THE DRY CELL

Moist paste of MnOs, NH4CI, and carbon black 

When the two terminals on the electrodes are connected, such as through an electrical circuit powering a portable radio, half-reactions occur at both electrodes. At the graphite electrode, the half-reaction is 

This reaction leaves a surplus of negatively charged electrons on the zinc electrode, so that it has a negative charge. It is an oxidation half-reaction. The electrode at which oxidation occurs in an electrochemical cell is always called the anode. If the two electrodes are coimected, electrons will flow from the zinc anode to the carbon cathode. Such a flow of electrons is an electrical current from which useful work can be extracted. Every time that 2Mn02 molecules are reduced, as shown in the first half-reaction, IZn is oxidized. Therefore, the overall oxidation-reduction reaction is obtained by multiplying everything in the reduction half-reaction at the 

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Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Dry cells (batteries) and fuel cells are the main chemical electricity sources. Diy cells consist of two electrodes, made of different metals, placed into a solid electrolyte. The latter facilitates an oxidation process and a flow of electrons between electrodes, directly converting chemical energy into electricity. Various metal combinations in electrodes determine different characteristics of the dry cells. For example, nickel-cadmium cells have low output but can work for several years. On the other hand, silver-zinc cells are more powerful but with a much shorter life span. Therefore, the use of a particular type of dry cell is determined by the spacecraft mission profile. Usually these are the short missions with low electricity consumption. Diy cells are simple and reliable, since they lack moving parts. Their major drawbacks are... 

Electrochemical cells are familiar—a flashlight operates on current drawn from electrochemical cells called dry cells, and automobiles are started with the aid of a battery, a set of electrochemical cells in tandem. The last time you changed the dry cells in a flashlight because the old ones were dead, did you wonder what had happened inside those cells Why does electric current flow from a new dry cell but not from one that has been used many hours We shall see that this is an important question in chemistry. By studying the chemical reactions that occur in an electrochemical cell we discover a basis for predicting whether equilibrium in a chemical reaction fa-... 

The dry cell was invented by Leclanche in the 1860s. This type of battery was developed in the 19th century. In the 1940s, Rubel achieved significant progress in alkaline-zinc batteries, and manufactured zinc powder with high surface area to prevent zinc passivation. 

FIGURE 12.21 A commercial dry cell. The dry cell is also called the Leclanche cell, for Georges Leclanche, the French engineer who invented it in about 1866. The electrolyte is a moist paste. 

It was confirmed, by separating the enzymes in the powder, that many S- and R-directing enzymes do indeed exist in the dried cells. The addition of a coenzyme and cyclopentanol stimulates only an S enzyme because the specific S enzyme can oxidize cyclopentanol (concomitantly reducing NAD(P) ), while other S or R enzymes cannot use cyclopentanol as effectively [14cj. This presents a very interesting case, where the experimental conditions of reduction with a cell having both S-and R-directing enzymes was modified and resulted in excellent S enantioselectivity. 

Figure 8.30 Reduction of ketones by the dried cell of C. candidum, NAD(P), and secondary alcohol [14],...

The Zinc-air battery is more expensive than the dry cell and deteriorates relatively quickly once it is exposed to air. High capacity and a cell potential that does not vary with use offset these disadvantages. Like the dry cell, a zinc-air battery uses zinc for the anode reaction. Uniquely among batteries in common use, this battery relies on molecular oxygen from the atmosphere for its cathode reaction. 

The polyuronic content (PUC) and the degree of esterification (DE) of the dry cell walls were determined according to the method of Gee (13). The results for these two parameters are given in terms of per cent of the total amount of cell walls. 

The Leclanche cell (also known as the dry cell) is frequently used to power flashlights, watches, calculators, and a number of other portable devices. Despite the name dry cell, this battery does contain an electrolyte solution but only in the form of dense paste. There are two versions of this cell, the acid version and the alkaline version. 

The resulting water helps to extend the life of the dry cell by providing moisture for the movement of the ionic species. A second reaction, that extends the usefulness of this cell, is the formation of an amine complex of zinc ions in the cell ... 

The progress of battery technology could not wait for the development of plastics. Scientists continued to look for ways of solving the problem of fussy liquids entirely. This came to fruition when battery chemistry was further modified to produce the dry cells. This breakthrough made... 

The more expensive alkaline cell, shown in Figure 11.8, is an improved, longer-lasting version of the dry cell. 

When the circuit containing the battery is closed, electrons from the oxidation of zinc flow from the negative terminal through the object being powered to the negative carbon rod that serves as an electron collector. Several reduction reactions occur at the collector one main reaction is represented earlier. The dry cell with the NH Cl electrolyte creates acidic conditions in the battery. The acid attacks the zinc, which accelerates its... 

Food yeast, molasses-grown, is dried to about 5% moisture and has the same chemical composition as bakers yeast. In terms of micrograms per gram of yeast, the vitamin content is 165 thiamine 100 riboflavin 590 niacin 20 pyridoxine 13 folacin 100 pantothenic acid 0.6 biotin 160 para-ainiiiobeuzoie acid 2710 choline and 3000 inositol. YeasL crude protein contains 80% amino acids 12% nucleic acids and 8% ammonia. The latter components lower the true protein content to 40% of the dry cell weight. 

An alkaline cell uses an alkaline electrolyte, with which the zinc electrode does not readily react when the battery is not in use. As a result, alkaline cells have longer lives than dry cells. The uses are the same as those for the dry cell, plus applications that require a long-lasting charge, such as smoke detectors and backup power supplies. 

Over 100 genes in E. coli participate in the synthesis if of the RNAs and proteins that constitute the enzymatic machinery for translation. The relevant gene products make up between 20% and 40% of the dry cell mass. In rapidly growing cells about 85% of the RNA is ribosomal, 10% is tRNA, and most of the remainder is mRNA. The various RNAs and proteins are produced according to need. For ribosomes and tRNAs, this results in a synthesis rate that is roughly proportional to the cell growth... 

First, let us define the terminologies we use for microbial growth. If we mention the cell concentration without any specification, it can have many different meanings. It can be the number of cells, the wet cell weight, or the dry cell weight per unit volume. In this text, the following nomenclature is adopted ... 

Heat is the most widely used means of sterilization, which can be employed for both liquid medium and heatable solid objects. It can be applied as dry or moist heat (steam). The moist heat is more effective than the dry heat, because the intrinsic heat resistance of vegetative bacterial cells is greatly increased in a completely dry state. As a result the death rate is much lower for the dry cells than for moist ones. The heat conduction in dry air is also less rapid than in steam. Therefore, dry heat is used only for the sterilization of glassware or heatable solid materials. By pressurizing a vessel, the steam temperature can be increased significantly above the boiling point of water. Laboratory autoclaves are commonly operated at a steam pressure of about 30 psia, which corresponds to 121°C. Even bacterial spores are rapidly killed at 121 °C. 

Perhaps the most familiar application of the dry cell is its use in flashlights. This battery consists of a zinc case that serves as the negative terminal. The inner walls are covered with a thin coating of porous paper or cloth, which keeps the zinc from coming into direct contact with the contents of the cell. A carbon rod placed in the center of the... 

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