Of coulometry

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Studies aimed at characterizing the mechanisms of electrode reactions often make use of coulometry for determining the number of electrons involved in the reaction. To make such measurements a known amount of a pure compound is subject to a controlled-potential electrolysis. The coulombs of charge needed to complete the electrolysis are used to determine the value of n using Faraday s law (equation 11.23). 

The following four experiments describe useful and accessible applications of coulometry. 

The techniques of voltammetry/polarography, atomic absorption, ICP, etc., have in most cases supplanted the coulometric approach for the determination of inorganic analytes. Coulometry and the use of coulometry in food analysis have recently been reviewed [473,476]. 

Precision of analytical techniques should only be described verbally or comparatively (e.g., the precision of coulometry is high or the precision of spectrophotometry is better then that of OES ). 

Figure 14 displays the coverages for Cd and Te deposited in each of the first few CdTe cycles with a program where the potentials remained constant. The coverages are the result of coulometry, 2 electrons for Cd and 4 electrons for Te. Note the excessive deposits for the first few cycles, significantly more than a monolayer. Steady... 

The determination of chloride using an instrument known as a chloride meter is probably the most common application of coulometry in biochemistry. The instrument is designed to generate silver ions electrolytically from a silver anode. These ions are removed from the solution as undissociated silver chloride, which is either deposited on the anode or precipitated in the solution. A low concentration of carrier electrolyte (nitrate ions) permits a small current... 

To learn that one of the most important areas of coulometry for the electroanalyst is stripping , in which analyte is allowed to accumulate on the surface of, e.g. a hanging mercury-drop electrode ( pre-concentration ), and then electro-oxidized ( stripped ). 

We will define the topic of coulometry from within the context of Faraday s laws (which were formulated in 1834 by the great Victorian scientist Sir Michael Faraday), as follows ... 

In this present section, we will look at more specific forms of coulometry. Other than electrolysis of a bulk solution, the most common technique is stripping, which is the method of choice when only a tiny amount of analyte is in solution. 

The most common type of errors found during coulometry is the incorporation of non-faradaic charge within the overall charge measured, e.g. as caused by double-layer charging or electrolytic side reactions. These aspects of coulometry have been discussed above. 

In the previous chapter, we encountered a form of coulometry known as stripping . We can combine both stripping and voltammetry in the powerful technique of stripping voltammetry. As we have seen, the potential of the working electrode is ramped during a voltammetric or polarographic experiment. The resultant current represents the rate at which electroactive analyte reaches the surface of the electrode, that is, current / a flux j. 

An example of a primary procedure to establish the mass fraction of a pure reference material is the use of coulometry to prepare an amount of an... 

This chapter is an overview of voltammetric methods used in analytical laboratories and as an example of coulometry the Karl Fischer method is presented. 

Detectors designed according to the principles of coulometry have innate possibilities for selectivity and quantitative capacity. Coulson and Cavanaugh (37) first implemented coulometry to determine chlorine and later sulfur. Martin advanced the state of the art by designing a detector capable of determining trace amounts... 

Coulogravimetry — A combination of -> coulometry and -> electrogravimetry, in which the weight of the deposited analyte and the passed charge are measured [i]. 

The methods of coulometry are based on the measurement of the quantity of electricity involved in an electrochemical electrolysis reaction. This quantity is expressed in coulombs and it represents the product of the current in amperes by the duration of the current flow in seconds. The quantity of electricity thus determined represents, through the laws of Faraday, the equivalents of reactant associated with the electrochemical reaction taking place at the electrode of significance. In the analytical chemistry sense, the process of coulometry, carried out to the quantitative reaction of the analyte in question, either directly or indirectly, will yield the number of analyte equivalents involved in the sample under test. This will lead to a quantitative determination of the analyte in the sample. Analytical coulometry can be carried out either directly or indirectly. In the former the analyte usually reacts directly at the surface of either the anode or cathode of the electrolysis cell. In the latter, the analyte reacts indirectly with a reactant produced by electrolytic action at one of the electrodes in the electrolysis cell. In either case, the determination will hinge on the number of coulombs consumed in the analytical process. 

Analytical Applications of Coulometry. The major advantage of coulometry is its high accuracy, because the reagent is electrical current, which can be well controlled and accurately measured. Coulometry is used for analysis, for generation of both unstable and stable titrants on demand , and for studies of redox reactions and evaluation of fundamental constants. With careful experimental technique, it is possible to evaluate the Faraday constant to seven significant figures, for example. 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures the potential of B12 vs. Br and, based on the measurement, automatically regulates the coulometric generation of the bromine. Coulometry is used in commercial instruments for the continuous analysis and process control of the production of chlorinated hydrocarbons. The chlorinated hydrocarbons are passed through a hot furnace, which converts the organic chloride to HCl. The latter is dissolved in water and the Cl titrated with Ag" ". The Ag" " is generated by coulometry from a sUver electrode, Ag°. It is necessary for the sample flow rate to be constant at all times. Integration of the coulometric current needed to oxidize the silver to silver ion results in a measurement of the Cl concentration. 

The use of coulometry as a detection technique has become of interest as the legal requirements for very low TOX values (TOX = total organic halogen). Mitsubishi has... 

Coulometry The basic information on the principle and practice of coulometry is presented elsewhere in this encyclopedia. In cHnical chemistry, coulometry is exclusively used for the determination of chloride concentration. For this application coulometry is sufficiently specific, because the concentration of other halides is usually very low in comparison to the chloride concentration ( 100 mmol 1 in serum). However, in case of long-lasting abuse of, e.g., hypnotics, which contain bromine (carbromal, bro-misoval), bromide concentrations higher than 5mmoll can be observed (bromide is eliminated very slowly (elimination half-life 300h) and will accumulate). The bromide concentration will add... 

Some typical important industrial applications of coulometry include the continuous monitoring of mercaptan concentration in the materials used in rubber manufacture. The sample continuously reacts with bromine, which is reduced to bromide. A third electrode measures... 

Several different methods have been utilized for measuring iron in these biological samples. However, spectrophotometry is the most widely used because it does not require unusual equipment and is readily amenable to automation. Atomic absorption spectrometry is effectively used for tissue and urine analyses [33-35], but unreliable results are obtained with serum due to sensitivity limitations as well as matrix and hemoglobin interferences [35]. Other methods utilizing inductively coupled plasma emission spectroscopy [36], coulometry [37], proton induced X-ray emission [38], neutron activation analysis [39], radiative energy attenuation [40], and radiometry with Fe [41] have been described but, with the exception of coulometry, have not become standard procedures in the clinical chemistry laboratory, inasmuch as sophisticated and expensive instrumentation is required in some instances. However, some of them, e.g., neutron activation, may be the method of choice for definitive accurate analysis. 

The fundamental principle of coulometry originates in Faraday s laws, which relate the quantity of a substance transformed in an electrolysis to the... 

The principles and applications of coulometry under conditions of controlled potential or controlled current have been discussed by Delahay " and by Chariot et... 

A measurement of the content of antioxidants in walnut, hazelnut and peanuts by the method of coulometry was made. It has showed fliat the content of antioxidants is an indicator of oxidative damage of the nut s fat during the process of storage. The main aim of our research is to study the dependence of oxidative damage of the nut s fat on the content of antioxidants. 

---