Electrogravimetry

In electrogravimetry [19], the analyte, mostly metal ions, is electrolytically deposited quantitatively onto the working electrode and is determined by the difference in the mass of the electrode before and after the electrolysis. A platinum electrode is usually used as a working electrode. The electrolysis is carried out by the con-trolled-potential or the controlled-current method. The change in the current-potential relation during the process of metal deposition is shown in Fig. 5.33. The curves in Fig. 5.33 differ from those in Fig. 5.31 in that the potentials at i=0 (closed circles) are equal to the equilibrium potential of the M +/M system at each instant. In order that the curves in Fig. 5.33 apply to the case of a platinum working electrode, the electrode surface must be covered with at least a monolayer of metal M. Then, if the potential of the electrode is kept more positive than the equilibrium potential, the metal (M) on the electrode is oxidized and is dissolved into solution. On the other hand, if the potential of the electrode is kept more negative than the equilibrium potential, the metal ion (Mn+) in the solution is reduced and is deposited on the electrode. 

In order to deposit more than 99.9% of the metal ion Mn+ by controlled-poten-tial electrolysis, the potential of the working electrode should be kept more negative than the equilibrium potential at [M +] = C0/1000 (C0 initial concentration). Thus, the electrode potential Ec (V) should be 

When the solution contains two metal ions, Mi1+ and M 2+ (initial concentrations =C10 and C2o, respectively), and when it is necessary to deposit more than 99.9% of Mi1+ on the electrode, leaving all of M22+ in the solution, the potential Ec should be 

When metal ion M + is deposited by the controlled-current method, the electrode potential during the electrolysis changes in the order T, 2, 3, 4, 5, 6 in Fig. 5.33 and the next reduction process occurs near the end of the electrolysis. If the solution is acidic and the next reduction process is hydrogen generation, its influence on the metal deposition is not serious. However, if other metal is deposited in the next reduction process, metal M is contaminated with it. In order that two metal ions M 1+ and M 21 can be separated by the controlled-current method, the solution must be acidic and the reduction of hydrogen ion must occur at the potential between the reductions of the two metal ions. An example of such a case is the separation of Cu2+ and Zn2+ in acidic solutions. If two metal ions are reduced more easily than a hydrogen ion (e.g. Ag+ and Cu2+), they cannot be separated by the controlled-current method and the controlled-potential method must be used. 

The technique of electrogravimetry consists of electroplating a metal (usually) onto a previously weighed electrode, and then reweighing to determine the amount of metal initially present in solution. Sufficient voltage is applied to the electrochemical cell for long enough to remove the metal quantitatively from solution. 

Many of the principles involved in electrogravimetry can be illustrated by considering the determination of copper. A simplified version of the apparatus used 

The total voltage, app. applied across the cell is given by 

The reversible potentials can be calculated from the appropriate Nernst equation. For instance, for copper at 25°C 

This equation neglects activity coeflicients— not a very good approximation under normal experimental conditions—but this is usually compensated for by simply applying a greater voltage than calculated. Equation 4.2 can be used to decide what potential the cathode must attain to eventually reduce the concentration of copper remaining in solution to an acceptable value. For instance, if the initial solution was 10 M in Cu and it was desired to plate 99.9% of the copper, the following calculation would pertain At the start of the electrolysis the cathode potential would be 

If a solution of a metaUic ion, such as copper, is electrolyzed between electrodes of the same metal (i.e., copper), the following reaction takes place  

The net result is that metal dissolves from the anode and deposits on the cathode. The phenomenon is the basis of electroplating (e.g., chromium plating of steel). Also, it is the analytical basis of an electrodeposition method known as electrogravimetry. This involves the separation and weighing of selected components of a sample. Most metal elements can be determined in this manner, usually deposited as the M species, although some metal elements can be deposited as oxides. The halides can be determined by deposition as the silver halide. Metals commonly determined include Ag, Bi, Cd, Co, Cu, In, Ni, Sb, Sn, and Zn. 

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In electrogravimetry the analyte is deposited as a solid film on one electrode in an electrochemical cell. The oxidation of Pb +, and its deposition as Pb02 on a Pt anode is one example of electrogravimetry. Reduction also may be used in electrogravimetry. The electrodeposition of Cu on a Pt cathode, for example, provides a direct analysis for Cu +. 

For a description of electrogravimetry, see the following resource. Tanaka, N. Electrodeposition, In Kolthoff, I. M. Living, P. J., eds. Treatise on Analytical Chemistry, Part I Theory and Practice, Vol. 4. Interscience New York, 1963. 

Electrogalvanizing Electrography Electrogravimetry Electrohydrodynamics Electro-Katadyn process Electrokinetics Electroless deposition Electroless nickel Electroless plating... 

Determination. The most accurate (68) method for the deterrnination of copper in its compounds is by electrogravimetry from a sulfuric and nitric acid solution (45). Pure copper compounds can be readily titrated using ethylene diamine tetracetic acid (EDTA) to a SNAZOXS or Murexide endpoint. lodometric titration using sodium thiosulfate to a starch—iodide endpoint is one of the most common methods used industrially. This latter titration is quicker than electrolysis, almost as accurate, and much more tolerant of impurities than is the titration with EDTA. Gravimetry as the thiocyanate has also been used (68). 

The quantitative execution of chemical reactions is the basis of the traditional or classical methods of chemical analysis gravimetry, titrimetry and volumetry. In gravimetric analysis the substance being determined is converted into an insoluble precipitate which is collected and weighed, or in the special case of electrogravimetry electrolysis is carried out and the material deposited on one of the electrodes is weighed. 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

SOME EXAMPLES OF METALS WHICH CAN BE DETERMINED BY ELECTROGRAVIMETRY... 

Electrical units 503, 519 Electrification due to wiping 77 Electro-analysis see Electrolysis and Electrogravimetry Electrochemical series 63 Electro-deposition completeness of, 507 Electrode potentials 60 change of during titration, 360 Nernst equation of, 60 reversible, 63 standard 60, (T) 62 Electrode reactions 505 Electrodeless discharge lamps 790 Electrodes antimony, 555 auxiliary, 538, 545 bimetallic, 575... 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

The main electroanalytical techniques are electrogravimetry, potentiometry (including potentiometric titrations), conductometry, voltammetry/polarography, coulometry and electrochemical detection. Some electroanalytical techniques have become very widely accepted others, such as polarography/voltammetry, less so. Table 8.74 compares the main electroanalytical methods. 

For polymer/additive analysis, electrogravimetry, potentiometry, conductometry and voltammetry have never played a major role. Because of many complications, which can arise by the use of conductometry for complicated matrices (such as most polymeric compounds), the technique is not extensively applied in this field. Conductometric measurements are mostly... 

Here the most important techniques are voltammetry, electrogravimetry and coulometry. 

In electrogravimetry, also called electrodeposition, an element, e.g., a metal such as copper, is completely precipitated from its ionic solution on an inert cathode, e.g., platinum gauze, via electrolysis and the amount of precipitate is established gravimetrically in the newer and more selective methods one applies slow electrolysis (without stirring) or rapid electrolysis (with stirring), both procedures either with a controlled potential or with a constant current. Often such a method is preceded by an electrolytic separation using a stirred cathodic mercury pool, by means of which elements such as Fe, Ni, Co, Cu, Zn and Cd are quantitatively taken up from an acidic solution whilst other elements remain in solution. 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

As the first experimental condition in electrogravimetry one makes a choice from two possibilities, viz. ... 

Eqn. 3.106 must be considered as an approximate relationship for at least two reasons first, the assumption of a rapid complete coverage of the Pt electrode surface by Ag right from the start of the electrolysis is certainly incorrect (cf., Bard and Faulkner150) second, at the end of the electrolysis the remaining Cu2+ solution is virtually in contact with a silver electrode instead of a copper electrode, for which E u2+, Cu = 0.340 V is valid. Practice has shown that by means of CPE, selective electro-deposition and thus electrogravimetry of silver in addition to copper is possible down to 10 8MAg+, as the above calculation indicates. 

In many instances electrogravimetry must be preceded by a separation between metals suitably this can be an electroseparation by means of constant-current electrolysis as previously described, but more attractively an electroseparation by means of controlled-potential electrolysis at a mercury pool or sometimes at an amalgamated Pt or brass gauze electrode. In this way one can either concentrate the metal of interest on the Hg or remove other metals from the solution alternatively, it can be a rougher separation, i.e., the concentration of a group of metals such as Fe, Ni, Co, Cu, Zn and Cd on the Hg whilst other metals such as alkali and alkaline earth metals, Be, Al, Ti and Zr remain in solution151. In all these procedures specific separation effects can be... 

Absolute measurements by fundamental quantities like Faraday constant and quotients of atomic and molar masses, respectively (coulom-etry, electrogravimetry, gravimetry, gas volumetry)... 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell current is employed, but in both cases selectivity is poor and they are generally used when there are... 

Laboratory apparatus represents an important application sector. Platinum crucibles, dishes, boats and electrodes for electrogravimetry have long been basic items in chemical laboratories. Today the more dimensionally stable alloys Pt97-Ir3, Pt95-Au5, Pt99.7-Ir0.3 are mainly used. Platinum components are essential in fluorine chemistry. 

A Methods that electrolyze the electroactive species under study completely Electrogravimetry Coulometry... 

Electrogravimetry and Coulometry - Methods that Completely Electrolyze Electroactive Species 1143... 

A method that completely electrolyzes the substances under study is used in electrogravimetry and coulometry. The method is also useful in electrolytic separations and electrolytic syntheses. Electrolysis is carried out either at a controlled potential or at a controlled current. 

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