, a potential more negative than —1 V versus the SHE is feasible at a Hg electrode—but not at a Pt electrode—even in a very acidic solution.
Platinum is the working electrode of choice when we need to apply a positive potential. The auxiliary electrode, which often is a Pt wire, is separated by a salt bridge from the analytical solution.
This is necessary to prevent the electrolysis products generated at the auxiliary electrode from reacting with the analyte and interfering in the analysis. The other essential need for controlled-potential coulometry is a means for determining the total charge.
Modern instruments use electronic integration to monitor charge as a function of time. The total charge at the end of the electrolysis is read directly from a digital readout. As we learned in Chapter 8 , we call an analytical technique that uses mass as a signal a gravimetric technique; thus, we call this electrogravimetry.
Controlled-current coulometry has two advantages over controlled-potential coulometry. First, the analysis time is shorter because the current does not decrease over time. A typical analysis time for controlled-current coulometry is less than 10 min, compared to approximately 30—60 min for controlled-potential coulometry.
Using a constant current presents us with two important experimental problems. To maintain a constant current we must allow the potential to change until another oxidation reaction or reduction reaction occurs at the working electrode. The second problem is that we need a method to determine when the analyte's electrolysis is complete. At the beginning of the analysis, the potential of the working electrode remains nearly constant at a level near its initial value.
This reaction is identical to a redox titration; thus, we can use the end points for a redox titration—visual indicators and potentiometric or conductometric measurements—to signal the end of a controlled-current coulometric analysis.
Controlled-current coulometry normally is carried out using a two-electrode galvanostat, which consists of a working electrode and a counter electrode. The working electrode—often a simple Pt electrode—also is called the generator electrode since it is where the mediator reacts to generate the species that reacts with the analyte. If necessary, the counter electrode is isolated from the analytical solution by a salt bridge or a porous frit to prevent its electrolysis products from reacting with the analyte.
Alternatively, we can generate the oxidizing agent or the reducing agent externally, and allow it to flow into the analytical solution. A solution that contains the mediator flows into a small-volume electrochemical cell with the products exiting through separate tubes.
Depending upon the analyte, the oxidizing agent or the reducing reagent is delivered to the analytical solution. Figure Although a modern galvanostat uses very different circuitry, you can use Figure There are two other crucial needs for controlled-current coulometry: an accurate clock for measuring the electrolysis time, t e , and a switch for starting and stopping the electrolysis.
The switch must control both the current and the clock so that we can make an accurate determination of the electrolysis time. A controlled-current coulometric method sometimes is called a coulometric titration because of its similarity to a conventional titration. There are other similarities between controlled-current coulometry and titrimetry. The assumption, however, is not important and does not effect our observation of the similarity between controlled-current coulometry and a titration.
Coulometry is used for the quantitative analysis of both inorganic and organic analytes. Examples of controlled-potential and controlled-current coulometric methods are discussed in the following two sections.
The majority of controlled-potential coulometric analyses involve the determination of inorganic cations and anions, including trace metals and halides ions. The actual species in solution depends on the analyte. For example, we can determine the composition of an alloy that contains Ag, Bi, Cd, and Sb by dissolving the sample and placing it in a matrix of 0. When electrolysis is complete, we use the total charge to determine the amount of silver in the alloy.
Finally, we determine cadmium following its electrodeposition on the working electrode at a potential of —0. We also can use controlled-potential coulometry for the quantitative analysis of organic compounds, although the number of applications is significantly less than that for inorganic analytes. One example is the six-electron reduction of a nitro group, —NO 2 , to a primary amine, —NH 2 , at a mercury electrode.
Solutions of picric acid—also known as 2,4,6-trinitrophenol, or TNP, a close relative of TNT—is analyzed by reducing it to triaminophenol. Another example is the successive reduction of trichloroacetate to dichloroacetate, and of dichloroacetate to monochloroacetate. We can analyze a mixture of trichloroacetate and dichloroacetate by selecting an initial potential where only the more easily reduced trichloroacetate reacts.
When its electrolysis is complete, we can reduce dichloroacetate by adjusting the potential to a more negative potential. The total charge for the first electrolysis gives the amount of trichloroacetate, and the difference in total charge between the first electrolysis and the second electrolysis gives the amount of dichloroacetate.
The use of a mediator makes a coulometric titration a more versatile analytical technique than controlled-potential coulometry. A coulometric titration of the protein is possible, however, if we use the oxidation or reduction of a mediator to produce a solution species that reacts with the protein.
Note: The electrochemically generated reagent and the analyte are shown in bold. The resulting reaction is identical to that in an acid—base titration. Coulometric acid—base titrations have been used for the analysis of strong and weak acids and bases, in both aqueous and non-aqueous matrices.
In comparison to a conventional titration, a coulometric titration has two important advantages. The first advantage is that electrochemically generating a titrant allows us to use a reagent that is unstable. Second, because it is relatively easy to measure a small quantity of charge, we can use a coulometric titration to determine an analyte whose concentration is too small for a conventional titration. A sample weighing 0. Electrolysis at a constant current of To analyze a brass alloy, a 0.
Electrolysis of a Adjusting the potential to —0. This is the Cu from a The best way to appreciate the theoretical and the practical details discussed in this section is to carefully examine a typical analytical method. The description here is based on Bassett, J. The endpoint of the titration is determined potentiometrically.
The electrochemical cell consists of a Pt working electrode and a Pt counter electrode placed in separate cells connected by a porous glass disk. Prepare a mediator solution of approximately 0.
Add 5. Bubble pure N 2 through the solution for 15 min to remove any O 2 that is present. Maintain the flow of N 2 during the electrolysis, turning if off momentarily when measuring the potential. Stir the solution using a magnetic stir bar. Adjust the current to 15—50 mA and begin the titration. Periodically stop the titration and measure the potential. Construct a titration curve of potential versus time and determine the time needed to reach the equivalence point.
Why is it necessary to remove dissolved oxygen by bubbling N 2 through the solution? In either case, the result is a positive determinate error. One useful application of coulometry is determining the number of electrons involved in a redox reaction.
To make the determination, we complete a controlled-potential coulometric analysis using a known amount of a pure compound. An exhaustive controlled-potential electrolysis of a What is the value of n for this reduction reaction? The Thus, reducing a molecule of tetrachloropicolinic acid requires four electrons. The overall reaction, which results in the selective formation of 3,6-dichloropicolinic acid, is.
A coulometric method of analysis can analyze a small absolute amount of an analyte. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the endpoint.
When using a visual end point, the smallest concentration of analyte that can be determined by a coulometric titration is approximately 10 —4 M. As is the case for a conventional titration, a coulometric titration using a visual end point is limited to major and minor analytes.
Dynamic techniques, in which current passes through the electrochemical cell and concentrations change, also are important electrochemical methods of analysis. In this section we consider coulometry. Voltammetry and amperometry are covered in Chapter Coulometry is based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is oxidized or reduced completely at the working electrode, or that it reacts completely with a reagent generated at the working electrode. There are two forms of coulometry: controlled-potential coulometry , in which we apply a constant potential to the electrochemical cell, and controlled-current coulometry , in which we pass a constant current through the electrochemical cell.
11.3: Coulometric Methods
Electrogravimetry is a method used to separate and quantify ions of a substance, usually a metal. In this process, the analyte solution is electrolyzed. Electrochemical reduction causes the analyte to be deposited on the cathode. The mass of the cathode is determined before and after the experiment, and the difference is used to calculate the mass of analyte in the original solution. Controlling the potential of the electrode is important to ensure that only the metal being analyzed will be deposited on the electrode. The process is similar to electroplating. Thus electrolysis of an electrolyte is possible only when this back EMF is overcome.