Internal electrolysis

Principles Immersion plating resulting from a displacement reaction involving the metal to be coated can continue only as long as the less noble substrate remains accessible to the plating solution, and therefore as plating proceeds, the quantity of A/, deposited, and of A/j dissolved, falls. Dissolution of A/j can be avoided by coupling it with a less noble metal A/, so that only A/j dissolves, i.e, by internal electrolysis. 

Internal electrolysis is the term applied by Sand1,2 to an electrogravimetric analysis proceeding spontaneously without the application of an external voltage, i.e., by the short-circuited galvanic cell. 

In case (a), the galvanic cell under non-faradaic conditions, one obtains an emf of 0.34 - (-0.76) = 1.10 V across the Cu electrode ( + pole) and the Zn electrode (- pole). In case (b), the galvanic cell with internal electrolysis, the electrical current flows in the same direction as in case (a) and the electrical energy thus delivered results from the chemical conversion represented by the following half-reactions and total reaction, repsectively ... 

One must realize that once complete metal deposition has been attained, the emf across the electrodes cannot be switched off before the cathode has been taken out of the solution and rinsed with water, otherwise the metal deposit may start to redissolv e in the solution as a consequence of internal electrolysis by the counter emf. After disconnection the electrode is rinsed with acetone and dried at 100-110° C for 3-4 min. The analytical result is usually obtained from difference in weight of the dry cathode before and after electrolysis. In a few instances a copper- or silver-plated Pt cathode or even an Ag cathode is used, e.g., Zn and Bi are difficult to remove entirely from Pt, as they leave black stains and on heating form an alloy with the noble metal for this and other reasons (see below) the experimenter should consult the prescriptions in handbooks149. 

The main reason for avoiding water as a solvent is the fact that the electrolysis of aqueous solutions of alkali and alkaline-earth metal salts commences at 1.7-2.0 volts (depending on the electrode material) and results in the evolution of O2 and H2. If the cell itself has a higher voltage, internal electrolysis can, but not always does occur, accompanied by the evolution of H2 and O2 and by self-discharge (117). However, this fact does not preclude attempts to create moist primary batteries with Li, Na or Ca, if the activity of H2O is kept sufficiently low. 

Electroanalysis is the application of electrochemistry to chemical analysis of various substances. It may be divided into electro-deposition(coulometry), internal electrolysis and electrosolution (Ref 16)... 

Note Electrodeposition of metals has found practical application in electroplating The term internal electrolysis was proposed by H.J.S. Sand, Analyst 55, 309- 12(1930) ... 

Internal electrolysis — Internal electrolysis, also known as spontaneous electrogravimetric analysis, is the deposition of a metal in an electrochemical cell for the purpose of gravimetric analysis without an external source of electrical energy by proper selection of the anode material. For example Ag can be determined in the presence of Pb, Cu, and Bi by use of a Cu anode. (See also electrogravimetry). 

This condition is met in a method called internal electrolysis (or spontaneous electrogravimetric analysis), first described by Ullgren in 1868, in which electrolysis occurs by spontaneous discharge of a galvanic cell. To illustrate the principle, consider two half-cells, comprising a zinc rod in a zinc sulfate solution and a copper rod in a copper sulfate solution. At open circuit, 25 °C, the reversible cell potential is related to the two standard electrode potentials (E°) ... 

In internal electrolysis, since the cell emf is distributed across the cell as the iR drop, the deposition rate is inversely proportional to R the maximal deposition rate is thus achieved by minimizing R. The progress of the reaction can be monitored via the cathode potential or current, although variation of R during the electrolysis distorts the simple exponential decay of the current. The determination itself can be based on spontaneous current measurement during internal electrolysis, although this is not normal practice. For example, determination of cyanide or fluoride in potable water can be based on empirical correlation of current and concentration. 

None of the examples below involves the internal electrolysis method. This is because, despite its simplicity, it suffers from slow deposition rates. This is less of a disadvantage when the amount of metal to be deposited is small (<50mg), as in determinations of impurities in more base metals or alloys. Determinations of copper and bismuth in lead-tin alloys or of bismuth in pig-lead fall within this category other examples are given in Table 3. 

If the products of an internal electrolysis are soluble, the quantity of electricity, instead of a gain in weight, will be measured, and Faraday s law then gives the number of moles reacted. 

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