K7 Electro-chemical actions of the current. Electro-motoric action of ions
If two strongly interrelated substances are combined chemically, there arises a large amount of heat, the equivalent of large work. If hydrogen and oxygen compound into exactly 1 kg of water, as much heat is formed as is required to lift one kilogram by 1600 km, provided there existed a steam engine which could convert all of it into mechanical work. Hence: oxygen and hydrogen contain as long as each is on its own in their chemical affinity a certain amount of energy. However, if they are combined, they continue to attract each other without being able to do work. In order to enable them to work, you must separate them again from each other, and this demands work.
Also work of this kind can be performed by an electric current, that is, it can separate the components of chemically compounded substances from each other - the process is therefore called electrolysis (Faraday 1834) - and revert them into the state when they can do work. The current only performs this work while it flows through the substance. Substances, which a current can disintegrate chemically, are conductors, They are called electrolytes, also conductors of the second kind, in contrast to those of the first kind, the metals. - (Just as the concept of the conductor is only relative, so is that of the electrolyte. There exist electrolytes which conduct current hardly noticeably, so that their products of decomposition cannot be detected by ordinary means of chemical analysis; there exist also electrolytes which do not conduct at ordinary temperatures, but at higher ones.) Acids, bases and salts are most readily decomposed. They are non-conductors, but conduct as soon as they are dissolved in water, for example, zinc chloride, lead chloride, potassium hydroxide.
We make such a solution into a part of a circuit (Fig.490). The locations A and B, where the metal conductor is interrupted, but which the liquid conductor connects, are called electrodes (Greek: odos = path). Also the fluid, which conducts best, conducts badly in comparison with a metal. For this reason, electrodes are made relatively large, compared with the cross-section of the metal conductor leading to them, in the form of plates connected to them. . While you call the entire plate an electrode - one the positive, the other the negative electrode; an electrode in the true sense, that is, entry- and exit-location of the current, is only that part of the plate, which is dipped into the fluid.
Naturally, you can interrupt the same conductor at several locations - in Fig. 491 at 1, 2, 3 - and introduce at each of them with the aid of electrodes a cell with conducting fluid, a decomposition cell. Through all of them, the same current passes at the same time. You say that cells in such an arrangement - the negative electrode of one cell connected to the positive electrode of the next cell - are connected in series. - The decomposition products deposit themselves at the electrodes and not inside the fluid. You can see that as you decompose a substance, the components of which are gases (hydrochloric acid). The gases only rise at the electrodes. (In order to avoid complications, which could originate in the chemical sensitivity of the electrodes, they are made out of carbon or platinum.) First of all, we learn: Irrespectively of the nature of the fluid conductor, there appears at the negative electrode that part of it, which Chemistry calls a base, at the positive electrode the acid or that part, which is an essential part of an acid. Whether the conductor is zinc chloride or potassium chloride or ammonia - chlorine and nitrogen appear at the positive, zinc and hydrogen at the negative electrode (whence you can detect with pole paper which of the ends of the conductor is positive or negative).
Since the products of the decomposition only appear at the electrodes, they must have moved there. Because of this motion, Faraday called them ions (Greek: iwn, iontos = walking): The ion which moves uphill to the positive electrode, he called anion (chlorine, nitrogen, acid) and which moves downhill the cation (hydrogen, zinc). He called the ends of the fluid conductor, that is, the locations where the anions and cations exit, anode and cathode. This terminology identifies the ends of the fluid conductor with the joining ends of the metallic conductor, the electrodes, and calls the positive electrode anode, the negative one cathode.
Certain changes in concentration in the vicinity of the electrodes have shown (Hittorf ), that two ions, which were interlinked, after their separation do not travel to their electrode at the same velocity. For example, if hydrochloric acid, HCl, is electrolyzed - other things being equal - the H-ion moves five times as fast as the Cl-ion (transference numbers). In a very diluted solution, every ion has a definite velocity (the largest has hydrogen among the cations, the hydroxyl group OH among the anions), independently of the ion with which it was connected and whether yet other ions are moving in the same or in the opposite direction through the fluid.Arrhenius 1886/87 tied his theory of electrolytic dissociation to the law of the independence of ion migration (Kohlrausch); it shows, in addition, how to compute the conductance of very diluted solutions.
How occurs the secretion of the
components of the electrolyte at the electrodes? It is explained by the theory of electrolytic dissociation. In certain aqueous solutions, the osmotic pressure is larger than corresponds to the concentration
of the solution and the molecular weight of the dissolved
substances, whence the number of dissolved particles is
apparently larger than one would expect. This leads to the
conclusion that the molecules of the dissolved substance are
partly dissociated, split into components. Experience
1. These apparent dissociated solutions are the same, which also conduct the current and are decomposed by it,
2. The deviation of the osmotic pressure from the computed value, that is, the number of the apparently split molecules rises to a certain limiting value as the solution is diluted.
Hence one concludes: The increase in the number of split molecules causes the increase in the conductivity and the dissolving process itself provokes both of them. Hence you have the following concept of the conduction of current in electrolytes and the deposition of their components: The electrolyte, for example, potassium chloride (KCl) is, while dissolvd in water, split into the components potassium (K) and chlorine (Cl) - that is, it is not that the current is the first to split it. At the electrodes, the components deposit themselves because - it is assumed - the one (potassium) is charged positive, the other (chlorine) negative, whence the one is attracted by the negative electrode, the other by the positive electrode (Fig. 492). We disregard for the moment from where the ions potassium and chlorine - denoted by K+ and Cl - - have their charges; moreover also what concept we should have of the charges of ions. Fig 492 shows: The ions transport the electricity through the fluid, which on its own does not conduct, and form in the process what we call an electric current. Potassium chloride molecules, which are not split - you must conceive them to be charged simultaneously with equally large amounts of positive and negative electricity - do not partake in the conduction of the current. - The splitting of the molecules explains also that each however small current strength deposits the components at the electrodes: They are already separated from each other, they are only moved towards the electrodes, and for that every arbitrarily small force is sufficient, provided it acts long enough.
Do only some or all molecules decompose during the formation of the solution? In this sense, you speak of the degree of dissociation of an electrolyte and understand by it the ratio of the numbers of electrolytically split molecules to their total number. You compute the degree of dissociation from the change of the freezing and boiling points, from the conductivity, from the EMF between a metal and the solution of one of its salts and from other, predominantly chemical processes. During the computation of the degree of dissociation (a), the electrolytes in an aqueous solution become sudivided into strong and weak ones. The strong ones have in diluted solutions a value a , which lies near 1 and drops only slowly with increasing concentration. Among them are most neutral salts, moreover strong mineral acids and bases like HCl, HNO3, H2SO4, NaOH, KOH, etc. The degree of dissociation of the weak electrolytes changes greatly with the concentration, but even at very strong dilution the ion formation is not very complete. Among them are most organic acids, carbonic acid, hydrogen sulphide and other weak acids as well as ammonia and many organic bases.
As regards electrolytic dissociation, one is dealing with the equilibrium between ions and undissociated molecules, which changes with the concentration of the solution. A decisive role in this has the dissociation constant k, linked to the concentration c and the degree of dissociation a by the equation a2·c/(1 - a) = k. However, for strong electrolytes (neutral salts, strong mineral acids, lyes), this dilution law fails; the expression on the left hand side turns out not to be constant, but increases in most cases as the concentration increases.
An attempt to explain this anomaly of the strong electrolytes caused the most recent development (in 1935) in the theory of dissociation. It was guided by the basic concept: The strong electrolytes are in their well conducting solutions always totally decomposed into ions, whence the degree of dissociation is independent of the concentration, that is, it always is equal to 1. The decrease of conductivity, osmotic and chemical effectiveness at rising concentration explains itself by the mutual electrostatic influence of the ions, which limits their movability.
The theory of electrolytic dissociation has led to a new kind of molecules - electrically charged ions - and their chemical ability to react. It has brought thereby new fertile concepts to the theory of chemical reactions. We see: The same substance can, according to circumstances, electrolytically decompose or split into unelectric molecules. If you dissolve ammonium chloride in much water, it dissociates itself electrolytically almost totally in the sense of the equation N+H4Cl- = N+H4 + Cl-; if we evaporate it at sufficiently small pressure, it decomposes into unelectric molecules in the sense of the equation NH4Cl-=NH3+Cl .
Electrolytic dissociation differs not only from the purely chemical one in that components are electrically charged, but also in that the components behave chemically completely differently. For example, potassium and chlorine ions have quite a different chemical behaviour compared with that of neutral potassium and neutral chlorine. Free chlorine in solution is odourless, the potassium ion does not react to water. This difference is explained as follows: The potassium ion and the chlorine ion contain different quantities of energy from neutral chlorine and neutral potassium.
The following agrees with this: In order to separate from each other the products of ordinary dissociation, no other work is required than to separate altogether the components of a mixture; in the case of products of electrolytic dissociation, the much larger work against the electric attractive forces of the oppositely charged ions has to be performed.
The process of solution has split the electrolytes; what is the work of the current? - The ions, attracted by the electrodes, travel through the fluid, one kind against the current, the other kind with the current - of course, with large friction: Overcoming these forces is one component of the work of the current. Once the ions have arrived at the electrodes, they must be relieved of their charges: The conversion of the ions into neutral atoms is the other component of this work.
How large are the quantities of the products of decomposition, which appear simultaneously at the two electrodes? Experience tells: They are in that ratio one to another which is yielded by the formula of the compound: In the hydrochloric acid (HCl) form pairs of the atoms of hydrogen and chlorine, 1 mg of hydrogen with 35.5 mg of chlorine, and in the same manner appear simultaneously at the anode for each 1 mg hydrogen 35.5 mg chlorine.
The absolute quantity of the secreted components increases in proportion to the current strength and its duration. A current of 1 Ampere secretes from nitrous silver in 1 second 1.118 silver, a current of 2 Ampere in 1 second 2·1.118 mg silver and in 2 seconds 2·2·1.118 mg, etc. But the mutual ratios of the quantities of secreted components is always the same - independent of the current strength and the duration of flow, the magnitude and the form of the decomposition cells, the size and mutual distance of the electrodes.
Faraday's electrochemical law of equivalence
Up till now, we have only talked about a single cell. What happens if the current passes through several cells in sweries (Fig. 491)? In each cell, there arise components corresponding to the chemical formula of the electrolyte, present in it. In a cell with zinc chloride, ZnCl2, 71 mg chlorine secret simultaneously for 65.4 mg zinc, in one with hydrochloric acid, HCl, 35.5 chlorine for 1 mg hydrogen simultaneously, etc. Experience now yields the law, which comprises the process in all consecutively connected cells: One of the most important laws (Faraday) of the entire theory of electricity, one of the foundations of the modern theory (Helmholtz). Briefly formulated, it employs the concept of valence.
We will explain this concept by means of examples. Copper sulphate CuSO4 and iron sulphate FeSO4 differ according to their formulae in that copper, Cu, and iron, Fe, are replaced by each other - one copper atom by one iron atom-, soda Na2CO3 and potassium carbonate K2CO3 by replacement of potassium, K, by sodium, Na - two atoms potassium by two atoms sodium. Hence one says that the copper atom is equivalent to the iron atom, the potassium atom to the sodium atom. However, copper and iron, on the one hand, and potassium and sodium, on the other hand, are not mutually equivalent. Two compounds like copper sulphate, CuSO4, and sodium sulphate, Na2SO4, confirm this. While sodium and copper take each other's place, two atoms Na are required in place of one atom Cu. One says therefore that copper is multi-valent compared to sodium.
You can subdivide the elements into groups: Elements of the same group are mutually equivalent, but the elements of different groups are not. There does not exist any element, in comparison to which hydrogen is multi-valent, when you ascribe to the hydrogen atom the lowest valence (1 valence) and call it and its equivalent elements (potassium. sodium, etc.) uni-valent. Correspondingly, an element, one atom of which replaces two uni-valent atoms, for example, Cu, Zn, Hg, is called bi-valent and ascribed 2 valences, etc.
The concept of valence of atoms and groups of atoms becomes valuable for the formulation of Faraday's law by the following considerations: One bi-valent atom, for example, the Zn atom, is equivalent to 2 hydrogen atoms. Since zinc has the atomic weight 65.4 and hydrogen 1, this means: 65.4 weight units zinc are equivalent to 2 weight units of hydrogen, whence, for example, 65.4 g zinc to 2g hydrogen and therefore 65.4/2 g zinc to 1 g hydrogen. The atomic weight of an element divided by its valence is called equivalent weight of a substance. The equivalent weight of an uni-valent element is equal to its atomic weight, that of the 2-, 3-, ···, n-valent elements to the 2-, 3-, ···, n-th part of the atomic weight.
Helmholtz has formulated Faraday's law as follows: The same amount of electricity, as it flow through an electrolyte, secrets always the same number of valences at the two electrodes. Hence, if the same current passes consecutively through several cells and secrets at the cathode of the first hydrogen, of the second silver, of the third gold, the fourth zinc, there arises for every gram hydrogen in the first cell: 107.88/1 = 107.88 g silver, in the second, 197/3 = 65.7 g gold, in the third, 65.4/2 = 32.7 g zinc in the fourth cell. What amount of electricity is required to secret at one electrode 1 g hydrogen or the equivalent weight of any other element, for example, 107.88 g silver or 32.7 g zinc? For example, 1 Coulomb, a current of 1 Ampere and 1 sec duration, secrets 0.001118 g silver at the cathode, whence it follows that the secretion of 107.88 g silver (or 32.7 g zinc or 65.7 g gold) demands 107.88/ 0.00118 = 96494 Coulomb.
However, if always the same amount of electricity frees equal valences at the electrodes, then there must be available for one valence a certain amount of electricity, if it is to neutralize its charge. Hence we conclude: Every ion contains, as long as it is in the fluid, for every one of its valences a corresponding charge of electricity and all the ions with equal valence (potassium-, silver-, hydrogen-ions) carry the same charge. Hence, at its arrival at the electrode, a potassium ion requires for neutralization of its charge and for becoming again a neutral atom the same amount of electricity as a silver ion or a hydrogen ion. However, only the size of the charge is the same for all of them; the tenacity by which they hold on to their charge differs (intensity of adhesion). Experience tells: In order to neutralize an ion, not only the quantity of electricity is important, which is at its disposal at an electrode, but also the difference of the potential at the electrode. In order to remove its charge from the potassium ion, more EMF is required than to remove it from the silver ion. This is expressed in the decomposition Voltage (or polarization Voltage), which must be maintained at the electrodes depending on which element is to be secreted electrolytically. One explains the differences in the chemical activity of the elements (one element is more strongly positive than another) by the difference in the adhesion intensity of the electric charge.
Apparently, univalent ions, hydrogen ion, potassium ion. etc. have the smallest charge in terms of electricity units. But one atom is the smallest mass which exists independently, the charge of an uni-valent atom is therefore the smallest quantity of electricity to which we can ascribe an independent existence. Hence we conclude: Positive as well as negative electricity are subdivided into certain, elementary quanta which behave like atoms of electricity (Helmholtz). The charge of an uni-valent atom can be computed: According to certain electrolytic results, 1 mg hydrogen charged with 96.49 Coulomb (1 Coulomb = 3·109 electrostatic units) and according to the kinetic gas theory 1 mg hydrogen contains about 1021 atoms. Hence 1021 atoms are charged by about 100 Coulomb, that is, 1 atom carries about 10-19 Coulomb (that is, 3·10-10electro-static units), This amount of electricity, bound to an uni-valent ion is called the electric, elementary quantum or elementary charge. Thus, 1 elementary charge = 3·10-10 electro-static units, whence an electrostatic unit equals about 3·109 elementary charges. This elementary charge is extremely small: The earlier, in terms of a mechanical process, explained absolute eletrostatic unit of the quantity of electricity is about 109 times as large. Recomputed in terms of oxygen atomic weight, the electric elementary quantum is 90.50:1.008 = 95.73 Coulomb.
Since each univalent ion has the same amount of electricity as charge, the ratio charge e : mass m must always be the same, also for a weighable number of such ions as for a single ion of the same kind. Hence we can draw conclusions from observations of weighable quantities of substances to certain properties of single atoms. We know that e/m - the specific charge - for hydrogen is 96494 Coulomb/gram, which corresponds to 9649, that is, about 104 absolute electromagnetic units. Naturally, the specific charge of a single hydrogen ion is the same. It is smaller for every other element, since hydrogen is uni-valent and has the smallest atomic weight. For silver, which is uni-valent, but 107.88 times as heavy as hydrogen, it is 104/107.88, for zinc, which is bi-valent, but 65.4 as heavy as hydrogen, it is 2·104/65.4. Hence the numerical value of e/m is for every element in the ratio valence:atomic weight smaller than for hydrogen.
Electrons are to be viewed as the atoms of electricity; their specific charge e/m is about 1800 times that of hydrogen, the absolute (negative) charge e of which is equal to that of the hydrogen ion: Compared with the hydrogen atom, the electron has a very small mass.
Electrolysis of water
As a rule, chemical changes occur at the electrodes (secondary processes) between the ions and the solvent, etc. The substances, secreted at the electrodes, are therefore not always identical to the ions of the electrolyte. If you acidify water, which effectively does not conduct at all, with sulphuric acid and decompose it between platinum electrodes, there secret according to the formula H2O hydrogen and oxygen in the ratio 2 : 1. But this so-called electrolysis of water is, in fact, electrolysis of the sulphuric acid, dissociated in the water. The process proceeds as follows: H2SO4 decomposes into H2+ and SO-4. The ion H2+ is freed, but the ion SO-4 recompletes itself at the expense of water into H2SO4 and thereby releases oxygen.
Applications of electrolysis
Metals, which precipitate at the electrodes, cover these mostly as solid layers. This process is employed for copper- and silver-plating, etc (electro-plating). You change the body, to be plated - its surface must conduct or (for example, be rubbed with graphite, to be made conducting) - into the solution of a metal salt and connect it to the cathode. You can also achieve deposits which can be detached from the electrode (galvano-plastic) and use them as metal imprints (clichés). Metallurgy employs electrolysis for the production of aluminium and aluminium bronze, of copper, of gold; it is also used in bleaching and tanning.
You can measure the strength of an electric current by means of electrolysis. The law of the proportionality between current strength and duration of current passage, on the one hand, and the amount secreted, on the other hand, is so strictly observed, that you can define the one Ampere strength of electric current by it: The Ampere is the unit of electric current strength; it is represented by the unchangeable electric current, which during its passage through an aqueous solution of silver nitrate secrets during one second 0.001118 g silver (the electrochemical equivalent of silver). You can now determine every strength of current in Ampere: You pass the current to be measured for a measured number of seconds through a solution of nitrate of silver, AgNO3, and weigh the secreted silver. The numbers of milligrams and seconds yield the strength of the current in Ampere.
A decomposition cell for current measurement is called a voltmeter, depending on the electrolyte used: Silver-, copper, water-volt-meter, etc. Treated professionally, the silver-voltmeter is most reliable (Fig. 494). The cathode is a platinum (or silver) container with the solution (20 - 40 % solution of AgNO3 in distilled water), the anode a silver bar. The current secrets in the vessel metallic silver; it secrets at the silver bar the nitric acid rest, which converts the silver into silver nitrate. - In the water-voltmeter (Kohlrausch), you decompose 10 - 20 % pure sulphuric acid between clean platinum electrodes. You read the amount of oxydrogen gas generated directly in cm², but must take into account barometer and temperature readings.
This voltmeter demands much time, effort and experience, whence it is only employed for the calibration of the scales of current meters (Ampere meters) in Ampere. You connect it with the instrument to be calibrated in series, so that the current in both is the same. The pointer of the ampere meter is then located in front of the same location of the scale, as long as the current does not change. With the voltmeter, you determine the number of ampere of the current.
The voltmeter also serves as electricity meter (electricity counter), that is, as partner of the gas-counter (gas-meter). A gas-counter indicates the number of cubic meters of gas which have passed through the system during a longer time, The voltmeter shows the corresponding information in electricity units. We know that 1.118 mg silver in the voltmeter indicate that 1 Ampere passed during one second or also 2 Ampere during 1/2 second or also 1/2 Ampere during 2 seconds, etc. The amount of electricity, which takes 1 Ampere during 1 second through the cross-section of the conductor, is called 1 Ampere-second; it is 3·109 electro-statically measured units - 1 Coulomb. Hence, in the voltmeter, 1.118 mg silver correspond to 1 Coulomb. For example, if we find 1118 mg silver in the voltmeter, 1000 Coulomb electricity or, what is the same thing, 1000 Ampere-seconds have passed through the cable; however, we do not find out whether 1000 Ampere passed for 1 second or 1 Ampere for 1000 seconds; this is immaterial, because the quantity of electricity is the same. - Most popular is an electrolytic counter, in which the current deposits at the cathode mercury (Friedrich Otto Schott 1851-1935), which secrets in a calibrated measuring tube and thus indicates the number of Ampere-hours (Stia-counter). Electrolytic counters can only be employed with direct current.
electro-motoric effect of ions (Nernst 1888/9)
Ions are carriers of electric charge, even enormously large ones: An uni-valent gram ion, for example, 1 g hydrogen, carries as many units of electricity as 27 Ampere pass during 1 hour through the cross-section of a conductor. This ownership of electricity makes ions under corresponding conditions into a source of electro-motoric forces. For example, a vessel (Fig. 495) with a strongly dissociated solution of hydrogen chloride gas in water contains positively charged hydrogen ions and negatively charged chlorine ions. If we could bring all the H+ to the one end of the tube and all the Cl- to the other end, the column of fluid would be charged positively at one end and negatively at the other end. Current would have to flow through a conductor connecting the two ends. We cannot realize this completely, however, far enough to become convinced of the truth of this conclusion. If you bring into contact strongly and weakly concentrated HCl solutions, they will diffuse into each other. However, the H+ migrate faster than the Cl-, whence there assemble at the one end of the tube more H+, at the other end more Cl-. If you place at both ends suitable electrodes, you can detect the potential difference.
However, ions can also arise in a way different from that of dissolution of a salt, a base or an acid in water. It you dip a metal, for example, a bar of zinc, into water, a little of it dissolves, much too little to be weighable, but sufficient to be detectable in another manner - by the potential difference,.which arises strangely enough between the zinc and the water. Nernst 1888/1889 explains this potential difference as follows:
According to the osmotic theory of solutions, the process of dissolution is analogous to that of vaporization. Every substance, however difficult it is to evaporate it, vaporizes from its surface molecules until the pressure, exerted on it by the vaporized molecules, equals its own tendency to evaporate, that is, is equal to the pressure at which it drives molecules into the surrounding space. Quite similarly, a body surrounded by a fluid ejects from its surface molecules. Once they enter the fluid, the molecules exert osmotic pressure, that is press also on the dissolving body. Hence the body dissolves only until the osmotic pressure of the molecules, which entered the fluid, is in equilibrium with the dissolution pressure, at which the solid ejects the molecule into the fluid.
You must exactly conceive in this way the process of dissolution of a metal in water. A metal soluble in water?! Everyone thinks that dissolution must be detectable immediately; he would have to see that the body dissolves, would have to see its volume decrease, or would have to be able to taste it in the solution, etc. However, this is incorrect! Our means of sensing are limited, very limited when compared with the performance of physical instruments, which serve the purpose of widening our senses. We cannot detect a quantity of 1/300000 mg cooking salt with our tongue; however, we can do so with a spectroscopic apparatus (Spectroanalysis). During the dissolution of a metal in water, one is concerned with quantities to which not even that apparatus reacts. However, an electrometer does. We can discover with the electrometer a potential difference between zinc and water. According to Nernst, its appearance can be understood as soon as it is interpreted as an action of the dissolution of the zinc in water, that is, viewed as a symptom of dissolution. Nernst assumes: The molecules, which the zinc ejects into the water, enter as ions, and indeed - that is the special feature signifying the dissolution of metals - as positive ions; the negative electricity, which arises simultaneously with the positive one, charges the zinc bar negatively. Hence the zinc bar and the fluid form a double layer of electric charge, whence there develops between them a potential difference. The negatively charged bar and the positive ions attract each other. Hence the ions press on the bar and, indeed, with an enormous force, because the electro-static, mutual attraction due to the great charge of the ions is very large. If the two electricities, with which the ions of 1 mg water are charged, were separated and placed on two spheres 1 km apart, they would cause an attraction between them which would be about 100000kg* (as Helmholtz stated in his Faraday address). Hence the dissolution of zinc ceases almost instantaneously; yet before the ions, ejected into the solution, are numerous enough to be detected spetroscopically, not to talk about weighing and even in our sense direct detection.
Electrolytic solution pressure
The pressure at which a substance tends to send its molecule into a solvent is called solution pressure; that of a metal, especially, electrolytic solution pressure, because its molecules enter the solution as ions, whence we can say: The metal ejects ions into the solution until the pressure, which they exert as a result of their charge on the metal, equals the magnitude of its electrolytic solution pressure. If the solvent is pure water, the electro-static pressure of the ions is the only pressure which counteracts the solution pressure. However, if the water dissolves an electrolyte - other soluble substances are here of no interest - an osmotic pressure is present in the water. It opposes the solution pressure of the metal, that is, tries to impede its dissolution or at least to reduce it. The solution pressure acts in the direction metal solution, the electrostatic pressure of the ions in the direction metal solution. Metal and solvent are in equilibrium only when
solution pressure = electrostatic pressure + osmotic pressure.
The magnitude of
the osmotic pressure in comparison with the solution pressure is
1. If it is just as large, the metal cannot emit ions into the solution; there does not develop a potential difference between the metal and the fluid.
2. If it is smaller, the metal starts to dissolve, emits positive ions into the fluid and charges itself negatively. However, it cannot emit as many ions into the solution as into pure water, because the osmotic pressure opposes it. Therefore the potential difference between the metal and solution becomes somewhat smaller than that between metal and pure water.
3. If the osmotic pressure is larger than the solution pressure, this excess pressure stops the metal from sending anything into the solution. Moreover, the metal surrounded by the electrolyte is then (solution and evaporation are analogous processes!) in the state of a fluid, touched by super-saturated steam. Some of the cations in the solution deposit themselves on the metal, that is, charge it positively, while the solution is charged negatively. This process ends as soon as the positively charged metal rejects further entering ions and keeps the osmotic pressure in equilibrium by rejection together with the electrolytic solution pressure. Also this process results (as the corresponding one of the solution) in a potential difference between the metal and the solution; the electrometer moves in the opposite direction as at first. For example, the first case (metal -, solution +) occurs when zinc is dipped into a solution of zinc sulphate, the second (metal +, solution -) when copper is dipped into a solution of copper sulphate. Hence we conclude: The solution pressure of zinc is larger (that of copper smaller) than the osmotic pressure of the zinc ions (copper ions). - Moreover, since the osmotic pressure of the zinc sulphate solution is equal to that of an equi-molecular solution of copper sulphate, the solution pressure of zinc is larger than that of copper.
Mechanism of galvanic element according to Nernst
Hence we can charge metals by contact with electrolytes positively and negatively, that is, we can generate with metals and electrolytes potential differences.This idea is realized by the galvanic element. A bar of zinc is dipped into the solution of a zinc salt, say, ZnSO4, and one of copper in the solution of a copper salt, say, CuSO4 (Fig. 496). At the instant of its submersion, the zinc charges itself negatively and the surrounding solution positively, the copper positively and the surrounding solution negatively; according to Nernst - we repeat! - because the solution tension of the zinc exceeds the osmotic pressure of the zinc sulphate solution and therefore ejects positive zinc ions into the solution; on the other hand, the osmotic pressure of the copper sulphate solution outweighs the solution pressure of the copper and therefore precipitates positive copper ions on the copper bar. The solution and deposition cease due to the electrostatic actions between the metals and the solutions before the dissolved; and precipitated quantities become weighable. However, at the same time, also the transport of electricity from the zinc into the zinc sulphate solution and from the copper into the copper sulphate solution ends. However, if you connect the metal bars (Fig. 497 dashed line), they adjust their charges with one another; and likewise the two solutions, if they are connected by a porous wall, which, while impeding direct mixing, admits through its pores contact of the solutions sufficient for conduction. As a result, the electro-static actions disappear, the zinc can again emits new ions into the solution, the copper sulphate ions can precipitate on the copper bar, that is, the bars of zinc and copper recharge, the first negative, the second positive. If their connection is permanent, these individual processes occur enduringly and all the time positive electricity flows through the cable from the copper to the zinc and in the fluid (as charge of zinc and copper ions) from the zinc to the copper. The group: zinc, zinc sulphate, copper sulphate, copper then supplies continuously electric current. In the process, the Zn-ion, sent into the solution, links up with the SO4-ion, which was freed by the emission of the Cu-ion. As a consequence, the concentration of the ZnSO4-solution increases and that of the CuSO4-solution decreases: The zinc electrode dissolves and the mass of the copper electrode increases. - The entire apparatus is called a galvanic element, which is closed in the state of Fig. 497, open when the connecting wire is absent. The copper and zinc are called the electrodes or also poles, the one positive, the other negative.
In fact, the galvanic element, as it has been described, did not arise out of a systematic application of Nernst's concept; it is the oldest method of current generation. The fact that metals and conducting fluids, when in contact with each other, charge themselves oppositely (contact electricity) was discovered by Volta 1794 and became the foundation for the construction of galvanic elements. However, the cause of the EMF was only satisfactorily explained by Nernst in 1889. The impetus to Volta's discovery was due to an accidental observation of the anatomist Luigi Galvani 1737-1798 1786 in Bologna during physiological examinations of the leg of a frog. After him, the Physics of contact electricity is called galvanism.
According to Nernst, the electrolytic solution pressure of zinc and the osmotic pressure of a copper sulphate solution are the cause of the potential difference between copper and zinc. Apparently, the solution pressure of zinc can develop the more energetically, the smaller is the osmotic pressure opposing it, that is, the less concentrated is the zinc sulphate solution. And the osmotic pressure of the copper sulphate solution is the stronger, the more concentrated is the solution. In other words, the potential difference between the copper bar and the zinc bar of the element must increase if the zinc sulphate solution is diluted and the copper sulphate solution becomes more concentrated. Experience has confirmed this conclusion. Nernst's theory also leads to computation of the potential difference and to total agreement between it and measurements.
The magnitude of the solution pressure of a metal and whether it is larger or smaller than the osmotic pressure of the solution is given by the potential difference and the side, to which the electrometer deflects. The metals, ordered according to decreasing potential differences, form the electric voltage sequence. The sign indicates whether the solution pressure of the metal is larger (+) or smaller than the osmotic pressure.
|Potential difference||solution pressure|
The element, presented schematically in Fig. 497 (John Frederic Daniell 1790-1845), is one of the most frequently employed elements. It is usually given the form of Fig. 498. A glass container A holds diluted ZnSO4 solution, a porous clay vessel B of concentrated CuSO4 solution Z and K stand in the vessels A and B. The clay cylinder is placed into the glass cylinder. The wall of B between the solutions stops the copper sulphate from moving to the zinc and causing changes which would soon make the element useless. The porosity of this wall mediates the conducting link between the solutions.
In order to extend as much as possible the path of the copper sulphate to the zinc, Helmholtz brought the copper (as a flat wire spiral) and the copper sulphate to the bottom of a glass cylinder, which otherwise was filled by a zinc sulphate solution; he fixed the zinc to a lid closing the cylinder. Only during weeks diffuses the copper sulphate upwards to the zinc cylinder. However, irrespectively of what means are employed, the copper sulphate reaches it eventually and causes changes which make the element useless.
There exist still many other elements, the most important of which are listed in the table below. All changes of their composition aim to make their electro-motoric force as large as possible and to maintain their action as constant as possible. Depending on whether the magnitude or the constancy of the EMF in a given case is more important and according to their cost, elements are selected. While the EMF of the Daniell element is smaller than that of the other models, it remains constant longer. These elements serve now only technical purposes in telegraphy and telephony, also in domestic telegraphy (bells, etc.) and have been replaced by the accumulator.
While the elements differ through their chemical nature, they have in common: Everyone has two different metals or one metal and carbon as electrodes and between them an electrolyte. The electrolyte is indispensable: The capacity to form with two different metals as electrodes an element is a characteristic of whether a substance is an electrolyte or not. Even glass is an electrolyte!
|Daniell||Cu||Zn||diluted H2SO4||concentrated CuSO4|
|amalgamated||or concentrated Cu(NO3)2||1|
|(dip-elelemnt)||"||K2Cr2O7||2 - 2.2|
Connection of several elements
The EMF of an element is only 1 - 2 Volts. In most applications, you want to overcome the resistance of a circuit and generate a given current intensity, for which you require very many more Volts. So you add the electro-motoric forces of several elements (Fig. 499). "The potential difference between the poles Zn1 and Cu1 is 1 Volt" means: The potential of the pole Cu1 lies by 1 Volt higher than the potential of the pole Zn1. Or, in equations:
pot.Cu1 = pot Zn1
+ 1 Volt
pot.Cu2= pot Zn2 + 1 Volt, etc.
If you now connect Cu1 directly to Zn,2 both will have the same potential, that is,
pot Zn2 = pot Cu1, whence pot Cu1 = pot Cu1 + 1Volt = pot Zn1 + 1Volt + 1 Volt.
This means that the potential of Cu2 is higher by 2 Volt than that of Zn1. If we now connect Cu2 with Zn3, the same potential is at both of them and between the copper pole of the third cell and the zinc pole of the first cell is a potential difference of 3 Volt, whence the three elements yield now the electro-motoric force as a single element of threefold EMF. This multitude of elements is called a galvanic battery and the elements are said to be connected in series.
Also the current intensity, which can be achieved with a single element, is limited. An element has an internal resistance through the resistance of the electrolyte, and since the electro-motoric force is 1 - 2 Volt, the maximal current intensity is fixed, which it can deliver. It is assumed here that the resistance of the external circuit is zero (the element is short circuited), the poles are interconnected, for example, by a thick copper bar. In practice, an external resistance is always present, whence the attainable current intensity is yet smaller. In order to increase it, you must reduce the inner resistance of the element, that is, make the copper and zinc plates and thereby the cross-section of the electrolyte as large as possible. However, the same goal can be achieved more comfortably: You can add the effective surfaces of the electrodes of arbitrarily many elements and make thereby the internal resistance of the battery arbitrarily small, if enough elements are available. You must treat the elements like the wires in Fig. 469, that is, corresponding ends connect, that is, the copper plates and the zinc plates are connect to each other (Fig. 500). The elements are then said to be connected in parallel. - Fig. 501 shows a battery, the elements of which are connected partly in series, partly parallel - the first in the interest of the required voltage, the second in the interest of the required current intensity.
In the dry column of Zamboni (Fig. 502), very thin metal foils, as electrodes (zinc and copper) between thin pieces of damp paper P, replace the electrodes. The foils and paper are heaped onto each other in thousands and form thus a battery of elements in sequel, the total tension of which can be hundreds of Volt. Of course, the internal resistance of such a battery or column is extraordinarily large, so that it is only employed electro-statically, for example, for use in the quadrant electrometer.