Electrogravimetric analysis

How do you know when electrolysis is complete One way is to observe the disappearance of color in a solution from which a colored species such as Cu2+ or Co21 is removed. 

Another way is to expose most, but not all, of the surface of the cathode to the solution during electrolysis. To test whether or not the reaction is complete, raise the beaker or add water so that fresh surface of the cathode is exposed to the solution. After an additional period of electrolysis (15 min, say), see if the newly exposed electrode surface has a deposit. If it does, 

To prevent the cathode potential from becoming so negative that unintended ions are reduced, a cathodic depolarizer such as NO can be added to the solution. The cathodic depolarizer is more easily reduced than H30+  

What cathode potential is required to reduce 99.99% of 0.10 M Cu2+ to C u(.v) Is it possible to remove this Cu2+ without reducing 0.10 M Sn2+ in the same solution  

Solution If 99.99% of Cu2+ were reduced, the concentration of remaining Cu2+ would be 1.0 X 10-5 M, and the required cathode potential would be 

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Internal electrolysis is the term applied by Sand1,2 to an electrogravimetric analysis proceeding spontaneously without the application of an external voltage, i.e., by the short-circuited galvanic cell. 

In electrogravimetric analysis, analyte is quantitatively deposited on an electrode by electrolysis. The electrode is weighed before and after deposition. The increase in mass tells us how much analyte was deposited. We can measure Cu2+ in a solution by reducing it to Cu(,v) on a clean Pt gauze cathode with a large surface area (Figure 17-5). 02 is liberated at the counter electrode. 

In electrogravimetric analysis, analyte is deposited on an electrode, whose increase in mass is then measured. With a constant voltage in a two-electrode cell, electrolysis is not very selective, because the working electrode potential changes as the reaction proceeds. 

Internal electrolysis — Internal electrolysis, also known as spontaneous electrogravimetric analysis, is the deposition of a metal in an electrochemical cell for the purpose of gravimetric analysis without an external source of electrical energy by proper selection of the anode material. For example Ag can be determined in the presence of Pb, Cu, and Bi by use of a Cu anode. (See also electrogravimetry). 

Electrogravimetric analysis involving control of the cathode potential is proposed as a means to separate Bi and Sn- in a solution that is 0.200 M in each ion and buffered to pH 1.50. 

Figure 11.2.2 Typical cells for bulk electrolysis, (a) Undivided cell for controlled-potential separations and electrogravimetric analysis at a solid cathode. [From J. J. Lingane, Anal Chim. Acta, 2, 584 (1948), with permission.] ib) Undivided cell for coulometric analysis at mercury cathode with a silver anode. [Reprinted with permission from J. J. Lingane, J. Am. Chem. Soc.,...

Tin can be deposited from solutions of Sn(II) in HCl or HBr media. " Metal deposits obtained are not suitable for electrogravimetric analysis. 

Okabayashi, K., R Goto, K. Abe, and T. Yoshida. 1989. In situ electrogravimetric analysis of polypyrrole and polyaniline positive electrodes in nonaqueous medium. J Electrochem Soc 136 1986. 

This condition is met in a method called internal electrolysis (or spontaneous electrogravimetric analysis), first described by Ullgren in 1868, in which electrolysis occurs by spontaneous discharge of a galvanic cell. To illustrate the principle, consider two half-cells, comprising a zinc rod in a zinc sulfate solution and a copper rod in a copper sulfate solution. At open circuit, 25 °C, the reversible cell potential is related to the two standard electrode potentials (E°) ... 

Describe the process of electrodeposition and how it is used for electrogravimetric analysis. 

One of the oldest electrolytic methods in quantitative analysis is electrogravimetric analysis, in which analyte is plated out on an electrode and weighed. For example, an excellent procedure for the measurement of copper is to pass current through a solution of a copper salt to deposit all of the copper on the cathode ... 

Electrogravimetric analysis would be simple if there were only a single analyte in an otherwise inert solution. In practice, there may be other electroactive species that interfere. Water decomposes to H2 at the cathode and to O2 at the anode at sufficiently high voltage. Gas bubbles at an electrode interfere with deposition of solid. Because of these complications, control of electrode potential is important for successful analysis. 

The name "Sand usually conjures up only a particular electrochemical equation. Older workers can recall the flow of papers from what is now the City of London Polytechnic, where Sand and his students made notable advances in the practice of electrogravimetric analysis. Actually Sand s Ph.D., obtained under Bamberger, was in organic chemistry. Nevertheless he soon began electrochemical work, but by no means restricted himself to this field. Studies on the dissociation pressures of alkali metal bicarbonates and on fermentation by yeast cells are examples of his diverse interests. He held several patents on glass working and the like. His Joint authorship of... 

Sand did the work that led to this equation before ny time. However, I was aroimd when Sand and his students were developing apparatus and techniques that greatly influenced electrogravimetric analysis. Like other gravimetric processes, electrogravimetry is now overshadowed by more rapid methods. This deposition method, capable of giving high precision and accuracy,... 

W. GIBBS (1822-1908) publishes the first electrogravimetric analysis (1864) Fresenius Z Anal Chem 3 334... 

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