Coulometric flow titration methods

Biological system Biosensor design Results and remarks Reference 

Acetylcholineesterase and choline oxidase Carbon paste electrode containing AChE and ChO and the electron transfer mediator tetrathiafulva-lene. The electrode was applied to the cyclic voltammetric determination of ACh in 0.1 M phosphate buffer at + 200 mV versus SCE. The calibration graph was linear up to 400 pM ACh and the detection limit was 0.5 pM. The tetrathiafulvalene-based sensor operated efficiently at low applied potential (0 to — 100 mV versus SCE). [21] 

Acetylcholineesterase and choline oxidase Carbon-fiber electrode having a recessed tip into which crystals of the conducting salts tetrathiafulva-lene—tetracyanoquinodimethane were galvanostatically deposited followed by AChE and ChO which were co-immobilized in the recess. No interference from uric acid. The electrode responded to 5 pM or 800 pM ACh within 4 s. Kinetic studies of the biosensor were reported. [22] 

Acetylcholineesterase (AChE) Glass microelectrode with AChE immobilized on active surface. Nernstian response. Detection limit is pH-dependent. At pH 8.5 detection limit was 10 pM. In cerebrospinal fluids it was 0.1 mM. [63] 

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Should any iron(II) reach the anode, it also would be oxidized and thus not require the chemical reaction of Eq. (4.13) to bring about oxidation, but this would not in any way cause an error in the titration. This method is equivalent to the constant-rate addition of titrants from a burette. However, in place of a burette the titrant is electrochemically generated in the solution at a constant rate that is directly proportional to the constant current. For accurate results to be obtained the electrode reaction must occur with 100% current efficiency (i.e., without any side reactions that involve solvent or other materials that would not be effective in the secondary reaction). In the method of coulometric titrations the material that chemically reacts with the sample system is referred to as an electrochemical intermediate [the cerium(III)/cerium(IV) couple is the electrochemical intermediate for the titration of iron(II)]. Because one faraday of electrolysis current is equivalent to one gram-equivalent (g-equiv) of titrant, the coulometric titration method is extremely sensitive relative to conventional titration procedures. This becomes obvious when it is recognized that there are 96,485 coulombs (C) per faraday. Thus, 1 mA of current flowing for 1 second represents approximately 10-8 g-equiv of titrant. 

Flow-through methods are also used for sample treatment prior to coulometric titration, for the purpose of cleanup or preconcentration on an ion-exchange column or for reduction to a desired oxidation state in reductor columns. Preadjustment of redox states or removal of interferents may also be carried out by an auxiliary electrochemical flowthrough cell located ahead of the analytical cell. 

The iodine then reacts with the water that is present. The amount of water titrated is proportional to the total current (according to Faraday s law) used in generating the iodine necessary to react with the water. One mole of iodine reacts quantitatively with 1 mol of water. As a result, 1 mg of water is equivalent to 10.71 C. Based on this principle, the water content of the sample can be determined by the quantity of current that flows during the electrolysis. For this reason, the coulometric method is considered an absolute technique, and no standardization of the reagents is required. 

As a final illustration of the use of computers in coulometric titrations, a method of automatic dilution of samples developed by Ruzicka, Christian, and co-workers is discussed. Flow injection (FI) involves the injection of a solution of analyte into a flowing carrier stream contained in narrow-bore tubing (e.g., 0.5 mm diameter) where controlled dispersion of the injected solution takes place... 

The simplest methods of HTSC analysis are based on the determination of the products of sample dissolution in acidic media. Potentiometric, amperometric, or coulometric titrations are frequently used (mainly for YBCO ceramics [525-527] and their analogs with other rare-earth elements [528, 529], and also for BSCCO [530]). We note particularly the method of potentiostatic coulometric analysis [531], which allows one to analyze thallium cuprate samples over a wide range of the Tl/Cu ratio, and also the method of flow-through coulometry for determining the effective valence of copper [532]. The polarographic determination of Cu content in the samples obtained by dissolving HTSCs in concentrated alkaline solutions with special... 

A special case in amperometric indication is the "Dead-Stop Method". Current can only flow at the indicator electrodes if both oxidizable and reducible substances are present. The potential difference may be very low (<100 mV). The equivalent point is characterized by the fact that the indicator current is zero, as no reversible ion pairs are present here (e.g. coulometric titration of Fe + with Ce "). 

The gas titration with coulometric SE cells can be subdivided into two different applications the batch-like titration into nearly stationary titrands [10] and the continuous titration into flowing titrants [11]. As the batch-like titration needs a certain amount of time depending on the volume to be titrated and the applicable current, it does not belong to the real-time methods. In contrast to that, the continuous titration delivers... 

Test Method B, Combustion and Microcoulometry—The washed naphtha fiaction of a crude oil specimen is injected into a flowing stream of gas containing about 80 % oxygen and 20 % inert gas such as argon, helium, or nitrogen. The gas and sample flow through a combustion tube maintained at about 800 C. The chlorine is converted to chloride and oxychlorides whidi then flow into a titration cell where they react with the silver ions in the titration cell. The silver ions thus consumed are coulometrically replaced. The total current required to replace the silver ions is a measure of the chlorine present in the injected samples. 

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