Bases amphoteric substances

Acids, bases and amphoteric substances can be purified by taking advantage of their ionisation constants. Ionisation constants and pK. 

In the two equations above, notice that water is acting as an acid in one instance and as a base in the other. Substances like water that can act as an acid or a base depending on the circumstances are called amphoteric substances. The word comes from the Greek prefix ampho-, which means both. Water is the most common amphoteric substance, but amino acids, proteins, and some metal oxides—such as aluminum oxide (Al203) and zinc oxide (ZnO), for example—can also act as amphoteric substances. 

The solubility of antibiotics, including CTC-HC1, was reported by Andrew and Weiss (17). CTC-HC1 is an amphoteric substance and consequently it is soluble in aqueous acid and base. However, it can rapidly degrade in these solvents. Its solubility in water is about 8 mg/ml and in methanol about 17 mg/ml. In higher molecular weight alcohols, the solubility of CTC-HC1 is considerably less than in methanol. For practical purposes, it is insoluble in many common solvents such as the aliphatic hydrocarbons, benzene, ether, and chloroform. It is readily soluble in pyridine and to the extent of about 5 mg/ml in formamide. Pyridine is an undesirable solvent because of its basicity, and formamide is not desirable because of the difficulty in obtaining and maintaining it as a stable solvent. 

The species Y is also probably non-existent in most of the enzyme catalysed reactions involving only one substrate. In acidic or basic reactions, Y and W do, however, play roles. In acid catalysed reactions, where C is an acid, transfer of proton to S takes place giving Y as a conjugate base of C. W is a basic or amphoteric substance which accepts a proton from X. In base catalysis, Y is a conjugate acid to the base C while W transfers a proton to X and may be the solvent or another acidic substance. With regard to the stability of the intermediate complex X, the two possibilities, which may be considered, are ... 

Water is an amphoteric substance, acting either as an acid or a base. 

Amphoteric substances such as water can act as both an acid and as a base. 

Information on chemical structure, molecular weight, nature of the drug substance (acid, base, amphoteric, or neutral), and dissociation constants (pKa(s))... 

Most biological processes in the cell take place in a water-based environment. Water is an amphoteric substance that is, it may serve as a proton donor (acid) or a proton acceptor (base). Equation 2.1 shows the ionic equilibrium of water. 

You will also see the term amphoteric. An amphoteric substance is a substance that reacts with both acids and bases. The term amphiprotic is more specific and refers to the ability both to donate and accept protons. 

The benzimidazoles, as we have seen, are weak bases that are soluble in aqueous acids but may be precipitated from them by the addition of a stronger base such as ammonium hydroxide. The benzimidazoles are amphoteric substances, with acidic properties also, as shown by the formation of copper, silver, and other insoluble salts they are soluble in an excess of a strong base such as sodium hydroxide and may be precipitated from solution in that base by the addition of carbon dioxide. 

Amphoteric substances are necessarily extremely weak both as acids and as bases, but they possess the property of reacting with strong acids on the one hand and with strong bases on the other hand. 

An amphoteric substance is a substance that acts as an acid in some reactions and a base in others. In the two reactions below, note how water donates a proton (making it an acid) in the first reaction, while in the second reaction it receives a proton (making it a base) ... 

Water is an amphoteric substance (acts as acid or base) depending on the substance that the water reacts with. Water molecules may dissociate as shown below ... 

This brings up an interesting situation. Isn t water a neutral substance How can it react as an acid in one reaction and as a base in another Water is a substance that can gain or lose a proton depending upon the environment it is in. Water is what is called an amphoteric substance because it can act as either an acid or a base. 

Amphoteric substances, or ampholytes, are able to engage in neutralization reactions both with acids and bases (more precisely, both with hydrogen and hydroxyl ions). Aluminium hydroxide, for example, reacts with strong acids, when it dissolves and aluminium ions are formed ... 

A substance is said to be amphoteric if it can behave either as an acid or as a base. Water is the most common amphoteric substance. We see this behavior in the autoionization of water, which involves the transfer of a proton from one water molecule to another to produce a hydroxide ion and a hy-dronium ion ... 

Amphoteric substance a substance that can behave either as an acid or as a base. (7.2)... 

Vacuum-still bottoms from the H-coal liquefaction process were separated into acid, neutral, and basic fractions by precipitation with acids or by extraction with bases. About one-third of the preasphaltene and one-sixth of the asphaltene fraction were precipitated by acids equivalent weights of the bases were in the range 1200-1800 for preasphaltenes and 600-800 for asphaltenes. The acidic components were obtained either by extraction with aqueous sodium hydroxide or by extraction with benzyltrimethylammonium hydroxide in methanol. About one-fifth of the asphaltene and one-fourth of the presasphaltene fractions were obtained as acids, and up to 10% as amphoteric substances. Nitrogen and sulfur were present in all fractions found. Deno axidation (CF3C02H, H202, H 04) gave dicarboxylic acids from malonic to adipic in addition to mono acids. 

From the table, the bases between water and the hydroxide ion are the weak bases. They are the ones that ionize to 10%. Note that in the case of bases, the other boundary limit is demarcated by the hydroxide ion rather than the hydronium ion. This is so, because the hydronium ion is not a base thus, it cannot form as a boundary for the bases. One the other hand, H2O is both an acid and a base. Thus, it consistently forms as a boundary limit in both the acids and the bases. Compounds that act both as an acid and a base are called amphoteric substances. H2O is an amphoteric substance. Above water in the table, the compounds do not exhibit any observable basic behavior. Water, then, is the very limit of basicity. Notice the arrows pointing downward from the weakest to the strongest bases. 

Effects of pH on the retention of ionizable compounds. Figure 3.9 shows the various equilibria contributing the retention of ionizable compounds in reversed phase systems. In general, the retention of a neutral form of an ionizable compound will be greater than the retention of the ionized form. Therefore, maximum retention will occur at pH < (p a-2) for weak acids, at pH > (p. a + 2) for weak bases, and at pH = pi for amphoteric substances. It also follows that retention will be sensitive to changes in pH only when pH = (p. a 2). 

The appropriate expressions for the retention of weak bases and amphoteric substances are given in equations (3.21) and (3.22), respectively. 

Lowry s ideas extended the notion of an acid to that of a substance capable of acting as a proton donator, so that, in his terminology, even pure water is an acid. On the other hand, the notion of a base was extended to that of a substance capable of acting as a proton acceptor, so that, in Lowry s terminology, water is a base as well. Hence, water turns out to be an amphoteric substance of central importance for catalytic processes. 

If you took a good look at the information just presented, you might think that I made a couple of mistakes. Did you notice that I included water (HzO) and ammonia (NH3) as examples of both Brpnsted-Lowry acids and Brpnsted-Lowry bases This is no error. Water and ammonia are examples of amphoteric substances. An amphoteric substance is a substance that acts as an acid in some cases and as a base in other cases. Look at the following sets of reactions to see examples of these substances acting as both acids and bases. 

Explain Water molecules can break apart to form H ions and OH ions. Water is known as an amphoteric substance, which is something that can act as an acid or a base. Explain how this can be so. 

Finally, let us discuss the acid-base properties of the solvent itself. Water is an amphoteric substance and behaves both like an acid and a base. 

Those substances which can act both as an add and a base are called amphoteric substances. 

Water is an example of an amphoteric substance. An amphoteric substance may act as either an acid or a base. There are other amphoteric substances. The hydrogen carbonate ion is also amphoteric. 

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