Recent Trend of Coulometry

Abstract

Culomtery is based on the exact analytical chemical reaction and recently developed techniques, but not so refered to a simple measurement of physical quantities as other instrumental analytical methods. Recently the controlled potential coulometry developed to be applied to analytical method for determination of actinide elements (U, Pu, etc.) and rare elements (Np, Ir, Ru, In, Tl, Tc, etc.) mostly in buffered tripolyphosphate medium, while the organic substances (olefin, carboxlic acids, aromatic amines, phenols, etc.) were successively determined by electrogenerated bromine according to coulometric titration method. Especially, the unstable reagents such as Cr(II) or Mn(III) are capable of being electrogenerated from Cr(III) by reduction or from Mn(II) by oxydation and are respectively employed in coulometric reduction or oxydation. I-Q (current-coulomb) recording coulometer is very available in all the coulometric methods, because the coulomb can be easily calculated by the intersection of extrapolated I-Q line to the electrolysis current axis. Another new technique in the coulometry is a coulogram method which uses a scanning coulometer and records a coulomb-potential curve of a reversible process under nearly equilibrium conditions. The circuit is shown in Fig. 1 and features a novel system for producing a scan rate that is an inverse function of electrolysis current. Application of the method was given to direct determination of Pu in irradiated nuclear fuel. Constant current potential limit coulometry (C.C.P.L.) has the simplicity of constant current method as well as the versatility of controlled potential method. In this methed the potential approaches to preset potential during the constant current electrolysis and then the electrolysis current is set forward low constant current. The block diagram is shown in Fig. 2. Operational amplifier was recently used in coulometer and potentiostat of coulometric apparatus, resulting the coulometry to develope as an exact and reliable method of analysis. The example is given in Fig. 3. For the purpose of increasing the sensitivity and precision, differential coulometric titration method is considered to be a versatile method of analysis. Moreover, high speed controlled potential coulometry which uses a large electrode area against solution volume and the stirring by ultrasonic device or nitrogen gas bubble is one of an interesting method for practical analysis. The trend of coulometer for coulometry is to use the integrating motor for measurement of coulomb. The coulometer using relaxation oscillator has not yet proved to show a linear relationship between electrolysis current and revoluton of the motor. Recently, voltage-to-frequency convertor type coulometer is available for rapid constant potential coulometry. The pulse which is proportional to the electrolysis current is read by electron counter. Pulse coulometry uses the constant pulse for electrolysis current and the coulomb cousumed for the electrolysis can be read by means of electron counter. The pulse generating circuit is shown in Fig. 4. The coulometric titration can be carried out in non-aqueous solution such as isopropanol. Recently, a coulometric acidimetry was proposed for determination of organic weak acids. The coulometric titration was also tried in fused medium of LiCl-RCl mixed salts. This coulometry has advantage in high sensitivity over polarography or chrono-potentiometry in fused salts medium. The constant potential coulometry with anodic stripping method to be available for determination of Zn Cd, or Ni fused salts bath. Stripping coulometry was applied to determination of organic compounds by means of using an acetyleneblack electrode which adsorbs or contains the organic compounds inn the apparatus shown in Fig.5 and strips coulometrically. Generally, the radical titration can not be available by conventional titration technique, but available by coulometric technique. Biphenyl in