Potentiometric Titration of Aminoalkylisothiuronium Salts

The reversibilities of the titration curves of 2-aminoethyl- and 3-aminopropylisothiuronium salts were discussed. The potentiometric titration was done at various titration speeds and at various temperatures. Though the titration curves of these two compounds showed different behaviors against the rate of alkali addition, it was confirmed that both compounds were transguanylated during the titration. The isothiuronium salt was ionized to the unstable conjugate base, which was in turn transguanylated rapidly to mercaptoalkylguanidine. In 2-aminoethylisothiuronium salt, however, the conjugate base was partly cyclized to 2-aminothiazoline. From the titration curves, the apparent ionization constants were determined graphically ; pKa values at 25°of 2-aminoethyl- and 3-aminopropyl-isothiuronium salts were 7.5 and 8.6, respectively.