
%0 Journal Article
%A Stunzi, H 
%T Derivatives of isatin in aqueous solution. I. Kinetics of ring opening of Isatin-5-sulfonate
%J Australian Journal of Chemistry
%D 1981/02/01
%V 34
%N 2
%P 365-371
%K 
%R https://doi.org/10.1071/CH9810365
%U https://www.publish.csiro.au/paper/CH9810365 
%X The reactions of isatin-5-sulfonate anion (si-) which cause a hysteresis in pH titrations were studied by pH-metric and n.m.r, spectroscopic methods. Rapid alkalimetric titrations [I 0.15 M (KNO3),37º] gave the pKa value corresponding to the addition of OH- to si- [pKa(ring) 9.55]. The slow ring opening to the sulfonatoisatate dianion (sia2-) led to a drift of the pH values towards an equilibrium buffer region. Its pKa, value [pKa(eq) 3.44] corresponds to the reaction si-+H2O &#8596; sia 2-+H+ Rapid back-titration gave the pKa value of the ring-opened species Hsia- [pKa(open) c. 1.3]. The rate law for the ring opening d[sia]/dt=k2 [siOH](OH)+k1*[si] was obtained from the rate of change of pH. N-Methylisatin-5-sulfonate behaves analogously. 
