Buffering inhibitors

There is another group of inhibitors which act by adsorption onto either the metal or the oxide. These are usually organic materials and the most effective are either alcohols or amines. They are mainly used in specialized applications such as inhibition of acid corrosion during pickling or in mitigation of corrosion in acid oil wells. The exact action of these inhibitors is beyond the scope of this chapter but they are discussed by Hackerman and others (14). Some buffering inhibitors, such as sodium benzoate may also act by adsorption on the surface. 

Alcohols Organic, inorganic, and organometallic pigments Polyol derivatives High molecular weight hydrocarbons None Amides or nitrogenous compounds (slip additives, buffers, inhibitors)... 

Amides or nitrogenous compounds (slip additives, buffers. Inhibitors)... 

Complexing agents, which act as buffers to help control the pH and maintain control over the free metal—salt ions available to the solution and hence the ion concentration, include citric acid, sodium citrate, and sodium acetate potassium tartrate ammonium chloride. Stabilizers, which act as catalytic inhibitors that retard the spontaneous decomposition of the bath, include fluoride compounds thiourea, sodium cyanide, and urea. Stabilizers are typically not present in amounts exceeding 10 ppm. The pH of the bath is adjusted. 

It is a consequence of the action of different pH values in the aeration cell that these cells do not arise in well-buffered media [4] and in fast-flowing waters [5-7]. The enforced uniform corrosion leads to the formation of homogeneous surface films in solutions containing Oj [7-9]. This process is encouraged by film-forming inhibitors (HCOj, phosphate, silicate, Ca and AP ) and disrupted by peptizing anions (CP, SO ") [10]. In pure salt water, no protective films are formed. In this case the corrosion rate is determined by oxygen diffusion [6,7,10]... 

FIGURE l.l Hydrophobic interaction and reversed-phase chromatography (HIC-RPC). Two-dimensional separation of proteins and alkylbenzenes in consecutive HIC and RPC modes. Column 100 X 8 mm i.d. HIC mobile phase, gradient decreasing from 1.7 to 0 mol/liter ammonium sulfate in 0.02 mol/liter phosphate buffer solution (pH 7) in 15 min. RPC mobile phase, 0.02 mol/liter phosphate buffer solution (pH 7) acetonitrile (65 35 vol/vol) flow rate, I ml/min UV detection 254 nm. Peaks (I) cytochrome c, (2) ribonuclease A, (3) conalbumin, (4) lysozyme, (5) soybean trypsin inhibitor, (6) benzene, (7) toluene, (8) ethylbenzene, (9) propylbenzene, (10) butylbenzene, and (II) amylbenzene. [Reprinted from J. M. J. Frechet (1996). Pore-size specific modification as an approach to a separation media for single-column, two-dimensional HPLC, Am. Lab. 28, 18, p. 31. Copyright 1996 by International Scientific Communications, Inc.. Shelton, CT.]... 

Commercial processes Commercial electroless nickel plating stems from an accidental discovery by Brenner and Riddell made in 1944 during the electroplating of a tube, with sodium hypophosphite added to the solution to reduce anodic oxidation of other bath constituents. This led to a process available under licence from the National Bureau of Standards in the USA. Their solutions contain a nickel salt, sodium hypophosphite, a buffer and sometimes accelerators, inhibitors to limit random deposition and brighteners. The solutions are used as acid baths (pH 4-6) or, less commonly, as alkaline baths (pH 8-10). Some compositions and operating conditions are given in Table 13.17 . 

Sometimes an aqueous extract of a luminous organism is nonlu-minous although it contains all the components necessary for light emission. If that is due to the presence of various inhibitors that are extracted together with the luminescent substances, the extract will become luminous by merely diluting the extract with water or a buffer solution. A means of reversible inhibition will be needed to purify such a luminous extract. 

For in vitro studies there are a number of compounds available to block protein phosphatase activity. Phosphate buffers inactivate all of these enzymes. Several naturally occurring toxins are potent inhibitors of PPPs, e.g., okadaic acid or microcystin, and are frequently used tools. PPM and PTP family members are not affected by these toxins. Vanadate containing solutions are competitive inhibitors of PTPs, pervanadate is an irreversible inhibitor of PTPs. 

Also, in severe cases in which lean or acid MU water is employed, or where glycol degradation occurs, the loss of inhibitor and associated pH buffer permits the BW pH to drop below 7.0 and acid corrosion occurs. 

Benzoate can be used as an anodic inhibitor and is effective at preventing waterline attack. Phosphate is also sometimes included. Sodium borate (borax) is usually added to the formulation to provide an alkalinity reserve buffer. Typically, a copper inhibitor such as benzo-triazole (BTA) or tolyltriazole (TTA) also forms part of a formulation and is used to provide yellow metal (copper and brasses) protection. 

Where glycols are added to both hot and cold water systems, it is common for the glycol to be supplied neat, preinhibited and buffered (although the glycol also may be supplied in a diluted and ready-to-use form). This formulation is an example of a type produced as an glycol inhibitor-pack concentrate ... 

A further longer term wet lay-up alternative is through the use of volatile corrosion inhibitors (VCIs) such as dicyclohexylamine acetate. These are dissolved in the water at a temperature below 60 °C, and the water is circulated for 4 to 5 hours. The boiler does not need to be completely filled because the VCI migrates to all parts of the boiler and reaches equilibrium in each of the void spaces. With traditional lay-up chemicals, the oxygen scavenger may become depleted easily (which is why the reserve usually is so high) and corrosion protection is quickly lost however, with VCI programs, there is always a volatile buffer available that maintains equilibrium and hence corrosion protection. 

The protected E-ring moiety of (S)-camptothecin has been prepared in enantio-merically enriched form by the enzymatic resolution of a triester with PLE in a pH 7 phosphate buffer-acetonitrile (5 1) solution (Figure 6.7). The alkaloid camptothecin is an inhibitor of the enzyme topoisomerase and some of its derivatives are anticancer drugs [52]. 

Inhibitor assay A suitable amount of inhibitor was preincubated with 0.2 ml of polygalacturonase and buffer in a total volume of 1 ml for 10 minutes at 37°C. Control without inhibitor was run simultaneously. The enzyme reaction was initiated by the addition of 1 ml of substrate solution (1% polygalacturonic acid). The decrease in PG activity was a measure of the inhibitory activity. Proper controls containing only Dieffenbachia extract and no fungal PG in the assay mixture were also run to account for the inherent PG activity, if any, of Dieffenbachia extract. One unit of inhibitor activity is defined as the amount of inhibitor that reduces the polygalacturonase activity under the assay conditions by one unit. Specific activity of the inhibitor is expressed as units per mg protein. 

Effect of preincubation of PG inhibitor with PG A suitable aliquot of the inhibitor was incubated with 0.2 ml of polygalacturonase and buffer in a total volume of 1 ml for various periods of time at 37°C. The reaction was initiated by addition of 1 ml of 1% polygalacturonic acid solution. It is apparent from Table 6 that the magnitude of inhibitory... 

However, there are two problems with these unconjugated lactones lack of selectivity and limited stability of the inhibitor in biological buffers. Coumarin carboxylates have been developed to improve selectivity toward a given serine protease (Section 11.4.1). On the other hand, the amide bond is chemically and enzymatically much more stable than the ester one. This raised the question of whether a starting functionalized lactam behaved like the previous lactones and generated in situ a quinonimine methide, the aza-analogue of the quinone methide (Section 11.5). 

The pancreatic juice is released through the ampulla of Vater into the duodenum to aid in the digestive process as well as buffer acidic fluid released from the stomach (Fig. 20-1). The pancreas contains a trypsin inhibitor to prevent autolysis. 

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