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Ion-Association a Review' Reagents ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 479 Reviews Ion-Association Reagents A Review' Kyoji TBEI Faculty of Liberal Arts and Science, Okayama University of Science, Ridaicho, Okayama 700 Ion-association reagents are widely used for analytical chemistry. The ion-associable cation or anion should be univalent, bulky and charge-dispersed. The reagents are classified by their shape: plane, sphere, chain and polymer types. The ion-associable cation and anion can combine with each other to form the ion-associate in water. This phenomenon has been used for gravimetric and volumetric analyses and for extraction-spectrophoto- metry of a trace component. Several examples of specific and sensitive extraction-spectrophotometries are illustrated. The polymer type reagent can react with the counter-charged poly-electrolyte to form a precipitate quantitatively, and it can be used for colloid titration. Metachromasy phenomenon is based on the formation of the ion-associate of colored ion-associable cation or anion with counter-charged colorless ion-association reagents to show distinguished color changes. Keywords Ion-association reagents, charged quinone theory, extraction-spectrophotometry, colloid titration, metachromasy with ion-associable cations and the cationic one reacts 1 Introduction with ion-associable anions to form the respective ion- associates in an aqueous solution. When the Many kinds of organic reagents have been used for concentration of the ion-associate is relatively high in analytical chemistry. Most of the reagents are water, it is precipitated. This fact is used for chelating reagents. On the other hand, there is an ion- gravimetric or volumetric analysis. When it is diluted, association reagent which does not belong to the it is extracted into an organic solvent which is chelating reagent. The chelating reagent is composed immiscible with water and the extract is used for of an organic compound and can react only with a spectrophotometry to determine a trace component in metal ion (cation) to form its chelate compound, the ion-associate. whereas the ion-association reagent is either organic or The ion-association occurs in water, which is a inorganic compound, so it can react with either cation highly polar and hydrogen-bonded solvent. The ion- or anion. The anionic ion-association reagent reacts associable ion is slightly hydrated, because the ion can 480 ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 attach water molecules by its charge but not by its associability and lithium tetraphenylborate is soluble in bulkiness. When the cation is combined with the anion water. to form the ion-associate, the charges are neutralized and at the same time the attached water molecules are 2.3 The charge of an ion-associable ion should be released. Therefore, the ion-associate is precipitated or distributed over the whole ion. extracted into an organic solvent immiscible with Picrate anion is one of the ion-association reagents water. for potassium ion. Although phenolate anion is bulky An ion-associate is usually composed of one cation and one and univalent, it does not show ion-associability. anion. When either is used as an analytical reagent, the Picrate anion, which has three nitro groups in reagent can be named as ion-pair reagent. The name of ion- phenolate ion, shows some ion-associability. The pair reagent is widely used, especially in HPLC. However, strongly electro-negative nitro groups distribute the the composition ratio of the ion-associate is sometimes 1:2 univalent charge over the whole ion; with bulky nitro or 1:3. Then the reagent is not suitable for the name of ion- groups, the ion becomes more ion-associable. pair reagent; therefore, the present author has used the term ion-assocition reagent. Ion-association reagent is a synonym In conclusion, to express the ion-associability, the of ion-pair reagent in many cases. "Ion-association reagents" ion-associable ion should be univalent and bulky and appeared first in English in "Photometric Determination of the charge should be distributed over the whole ion. Traces of Metals: General Aspect" by E. B. Sandell and Hiroshi Onishi, John Wiley & Sons (1978). 3 Classification of Ion-Association Reagents 2 Characteristics of Ion-Association Reagents Some representative ion-association reagents which are actually used in analytical chemistry are shown in The characteristics of ion-association reagents are as Table 1 and Fig. 1. They are classified by their shape. follows: 3.1 Plane type reagents 2.1 An ion-associable ion should be univalent. The plane type reagents are all colored. Methylene For example, perchlorate ion (C104) is one of the Blue (1), Rhodamine B (2), Malachite Green (3) and ion-association reagents for potassium ion. It is Crystal Violet (4) are all univalent cations and have univalent and reacts with univalent potassium cation quinone structures. The positive charge is not fixed on (K+) to form a precipitate (KC104), because it has ion- the nitrogen atom as shown in Fig. 1. For example, associability. Sulfate ion (SO42~) or phosphate ion Methylene Blue (1) in Fig. 1 has the positive charge at (P043-) is almost the same size as perchlorate ion, but they cannot form a precipitate with potassium ion. Univalent cations: K+, Rb+ and Cs+ have ion- associability, while divalent cations: Cat, Cu2+or Zn2+ Table 1 Classification of Ion-Association Reagents and tervalent A13+or Fe3+do not. On the other hand, a bulky divalent cation, [Fe(phen)3]2+ (9) in Table 1 has enough ion-as- sociability to form the ion-associate with two perchlo- rate ions which can be extracted into nitrobenzene, but the ion-associability is not so high compared with univalent ion-association reagents. 2.2 An ion-associable ion should be bulky. To have ion-associability, it is necessary but not sufficient for the ion to be univalent. For example, small fluoride ion (F-) has no ion-associability, but chloride ion (Cl-) has some, because chloride ion is larger than fluoride. As perchlorate ion is larger than chloride ion, the ion-associability of perchlorate is stronger than that of chloride and is used as one of ion-association reagents for potassium ion. Tetra- phenylborate anion (10) in Table 1 is much larger and nowadays it is widely used as the quantitative precipitant for potassium ion. The radii of potassium (K+), rubidium (Rb+) and caesium (Cs) ions increase in this order and the solubilities of their tetraphenyl- borates in water decrease in the same order. However, lithium ion (Li+) is so small that it has no ion- ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 481 Fig. 1 Structural formulas of ion-association reagents. The number in parenthesis corresponds to the number in Table 1. The counter ions of (8) and (9) are mostly sulfate anion and that of (11) is mainly sodium cation. 482 ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 the right side nitrogen, but the charge can exist on the to form precipitate. Polydiallyldimethylammonium left side and the sulfur atom can also show the positive chloride (17) and polyvinylsulfate (18) are constantly charge. So the positive charge of Methylene Blue (1) is positive and negative, respectively, between pH 0 and distributed over the whole ion. Similarly, the charge of 14. They are used as titrants for the colloid titration Rhodamine B (2), Malachite Green (3) or Crystal which will be discussed later. Violet (4) is also distributed over the entire ion. Tetrabromophenolphthalein ethyl ester (5) and hexani- trodiphenylamine (6) have strongly electro-negative 4 Application of Classification Table bromo and nitro groups, respectively, and the negative charge is distributed over the entire ions. These cations 4•l New scheme of ion-association reagents and anions have the characteristic properties of ion- Ion-association reagents can be classified into three association reagents. classes by their shape. It may be possible to propose The reagents having charged quinone structure show new reagents which have two different shapes in one a molar absorptivity as high as 105 dm3 mol-' cm'. molecule. They include plane-sphere, sphere-chain and Thus, highly sensitive extraction-spectrophotometry chain-plane. For example, when dimethylamino group can be carried out by using them. in Methylene Blue (1) is replaced by di-butylamino group, it is a new chain-plane type compound. This 3.2 Sphere type reagents combines with anionic surfactants (14) and (15) firmly Among the sphere type reagents, tetraphenylarson- to form the ion-associates extracted into toluene ium (7) and tetraphenylborate (10) are colorless, while quantitatively.2 The method is far better than Cu(I) neocuproine chelate cation (8) and Fe(II) 4- Methylene Blue-chloroform extraction described in chloro-2-nitrosophenol chelate anion (11) are colored. JIS (Japanese Industrial Standard) K 0102-1981, If the colored chelate ion and an oppositely charged because the compound has a molar absorptivity as high ion-association reagent have the same Amax(wavelength as that of Methylene Blue and also has higher ion- at maximum absorption), the sensitivity of the associability by the long chain. extraction-spectrophotometry will be greatly increased. New ion-association reagents such as plane-sphere or Therefore, it is desirable that such a new colored sphere-chain will appear in future. chelate cation or anion appears. 4.2 Various kinds of ion-associates 3.3 Chain type reagents As listed in Table 1, ion-association reagents are The chain type reagents all have a long alkyl group. classified into three kinds of cations and anions by Zephiramine (12) and dodecylbenzenesulfonate (15) are their shapes. By the combination between these cations bulky and univalent, but their charges are not and anions, various kinds of ion-associates are distributed over the entire ions. However, above a possible. For example, Zephiramine (12) belongs to certain concentration in water they can form micella the chain type.
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