Zinc-mercuric oxide system

Power sources based on the zinc-mercuric oxide system are particularly suited to a wide range of applications, mainly concerned with miniature portable electronic equipment, where a relatively constant voltage is required throughout long discharge periods. In addition, such cells are used as voltage reference standards in regulated power supplies, potentiometers, chart... 

Replacing zinc with cadmium produces a cell with an OCV of 0.90 V, with characteristics very similar to those of the zinc-mercuric oxide system described above, but which is able to be stored and operated at extreme temperatures (—55 to 80°C) due to the low solubility of cadmium oxide even in concentrated KOH. Cells have been successfully operated at 180°C. Note that hydrogen generation does not occur at a cadmium anode. Because of cost and disposal problems, such cells are used only for applications where their special properties can be exploited, e.g. telemetry from internal combustion, jet or rocket engines. 

The main features of zinc-silver oxide cells are similar to those of the zinc-mercuric oxide system, except for a higher OCV and significantly increased cost. The overall cell reaction is... 

Zinc-mercuric oxide, cadmium-mercuric oxide, zinc-silver oxide and related systems... 

The primary objective of miniature battery design is to maximize the energy density in a small container. A compromise must be reached, however, since volumetric energy density decreases as cell volume decreases and the dead volume due to containers, seals, etc. becomes increasingly significant. A plot of energy density as a function of total volume is given in Fig. 3.28 for the zinc-mercuric oxide and zinc-silver oxide systems. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

Although die zinc-mercuric oxide battery has many excellent qualities, increasing environmental concerns have led to a deemphasis in the use of this system. The main environmental difficulty is in the disposal of the cell. Both the mercuric oxide in the fresh cell and the mercury rcducrion product in the used cell have long-term toxic effects. 

Cadmium/Mercuric Oxide Battery. The substitution of cadmium for the zinc anode (the cadmium/mercuric oxide cell) results in a lower-voltage but very stable system, with a shelf life of up to 10 years as well as performance at high and low temperatures. Because of the lower voltage, the watthour capacity of this battery is about 60% of the zinc/mercuric oxide battery capacity. Again, because of the hazardous characteristics of mercury and cadmium, the use of this battery is limited. 

Volumetric energy density is, at times, a more useful parameter than gravimetric specific energy, particularly for button and small batteries, where the weight is insignificant. The denser batteries, such as the zinc/mercuric oxide battery, improve their relative position when compared on a volumetric basis, as shown in Table 7.4 and Fig. 7.9. The chapters on the individual battery systems include a family of curves giving the hours of service each battery system will deliver at various discharge rates and temperatures. 

The alkaline zinc/mercuric oxide battery is noted for its high capacity per unit volume, constant voltage output, and good storage eharacteristics. The system has been known for over a century, but it was not until World War II that a practical battery was developed by Samuel Ruben in response to a requirement for a battery with a high capacity-to-volume ratio which would withstand storage under tropical conditions. - ... 

Zinc/mercuric oxide batteries have good storage characteristics. In general they will store for over 2 years at 20°C with a capacity loss of 10 to 20% and 1 year at 45°C with about a 20% loss. Storage at lower temperatures, such as down to -20°C, will, as with other battery systems, increase storage life. 

The more familiar types of primary alkaline systems are the zinc/manganese dioxide, zinc/ mercuric oxide, and zinc/silver oxide batteries. These, typically, use potassium or sodium hydroxides, in concentrations from 25 to 40% hy weight, as the electrolyte, which functions primarily as an ionic conductor and is not consumed in the discharge process. In simple form, the overall discharge reaction for these metal oxide cells can be stated as... 

The performance advantages of several types of lithium batteries compared with conventional primary and secondary batteries, are shown in Secs. 6.4 and 7.3. The advantage of the lithium cell is shown graphically in Figs. 7.2 to 7.9, which compare the performance of the various primary cells. Only the zinc/air, zinc/mercuric oxide, and zinc/silver oxide cells, which are noted for their high energy density, approach the capability of the lithium systems at 20°C. The zinc/air cell, however, is very sensitive to atmospheric conditions the others do not compare as favorably on a specific energy basis nor at lower temperatures. 

Because the introduction of lithium power sources to the electronics industry is so recent, many potential users are not aware that lithium batteries are not all alike. Lithium is only the first name of any lithium power source. Just as there are many zinc batteries available (zinc-carbon, zinc-silver cxide, zinc-mercuric oxide) there are many varieties of lithium system, each with its own peculiar internal chemistry and construction. Several of these systems are briefly described below. 

The button cells that provide the energy for watches, electronic calculators, hearing aids, and pacemakers are commonly alkaline systems of the silver oxide-zinc or mercuric oxide-zinc variety. These alkaline systems provide a vei y high energy density, approximately four times greater than that of the alkaline zinc-manganese dioxide battery. 

Some battery-producing companies prefer purchasing pure, nonamalgamated zinc powder to apply their own proprietary corrosion protection system. The general trend is to keep the anodes of all the consumer cells mercury-free (usually indicated by a "green label) and to make them disposable with the regular household trash. The exceptions to this rule are those cells where this makes no sense, such as cells with a mercuric oxide cathode. 

This system, commonly known as the mercury cell , is based on an amalgamated zinc anode, a concentrated aqueous potassium hydroxide electrolyte - saturated with zincate ion by zinc oxide - and a mercuric oxide/graphite cathode ... 

The term primary battery is used to describe any single use battery system. These include, amongst others, alkaline-manganese, zinc-carbon, lithium, mercuric oxide and zinc-air chemistries. Primary batteries are lightweight and convenient, relatively inexpensive and eonsequently are used by households throughout the world to power portable electrical and electronic devices, radios, torches, toys and a whole host of other every day appliances. 

Miniature button-type batteries, using the same zinc/alkaline-manganese dioxide chemistry as cylindrical cells, compete with other miniature battery systems such as mercuric oxide, silver oxide, and zinc/air. Table 10.2 shows the major advantages and disadvantages of miniature alkaline-manganese dioxide batteries in comparison to other miniature batteries. 

Miniature applications have become more important in recent years with the general aeeeptance of the behind-the-ear hearing-aid and the advent of the electronic watch. High energy density per unit volume is the prime requirement for a battery in these products. The mercuric oxide-zinc, silver oxide-zinc, zinc-air and lithium-based systems appear to be likely contenders for this market. Although the last two types of battery have been produced in sizes suitable for miniature applications, they are not widely available in this format. These systems will therefore be discussed later in their usual cylindrical form, and the conclusions drawn then may explain the difficulties that have prevented their wide acceptance. 

The volumetric ampere hour capacity of mercuric oxide-zinc cells is higher than that of lithium-based systems. However, in many cases using two lithium cells in parallel or one larger lithium cell will give the same ampere hour capacity that can be achieved in an equal or even smaller volume than an equivalent two-cell series mercury-zinc battery of similar voltage. This is illustrated in Table 2.7, which gives a... 

P3o-idine-I-oxides are comparatively resistant to reduction because of resonance stabilization by the aromatic system. Typical reagents that have been used for the formation of pyridones and pyridinols are Raney Nickel in methanol, palladium-on-charcoal, phosphorous trichloride, or phosphorus oxychloride in ethyl acetate. The N-oxides of pyridoxine, pyridoxal, and pyridoxamine have been deoxygenated catalytically. 4-Alkoxy-3-halopyri-dine-1-oxides are A-deoxygenated by phosphorous trichloride in chloroform. 2-Amino-3-pyridinol can be prepared ffom2-nitro-3-pyridinol-l-oxide (X1I450) in acetic acid by treatment with iron and mercuric chloride and then with zinc. 2-Halo-3-pyridinols can be prepared from XII-450 by treatment with phosphorous trihalides in chlorofiMm ... 

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