Specifications mercury cells

Membrane Cells. Membrane cells are not subject to the electrode poisoning suffered by mercury cells. They are in this respect similar to diaphragm cells, but the membranes themselves are exceptionally sensitive to brine impurities [77], and brine specifications for membrane cells are more onerous. Section 4.8 discussed the structure and performance of membranes and explained the reasons for this sensitivity. Certain impurities can affect cell performance and the service life of the membranes even when present at ppb levels. Their concentrations in the brine must be rigidly controlled. When this is done successfully and ultra-pure brine is consistently available, service life can be quite long, and test cells have operated well for up to 9 years [78]. 

Each type of cell has its own requirements. Diaphragm cells are perhaps the most forgiving of impurities. While mercury cells can tolerate a higher total concentration because of their lesser sensitivity to the major impurities, they are susceptible to damage by small quantities of certain heavy metals. Membrane cells have the most exacting brine specification and are distinguished front the others by their very low hardness specification. 

Properly treated and filtered brine is suitable for use in diaphragm or mercury cells. Before it can be used in membrane cells, its hardness must be reduced to ppb levels. The more rigorous specification is necessary because multivalent cations are able to travel into the membranes along with Na" " and K" ". Section 4.8.6 discusses the mechanisms by which these ions damage the membranes. 

Chloride concentrations in the catholyte are measured in tens of parts per million. These concentrations are not so low as those in the mercury cell and are of concern in certain applications. The same can be said of chlorate concentrations. The concentrations of these impurities in the product are functions of their speed of transport through the membranes relative to the speed of die cation. At lower current densities, these relative rates are higher. Moreover, at shutdown there is still a slow transfer of anions across the membranes. These effects can lead to problems in maintaining tight specifications when production is curtailed. They are particularly troublesome in the production of KOH. 

Let us adopt the Guggenheim description for the interphase, and apply these concepts to a specific experimental cell consisting of a dropping mercury electrode (Hg ) in contact with an aqueous electrolyte (KCl) and calomel reference electrode ... 

There are three basic processes for the electrolytic production of chlorine, the nature of the cathode reaction depending on the specific process. These three processes are (1) the diaphragm cell process (Griesheim cell, 1885), (2) the mercury cell process (Cast-ner-Kellner cell, 1892), and (3) the membrane cell process (1970). 

In some cases, particularly with iaactive metals, electrolytic cells are the primary method of manufacture of the fluoroborate solution. The manufacture of Sn, Pb, Cu, and Ni fluoroborates by electrolytic dissolution (87,88) is patented. A typical cell for continous production consists of a polyethylene-lined tank with tin anodes at the bottom and a mercury pool (ia a porous basket) cathode near the top (88). Pluoroboric acid is added to the cell and electrolysis is begun. As tin fluoroborate is generated, differences ia specific gravity cause the product to layer at the bottom of the cell. When the desired concentration is reached ia this layer, the heavy solution is drawn from the bottom and fresh HBP is added to the top of the cell continuously. The direct reaction of tin with HBP is slow but can be accelerated by passiag air or oxygen through the solution (89). The stannic fluoroborate is reduced by reaction with mossy tin under an iaert atmosphere. In earlier procedures, HBP reacted with hydrated stannous oxide. 

Silver and mercury salts have a long history of use as antibacterial agents.241-243 The use of mercurochrome ((40), Figure 18) as a topical disinfectant is now discouraged. Silver sulfadiazene (38) finds use for treatment of severe burns the polymeric material slowly releases the antibacterial Ag+ ion. Silver nitrate is still used in many countries to prevent ophthalmic disease in newborn children.244 The mechanism of action of Ag and Hg is through slow release of the active metal ion—inhibition of thiol function in bacterial cell walls gives a rationale for the specificity of bacteriocidal action. 

Mercury can influence ion, water, and nonelectrolyte transport in different cells [ 14, 77]. The cell membrane is believed to be the first point of attack by heavy metals however, intracellular enzymes and metabolic processes may also be inhibited [70, 78, 79]. The attachment of heavy metals to ligands in or on the plasma membrane may result in changes in passive permeability or selective blockage of specific transport processes. Many membrane transport systems are known to be sensitive to sulphydryl-group modification [ 14, 80, 81]. 

Certain organic forms of mercury can elicit specific damage in the main cell body of peripheral neurons. Similar responses are associated with certain natural products called vincristine and vinblastine, both of which have been used as antileukemic medicines. The deadly botulinum toxins, mentioned earlier in this chapter, block transmission of nerve impulses at the synapses of motor neurons. This blockage results in muscular paralysis which, if sufficiently severe, can lead to death, usually because respiration is impaired. The once widely used pesticide, DDT, is an organic chemical that also acts on the nervous system at this site, although it can also mount an attack on areas of the CNS. 

There are several such toxic agents that cause considerable medical, public and political concern. Two examples are discussed here the heavy metal ions (e.g. lead, mercury, copper, cadmium) and the fluorophosphonates. Heavy metal ions readily form complexes with organic compounds which are lipid soluble so that they readily enter cells, where the ions bind to amino acid groups in the active site of enzymes. These two types of inhibitors are discussed in Boxes 3.5 and 3.6. There is also concern that some chemicals in the environment, (e.g. those found in industrial effluents, rubbish tips and agricultural sprays), although present at very low levels, can react with enhanced reactivity groups in enzymes. Consequently, only minute amounts concentrations are effective inhibitors and therefore can be toxic. It is suggested that they are responsible for some non-specific or even specific diseases (e.g. breast tumours). 

Concerning AFS, the atomiser can be a flame, plasma, electrothermal device or a special-purpose atomiser e.g. a heated quartz cell). Nowadays, commercially available equipment in AFS is simple and compact, specifically configured for particular applications e.g. determination of mercury, arsenic, selenium, tellurium, antimony and bismuth). Therefore, particular details about the components of the instrumentation used in AFS will not be given in this chapter. 

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