Water, hydroxide ion

Figure 1 Simplified scheme illustrating in space-energy coordinates, the photogeneration, the bulk and surface recombination, the reaction with dioxygen, hydroxide ions, water and electron-donor pollutants, of charge carriers in an n-type semiconductor such as Ti02.

Barium hydroxide + water — barium + 2 hydroxide ions + water... 

Since acids and bases produce hydrogen ions and hydroxide ions, water is an inevitable result of acid-base reactions. 

The SjSii ring system is susceptible to attack by nucleophiles, as is illustrated for 636. Examples of nucleophiles are alcohols (best with primary and secondary), 1563-1538 phenols,hydroxide ion, water, " and carboxylic acids. The boron compound 641 reacts with 636 to give the five-membered heterocycle 642. The electrophilic reagents, acetyl chloride and benzoyl chloride, react with tetraphenylcyclodisiladithianes to give the diacyl sulfide and dichlorophenyl-silane. ... 

Degradation rates of drug substances are generally affected by pH because most degradation pathways are catalyzed by hydronium and/or hydroxide ions. Water itself is also a critical reactant. If the critical path in a reaction involves a proton transfer or abstraction step, other acids and bases present in solution (usually buffer species) can affect the reaction rate. These reactions will also be pH-dependent because the fraction of any species present in its acid or base form will be dependent on its dissociation constant and the solution pH. Also, for ionizable drugs, the fraction of drug present in any particular form will depend on the solution pH. Therefore, if the reactivity of the drug depends on its form, its reactivity will be pH-dependent. 

Water (DNsbCU lS) may be considered as a solvent on the border line between solvents of medium and high donor properties and is therefore widely used as a solvent also in coordination chemistry. Due to the high reactivity of the solvent and the availability of the very strongly coordinating hydroxide ions, water will convert many acceptor compounds to hydroxo-complexes and thus prevent the formation of various other complex species. 

VX in various media are of particular importance. The degradation of such compounds is suggested to be initiated by electron addition to phosphorus via reachon with anionic nucleophiles (Yang, 1999), such as hydroxide ion, water, alcohols, amines, or unsaturated organic compounds. The problem of determinahon of RVX in complex matrices and idenhficahon of RVX transformation products is still more complicated by the scarcity of available reference information. 

One anomaly inmrediately obvious from table A2.4.2 is the much higher mobilities of the proton and hydroxide ions than expected from even the most approximate estimates of their ionic radii. The origin of this behaviour lies in the way hr which these ions can be acconmrodated into the water structure described above. Free protons cannot exist as such in aqueous solution the very small radius of the proton would lead to an enomrous electric field that would polarize any molecule, and in an aqueous solution the proton inmrediately... 

In the strongly basic medium, the reactant is the phenoxide ion high nucleophilic activity at the ortho and para positions is provided through the electromeric shifts indicated. The above scheme indicates theorpara substitution is similar. The intermediate o-hydroxybenzal chloride anion (I) may react either with a hydroxide ion or with water to give the anion of salicyl-aldehyde (II), or with phenoxide ion or with phenol to give the anion of the diphenylacetal of salicylaldehyde (III). Both these anions are stable in basic solution. Upon acidification (III) is hydrolysed to salicylaldehyde and phenol this probably accounts for the recovery of much unreacted phenol from the reaction. 

According to the Arrhenius definitions an acid ionizes m water to pro duce protons (H" ) and a base produces hydroxide ions (HO ) The strength of an acid is given by its equilibrium constant for ionization m aqueous solution... 

Although acetylene and terminal alkynes are far stronger acids than other hydro carbons we must remember that they are nevertheless very weak acids—much weaker than water and alcohols for example Hydroxide ion is too weak a base to convert acety lene to its anion m meaningful amounts The position of the equilibrium described by the following equation lies overwhelmingly to the left... 

IS a two step process m which the first step is rate determining In step 1 the nucleophilic hydroxide ion attacks the carbonyl group forming a bond to carbon An alkoxide ion is the product of step 1 This alkoxide ion abstracts a proton from water m step 2 yielding the gemmal diol The second step like all other proton transfers between oxygen that we have seen is fast... 

The role of the basic catalyst (HO ) is to increase the rate of the nucleophilic addi tion step Hydroxide ion the nucleophile m the base catalyzed reaction is much more reactive than a water molecule the nucleophile m neutral solutions... 

Alkoxide ion Water intermediate Gemmal diol Hydroxide... 

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

Water Alkoxide ion ft om nucleophilic addition Hydroxide ion Aldol... 

In the reaction of phenol with hydroxide ion the stronger acid (phenol) is on the left side of the equation and the weaker acid (water) is on the nght... 

Base According to the Arrhenius definition (Section 1 12) a substance that ionizes in water to produce hydroxide ions According to the Brpnsted-Lowry definition (Section 1 13) a substance that accepts a proton from some suitable donor According to the Lewis definition (Section 1 17) an electron pair donor... 

At the cathode, water molecules are discharged yielding gas and hydroxide ions, OH. Some of the caustic generated in the cathode compartment back-migrates to the anode compartment and reacts with dissolved chlorine (Cl2, ) to form chlorate as follows... 

DeioniZa.tlon, The removal of cations and anions from water and replacement of them with hydrogen and hydroxide ions is called deionization. The completeness of the ionic removal is dependent on resin selection, design of the system, operating conditions, and the quaUty of treated water required. In general, systems become more complex as quaUty requirements increase. 

Demineralizers are often used to treat raw makeup water or condensate where high purity is required, such as in large central station boHers that operate at high steam pressures. Demineralizers employ a combination of cation and anion exchange to remove additional material, including sodium and ammonium cations. VirtuaHy aH salt anions, such as bicarbonate, sulfate, and chloride, are removed and replaced by hydroxide ions in the demineralizer. 

Ion Product. The ion product of water is the product of the molality of the hydrogen and hydroxide ions, oh - temperature... 

Alkalinity. The alkalinity of a water sample is its acid-neutrali2ing capacity. Bicarbonate and carbonate ions are the predominant contributors to alkalinity in most waters, and their chemical equiUbria generally maintain the pH of 5—9. The presence of enough hydroxide ion to affect the alkalinity determination in natural waters is rare. SiUca, borate, or phosphate do contribute to the overall alkalinity if present in large enough quantities. 

Neutralizing Amines. Neutralizing amines are used to neutralize the acid (H" ) generated by the dissolution of carbon dioxide or other acidic process contaminants in the condensate. These amines hydrolyze when added to water and generate the hydroxide ions required for neutralization ... 

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

---