Coulometry

Last updated December 01, 2024

In analytical electrochemistry, coulometry is the measure of charge (coulombs) transfer during an electrochemical redox reaction.^[1] It can be used for precision measurements of charge, but coulometry is mainly used for analytical applications to determine the amount of matter transformed.^[2]

History

The term coulometry was introduced in 1938 by Hungarian chemist László Szebellédy and Zoltan Somogyi.^[3] Coulometry is the measure of charge, thus named after its unit the coulomb.

Michael Faraday, known for his work in electricity and magnetism, made critical contributions to the field of electrochemistry. He discovered the laws of electrolysis, and in his recognition is the eponym of the Faraday constant. In the earliest developments of coulometry, Faraday proposed the first instrument to measure charge by utilizing the electrolysis of water.^[4]

Surface coulometry, the method of determining metallic layers or oxide films on metals, was first applied by American Chemist G. G. Grower in 1917 by checking the quality of tinned copper wire.^[5]

Coulometric methods were used widely in the middle of the twentieth century but voltammetric methods and non-electrochemical analytical methods took over decreasing the use for coulometry, but one method widely used today is the Karl Fischer method.^[6]

Potentiostatic coulometry

Potentiostatic coulometry utilizes a constant electric potential and is a technique most commonly referred to as "bulk electrolysis". Also called direct coulometry, the analyte is oxidized or reduced at the working electrode without intermediate reactions.^[6] The working electrode is kept at a constant potential and the current that flows through the circuit is measured. This constant potential is applied long enough to fully reduce or oxidize all of the electroactive species in a given solution. As the electroactive molecules are consumed, the current also decreases, approaching zero when the conversion is complete. The sample mass, molecular mass, number of electrons in the electrode reaction, and number of electrons passed during the experiment are all related by Faraday's laws. It follows that, if three of the values are known, then the fourth can be calculated.

Bulk electrolysis is often used to unambiguously assign the number of electrons consumed in a reaction observed through voltammetry. It also has the added benefit of producing a solution of a species (oxidation state) which may not be accessible through chemical routes. This species can then be isolated or further characterized while in solution.

The rate of such reactions is not determined by the concentration of the solution, but rather the mass transfer of the electroactive species in the solution to the electrode surface. Rates will increase when the volume of the solution is decreased, the solution is stirred more rapidly, or the area of the working electrode is increased. Since mass transfer is so important the solution is stirred during a bulk electrolysis. However, this technique is generally not considered a hydrodynamic technique, since a laminar flow of solution against the electrode is neither the objective nor outcome of the stirring.

The extent to which a reaction goes to completion is also related to how much greater the applied potential is than the reduction potential of interest. In the case where multiple reduction potentials are of interest, it is often difficult to set an electrolysis potential a "safe" distance (such as 200 mV) past a redox event. The result is incomplete conversion of the substrate, or else conversion of some of the substrate to the more reduced form. This factor must be considered when analyzing the current passed and when attempting to do further analysis/isolation/experiments with the substrate solution.

An advantage to this kind of analysis over electrogravimetry is that it does not require that the product of the reaction be weighed. This is useful for reactions where the product does not deposit as a solid, such as the determination of the amount of arsenic in a sample from the electrolysis of arsenous acid (H₃AsO₃) to arsenic acid (H₃AsO₄).

Coulometric titration

Coulometric titrations under a constant current system quantifies the to analyte by measuring the duration that current passes through the sample. In indirect or secondary coulometry, the working electrode produces a titrant that reacts with the analyte. When the analyte is completely consumed, endpoint detection is employed, preferably with an instrumental method for higher precision.^[6] The total charge that has flowed through the sample can be determined from the magnitude of the current (in amperes) and the duration of the current (in seconds). Using Faraday's Law, total charge can be used to determine the moles of the unknown species in solution. When the volume of the solution is known, the molarity of the unknown species can be determined.

Advantages of Coulometric Titration

Coulometric titration has the advantage that constant current sources for the generation of titrants are relatively easy to make.

The electrochemical generation of a titrant is much more sensitive and can be much more accurately controlled than the mechanical addition of titrant using a burette drive. For example, a constant current flow of 10 μA for 100 ms is easily generated and corresponds to about 10 micrograms of titrant.
The preparation of standard solutions and titer determination is no longer necessary.
Chemical substances that are unstable or difficult to handle because of their high volatility or reactivity in solution can also very easily be used as titrants. Examples are bromine, chlorine, Ti3+, Sn2+, Cr2+, and Karl Fischer reagents (iodine).
Coulometric titration can also be performed under inert atmosphere or be remotely controlled e.g. with radioactive substances.
Complete automation is simpler.^[6]

Applications

Karl Fischer reaction to determine water content

The Karl Fischer reaction uses a coulometric titration to determine the amount of water in a sample. It can determine concentrations of water on the order of milligrams per liter. It is used to find the amount of water in substances such as butter, sugar, cheese, paper, and petroleum.

The reaction involves converting solid iodine into hydrogen iodide in the presence of sulfur dioxide and water. Methanol is most often used as the solvent, but ethylene glycol and diethylene glycol also work. Pyridine is often used to prevent the buildup of sulfuric acid, although the use of imidazole and diethanolamine for this role are becoming more common. All reagents must be anhydrous for the analysis to be quantitative. The balanced chemical equation, using methanol and pyridine, is:

${\ce {[C5H5NH]SO3CH3 + I2 + H2O + 2 C5H5N -> [C5H5NH]SO4CH3 + 2 [C5H5NH]I}}$

In this reaction, a single molecule of water reacts with a molecule of iodine. Since this technique is used to determine the water content of samples, atmospheric humidity could alter the results. Therefore, the system is usually isolated with drying tubes or placed in an inert gas container. In addition, the solvent will undoubtedly have some water in it so the solvent's water content must be measured to compensate for this inaccuracy.

To determine the amount of water in the sample, analysis must first be performed using either back or direct titration. In the direct method, just enough of the reagents will be added to completely use up all of the water. At this point in the titration, the current approaches zero. It is then possible to relate the amount of reagents used to the amount of water in the system via stoichiometry. The back-titration method is similar, but involves the addition of an excess of the reagent. This excess is then consumed by adding a known amount of a standard solution with known water content. The result reflects the water content of the sample and the standard solution. Since the amount of water in the standard solution is known, the difference reflects the water content of the sample.

Determination of film thickness

Coulometry can be used in the determination of the thickness of metallic coatings. This method is called surface coulometry and is performed by measuring the quantity of electricity needed to dissolve a well-defined area of the coating. The film thickness $\Delta$ is proportional to the constant current $i$ , the molecular weight $M$ of the metal, the density $\rho$ of the metal, and the surface area $A$ :

$\triangle \propto {\frac {iM}{A\rho }}$

The electrodes for this reaction are often platinum electrode and an electrode that relates to the reaction. For tin coating on a copper wire, a tin electrode is used, while a sodium chloride-zinc sulfate electrode would be used to determine the zinc film on a piece of steel. Special cells have been created to adhere to the surface of the metal to measure its thickness. These are basically columns with the internal electrodes with magnets or weights to attach to the surface. The results obtained by this coulometric method are similar to those achieved by other chemical and metallurgic techniques.

Coulometry in Healthcare

Determination of Chloride Levels

A type of clinical chemistry is measuring chloride levels in blood samples through a Cotlove chloridometer. Kidneys are responsible for the reabsorption of chloride to maintain electrolyte homeostasis. Measuring chloride levels allows for electrolyte stability, without this feature diseases such as hyperchoremia and hypochloremia would be harder to detect leaving body functions compromised.^[7]

Determination of Antioxidant Capacity in Human Blood

Coulometry can be used to measure the total antioxidant capacity (TAC) in blood and plasma through electrogenerated bromide. A method was developed that used TAC blood sampled from patients with chronic renal disease going through hemodialysis to research changes in TAC levels that could then be applied in clinics. ^[8]

Coulometers

Electronic coulometer

The electronic coulometer is based on the application of the operational amplifier in the "integrator"-type circuit. The current passed through the resistor R1 makes a potential drop which is integrated by operational amplifier on the capacitor plates; the higher current, the larger the potential drop. The current need not be constant. In such scheme V_out is proportional of the passed charge. Sensitivity of the coulometer can be changed by choosing of the appropriate value of R₁.

Electrochemical coulometers

There are three common types of coulometers based on electrochemical processes:

"Voltameter" is a synonym for "coulometer".

Coulometric Microtitrators

An acid-base microtitorator utilizes the electrolysis of water, where protons or hydroxide ions are produced at the working electrode. The analyte reacts with the generated reagent, buffering the overall rate of reagent generation. A pH gradient forms from the diffusion of these reagents, where a pH sensor will determine the endpoint.^[6]

Some advantages of using a microtitrator include the fast completion time of the titration due to the micro-scale. Additionally, a negligibly small amount of the sample is consumed, so titrations can be repeatedly analyzed with the same sample. On the contrary, microtitrators require calibration because diffusion is variable, and thus this method is not absolute.^[6]

Related Research Articles

Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separation isolates analytes. Qualitative analysis identifies analytes, while quantitative analysis determines the numerical amount or concentration.

Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase between electrodes separated by an ionically conducting and electronically insulating electrolyte.

Titration is a common laboratory method of quantitative chemical analysis to determine the concentration of an identified analyte. A reagent, termed the titrant or titrator, is prepared as a standard solution of known concentration and volume. The titrant reacts with a solution of analyte to determine the analyte's concentration. The volume of titrant that reacted with the analyte is termed the titration volume.

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity."

In analytical chemistry, Karl Fischer titration is a classic titration method that uses coulometric or volumetric titration to determine trace amounts of water in a sample. It was invented in 1935 by the German chemist Karl Fischer. Today, the titration is done with an automated Karl Fischer titrator.

In electrochemistry, cyclic voltammetry (CV) is a type of voltammetric measurement where the potential of the working electrode is ramped linearly versus time. Unlike in linear sweep voltammetry, after the set potential is reached in a CV experiment, the working electrode's potential is ramped in the opposite direction to return to the initial potential. These cycles in potential are repeated until the voltammetric trace reaches a cyclic steady state. The current at the working electrode is plotted versus the voltage at the working electrode to yield the cyclic voltammogram. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution or of a molecule that is adsorbed onto the electrode.

In electrochemistry, a mercury coulometer is an analytical instrument which uses mercury to perform coulometry on the following reaction:

Amperometric titration refers to a class of titrations in which the equivalence point is determined through measurement of the electric current produced by the titration reaction. It is a form of quantitative analysis.

<span class="mw-page-title-main">Potentiostat</span> Electronic system controlling a three electrode cell

A potentiostat is the electronic hardware required to control a three electrode cell and run most electroanalytical experiments. A Bipotentiostat and polypotentiostat are potentiostats capable of controlling two working electrodes and more than two working electrodes, respectively.

Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied. The analytical data for a voltammetric experiment comes in the form of a voltammogram, which plots the current produced by the analyte versus the potential of the working electrode.

<span class="mw-page-title-main">Chronoamperometry</span> Analytical method in electrochemistry

In electrochemistry, chronoamperometry is an analytical technique in which the electric potential of the working electrode is stepped and the resulting current from faradaic processes occurring at the electrode is monitored as a function of time. The functional relationship between current response and time is measured after applying single or double potential step to the working electrode of the electrochemical system. Limited information about the identity of the electrolyzed species can be obtained from the ratio of the peak oxidation current versus the peak reduction current. However, as with all pulsed techniques, chronoamperometry generates high charging currents, which decay exponentially with time as any RC circuit. The Faradaic current - which is due to electron transfer events and is most often the current component of interest - decays as described in the Cottrell equation. In most electrochemical cells, this decay is much slower than the charging decay-cells with no supporting electrolyte are notable exceptions. Most commonly a three-electrode system is used. Since the current is integrated over relatively longer time intervals, chronoamperometry gives a better signal-to-noise ratio in comparison to other amperometric techniques.

In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency. In an electrolytic cell the existence of overpotential implies that the cell requires more energy than thermodynamically expected to drive a reaction. In a galvanic cell the existence of overpotential means less energy is recovered than thermodynamics predicts. In each case the extra/missing energy is lost as heat. The quantity of overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction. Overpotential is experimentally determined by measuring the potential at which a given current density is achieved.

<span class="mw-page-title-main">Voltameter</span> Instrument for measuring electric charge

A voltameter or coulometer is a scientific instrument used for measuring electric charge through electrolytic action. The SI unit of electric charge is the coulomb.

A thermometric titration is one of a number of instrumental titration techniques where endpoints can be located accurately and precisely without a subjective interpretation on the part of the analyst as to their location. Enthalpy change is arguably the most fundamental and universal property of chemical reactions, so the observation of temperature change is a natural choice in monitoring their progress. It is not a new technique, with possibly the first recognizable thermometric titration method reported early in the 20th century. In spite of its attractive features, and in spite of the considerable research that has been conducted in the field and a large body of applications that have been developed; it has been until now an under-utilized technique in the critical area of industrial process and quality control. Automated potentiometric titration systems have pre-dominated in this area since the 1970s. With the advent of cheap computers able to handle the powerful thermometric titration software, development has now reached the stage where easy to use automated thermometric titration systems can in many cases offer a superior alternative to potentiometric titrimetry.

In analytical chemistry, potentiometric titration is a technique similar to direct titration of a redox reaction. It is a useful means of characterizing an acid. No indicator is used; instead the electric potential is measured across the analyte, typically an electrolyte solution. To do this, two electrodes are used, an indicator electrode and a reference electrode. Reference electrodes generally used are hydrogen electrodes, calomel electrodes, and silver chloride electrodes. The indicator electrode forms an electrochemical half-cell with the interested ions in the test solution. The reference electrode forms the other half-cell.

Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. These methods can be broken down into several categories depending on which aspects of the cell are controlled and which are measured. The three main categories are potentiometry, amperometry, coulometry.

Bulk electrolysis is also known as potentiostatic coulometry or controlled potential coulometry. The experiment is a form of coulometry which generally employs a three electrode system controlled by a potentiostat. In the experiment the working electrode is held at a constant potential (volts) and current (amps) is monitored over time (seconds). In a properly run experiment an analyte is quantitatively converted from its original oxidation state to a new oxidation state, either reduced or oxidized. As the substrate is consumed, the current also decreases, approaching zero when the conversion nears completion.

In electrochemistry, the Randles–Ševčík equation describes the effect of scan rate on the peak current for a cyclic voltammetry experiment. For simple redox events where the reaction is electrochemically reversible, and the products and reactants are both soluble, such as the ferrocene/ferrocenium couple, $i p$ depends not only on the concentration and diffusional properties of the electroactive species but also on scan rate.

Base Number (BN) is a measurement of basicity that is expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample. BN is an important measurement in petroleum products, and the value varies depending on its application. BN generally ranges from 6–8 mg KOH/g in modern lubricants, 7–10 mg KOH/g for general internal combustion engine use and 10–15 mg KOH/g for diesel engine operations. BN is typically higher for marine grade lubricants, approximately 15-80 mg KOH/g, as the higher BN values are designed to increase the operating period under harsh operating conditions, before the lubricant requires replacement.

A chloridometer is a measuring instrument used to determine the concentration of chloride ions (Cl^–) in a solution. It uses a process known as coulometric titration or amperostatic coulometry, the accepted electrochemistry reference method to determine the concentration of chloride in biological fluids, including blood serum, blood plasma, urine, sweat, and cerebrospinal fluid. The coulometry process generates silver ions, which react with the chloride to form silver chloride (AgCl).

References

↑ Kies, H. L. (1962-11-01). "Coulometry". Journal of Electroanalytical Chemistry (1959). 4 (5): 257–286. doi:10.1016/S0022-0728(62)80068-0. ISSN 0368-1874.
↑ DeFord, Donald D. (1960). "Electroanalysis and Coulometric Analysis". Analytical Chemistry. 32 (5): 31–37. doi:10.1021/ac60161a604.
↑ Szebellédy, L.; Somogyi, Z. (1938-09). "Die coulorretrisehe Analyse als Präzisionsmethode. III". Fresenius' Zeitschrift für analytische Chemie (in German). 112 (9–10): 332–336. doi:10.1007/BF01383887. ISSN 0016-1152.{{cite journal}}: Check date values in: |date= (help)
↑ Lubert, Karl-Heinz; Kalcher, Kurt (2010). "History of Electroanalytical Methods". Electroanalysis. 22 (17–18): 1937–1946. doi:10.1002/elan.201000087. ISSN 1521-4109.
↑ Grower, G.G. (1917). Proc. Am. Soc. Test. Matter. 17: 129.{{cite journal}}: Missing or empty |title= (help)
1 2 3 4 5 6 Hauser, P. C. (2005-01-01), "COULOMETRY", in Worsfold, Paul; Townshend, Alan; Poole, Colin (eds.), Encyclopedia of Analytical Science (Second Edition), Oxford: Elsevier, pp. 234–240, doi:10.1016/b0-12-369397-7/00104-7, ISBN 978-0-12-369397-6 , retrieved 2024-11-04
↑ Morrison, Gail (1990), Walker, H. Kenneth; Hall, W. Dallas; Hurst, J. Willis (eds.), "Serum Chloride", Clinical Methods: The History, Physical, and Laboratory Examinations (3rd ed.), Boston: Butterworths, ISBN 978-0-409-90077-4, PMID 21250151 , retrieved 2024-11-04
↑ Ziyatdinova, Guzel K.; Budnikov, Herman C.; Pogorel’tzev, Valery I.; Ganeev, Talgat S. (2006-01-15). "The application of coulometry for total antioxidant capacity determination of human blood". Talanta. 68 (3): 800–805. doi:10.1016/j.talanta.2005.06.010. ISSN 0039-9140.

Bibliography

A. de Agostini (2002). Coulometric Titration. Switzerland: Mettler-Toledo GmbH. p. 3.

External links

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[1] Kies, H. L. (1962-11-01). "Coulometry". Journal of Electroanalytical Chemistry (1959). 4 (5): 257–286. doi:10.1016/S0022-0728(62)80068-0. ISSN 0368-1874.

[2] DeFord, Donald D. (1960). "Electroanalysis and Coulometric Analysis". Analytical Chemistry. 32 (5): 31–37. doi:10.1021/ac60161a604.

[3] Szebellédy, L.; Somogyi, Z. (1938-09). "Die coulorretrisehe Analyse als Präzisionsmethode. III". Fresenius' Zeitschrift für analytische Chemie (in German). 112 (9–10): 332–336. doi:10.1007/BF01383887. ISSN 0016-1152.{{cite journal}}: Check date values in: |date= (help)

[4] Lubert, Karl-Heinz; Kalcher, Kurt (2010). "History of Electroanalytical Methods". Electroanalysis. 22 (17–18): 1937–1946. doi:10.1002/elan.201000087. ISSN 1521-4109.

[5] Grower, G.G. (1917). Proc. Am. Soc. Test. Matter. 17: 129.{{cite journal}}: Missing or empty |title= (help)

[:0-6] 1 2 3 4 5 6 Hauser, P. C. (2005-01-01), "COULOMETRY", in Worsfold, Paul; Townshend, Alan; Poole, Colin (eds.), Encyclopedia of Analytical Science (Second Edition), Oxford: Elsevier, pp. 234–240, doi:10.1016/b0-12-369397-7/00104-7, ISBN 978-0-12-369397-6 , retrieved 2024-11-04

[7] Morrison, Gail (1990), Walker, H. Kenneth; Hall, W. Dallas; Hurst, J. Willis (eds.), "Serum Chloride", Clinical Methods: The History, Physical, and Laboratory Examinations (3rd ed.), Boston: Butterworths, ISBN 978-0-409-90077-4, PMID 21250151 , retrieved 2024-11-04

[8] Ziyatdinova, Guzel K.; Budnikov, Herman C.; Pogorel’tzev, Valery I.; Ganeev, Talgat S. (2006-01-15). "The application of coulometry for total antioxidant capacity determination of human blood". Talanta. 68 (3): 800–805. doi:10.1016/j.talanta.2005.06.010. ISSN 0039-9140.

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v t e Electroanalytical methods
Techniques	Adsorptive stripping voltammetry Amperometric titration Anodic stripping voltammetry Bulk electrolysis Cathodic stripping voltammetry Chronoamperometry Coulometry Cyclic voltammetry Differential pulse voltammetry Electrogravimetry Hydrodynamic technique Linear sweep voltammetry Normal pulse voltammetry Polarography Potentiometry Rotated electrode voltammetry Squarewave voltammetry Staircase voltammetry Voltammetry
Instrumentation	Amperostat Auxiliary electrode Dropping mercury electrode Electrode Electrolytic cell Galvanic cell Hanging mercury drop electrode Ion selective electrode Mercury coulometer pH meter Potentiostat Reference electrode Rotating disk electrode Rotating ring-disk electrode Salt bridge Saturated calomel electrode Silver chloride electrode Standard hydrogen electrode Ultramicroelectrode Voltameter Working electrode
Theory	Activity coefficient Butler–Volmer equation Cell notation Cottrell equation Debye–Hückel equation Double layer Faraday's laws of electrolysis Half-reaction Ionic strength Nernst equation
Analytical Chemistry