Water amphotericity

Amphoteric water-soluble polymers, 20 475 Ampicillin, registered for use in aquaculture in Japan, 3 221t AmpliChip CYP450 Test, 16 392 Amplifiers, optical, 11 145-146 Amrinone, 5 186... 

Aqueous ammonia is a weak base, so relatively few hydroxide ions form. The position of equilibrium lies to the left. In the forward reaction, the water molecule gives up a proton and acts as an acid. A substance that can act as a proton donor (an acid) in one reaction and a proton acceptor (a base) in another reaction is said to be amphoteric. (Water acts as an acid in tbe presence of a stronger base, and as a base in tbe presence of a stronger acid. 

If a substance can behave both as a Bronsted-Lowry acid and as a Bronsted-Lowry base then it is called amphoteric. Water has this ability. As well as reacting with acids (above) it can also react with Bronsted-Lowry bases such as ammonia in the following way to form the base OH ... 

Water can act as either an acid or a base, depending on the circumstances. This ability to act as either an acid or a base is referred to by stating that water is amphoteric. Water serves as a base in (17-3) and as an acid in (17-4). Note that the bare H+ (a proton) becomes the hydronium ion, H30+, which is a hydrated proton (H30+ is H+ + H2O) because the bare proton does not really exist in solution. When we write the equilibrium constant expression for an aqueous equilibrium, we can use either the hydrogen ion, H+, or the hydrated form, H30+. Although the proton is hydrated in aqueous solution (as is the hydroxide), the use of H+ and H30+ is up to the style of the person working the problem and the problem itself. More often than not, leaving out water on both sides of the equation is used to keep the solutions to the problems visually simple. So long as water is in its standard state (liquid), it is not included in the K expression and, therefore, not necessary in the chemical equation. 

Some substances act as either an acid or a base, depending upon their environment. They are called amphoteric. Water is a good example. In the reaction above, water acts as a base accepting a proton. Water can also act like an acid by donating a proton. 

An amphoteric, water-soluble polymer support functionalized with a bidentate phosphine has been reported (Scheme 46). Starting from poly(maleic anhydride)-f-poly(methylvinylether), 91 was prepared by reaction with a bis(diphenylphosphino)ethylamine. Polymers with different phosphine loadings were made and soluble Rh complexes formed by reaction with [Rh(COD)] - - OTf. These supported metal complexes were then screened for activity in hydrogenation reactions. Recovery of the catalyst was effected by changing the pH of the reaction mixture to <7.5. 

Acid-Base Disproportionation of Water Finally, we will take a look at the acid-base disproportionation of water. It has already been demonstrated that amphoteric water can function as an acid as well as a base. For this reason, proton transfer between the water molecules can take place where oxonium and hydroxide ions are formed even when there are no other acids or bases present ... 

Based on the water dissociation constant, K (see Make up your mind Amphoteric water, earlier in this chapter), in pure water the [H3O ] equals 1.0 X10". Using this mathematical relationship, you cem calculate the pH of pure water ... 

Amphoteric species may also act as buffers by reacting with an acid or a base. (For an example of an amphoteric species, see "Make up your mind Amphoteric water, earlier in this chapter) The bicarbonate ion (HCO3 ) and the monohydrogen phosphate ion (HP04 ) are amphoteric species... 

PolyzwittGrionS. Amphoteric water-soluble pol5uners are polymeric systems containing both anionic and cationic charges. Such polyzwitterions may be subdivided into two major families, the polyampholytes and the polybetaines (239). Each of these groups may be further subdivided into specific types of each (see below). Figure 44 shows a number of monomers which have been used in pairs (cationic with anionic) to form polyampholytes. 

Scandium is not an uncommon element, but is difficult to extract. The only oxidation state in its compounds is -I- 3, where it has formally lost the 3d 4s electrons, and it shows virtually no transition characteristics. In fact, its chemistry is very similar to that of aluminium (for example hydrous oxide SC2O3, amphoteric forms a complex [ScFg] chloride SCCI3 hydrolysed by water). 

Titanium tetrachloride is hydrolysed by water, to give a mixture of anions, for example [Ti(OH)Cl5]" and [TiClg] , together with some hydrated titanium dioxide (Ti02.4H2O is one possible hydrate, being equivalent to [Ti(0H)4(H20)2]). This suggests that titanium dioxide is amphoteric (see below). 

Arylarsonic acids have usually a very low solubility in cold water. They are however amphoteric, since with, for example, sodium hydroxide they form sodium salts as above and with acids such as hydrochloric acid they form salts of the type [CaHjAsCOHljlCl. Both types of salt are usually soluble in water, and to isolate the free acid the aqueous solution has to be brought to the correct pH for most arsonic acids this can be achieved by niaking the solution only just acid to Congo Red, when the free acid will usually rapidly separate. 

Table 1 Hsts many of acetamide s important physical properties. Acetamide, CH2CONH2, dissolves easily ia water, exhibiting amphoteric behavior. It is slow to hydroly2e unless an acid or base is present. The autodissociation constant is about 3.2 x 10 at 94°C. It combines with acids, eg, HBr, HCl, HNO, to form soHd complexes. The chemistry of metal salts ia acetamide melts has been researched with a view to developing electroplating methods. The hterature of acetamide melts and complexes, their electrochemistry and spectroscopy, has been critically reviewed (9).

Lead Monoxide. Lead monoxide (litharge), PbO, occurs as a reddish alpha form, which is stable up to 489°C where it transforms to a yellow beta form (massicot). The latter is stable at high temperatures. The solubihty of a-PbO ia water is 0.0504 g/L at 25°C the solubihty of the p-PbO is 0.1065 g/L at 25°C (40). Lead monoxide is amphoteric and dissolves ia both acids and alkahes. In alkahes, it forms the plumbite ion PbO - The monoxide is produced commercially by the reaction of molten lead with air or oxygen ia a furnace. Black or gray oxide is manufactured by the Barton process, by the oxidation of atomized molten lead ia air, as well as by the ball mill process, ia which metallic lead balls of high purity are tumbled ia the mill to form partially oxidized lead particles. 

The hide proteins differ in amino acid composition and physical stmcture. The principal amino acids (qv) of the hide proteins are hsted in Table 1. Of particular importance is the difference in the water solubiUty of the proteins. AH of the proteins are soluble in water when heated, and upon the addition of either strong acids or bases. Proteins (qv) are amphoteric, possessing both acid and base binding capacity. 

Manganese Hydroxide. Manganese hydroxide [18933-05-6] is a weaMy amphoteric base having low solubihty in water. Mn(OH)2 crystals are reported to be almost pure white and darken on exposure to air. Manganese dihydroxide occurs in nature as the mineral pyrochroite and can also be prepared synthetically by reaction of manganese chloride and potassium hydroxide that is scmpulously free of oxygen. The entire reaction is conducted under reducing conditions (36). 

Nloha.tes, Niobic acid is amphoteric and can act as an acid radical in several series of compounds, which are referred to as niobates. Niobic acid is soluble in solutions of the hydroxides of alkaH metals to form niobates. Fusion of the anhydrous pentoxide with alkaH metal hydroxides or carbonates also yields niobates. Most niobates are insoluble in water with the exception of those alkaH metal niobates having a base-to-acid ratio greater than one. The most weU-known water-soluble niobates are the 4 3 ad the 7 6 salts (base acid), having empirical formulas MgNb O c, (aq) and M24Nb2202y (aq), respectively. The hexaniobate is hydrolyzed in aqueous solution according to the pH-dependent reversible equiHbria (130), when the pH is ca 9. 

Like the natural gums, starches need to be cooked in water to form dispersions for addition to the papermaking system. Various techniques have been developed for cooking starches rapidly (see Starch). In general, anionic starches are used with alum, which aids in starch retention. The cationic and usually the amphoteric starches are self-retaining. 

Anionic surfactants are the most commonly used class of surfactant. Anionic surfactants include sulfates such as sodium alkylsulfate and the homologous ethoxylated versions and sulfonates, eg, sodium alkylglycerol ether sulfonate and sodium cocoyl isethionate. Nonionic surfactants are commonly used at low levels ( 1 2%) to reduce soap scum formation of the product, especially in hard water. These nonionic surfactants are usually ethoxylated fatty materials, such as H0CH2CH20(CH2CH20) R. These are commonly based on triglycerides or fatty alcohols. Amphoteric surfactants, such as cocamidopropyl betaine and cocoamphoacetate, are more recent surfactants in the bar soap area and are typically used at low levels (<2%) as secondary surfactants. These materials can have a dramatic impact on both the lathering and mildness of products (26). 

Zinc forms salts with acids but since it is amphoteric, it also forms zincates, eg, [Zn(OH)2 H20] and Z.n([7) ). The tendency of zinc to form stable hydroxy complexes is also important because some basic zinc salts are only slightly soluble in water. Examples are 3Zn(OH)2 ZnSO [12027-98-4] and 4Zn(OH)2 ZnCl2 [11073-22-6] which may precipitate upon neutralization of acidic solutions of the salts. 

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