Sulfite, oxygen scavenger

In a typical experiment for CD-RTP analysis of PAHs an aliquot of the compovmd of interest is added to a flask and the solvent is evaporated gently on a hot plate. An aliquot of 1,2-dibromomethane or 2-bromoethanol (heavy-atom species) is then added, followed by an aliquot of 0.1 mol 1 sodium sulfite (oxygen scavenger), and final dilution with 0.01 mol 1 aqueous CD solution. The solution is shaken vigorously by hand. Some precipitation, usually due to inclusion of excess heavy atom by CD, may cause cloudiness in the solution, which does not affect the reproducibility and quality of phosphorescence spectra. Sample preparation time is less than 5 min in this method. Cline Love and colleagues reported RTP detection limits for several polynuclear aromatic compounds (naphthalene, biphenyl, phenanthrene, etc.) in CD solution in the range 10 -10 moll . ... 

Another group of compounds called oxygen scavengers retard oxidation by reducing the available molecular oxygen. Products in this group are water soluble and include erythorbic acid [89-65-6] C HgO, and its salt sodium erythorbate [6381-77-7] C HgO Na, ascorbyl pahnitate [137-66-6] 22 38 7 ascorbic acid [50-81-7] C HgO, glucose oxidase [9001-37-0] and sulfites (23). 

Oxygen scavengers other than hydrazine are also used, especially catalyzed sodium sulfite, which reacts rapidly with oxygen even at room temperatures to form sodium sulfate. Catalyzed hydrazine formulations are now commercially available that react with oxygen at ambient temperatures at rates comparable to catalyzed sulfite (189). At elevated temperatures, the reaction rates are all similar. Table 14 Hsts the standard hydrazine solution products offered by Olin Corp. for sale to the water-treatment market. Other concentrations are available and other companies offer similar products. 

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

Sodium Sulfite Solution 25-9 in Oxygen Scavenging, Technical Information Bulletin TlR-13, Rhtjane-Poulenc Basic Chemicals Co., Shelton, Conn., 1991. R. L. Miron, Mater. Peform., 45 (June 1981). 

There are several oxygen scavengers commercially available, such as hydrazine and sulfites, with the most cost-effective being sulfites. Sulfites react with oxygen in the following manner ... 

The boiler water must at all times contain a positive residual of oxygen scavenger (usually sodium sulfite). 

Deaerators not only effectively remove dissolved oxygen and other noncondensable gases but also provide the benefit of FW heating. Also, deaerators do not add dissolved solids to the FW, as happens with sulfite-based oxygen scavengers. 

All large boiler plants and many smaller units employ mechanical deaerators to remove oxygen. In addition, various oxygen-scavenging chemicals are employed as consumable treatments such as sodium sulfite-bisulfite, hydrazine, various novel chemistry organics (hydrazine replacements), and certain tannins. 

Where water softening is provided and there is no reduction in system water TDS, treatments are primarily based on inorganic corrosion inhibitor blends (nitrite, molybdate, etc.). Under these circumstances, there is no benefit in using an expensive organic oxygen scavenger to keep the TDS level low, and a common chemical such as catalyzed sodium sulfite may be used. 

Where sulfite or bisulfite oxygen scavengers are employed, a catalyst (such as a cobalt salt or an ethorbate) is vital to speed up the rate of deaeration. 

Oxygen scavengers include inorganics such as sodium sulfite and organics such as hydrazine. 

Also, these programs seldom contain only organic ingredients, as sodium or potassium hydroxide usually is present, and some multifunctional programs contain sulfite as an oxygen scavenger. 

Maintain adequate chelant reserve. Use sulfite or alternative oxygen scavenger, plus amine for program support. 

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

Where larger, semibulk quantities of sulfite-based oxygen scavengers are required, a 20 to 25% bisulfite solution is recommended. It is sufficiently concentrated to provide reasonable freeze protection. The overall oxygen scavenging reaction is ... 

Where dry, catalyzed sodium sulfite is used as the scavenger source, the provision of 2 to 3% metabisulfite into the day-tank batch provides sufficient pH level reduction to ensure the cobalt catalyst does not precipitate. The overall oxygen scavenging reaction is as follows ... 

The hydrazine-oxygen scavenging reaction is pH-dependent (like sulfite), and an increase in pH from 8 to 9 produces a threefold increase in reaction rate and a further three fold increase from pH 9 to 10. 

In its catalyzed form [catalyzed with hydroquinone (HQ), benzo-quinone, or copper], DEHA has a very fast reaction rate, almost as fast as catalyzed sulfite. Hydroquinone is the most popular catalyst for DEHA, and it is likely that the rapid reaction rate is, in part at least, due to the catalyst simply acting as an oxygen scavenger in its own right. 

NOTE Sulfite, when catalyzed, is probably the fastest of all oxygen scavengers. 

Various combination oxygen scavenger products exist that use erythorbate with, for example, tannins and sulfite, to obtain safe products having the benefits of low-cost scavenging and good passivation. 

Amines can be blended into multifunctional product formulations containing alkali, polymer, and phosphate and where sulfite- or hydrazine-based oxygen scavengers are part of the formula, but not where tannins are employed. Amines must be fed separately from tannins. 

Dissolved oxygen (DO) in the FW should be reduced to the lowest practical level before adding an oxygen scavenger. Hydrazine is recommended as an alternative to sodium sulfite only when it is dosed to the FW at a substantial distance from the boiler and where the FW is sufficiently hot to enable the removal of oxygen to be 90% complete before entry to the boiler. 

Oxygen scavenger (note 1) Sodium Sulfite 10 to 20 10 to 20 (note 1)... 

BSI recommended water characteristics for coil boilers note that it is imperative that the oxygen scavenger used in coil boilers remove all the oxygen in the FW, so the use of a proportionally dosed, fully catalyzed chemical scavenger is essential. Sulfite may decompose at pressures higher than 50 bar and produce hydrogen sulfide or sul-fure dioxide in the steam. 

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