User:Pestogreen/Coulometry

Coulometric titrations use a constant current system to accurately quantify the analyte by measuring the duration that current passes through the sample. The applied current is equivalent to the titrant in volumetric analyses. Current is applied to the unknown solution until all of the unknown species is either oxidized or reduced to a new state. Similar to volumetric titrations, the endpoint requires detection methods, some are . The magnitude of the current (in amperes) and the duration of the current (seconds) can be used to determine the moles of the unknown species in solution. When the volume of the solution is known, then 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

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Lead

In analytical electrochemistry, coulometry determines the amount of matter transformed during an electrolysis reaction by measuring the amount of electricity (in coulombs) consumed or produced. It can be used for precision measurements of charge, and the amperes even used to have a coulometric definition. However, today but coulometry is mainly used for analytical applications. The name coulometry is derived from coulombs,  It is named after Charles-Augustin de Coulomb. He introduced the attraction and repulsion between electric charges leading to a discovery of coulombs allowing for electrochemical research.

Micheal Faraday advanced the electrochemistry field by creating an instrument for coulometry to measure the electrolysis of water.  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 Karl Fischer method.

There are two basic categories of coulometric techniques. Potentiostatic coulometry involves holding the electric potential constant during the reaction using a potentiostat. The other, called Amperostatic coulometry keeps the current (measured in amperes) constant using an amperostat.

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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. 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.

Coulometric titrations under a constant current system quantifies the 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.The applied current is equivalent to the titrant in volumetric analysis. Current is applied to the unknown solution until all of the unknown species is either oxidized or reduced to a new state. When the analyte is completely consumed, endpoint detection is employed, preferably with an instrumental method for higher precision. The total charge that has flowed through the sample can be determined from the magnitude of the current (amperes) and the duration of the current (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, then the molarity of the unknown species can be determined.

Karl Fischer titration to determine water content

Coulometry in Heathcare

Determination of Chloride Levels

A type of clinical chemistry is measuring chloride levels in blood samples through a cotlove coulometric chloride titrator. 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.

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.

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.

Some advantages to using a microtitrator include the fast completion time of the titration due to the microscale. 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.

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