Materials and Methods
Chemicals and standard solution
All the chemicals used in this experiment were of analytical reagent grade purity. Triple distilled deionized water was used for the preparation of all the solutions. Solutions of 0.1 moldm-3 of Pb(II), Cd(II), Hg(II) nitrates (E-Merck, Germany) were prepared maintaining 0.05 mol dm-3 nitric acid to suppress the hydrolysis of metal salts. A solution (0.05moldm-3) of L-phenylalanine and maleic acid (GR grade, Loba, India) was prepared by maintaining a 0.05mol dm-3 nitric acid concentration to increase its solubility. Dimethylformamide (Qualigens, India) was used as received. Solutions of 0.2 moldm-3 nitric acid and 0.4 moldm-3 sodium hydroxide (Merck, India) were prepared. A solution of 2.0 moldm-3 sodium nitrate (Merck, India) was prepared to maintain the ionic strength in the titrand. All the solutions were standardized by the usual oxalic acid and potassium hydrogen phthalate solutions, while the normality of nitric acid was determined by using the standardized sodium hydroxide and the primary borax solutions [26]. The concentration of the metal ions were determined complexometrically by titrating against a standard solution of EDTA using Xylenol orange indicator and hexamine powder as buffer to maintain the pH at 5-6 [27]. So as to assess the errors that might have crept into the determination of the concentrations, the data were subjected to one-way analysis of variance (ANOVA) by using the computer program, COST [28]. The carbonate contamination in the sodium hydroxide solutions was determined by using the Gran Plot method [29].
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