CRC Handbook of Organic Analytical Reagents

Kuang Lu Cheng, Keihei Ueno, Toshiaki Imamura

• Publisher: CRC Press
• Year: 2017
• eBook ISBN:
  9781351457217
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

The Handbook of Organic Analytical Reagents, 2nd Edition, is an indispensable source book of physico-chemical properties, preparation, and analytical applications of the most commonly used organic reagents. Updated from the 1st Edition, this volume includes data on 40 new reagents (such as ultra-high sensitive azo dyes, fluorescent calcium indicators, and chromogenic crown ethers and porphyrin reagents), a new Reagent Index listing reagents according to the elements to be assayed, and completely updated references. Each entry contains information on synonyms, sources and methods of synthesis, analytical applications, complexation reactions and the properties of complexes, purification and purity of the reagent, and other regeants with a related structure. The Handbook of Organic Analytical Reagents, 2nd Edition, is an invaluable bench-side reference for professional analytical chemists and graduate students.

O,N-Donating Chelating Reagents

K. Ueno and T. Imamura

o,o′-DIHYDROXYARYLAZO COMPOUNDS

Synonyms

Azo compounds covered in this section are listed in Table 1, together with their synonyms and molecular formula.

Source and Method of Synthesis

All compounds are commercially available. These azo dyes are synthesized by the standard procedure of azo coupling reaction. Namely, (1) from diazotized l-amino-6-nitro-2-naphthol-4-sulfonic acid and 1-naphthol,¹,² (2), (3), and (4) from diazotized 1-amino-2-naphthol-4-sulfonic acid and 2-hydroxynaphthoic acid,³ p-creso),⁴,⁷ and 2-naphthol-3,6-disulfonic acid, respectively.

Analytical Uses

Azo dyes of this class are widely accepted as a metal indicator in the chelatometry, especially for the EDTA tritration of alkaline earth metals. They are sometime used as a photometric reagent for Ca and Mg in aqueous solution or after extraction into immiscible solvent.

Properties of Reagents

Eriochrome® Black T (1) is usually supplied as a monosodium salt, is dark violet powder with faint metallic sheen, and is easily soluble in water and alcohol, but insoluble in common organic solvents. Aqueous solution is red at pH <6, blue at pH 7 to 11, and orange at pH >12. In an alkaline solution, it is easily oxidized to become colorless, pKa (H₂L^-) = 6.3, and pKa (HL^2-) = 11.55 (μ = 0.08 ~ 0.008, 18 ~ 20°).³

Calcon carboxylic acid (2) is usually supplied as a free acid, is dark violet powder, and is slightly soluble in water and alcohol to give a pink solution at pH < 8. It turns to blue at pH 10 to 13 and faint pink in strong alkali,³ pKa (H₂L^2-) = 9.26, and pKa (HL^3-) = 13.67 (µ = 0.1 KCI,24°).⁶

Calgamite® (3) is usually supplied as a free acid, is dark violet powder, and is easily soluble in water to give a bright red solution at pH < 7. It turns to blue at pH 9.1 to 11.4 and reddish-orange at pH > 13.⁴ Aqueous solution is most stable among the azo dyes listed in Table 1, pKa (H₂L^-) = 7.92, and pKa (HL^2-) = 12.50 (µ = 0.1. KNO₃, 25°).⁸,⁹

Hydroxynaphthol Blue (4) is usually supplied as a trisodium salt, is dark violet hygroscopic powder, and is easily soluble in water and aqueous alcohol to give a red-violet solution at pH < 6. It turns to blue at pH 7 to 12 and pink at pH > 13, pKa (H₂L^3-) = 6.44, and pKa (HL^4-) = 12.91 (µ = 0.1, KC1, 24°),⁶,¹⁰

Absorption spectra of the aqueous solution of these dyes at different stages of deprotonation are illustrated in Figures 1 to 4.

Table 1 O,O′-DIHYDROXYARYLAZO COMPOUNDS

FIGURE 1 Absorption spectra of Eriochrome® Black T(1) and Mg chelate in water. (1) pH 5 (H₂L^-); (2) pH 8 (HL^2-); (3) pH 13 (L^3-); (4) pH 10 (MgL^-). (From Schwarzenbach, G. and Flaschka, H., Die Komplexometrische Titration, 2nd ed., Ferdinard Enke, Stuttgart, 1965. With permission.)

Complexation Reaction and Properties of Complexes

In the presence of metal ions other than alkali metals, the blue species of dyes turn to reddish in the aqueous solution, and such color reactions are utilized in the photometry of metal ions or in the chelatometry as metal indicators.

FIGURE 2 Absorption spectra of Calcon carboxylic acid (2) and Ca chelate in water. (1) pH 3.7 (H₃L^3-); (2) pH 9.9 (HL^3-); (3) pH 13 (CaL^3-). Dye, 2.4 x 10⁵ M. (From Itoh, A. and Ueno, K., Analyst. 95. 583. 1970. With permission.)

FIGURE 3 Absorption spectra of Calmagite® (3) in water. (1) pH 2.14 (H₂L^-); (2) pH 10.29 (HL^3-); (3) pH 13.8 (L^3-). Dye, 9.1 x 10^3- M (Reprinted with permission from Lindstrom, F., Anal. Chem., 32, 1124, 1960. Copyright 1960 American Chemical Society.)

FIGURE 4 Absorption spectra of Hydroxynaphthol Blue (4) and Ca chelate in water. (1) pH 3.3 (H₃L^3-); (2) pH 9.8 (HL^4-); (3) pH 13.8 (L^5-); (4) pH 13 (CaL^3-). Dye, 2.4 x 10^5- M. (From Itoh, A. and Ueno, K., Analyst 95, 583, 1970. With permission.)

Table 2 Color Reaction of Eriochrome® Black T With Metal Ion at pH 9

The color reactions of (1) with various metal ions were investigated in some detail and are summarized in Table 2. The structural change of the reagent accompanied by the color reaction may be schematically shown as below:

Stability constants of the dyes with several metal ions have been determined mostly by spectrophotometric method. Those values are summarized in Table 3. The conditional stability constants (log K′_ML) of (1), (3), and (4) at pH 10 are also reported as follows: (1), Ca (3.8), Mg (5.4); (3), Ca (3.67), Mg (5.69);¹⁰⁷ (4), Be (3.63), Mg (3.43), Ca (2.82), Sr (2.05), Ba (1.75), La (3.57), Pr (4.03), Nd (4.13), Sm (4.09), Eu (4.10), Ga (3.76), Tb (3.77), Dy (3.71), Ho (3.68), Er (3.60), Yb (3.50),¹¹,UO₂^2- (4.10 or 3.99).¹³

Absorption spectra of some representative metal chelates are illustrated in Figures 1, 2, 4, and 5. The colored chelates can be extracted into polar organic solvents such as n-amyl alcohol for the subsequent photometry. As example, the absorption spectra of Eriochrome® Black T (1) and its Mg chelate in n-amyl alcohol are shown in Figure 6. The anionic chelates can also be extracted with the long chain quaternery ammonium ion such as Zephiramine® ¹⁵ or Aliquat® 336 S¹⁶ into organic solvent like chloroform or 1,2-dichloroethane.

Table 3 Chelate Stability Constants of o,o′-Dihydroxyarylazo Dyes

FIGURE 5 Absorption spectra of Ca and Mg Calmagite® chelate at pH 10; Dye, 3.5 × 10^-1 M. (1) Ca in excess; (2) Mg in excess. (Reprinted with permission from Lindstrom, F., Anal. Cham., 32, 1124, 1960. Copyright 1960 American Chemical Society.)

FIGURE 6 Absorption spectra of Eriochrome® Black T and Mg chelate in n-amyl alcohol. (1) In n-amyl alcohol; (2) in n-amyl alcohol shaken with pH 11.5 solution not containing Mg; (3) shaken with pH 11.5 solution containing 0.1 ppm Mg; (4) shaken with pH 11.5 solution containing 0.2 ppm Mg. Reference, water. (From Nishimura, M. and Nakaya, S., Bunseki Kagaku, 16, 463, 1967. With permission.)

Purification and Purity of Reagents

Commercial samples are pure enough for use as metal indicators in the chelatometry. However, they contain a substantial amount of in...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Table of Contents
• A Theoretical SurVey of Organic Analytical Reagents
• O,O-Donating Chelating Reagents
• O,N-Donating Chelating Reagents
• N,N-Donating Chelating Reagents
• Chelating Reagents with Sulfur Functions
• Non-Chelating Reagents
• Non-Coordinating Reagents
• Organic Reagents For Anions
• Reagent Index for Elements
• Index