Water reduction additives

Catalytic Reduction. Catalytic reduction usually takes place in solution, emulsion, or suspension in autoclaves or pressurized vessels after the catalyst is added, the vessel is pressurized with hydrogen (32,33). Water and methanol are the preferred solvents. In water the addition of alkaU hydroxide (34), alkah carbonate (35), or acid (36) has been recommended. 

To improve magnesium reduction, which also improves siHca reduction in cold process softening, sodium aluminate may be used. The sodium aluminate provides hydroxyl ion (OH ) needed for improved magnesium reduction, without increasing calcium hardness in the treated water. In addition, the hydrolysis of sodium aluminate results in the formation of aluminum hydroxide, which aids in floe formation, sludge blanket conditioning, and siHca reduction. 

For practical applications it is important to minimize the production of the intermediate peroxide, and to ensure that the reaction goes all the way to water. Sometimes this can be ensured by the addition of a suitable catalyst. A case in point is oxygen reduction on gold from alkaline solutions. At low and intermediate overpotentials the reaction produces only peroxide in a two-electron process at high overpotentials the peroxide is reduced further to water. The addition of a small amount of Tl+ ions to the solution catalyzes the reaction at low overpotentials, and makes it proceed to water. Thallium forms a upd layer at these potentials it seems that a surface only partially covered with T1 is a good catalyst, but the details are not understood [3]. 

As noted before, many conventional methods rely on hydrolysis/precipi-tation chemistry to deposit Pt and other metals onto carbon, followed by chemical or gas-phase reduction. One common method is the use of metal sulfito chemistry. This method involves the preparation of metal sulfito complexes (e.g., Na3[Pt(S03)2(0H)2]) in water, the addition of carbon, and precipitation of metal by oxidation to deposit metal oxide particles. The route has advantages in that alkali metals and halides are excluded from the preparation. The method has been extended to bimetallics such as PtRu. 

Waste water Addition of isotopically-labeled standard, extraction with methylene chloride at pH 12-13, then at pH <2, removal of water, volume reduction, addition of internal standard. GC/IDMS (EPA Method 1625) 50 g/L 106 (25% RSD) at 100 g/L EPA 1984a ... 

Principally the same, but chemically simpler, sequence was used to prepare arylnitro anion-radicals from arylamines, in high yields. For instance, aqueous sodium nitrite solution was added to a mixture of ascorbic acid and sodium 3,5-dibromo-4-aminobenzenesulfonate in water. After addition of aqueous sodium hydroxide solution, the cation-radical of sodium 3,5-dibromo-4-nitro-benzenesulfonate was formed in the solution. The latter was completely characterized by its ESR spectrum. Double functions of the nitrite and ascorbic acid in the reaction should be underlined. Nitrite takes part in diazotization of the starting amine and trapping of the phenyl a-radical formed after one-electron reduction of the intermediary diazo compound. Ascorbic acid produces acidity to the reaction solution (needed for diazotization) and plays the role of a reductant when the medium becomes alkaline. The method described was proposed for ESR analytical determination of nitrite ions in water solutions (Lagercrantz 1998). 

Table 1.4 Typical water reductions of cement paste at normal addition levels of admixture to maintain paste viscosity...

The amount of water reduction possible is also a function of the way in which an admixture is added to the concrete if a period between mixing with water is allowed prior to the addition of the admixture, greater adsorption of the admixture on to the initial hydrates is obtained and a higher workability or alternatively a greater reduction in water-cement ratio is obtained, as can be seen from Table 1.14 [73]. 

Table 1.13 Effect of addition level of water-reducing admixtures on the water reduction ...

Table 1.14 Effect of varying the point of addition on workability and/or water reduction (after Dodson) ...

Method of addition of retarder (0.225% calcium lignosulfonate by wt cement) Water-cement ratio Slump Water reduction (mm) (%)... 

In addition to the two basic groups of antifreeze admixtures, others containing superplastizisers such as sulfonated naphthalene formaldehyde (SNF) and sulfonated melamine formaldehyde (SMF) have been developed. The main advantage afforded by such admixtures is that they produce significant water-reduction. 

The relationship of the concentration of the admixture to the water reduction produced has been found to be linear (Fig. 7.6). Two conclusions were reached from the study which included several brands of cement [15] (1) it is the addition rate of the admixture which is important, and (2) the chemical nature of the admixture does not play a part in water reduction, but is an important factor in determining the retention (or loss) of slump in concrete with age. For example, the extended workability of the more... 

Fig. 7.6 Influence of the addition rate of dispersant on the degree of water reduction (Dodson [15]).

The effectiveness of each admixture may vary, depending on its concentration in the concrete, the time of addition in the mixing cycle and various constituents of the concrete. Although each class of admixture is defined by its main effect (i.e. water reduction, set acceleration), it may have one or more secondary effects (retardation of set, increased bleeding, air entrainment) and its use may result in side effects. Side effects are those modifications of properties produced in the concrete that, even though unsought, are both inevitable and independent of an admixture s main function. Prior to selecting an admixture for an intended application, these... 

According to Lunsford, most of the observations on methane oxidation over oxide catalysts may be interpreted in terms of methyl radical chemistry.41 Most experimental data support the role of surface O- ions in the formation of methyl radicals. The latter are transformed by reductive addition to methoxide ions, which decompose to formaldehyde or react with water to form methanol. Methyl radicals may desorb to the gas phase and participate in free-radical reactions to yield non-selective oxidation products. 

Water-Soluble Sulfur Dyes (C.I. Solubilised Sulphur). Water-soluble sulfur dyes are available in the form of Bunte salts (.S -arylthiosulfuric acid salts) of sulfur dyes and can be dissolved in hot water. The addition of alkali and reducing agent gives them an affinity for the fiber. If NaSH or another reducing agent is used, alkali must be added to trap H+ ions resulting from reduction. 

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