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-spectrophotometry 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-associability to form the ion-associate with two perchlorate 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. This reacts with tetrabromophenol- which gather alkyl groups together to constitute a phthalein ethyl ester (5), plane type, to form the ion- sphere. The spherical surface is crowded around by associate extracted into organic solvent quantitatively, cationic or anionic charges. and it reacts with tetraphenylborate (10), sphere type, The cations are made of ammonium groups. Water to form the ion-associate which is precipitated molecules can not approach the cationic nitrogen, quantitatively. This is also combined with dodecylben- because the nitrogen atom is surrounded by bulky alkyl zenesulfonate (15), chain type, to form an ion-associate groups. Therefore, the ion-associability is large. precipitate. Thus, according to Table 1, nine kinds of On the other hand, the ion-associability of dodecyl- ion-associates are possible. Which kind of ion- sulfate (14) and dodecylbenzenesulfonate (15) is associate is most precipitable or most extractable into relatively weak, because their functional groups are organic solvent? The answer will be obtained by future jutting out into water and are slightly hydrated. studies. Generally speaking, for a small ion-associable However, these groups have a single negative charge ion, e.g. K+ or Cl-, to form a stable ion-associate, the between pH 0 and 14; the bulkiness is relatively large largest counter-charged ion-associable ion should be and the single negative charge is distributed to three chosen. oxygen atoms equally. Therefore, an organic compound having sulfate or sulfonate group has some ion- 4.3 Classification of published results associability, as shown later. Many studies on ion-association have been published hitherto. All of them can be classified by Table 1. For 3.4 Polymer type reagents example, metallohalogeno complex anions, AuC14-, The polymer type reagents do not correspond to ion- SbCl6-, GaCl4-, BF4- etc. or metal chelate anions can association reagents, but the property resembles closely react with a colored ion-association cation to form the that of ion-association reagents. For instance, ion-associates which are extractable into organic polyvinylsulfate (18) can react quantitatively with solvent. This method is called a ternary complex Zephiramine (12) or hexadecylpyridinium chloride (13) extraction method. ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 483

On the contrary, there are only a few examples of chelate has a charged quinone structure and the ion-association reactions by colored ion-associable fluorescence strength is 20 times larger than that of anions for metal chelate cations. While so many kinds o,o'-dihydroxyazobenzene (DH AB) aluminum chelate. of colored ion-associable cations are known and the This indicates that the charged quinone theory can be ion-associability is fairly strong, colored ion-associable extended to fluorescence intensity. anions are few and their ion-associability is relatively Nitrosophenol and nitrosonaphthol derivatives are weak. well known to be specific reagents for cobalt. 2- Nitroso-5-dimethylaminophenol can react with cobalt 5 Charged Quinone Theory" to form the 3:1 chelate. The molar absorptivity is as large as 6X104 dm3 mol-' cm' at 456 nm in chloroform, Methylene Blue (1), Rhodamine B (2), Malachite because the charged quinone structure is formed by the Green (3) and Crystal Violet (4) in Fig. 1 are all electron-donating dimethylamino group at para posi- univalent, bulky cations. These structures have tion of the nitroso group.5 quinone form and the nitrogen atoms of the quinones Generally speaking, studies on spectrophotometric or have positive charge. So they are charged quinone fluorometric chelating reagents have focussed on the compounds. The molar absorptivities of these chelate ring itself and the substituent at para position compounds are as large as I05 dm3 mol-' cm'. has not been given much attention, but it plays an Tetrabromophenolphthalein ethyl ester (5) has also the important part. quinone structure on one side and a negative charge of Recently, on the basis of this concept, a new phenol on the other side. If we consider the resonance compound, 1-(2-hydroxy-4-diethylamino-l-phenylazo)- form, the negative charge can be divided equally to the 2-naphthol-3,6-disulfonate appeared. The reagent two oxygen atoms, both benzene nuclei will have half- reacts with magnesium and calcium to form its quinone structure, and the molar absorptivity will chelates; these have the highest molar absorptivity become large. among magnesium and calcium chelates of o, o'- Accordingly, when a compound has quinone struc- dihydroxyazo compounds, because the chelates can ture and the nitrogen or oxygen atom composed of the take the charged quinone structure by diethylamino quinone is positive or negative, respectively, the molar group at the para position of azo group.6 absorptivity becomes as large as 105 dm3 mol-' cm'. If a chelating reagent can form its chelates with This empirical rule is called the "Charged Quinone various metal ions and they are also quite stable, the Theory". By using this theory, the sensitivity of chelates can be separated and determined spectro- extraction-spectrophotometry of ion-associates is extre- photometrically by high performance liquid chroma- mely improved. tography. As such a chelating reagent, 2-(8- The theory is not limited to the ion-association quinoylazo)-5-N, N-diethylaminophenol is proposed. reagents, but can be extended to the chelating reagents. The chelates will have high molar absorptivity because For instance, cobalt ion reacts with 2-pyridylazo-2',4'- of the charged quinone structure due to the electron- diaminobenzene (PADAB) at neutral pH to form the donating diethylamino group. The chelate also will 1:2 complex, whose molar absorptivity becomes more become more stable by the same electron-donating than 105 dm3 mol-' cm' after the addition of sulfuric effect.' acid, because the complex changes into the charged quinone structure as follows3: 6 Instances of Selective and Sensitive Spectro- photometry by Ion-Association Reagents

Sensitive spectrophotometry can be carried out by using plane type cationic or anionic ion-association reagents of Table 1. The selectivity is achieved by a specific reaction which depends mainly upon chelating reagents. When the ion-associate is extracted into organic solvent, the sensitivity also increases due to the concentration effect and at the same time the disturbance by co-existing materials can be eliminated through the phase separation. Typical examples are shown below.

6.1 Extraction-spectrophotometry of anionic and non-ionic surfactants Shigematsu and Nishikawa4 show that 4-(2-hydroxy Dodecylbenzenesulfonate (15) and dodecylsulfate phenylazo)-1,3-benzenediol (THAB) reacts with alu- (14) are anionic ion-association reagents. Therefore, minum to form its chelate, as shown above. Thf they can combine tightly with Ethyl Violet (4) to form 484 ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 ion-associates which are extracted into toluene. The on the cation, the extractability is increased. Extrac- anionic surfactants can be determined by measuring the tion solvents for the ion-associate (18-crown-6, potas- absorbance at 611 nm in toluene, and the molar sium and picrate) were investigated and the distribution absorptivity is I .1X l0s dm3 mol-' cm-'.8 The method is ratio of the ion-associate is correlated closely with the much better than the Methylene Blue-chloroform solubility parameter and Dimroth's Er (solvent pola- extraction method. Anionic surfantants in water were rity)." determined by a spectrophotometric flow injection Potassium in river water was determined by technique coupled with solvent extraction. The ion- extracting the ion-associate of dibenzo-l8-crown-6 associate which formed between an anionic surfactant potassium complex with 4-[4-(phenylamino)phenyl]- and 1-methyl-4-(4-diethylaminophenylazo)-pyridinium azo-2,5-dichlorobenzenesulfonate into benzene. '8 The was extracted into chloroform and the absorbance at principle is applied to a spectrophotometric flow 564 nm was measured.9 injection analysis. The ion-associate thus formed was A non-ionic surfactant has a bulky hydrophobic extracted into benzene and its absorbance at 430 nm group and polyethyleneoxide chain. In the presence of was measured. Potassium in river water was thus a large amount of potassium ion, the polyether chain of determined.19 the surfactant can coordinate to potassium ion to form an ion-associable cation. Tetrabromophenolphthalein 6.4 Extraction-spectrophotometry of borate ethyl ester (5) reacts with the cation to form the ion- Borate reacts with two organic reagent molecules, associate which is extracted into dichlorobenzene. The each having two hydroxyl groups, to form the surfactant is determined by measuring the absorbance univalent complex anion which is extracted with a at 620 nm in the extract. 10 cationic ion-association reagent into organic solvent. Since boron cation(III) in water is strongly hydrated, it 6.2 Determination of iron(II) with nitrosophenols and is difficult to form the boron complex anion in water. Rhodamine B So the sample solution is evaporated to dryness with Iron(II) reacts with nitrosophenol (HR) to form sodium acetate and mannitol, and borate reacts in FeR3- chelate anion, which is associated with Rhoda- acetic acid with two 2,4-dinitro-l,8-naphthalenediols to mine B (2) cation and extracted into benzene or form the complex anion, which reacts with Brilliant toluene. Then iron(II) can be determined measuring Green (3) to form the ion-associate, and this is the absorbance of Rhodamine B in the solvent. The extracted into toluene. The excess Brilliant Green co- extractability of the ion-associate depends greatly upon extracted into toluene is removed by washing the the substituent on nitrosophenol. The extractability organic phase with 2 M hydrochloric acid, and the becomes larger due to an electron-withdrawing group absorbance at 637 nm is measured. The molar such as Cl or Br, because the negative charge of the absorptivity is 10.3X104 dm3 mol' cm-'. Boron in chelate anion is distributed over the whole ion and the natural water20 and iron samples21 is determined by this anion becomes strongly ion-associable. On the other method. hand, a nitrosophenol chelate anion having electron- On the contrary, 2,6-dihydroxybenzoic acid can react donating group such as OH or N(CH3)2 is not with borate in water to form the 2:1 complex anion; it extracted into benzene at all with Rhodamine B, is extracted into chlorobenzene with Malachite Green because the negative charge of the chelate anion can (3). The organic solvent is washed with 1 M H2SO4 not be distributed over the whole anion. Thus 4- and the absorbance at 628 nm is measured, the molar chloro-2-nitrosophenol iron(II) chelate (11) was chosen. absorptivity being 9.5X104 dm3 mol' cm'. Borate in The molar absorptivity is 9X104 dm3 mol' cm' at 558 spring water and seawater was determined.22 nm in toluene. By this method iron in river water" and Boric acid and 3,5-di-t-butylcatechol form a complex seawater12 was determined successfully. anion at pH 6.3-8.7, which is extracted into toluene as ion-associate with Ethyl Violet (4); the excess of Ethyl 6.3 Extraction of potassium ion Violet co-extracted into toluene is removed by washing The small ion-associable potassium cation can form with 3% H2O2. The absorbance is measured at 610 nm the ion-associate with large hexanitrodiphenylaminate and the molar absorptivity is 10.5X104 dm3 mol' cm-'. anion (6). It can be extracted into nitrobenzene'3, and Boron in seawater was determined.23 is relatively associated in water, whereas it is relatively dissociated in nitrobenzene.14 6.5 Spectrophotometric determination of phosphate Various kinds of organic anions were tested for the Phosphate reacts with molybdate in acidic solution extraction of potassium. Bulky, charge-dispersed and to form molybdophosphate, H3PMo12040. On the univalent anions are most suitable for the extraction of other hand, in acidic solution Malachite Green, MG+ potassium.'s (3), is present as yellow, protonated species, HMG2+, Dibenzo- l 8-crown-6 potassium complex cation and in the presence of molybdophosphate the green reacts with picrate anion to form the ion-associate ion-associate is formed as follows: which is extracted into benzene.16 As the crown HMG2++H3PMo,20ao --i MG'H2PMo,20ao+2H+ coordinates to potassium to expel the water molecule yellow green ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 485

By measuring the absorbance at 650 nm, phosphate can form blue ion-associates with quaternary am- can be determined indirectly. In this case, polyvinyl- monium compounds (~max 610 nm) and red charge alcohol is added immediately after the coloration to transfer complex with aliphatic and aromatic amines stabilize it. The molar absorptivity is 8X104 dm3 mol-' (Amax 540 - 585 nm) in l,2-dichloroethane (DCE). cm'. The method is used for the determination of Molar absorptivities of the former are (9.4 - 9.8)X l04 phosphate in river water.24 The principle is applied to dm3 mol-' cm 1 and those of the latter, (2-6)X104 dm3 flow injection analysis and several ng/ ml amounts of mol-' cm'. It was found that the red color of the phosphate can be determined at a rate of 40 samples charge transfer complex in DCE disappeared with the per hour.25 elevation of temperature, while the absorbance of the Toluene-MIBK (1: 3 v/ v) mixture solvent can blue ion-associate was unchanged at 60° C. The extract the ion associate [(MG+)3PMo120403-]quantita- thermochromism allows the sparteine (quaternary tively. By measuring the absorbance of the organic ammonium ion type) to be determined successfully at phase at 630 nm, phosphate can be determined. The 60° C without the interference of co-existing amines.3o molar absorptivity is 2.3X105 dm3 mol-' cm~' and the Diphenhydramine and chlorpheniramine (amines) sensitivity is 3 times larger than that of the aqueous are determined by the thermochromism in the presence solution method using Malachite Green. It is possible of quaternary ammonium. The absorbance differences to condense the ion-associate 20 times by extraction; (AA) of the amines between 25° C and 50° C (tempera- then 0.1 ng/ ml phosphate can be determined without ture difference: Ot) at 573 nm in DCE are measured. difficulty.26 Linear graphs of DA / ~t versus concentration of the amines are obtained, while LA/At of the quaternary 6.6 Extraction of anionic chelate having sulfo groups ammonium is negligible.31 Moreover, each amine in A chelating reagent containing sulfonic acid group, two component mixture or each amine and a 2-nitroso-l-naphthol-4-sulfonic acid (H2R), reacts with quaternary ammonium compound in a three-com- cobalt to form CoR33- chelate anion, which reacts with ponent mixture can be determined with an error not bulky quaternary ammonium ion (QA) to form the exceeding about 4%, without separation.32 ion-associate (QA+)3COR33 it is extracted into chloro- The maximum absorbance of the dibucaine (amine)- form. In this case, if the quaternary ammonium ion is TBPE associate occurs at 555 nm and that of the too large, the excess chelating reagent is also extracted chlorpheniramine (amine) associate at 575 nm; DA/At as QA+HR- or (QA)2R2_ and the absorbance of the values are characteristic for each. The thermochro- reagent blank becomes large. Then, the spectrophoto- mism is applied to the simultaneous determination of metry is led to an undesirable result. When two amines. The method is highly selective, sensitive tetrabutylammonium (QA) is used, only the cobalt and precise.33 chelate anion is extracted and the excess chelating Quinine reacts with diprotic Bromophenol Blue reagent remains in water, because the extractability of (BPB) to form a 1: I complex anion which is extracted the chelate anion is much larger than that of the into chloroform with berberine (NR4+) at pH 6.7 as chelating reagent itself. An advantage of this method is follows: that the excess reagent remains in water to lower the BPB2-+HQuinine++NR4+_ [BPB•HQunine]-[NR4]+ reagent blank absorption in chloroform. Therefore, the spectrophotometry can be carried out at the highest Berberine and benzethonium are determined by sensitivity and the molar absorptivity is 6.5X104 dm3 measuring the absorbance at 610 nm in chloroform; mol-' cm' at 307 nm.27 many kinds of amines do not interfere.34 Instead of tetrabutylammonium, a colored azo compound, 1-propyl-4-(4-diethylaminophenylazo)-pyri- 6.8 Extraction of various ion-associable anions by dinium (AD), having the same extractability as ferroin and its derivatives35 tetrabutylammonium was synthesized and used. This The ion-associability of a divalent ion is weak, compound reacts with CoR33- to form the ion- though it is bulky and the charge is distributed over the associate, (AD+)3CoR33-, which is extracted into whole ion. One example is [Fe(phen)3]2+ (9). It can chloroform. By measuring the absorption of the ion- form the ion-associate only with strongly ion-associable associate, cobalt can be determined indirectly and the anions; this can be extracted into nitrobenzene, but not molar absorptivity is 1.66X 105 dm3 mol-' cm' at 566 into the other solvents, because nitrobenzene is the best nm.28 solvent for weak ion-associates. Therefore, ordinary Similarly the iron(II) chelate anion, Fe(II)R34, can anions and cations, e.g. chloride, nitrate, sodium, form the ion-associate (AD+)4Fe(II)R34- which is potassium and so on do not disturb the extraction at extracted into chloroform. The molar absorptivity is all. Bulky, univalent anions can be determined by 2.1X 105 dm3 mol-' cm ' at 555 nm.29 measuring the absorbance of [Fe(phen)3]2+ in nitrobenzene at 516 nm; examples include perchlorate, 6.7 Thermochromism of ion-associates with tetrabro- perrhenate, thiocyanate, metallohalogeno anions, mo- mophenolphthalein ethyl ester lybdophosphate, trichloroacetate, dehydroacetate and Tetrabromophenolphthalein ethyl ester (TBPE) (5) salicylate. 486 ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3

The ion-associability of [Cu(I)(neocup)2]+ (8) is back-titrated by a standard polyvinylsulfate solution strong, because the ion is univalent. The ion-associate after the addition of excess PDADMA (17). of [Cu(I)(neocup)2]+NO3_ can be extracted into MIBK As a standard substance for polyvinylsulfate solu- to determine N03- by measuring the absorbance at 456 tion, hexadecylpyridinium chloride monohydrate (13) nm in the solvent. was suggested.42 As an indicator for PDADMA (17), 3-(2-hydroxy-3-carboxyanilide-1-azonaphthalene)-4-hy- 6.9 Column separation of ion-associates droxybenzenesulfonate (NBS) was proposed: the color By using reverse phase partition column (e.g. ODS change was from red-violet to blue at the end point.43 column), ion-associates can be separated clearly. In Cationic polymeric flocculants such as PDADMA are this case the surface of column packings can function widely used in waste water treatment and trace as an organic solvent. In the column, the ion- amounts of the flocculant can be determined spectro- associates are extracted and eluted repeatedly, and the photometrically with NBS at pH 10 by flow injection separation can be carried out successfully even though analysis.44 the differences between their extraction coefficients are In addition, conductometry42, ion-electrode method45 small. Such a separation mode was called soap and turbidimetry46 are suggested for the end point chromatography or ion-pair chromatography, but has detection. recently been named ion-association chromatography.36 When colloid titration values (milliequivalents per Boric acid can react with chromotropic acid (H4R) gram) obtained at various pH ranges are expressed as in an aqueous solution to form the complex anion ordinate and pH as abscissa, the titration result proves (BR25-). In the presence of the excess reagent, the the dissociation state of the polyelectrolyte. For complex anion can not be determined spectrophoto- example, heparin has three 0503, two NHSO3 and metrically, because the absorption curve of the reagent two COOH groups per four hexose units. The titration resembles that of the boron chelate. The ion-associates result is shown in Fig. 2.47 Sulfate and aminosulfate of the reagent and the chelate anions with a counter groups are dissociated at every pH, but carboxyl group cation have different extractabilities into organic is not dissociated at lower pH but above pH 6 it is solvents, and the chelate ion-associate is more dissociated completely. The result shows that two extractable than that of the reagent.37 By using ODS carboxyl groups are dissociated for every three column and tetrabutylammonium as an eluent, the sulfonates and two aminosulfonates estimating from excess reagent and the boron-complex are separated the equivalent value. clearly, thus boron can be determined.38 In a similar The surface charges of some bacteria were deter- manner, the cobalt complex anion of 2-nitroso-l- mined at various pH ranges by colloid titration; after naphthol-4-sulfonate (1:3) can be separated from the the treatment with formalin the same titration was reagent and the other complex anions, so even minute carried out. From the two titration results, amounts of amounts of cobalt in nickel salts can be successfully amino and guanidyl groups were estimated.48 determined Dodecylbenzenesulfonate (15), an ion-association reagent, can not be titrated by PDADMA (17) solution

7 Colloid Titration

Colloid titration was invented by Terayama40 in 1948. The principle is based on the fact that polycation solution reacts quantitatively with polyanion solution to form precipitate. As the polycation, methylglycol- chitosan (16) or polydiallyldimethylammonium chloride (PDADMA) (17) is used and as polyanion, polyvinylsulfate (18). Polyethyleneimine and glycolchi- tosan, polycations, are titrated by a standard polyvinylsulfate solution directly. The end point is detected by the color change from blue to red-violet of Toluidine Blue as indicator.41 Polyanions, chondroitinsulfate, carboxymethylcell- ulose, alginate, carrageenan, ligninesulfonate etc. are

Fig. 2 Colloid titration results of heparin and dissociation state of its dissociative groups. ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3 487

directly, because the combination between them is not These are all examples of metachromasy. As another strong enough. A polycation having a long chain alkyl example of metachromasy, sphere type cation, Ag- group, poly(4-vinyl-l-pentylpyridinium bromide), was (phen)2+, can react with plane type anion, Bromopyro- synthesized and used for the determination of the gallol Red to show the color change from orange to purity of dodecylbenzenesulfonate on the market: the blue in aqueous solution.56 result was 81- 84%.49 This compound is a new chain- In conclusion, metachromasy occurs between ion- polymer type reagent (Table 1). associable colored cation or anion and oppositely By using hexadecylpyridinium chloride (13) standard charged ion-associable colorless sphere, chain or solution, polyanions, carrageenan and cellulosesulfate polymer type anion or cation in aqueous solution. can be titrated quantitatively.5o Thus, metachromasy is a specific phenomenon between The above results show that polyelectrolytes are ion-associable dyes and counter-charged ion-associable closely correlated with ion-association reagents. colorless ions.

Studies on ion-association reagents were carried out with 8 Metachromasy my colleagues and students in Okayama University, to whom I express my heartfelt thanks. In colloid titration, the color of Toluidine Blue turns from blue to red-violet by polyvinylsulfate (18) solution. The phenomenon is named as metachromasy. References A polymer type anion, polyvinylsulfate, in Table 1 can react with a plane type cation, Toluidine Blue, to 1. This is partly quoted from "Shin Jikken Kagaku Koza, show metachromasy. Also the chain type anion in No. 9 (Analytical Chemistry)", ed. The Chemical Society Table 1 can show metachromasy in aqueous solution. of Japan p. 537-560, Maruzen Co. Ltd. (1977). Dodecylbenzenesulfonate (15) reacts with 1-(4-N- 1' ibid., p. 549. 2. K. Toei and H. Fujii, Anal. Chim. Acta, 90, 319 (1977). methylpyridiniumazo)-4-(4-aminophenylazo) naphthalene 3. S. Shibata and M. Furukawa, Bunseki Kagaku, 23, 1412 to lower the absorbance at the maximum wavelength (1974). (594 nm)51, and also it reacts with 1-(1Vmethylpyri- 4. T. Shigematsu and Y. Nishikawa, Saishin no Bunseki dinium-4-ylazo)-4-(4-diethylaminophenylazo) naphtha- Kagaku, 26, 35 (1975). lene in the presence of iodide ion to show color change 5. K. Toei and S. Motomizu, Analyst [London], 101, 497 from red-violet to blue in aqueous solution52; the (1976). anionic surfactant can be determined by these 6. H. Wada, G. Nakagawa and K. Oshita, Anal. Chim. Acta, phenomena. 159, 289 (1984). Tetraphenylborate (10), sphere type anion, in Table 1 7. H. Hoshino and T. Yotsuyanagi, Bunseki Kagaku, 31, and its trifluoromethyl derivatives react with Rhod- E435 (1982). amine 6G, plane type cation, to lower the absorbance 8. S. Motomizu, S. Fujiwara, A. Fujiwara and K. Toei, Anal. Chem., 54, 392 (1982). at ).max 527 nm, and Crystal Violet (4) behaves 9. S. Motomizu, Y. Hazaki, M. Oshima and K. Toei, Anal. similarly.53 Sci., 3, 265 (1987). In acidic solution, spherical molybdophosphate 10. K. Toei, S. Motomizu and T. Umano, Talanta, 29, 103 anion, [PO412MoO3]3-, reacts with Malachite Green (3) (1982). to show color changes from yellow to green.24 These 11. T. Korenaga, S. Motomizu and K. Toei, Anal. Chim. phenomena are all regarded as examples of metachro- Acta, 65, 335 (1973). masy. 12. T. Korenaga, S. Motomizu and K. Toei, Talanta, 21, 645 On the other hand, PDADMA (17), polymer type (1974). cation, reacts with 3-(2-hydroxy-3-carboxyanilide-l- 13. T. Iwachido and K. Toei, Bull. Chem. Soc. Jpn., 37, 1276 azonaphthalene)-4-hydroxybenzenesulfonate, plane type (1964). anion, to show color change from red-violet to blue.43 14. T. Iwachido, Bull. Chem. Soc. Jpn., 45, 1746 (1972). 15. T. Iwachido, Bull. Chem. Soc. Jpn., 44, 1835 (1971). This is also metachromasy. 16. A. Sadakane, T. Iwachido and K. Toei, Bull. Chem. Soc. Zephiramine (12), chain type cation in Table 1, Jpn., 48, 60 (1975). reacts with Chlorophenol Red, plane type anion, to 17. T. Iwachido, M. Minami, H. Naito and K. Toei, Bull. express color change from yellow to red in aqueous Chem. Soc. Jpn., 55, 2378 (1982). solution. 54 18. T. Iwachido, M. Tajiri and K. Toei, Bunseki Kagaku, 34, Pyrocatechol Violet, Bromopyrogallol Red, Chro- 579 (1985). mazurol S, Eriochrom Cyanine R and so on, plane 19. T. Iwachido, M. Onoda and S. Motomizu, Anal. Sci., 2, type anions, react with some metal ions to form their 493 (1986). chelates. In the presence of quaternary ammonium 20. K. Kuwada, S. Motomizu and K. Toei, Anal. Chem., 50, such as Zephiramine (12), a pronounced bathochromic 1788 (1978). 21. K. Toei, S. Motomizu, M. Oshima and H. Watari, shift and an intensification of the absorption band are Analyst [London], 106, 776 (1981). observed. The phenomena are utilized for the highly 22. M. Oshima, S. Motomizu and K. Toei, Bunseki Kagaku, sensitive photometry to determine trace metals.55 32, 268 (1983). 488 ANALYTICAL SCIENCES DECEMBER 1987, VOL. 3

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