The acid dissociation constants of the hydroxyl groups in tetrasodium 25,26,27,28-tetrahydroxycalix[4]arene-5,11,17,23-tetrasulfonate (CALX-S4) were determined at 25 °C by potentiometric and spectrophotometric titration methods. The first acid dissociation constant(pKa1) was found to be 3.26 ± 0.02 (μ = 0.1 with KNO3), which demonstrated a remarkable pKa shift due to intramolecular hydrogen-bonding interactions among the hydroxyl groups in CALX-S4. On the other hand, considerably weak acidities were observed on the residual three hydroxyl groups of CALX-S4 (pKa2 = 11.8 ± 0.3 (μ = 0.1 with KNO3), 11.3 ± 0.3 (μ = 2.0 with KCl), pKa3 = 12.8 ± 0.3 (μ = 2.0 with KCl), and pKa4= ca. 14 (μ = 2.0 with KCl)).

Calix[4]arene-5,11,17,23-tetrasulfonate (1) reacts with cerium(III) ion in alkaline solution to form a colored complex (λmax = 417 nm, ε = 1.09 × 104 1 mol−1 cm−1). The max coloration reaction is specific for cerium among rare-earth elements. The coloration mechanism was discussed on the basis of the stability constants.

The concentration of sulfate ions in rain water was determined by a flow-injection method using a reaction column packed with barium chloranilate powder. Aqueous ammonium chloride (0.5%) containing 50% ethanol was used as a carrier at a flow rate of 1.9 ml/ min. The chloranilate ion concentration, which corresponds to the sulfate ion concentration, in the carrier from the reaction column was monitored by a photometric detector at 530 nm. Interferences by multivalent cations were eliminated with the use of a cation-exchange column inserted in the upper stream of the reaction column. The concentrations of sulfate ions in rain water (4 -100 ppm) were quickly determined (one sample per minute) with high reproducibility. The results agreed well with those obtained by the standard manual method.

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The first and second protonation constants of 2-nitroso-5-(N-propyl-3-sulfopropylamino)phenol (NPSAP, H2l) and the stability constants of the metal ion complexes formed by NPSAP with some divalent and tervalent metal ions such as magnesium(II ), calcium(l l ), cobalt(il ), nickel(I I ), copper(I (), zinc(l l ), aluminium(II I ), iron(l I I) and gallium-(Ill) have been determined potentiometrically in aqueous solution at 25°C (µ=0.1 with KNO3).Keywords Stability constant, metal ion complexes, o-nitrosophenol, potentiometric titration o-Nitrosophenols are useful as selective chromogenic reagents for cobalt(II) ion.' However, little is known about their stability constants with metal ions due to the poor solubility of these reagents in water. Recently, 2-nitroso-5-(N-propyl-3-sulfopropylamino)-phenol (NPSAP, H2L) has been synthesized as a water soluble derivative of o-nitrosophenol, and was reported to be a highly sensitive chromogenic reagent for iron(II)2 and cobalt(II)3 in aqueous samples. This paper reports the results of a potentiometric study on the protonation constants of NPSAP and the stability constants of its complexes with some metal ions such as magnesium(II), calcium(II), cobalt(II), nickel(II), copper(II), zinc(II), iron(III), aluminium(III) and gallium(III) in comparison with other o-nitrosophenols and o-nitrosonaphthols. ExperimentalReagents NPSAP was kindly donated by Dojindo Laboratories. It was used without further purification by dissolving it in water to make 0.02 M (1 M=l mol dm-3) stock solution.The concentration of stock solution was standardized by titrating with a standard KOH solution. The solution was kept in a refrigerator and used within 5 days after being produced.The stock solutions of metal ions were prepared from the corresponding metal nitrates (analytical grade), and were standardized by EDTA titration.4The decarbonated standard potassium hydroxide solution was prepared according to the conventional procedure, by treating 0.25 M potassium hydroxide solution with barium hydroxide to precipitate barium carbonate, followed by the treatment of supernatant solution with a cation exchange column to remove excess barium (II) ion. The final concentration of potassium hydroxide solution was nearly 0.1 M; the solution was standardized against potassium hydrogen phthalate. TitrationThe solutions of 1:2 and 1:3 metal to ligand ratio, were titrated with 0