The solubility of folic acid was determined at 25 °C in 1.00 mol dm ⁻³ and in 0.15 mol dm ⁻³ NaCl (physiological solution) spectrophotometrically by measuring the absorbance of saturated solution at different hydrogen ion concentrations.

Stability and structure of complexes between silver (I) and l-methionine (L) deduced from potentiometric and ¹H NMR measurements allow to assume the prevailing of several protonated species. The experimental data are compatible with the formation of the following complexes: AgL, AgL₂, AgH₂L, AgH₁L₂, AgH₂L₂, AgH₃L₂ and AgH₄L₂. The coordination sites are obtained by ¹H NMR spectra, showing that only the bond between the methylthioether sulfur atom and silver (I) is responsible of the complex stability. The system is studied potentiometrically with silver and glass electrodes at 25 °C and 1.00 mol·dm⁻³ NaClO₄ as ionic medium. Amino acids containing sulfur are few and not extensively studied. In particular, l-methionine, even if it is the most important, enantiomer and their complexes with silver (I) present anticarcinogenic properties, is quite not investigated. In the same experimental conditions, l-methionine protonation constants are determined. The ¹H NMR data allow one to assume that, in moderately alkaline solution, silver (I) is bond with six membered chelate rings with sulfur and amino nitrogen, while carboxylic groups are not involved. No polynuclear species are present. The high stability of the complex with ratio 1:2 (silver (I)/l-methionine), involving also two hydrogen ions, predominating in a wide range of hydrogen ion concentration suggests to propose a study for the preparation of an electrode to measure the deprotonated l-methionine concentration.