Metals in Salt Solutions

Problem In the previous reactions, electrons are directly transferred from one particle to the other - an electric current cannot take place. If the electrons are diverted back from one metal electrode to the other electrode, both dipped into a salt solution, it is possible to measure, first the voltage with a voltmeter or second to demonstrate the current using a light bulb or an electric motor. Recorded voltages are not normalized because the distances between electrodes 

Material Glass Beaker, voltmeter (multimeter), cable and alligator clips, light bulb, electric motor various metal strips or rods, sodium chloride solution. 

Procedure Fill a beaker two-thirds with salt solution, dip a copper strip to one side of the beaker and magnesium ribbon to the other side. Attach both metals to a multimeter, record the voltages. Check formation of an electric current with a light bulb or an electric motor. Replace magnesium by other metals like zinc or iron, or even with copper record the values measured. 

Observation For the metals copper and magnesium, a voltage of about 1.7 V is recorded. The electric motor starts to move and runs for a while. The other voltages are smaller the motor no longer runs. If the same metals are combined, then no voltage is measurable. 

Tip In order to attain a spectacular voltage out of a lemon , two different metal strips could be placed inside the lemon and the voltages measured the juice of the acidic cell serves as an electrolyte solution in this case. If the voltage or the current is too weak to start the motor or the light bulb, the multimeter can be used to measure the electric current. 

---

Evans and co-workers 1931-1934 Electric currents due to corrosion of metal in salt solutions were measured... 

May 29, 1905, Reichenbach, Germany - Dec. 4, 1983, Berlin, Germany) Schwabe studied chemistry from 1924 to 1927 at Technische Universitat Dresden, completed his diploma thesis in 1927, his Ph.D. thesis on anodic behavior of metals in 1928, and habilitated in 1933 on the anodic behavior of metals in salt solutions. He was Professor at the Technische Universitat Dresden from 1939-1940 and 1949-1970, and from 1949-1983 head of an electrochemical research institute in Meinsberg, Germany. His major contributions concern pH measurements, corrosion, and concentrated electrolyte solutions. 

As metal ion concentration increases in the crevice, a net positive charge accumulates in the crevice electrolyte. This attracts negatively charged ions dissolved in the water. Chloride, sulfate, and other anions spontaneously concentrate in the crevice (Figs. 2.4 and 2.5). Hydrolysis produces acids in the crevice, accelerating attack (Reactions 2.5 and 2.6). Studies have shown that the crevice pH can decrease to 2 or less in salt solutions having a neutral pH. 

Lower oxidation states are rather sparsely represented for Zr and Hf. Even for Ti they are readily oxidized to +4 but they are undoubtedly well defined and, whatever arguments may be advanced against applying the description to Sc, there is no doubt that Ti is a transition metal . In aqueous solution Ti can be prepared by reduction of Ti, either with Zn and dilute acid or electrolytically, and it exists in dilute acids as the violet, octahedral [Ti(H20)6] + ion (p. 970). Although this is subject to a certain amount of hydrolysis, normal salts such as halides and sulfates can be separated. Zr and are known mainly as the trihalides or their derivatives and have no aqueous chemistry since they reduce water. Table 21.2 (p. 960) gives the oxidation states and stereochemistries found in the complexes of Ti, Zr and Hf along with illustrative examples. (See also pp. 1281-2.)... 

Pure tin is completely resistant to distilled water, hot or cold. Local corrosion occurs in salt solutions which do not form insoluble compounds with stannous ions (e.g. chloride, bromide, sulphate, nitrate) but is unlikely in solutions giving stable precipitates (e.g. borate, mono-hydrogen phosphate, bicarbonate, iodide) . In all solutions, oxide film growth occurs and the potential of the metal rises. Any local dissolution may not begin for several days but, once it has begun, it will continue, its presence being manifested... 

There is a third type of solution preparation problem that could be encountered with the parts per million unit. This is the case in which the solution of a metal is to be prepared by weighing a metal salt, rather than the pure metal, when only the parts per million of the metal in the solution is given. In this case, the weight of the metal needs to be converted to the weight of the metal salt via a gravimetric factor (see Section 3.6.3) so that the weight of the metal salt is known. 

Using the plan from your Pre-Lab, construct voltaic cells using the four metals and 1 mL of each of the solutions. Remember to minimize the use of solutions. Put the metals in the wells that contain the appropriate solution (for example, put the zinc metal in the solution with zinc nitrate). Use a different salt bridge for each voltaic cell. If you get a negative value for potential difference, switch the leads of the probe on the metals. 

The nitrate salt prepared by this method is hydrated. It cannot be dehydrated fully without decomposition. Anhydrous CuNOs may be prepared by dissolving copper metal in a solution of dinitrogen tetroxide, N2O4, in ethyl acetate. Upon crystaUization, an N2O4 adduct of Cu(N03)2 that probably has the composition [NO [Cu(N03)3] is obtained. This adduct, on heating at 90°C, yields blue anhydrous copper(II) nitrate which can be sublimed in vacuum at 150°C and coUected. 

The first batteries date from the early 1800s. They consisted of a stack of disks made of two different metals, arranged alternately, with pads of cloth soaked in salt solution in-between each layer. A pile of nickel and copper coins separated by blotting paper that has been dipped in salty water will do just as well. Electrons will flow through the pile, from nickel to copper, but cannot escape until the top and bottom are connected by a wire. 

Solutions containing a high concentration of excess electrons display a transition to the metallic state. Thus, for sodium-ammonia solutions in the concentration region 1-6 M the specific conductance increases by about three orders of magnitude, and the temperature coefficient of the conductance is very small (27). Similar behavior is exhibited by other metal-ammonia solutions (but surprisingly, not by concentrated lithium-methylamine solutions ) (10) and by metal-molten salt solutions (17). 

Causo, M.S., Ciccotti, G., Montemayor, D., Bonella, S., Coker, D.F. An adiabatic linearized path integral approach for quantum time correlation functions electronic transport in metal-molten salt solutions. J. Phys. Chem. B 109 6855... 

Both formulations stumble when the materials are real conductors such as salt solutions or metals. In these cases important fluctuations can occur in the limit of low frequency where we must think of long-lasting, far-reaching electric currents. Unlike brief dipolar fluctuations that can be considered to occur local to a point in a material, walls or discontinuities in conductivity at material interfaces interrupt the electrical currents set up by these longer-lasting "zero-frequency" fields. It is not enough to know finite bulk material conductivities in order to compute forces. Nevertheless, it is possible to extend the Lifshitz theory to include events such as the fluctuations of ions in salt solutions or of electrons in metals. 

In the absence of trace H20 and 02 in esters, ethers and alkyl carbonate solutions, the initial voltammograms obtained with noble metals in these solutions are featureless and show the irreversible reduction wave with an onset at 1.5 V (Li/Li+), which corresponds to salt anion and solvent reduction. In addition, in the absence of H20 and 02, the peaks related to Li UPD shown in Figure 6 and the peaks related to the Au/Au(OH)3 [or Au/Au(OH)ads] couple described in the previous section do not appear. This is demonstrated in Figure 10. 

Electrowinning of metals in aqueous solutions is applicable to those metals that possess high electrochemical reduction potentials, such as silver, copper, cadmium, and zinc. Magnesium, aluminum, and sodium, like other reactive metals, are electro-produced from molten salt baths, such as NaCl/CaCh mixture at ca. 600 °C for sodium and MgCl2/NaCl/CaCl2 eutectic mixture at ca. 750 °C for magnesium. 

Red phosphorus is less soluble than white in all solvents. In water and alcohol it is almost insoluble. It is somewhat soluble in ether and in hot acetic acid, from which it is reprecipitated by water. It is slightly soluble in phosphorus trichloride. These solubilities refer to the ordinary preparation, which, as shown on p. 32, usually contains residual quantities of the white form. Red phosphorus is able to reduce salts, especially those of the noble metals, in aqueous solution on boiling. Salts of mercury are reduced to the metal those of gold and silver give insoluble phosphides while ferric and stannic salts are reduced to ferrous and stannous respectively.5... 

---