K8 Electro-chemical actions of the current. Electromotoric action of ions

Normal elements

All ordinary elements lose in the course of time their EMF (Daniell's most slowly). However, you can also produce elements which with proper treatment retain almost unchanged their EMF. Once their EMF has been determined, they serve as normal elements: They become for tension measurements what meter rulers are for length measurements. The most frequently used types (Fig. 503) are the Clark-element (mercury, sulphate of mercury, mercurous oxide, zinc sulphate, zinc) with 1.423 Volt at 15ºC and the Weston-element (like the Clark-element with cadmium instead of zinc) with 1.0185 Volt at 15ºC. The EMF of the Weston-element is almost temperature independent, that of the Clarke-element drops noticeably with increasing temperature. During tension measurments with normal elements, you must not draw current from them since otherwise the tension is not constant (compensation method).

Galvanic polarization

We return to electrolysis, in order to still present material which would have interrupted the earlier presentation.

The work performed by current during electrolysis comprises largely tearing away of the charges from ions. The instant, at which the ions relase them, is indicated by their appearance at the electrodes. Only then the current passes through the decomposition cell. However, in order to take the electrolyte that far, you must generate between the electrodes a certain potential: The decomposition tension of electrolytes (Nicolaus Le Blanc 1780-1850) A smaller EMF only drives a current impulse through the cell. While a galvanometer in the circuit moves, it returns immediately to almost zero. If you increase the potential difference at first just by a little, it moves more strongly, but at first only a little; eventually, a potentail differnec is reached, at which it suddenly reacts strongly and stays there - this means that the decomposition tension has been attained, the passage of current and decomposition continue.

Hence the decomposition cell behaves as if it has a resistance, which must at first be overcome so that the current impulse arises, then increases and indeed to a resistance, which only the decomposition tension overcomes. - In fact, it is like follows: From the instance of the first current impulse, the cell acts as a galvanic element and attempts to induce a current, which has the opposite direction to the electrolyzing current. Or in other words: From the instant of the current impulse, the cell develops an electromagnetic counter force (against the EMF, driving the electrolyzing current). Thr fact that the counterforce really exists is shown the experiment of Johann Wilhelm Ritter 1776-1810 1803: When you decompose acidified water between platinum electrodes, A as anode, K as cathode (Fig. 504), then interrupt the current and connect K and A outside the cell to each other, the cell acts like agalvanic element. A galvanometer in the circuit indicates a current, which flows in the water from K to A, that is, in the water, the electrolyzing current flows in the opposite direction. The electrode, which was the cathode, is now the negative, the other electrode, which was the anode, the positive pole. You call the current from K to A in the water the secondary current, the state, into which the primary current has put the electrodes, polarization, the electrodes polarized, and the seconday current (because it removes the polarization) depolarizing, the EMF between the polarized electrodes the electromotoric counter force of polarization.

Electrolysis is always accompanied by polarization, whence the depolarizing current offers means for detection of the slightest trace of receding decomposition; however, the small quantities of the decomposition products, generated by the electrolysis, must not have been destroyed by the oxygen, which is always dissolved in water. Helmholtz (in his Faraday Lecture) has discovered with a specially prepared cell "that you can observe polarization which in a few seconds generates a current which would demand 1 century for the decomposition of 1 mg water."

The counterforce of polarization develops always when, and only when the electrolysis has altered the electrodes on their surface physically or chemically or has changed the electrolyte close to the electrodes. It is absent when CuSO₄ decomposes between Cu-electrodes or ZnSO₄ between Zn-electrodes. During this process, the electrodes do not change chemically (unpolarizable), at the cathode precepitates the same metal, which was dissolved at the anode; it solely wanders from the anode to the cathode. Also the concentration of the electrolyte does not change, unless the current is very strong and does not pass through the cell for a very long time.

An electrode polarizes itself already, as it covers itself with gas, as the elctrolysis of water tells (Fig. 504). This is important for galvanic elements; those elements, the activity of which develops hydrogen at one electrode, would soon become useless unless it is made harmless by oxidation agents (chromic acid, nitric acid, etc.); it could impede polarization (Depolarisators are list in an earlier table).

The generation of the electromotoric counterforce in the decomposition cell becomes clear by application of the concept of solution pressure to the process in it. The essence of the theory of Nicolaus Le Blanc 1780-1850 follows: Metals in contact with an electrolyte tend to ionize (go into solution as ions). Once ionized, they tend to remain so. Also the ions in the decomposition cell tend to remain ions,, that is, not to secret at the elctrodes, not to deionize. Therefore, during electrolysis, two forces face each other at an electrode:
1. The electrode's electostatic attraction of the ions - it tends to tear away their charges and to precipitate them neutralized;
2. The solution pressure of the metal, the released molecules of which tend to remain ions, that is, in solution. The first force is directed towards, the second away from the electrode.
The consequences depends on their relative magnitudes. However, the magnitude of the solution pressure is limited, while we can raise the magnitude of the electrostatic attraction arbitrarily, that is, we can always force precipitation.

Ions - any metal ions - are attracted to the cathode and to start with lie there. Some of them precipitate, while the EMF lasts until the cations form a coherent layer of a certain minimal thickness. However, as soon as some metal has been precipitated, its solution pressure acts against the electrostatic attraction and you must raise the EMF at the electrodes, in order to overcome the solution pressure and precipitate more metal. With the additional precipitation, also the solution pressure rises again, and this struggle continues until the metal has formed on the electrode a layer of a thickness (that "concentration") at which the layer has the same solution pressure which the elctrode would have, if it consisted altogether of the metal in question. From there on, the solution pressure does not rise any further and the least increase in the EMF at the elctrodes is sufficient to effect lasting precipitation. At that stage, the decomposition tension has been attained and the electrolytic solution pressure due to electrostatic attraction is finally overcome.

Hence, the EMF of the decomposition cell is also explained by the solution pressure of metals. The decomposition tension can therefore also be computed - like the EMF of an element - from the potential difference of the polarized electrode in faceof a corresponding solution of the electrolyte, into which it is dipped, that is, from the potential difference which is characteristic for the solution pressure. The decomposition tensions of normal concentrations (in Volt) are, for example, for: ZnSO₄: 2.64, CuSO₄: 2.24, HCl: 1.31, H₂SO₄: 1.67.

Accumulator

The EMF of the polarization forces us to perform more work than is required to decompose the electrolyte. Hence, polarization seems to be a loss of energy. However, this apparently lost energy has only been converted into another form. It reappears when you employ a decomposition cell as a galvanic element. You call an element, which owes its EMF to polarisation a seondary element, mostly an accumulator, because it stores energy. Among the galvanic elements, only accumulators can become technical sources of current. However, for this purpose, not every decomposition cell can be used; in general, the strength of the depolarizing current decreases very quickly, because, as it flows through the cell, it destroys the changes at the electrodes and in the electrolyte, to which it owes its existence. The technically most important accumulator is the lead accumulator. (Sinsteden discovered the accumulation principle of peroxide of lead, lead and sulphuric acid in 1854. Planté built the first practically useful lead accumulator in 1859. Faure proposed in 1882 an improved version.) A lead plate, covered with peroxide of lead, and a lead plate, covered with specially prepared porous lead serve as electrodes in diluted sulphuric acid (Fig. 505/6). In this state (cf. the table below), the plates are charged, polarized. The cell serves as an element, the plate with the peroxide of lead as positive pole, the plate with the porous lead as negative pole. During the delivery of current, both plates cover themselves gradually with sulphate of lead. When the coverage is complete, the cell can no longer serve as element, it is discharged. In order to recharge it, you treat it as a decomposition cell and make the positive pole into the anode, the other pole into the cathode and feed current through it. The electrolysis converts the sulphate of lead at the anode back into peroxide of lead and reduces it at the cathode to lead. The cell can then again serve as an element.

Chemical symbols with + and - represent the ions, which split off during the passage of current and partake in the chemical conversion.

The operation of the accumulator: It comsumes the energy of the charging current and in the process undergoes a chemical change, which increases its content of energy. It enables the accumulator to deliver current, to perform work. While it does so - discharges - the chemical changes reverse and when they have been totally completed, its electromotoric effectiveness is finished. - Hence the accumulator is a store of energy, a very spacious one, if its plates have been made large enough. For this purpose, you interconnect a larger number of plates in parallel. Fig. 506 shows its scheme. It is used as a storage, to which energy can be brough and from which it can be drawn as needed. If you use a dynamo during the day less, in the evening more than it can yield, you employ during the day its excess energy for loading accumulators; you discharge them at night in support of the dynamo. You employ accumulators, which were charged during the day, at night instead of the dynamo, etc. Since they are readily transported, they serve the operation and illumination of electrically moved vehicles (motor cars, trams, boats). Every cell demands for its charge 2.65 - 2.75 Volt, whence you can charge with 120 Volt 45 cells, connected in series. During its discharge, a cell yields at first 2 Volt, it drops quickly to 1.95 Volt, than more slowly to 1.8 Volt, when it must be recharged. During slow drawing of current, you can count on 20 Ampere hours for every kg of electrode material. Accumulators must be treated with care.

In order to avoid the use of lead (due to itsm echanical properties), other accumulators have been made. Only the nickle-iron-accumulator of Edison has proved itself. It active ingedients are nickel superoxide (Ni₂O₃) as positive pole and finely distributed iron in potash lye (specific weight 1.2) as negative pole. The plate bodies are best made out of nickel-plated steel. A charged, rested accumulator has 1.36 Volt at 18ºC, that is, much less than the lead accumulator; however, it offers some advantages as regards servicing and sensitivity, mainly for vehicles.

continue stepback