Concentration cell prevention

The use of dispersants is highly recommended in systems containing silt, sand, oil, grease, biological material, and/or other foreign material. Not only does increased dispersion generally increase the effectiveness of chemical inhibition, it also prevents nucleation of oxygen concentration cells beneath foulants. 

To prevent or minimize problems associated with concentration cells, the following methods should be considered in general ... 

A number of 5-nitro-2-furaldehyde derivatives, called nitrofurans, are used in the treatment and/or prophylaxis of microbial infections, primarily in the urinary tract. Recent evidence suggests that the reduction of the 5-nitro group to the nitro anion results in bacterial toxicity. Intermediate metabolites modify various bacterial macromolecules that affect a variety of biochemical processes (e.g., DNA and RNA synthesis, protein synthesis) this observation may explain the lack of resistance development to these drugs. Evidence also indicates that the nitro anion undergoes recycling with the production of superoxide and other toxic oxygen compounds. It is presumed that the nitrofurans are selectively toxic to microbial cells because in humans, the slower reduction by mammalian cells prevents high serum concentrations. 

Cell interactions leading to kidney-tubule determination are tunica-mycin-sensitive. When tunicamycin was applied at concentrations that prevent induction of differentiation, cells did not contain laminin, a glycoprotein that is detected in early stages of differentiation. As tunicamycin did not prevent differentiation when applied later during the morphogenetic period, the authors considered it unlikely that the drug interferes with tubule formation by inhibiting the secretion of laminin.493,494... 

We distinguish between concentration cells without and with transference. In the first type the solutions surrounding both electrodes arc not brought into direct contact, while in concentration cells with transference two solutions arc in direct contact. The name cell with transference originates from the fact that during flow of the current a simultaneous transfer of the electrolyte takes place owing to the different ionic mobility. In the case of cells without transference the direct transfer of the electrolyte from one solution to the other is prevented in this instance the transport of the electromotive active substance proceeds exclusively as a result of reactions taking place at the electrodes. 

Dispersants may also be used in connection with fouling problems other than simple particulate deposition. For instance in corrosion prevention they may be used to prevent solids deposition and subsequent formation of oxygen concentration cells (see Chapter 10). In the contamination of surfaces by microorganisms (see Chapter 12) dispersants (often called biodispersants) are used to prevent or restrict, the approach of micro-organisms towards heat exchanger surfaces. 

The extracellular fluid of animal cells has a salt concentration similar to that of sea water. However, cells must control their intracellular salt concentrations to prevent unfavorable interactions with high concentrations of ions such as calcium and to facilitate specific processes. For instance, most animal cells contain a high concentration of K and a low concentration of Na" " relative to the external medium. These ionic gradients are generated by a specific transport system, an enzyme that is called the Na -K pump or the... 

Several different phospholipases can be distinguished on the basis of the site at which they hydrolyze phosphohpids (Figure 21.2). Phospholipase A2 is widely distributed in nature it is also being actively studied by biochemists interested in its structure and mode of action, which involves hydrolysis of phospholipids at the surface of micelles (Section 2.1). Phospholipase D occurs in spider venom and is responsible for the tissue damage that accompanies spider bites. Snake venoms also contain phospholipases the concentration of phospholipases is particularly high in venoms with comparatively low concentrations of the toxins (usually small peptides) that are characteristic of some kinds of venom. The lipid products of hydrolysis lyse red blood cells, preventing clot formation. Snakebite victims bleed to death in this situation. 

Irritation is induced by active substances that chemically damage cells of the skin. The strength of the effect depends on the concentration of the irritating substance. Irritation is expressed by redness, itching and sometimes erosion of the skin. Decreasing the concentration can prevent irritation. However, the therapeutic effect will be reached later as well. As irritation also depends on the individual sensitivity of the patient s skin the tolerance differs individually. An... 

The loss of HGPRT and APRT activities by freezing and thawing of the cells has been dealt with previously and can be prevented by the use of concentrated cell suspensions and the addition of PRPP (1). The presence of nucleotidase activity in the cell extracts causes the rapid degradation of the nucleotides, produced in the assay, to their corresponding nucleosides. 

If an electrolsrte can enter the crevice formed by the faying surfaces of two almninmn surfaces an oxygen concentration, and subsequently chemical concentration, cell can form and cause accelerated locahzed attack. As such, corrosion protection often is required in joints, even when not needed on the freely exposed almninmn. The severity of crevice corrosion depends on the electrolyte and how readily it is replenished. It also is influenced by the geometric shape of the crevice, and the ratio of active crevice cathode area to the adjacent external cathode area. The best protective measures are to design so that crevices will drain, and to effectively seal crevices to prevent ingress of the electrolyte. 

As the reactions proceed, the dissolved oxygen in the small volume of stagnated solution inside the crevice is consumed. However, this does not prevent the dissolution reaction inside the crevice because the electrons reach outside the crevice through the metal, where plenty of oxygen is available for reduction. A concentration cell (differential aeration) is set up between the crevice area and the area outside the crevice. 

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