Spectrophotometric study on the stability of dopamine and the determination of its acidity constants A.E. Sa ´nchez-Rivera a , S. Corona-Avendan ˜o a , G. Alarco ´ n-Angeles a , A. Rojas- Herna ´ndez a , M.T. Ramı ´rez-Silva a, *, M.A. Romero-Romo b a Departamento de Quı ´mica, Universidad Auto ´noma Metropolitana (Iztapalapa), Av. San Rafael Atlixco #186, Col. Vicentina, C.P. 09340, Me ´xico, D.F., Mexico b Departamento de Materiales, Universidad Auto ´noma Metropolitana (Azcapotzalco), Av. San Pablo #180, Col. Reynosa-Tamaulipas, C.P. 02200, Me ´xico, D.F., Mexico Received 13 February 2003; received in revised form 13 February 2003; accepted 26 March 2003 Abstract The interest in determining the acidity constants of the catecholamines stems from the fact that they play rather an important biological role. The present work reveals the effect of different parameters such as oxygen, light, analysis time and pH on the dopamine oxidation process, where oxygen has an effect on the dopamine oxidation of 40% and up to 20% is attributed to exposure to light as a function of the pH. The application of adequate control on the said parameters (which ensured stability of the dopamine) facilitated the determination of the corresponding three acidity constants, 9.0469 /0.147, 10.5799 /0.148 and 12.0719 /0.069. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Dopamine; Stability constants; Point by point analysis; Light exposure 1. Introduction The catecholamines are bioenergetic amines that play quite an important role as neurotransmitters in the central nervous system (CNS). It is well known that the dopamine (3,4-dihydroxipheny- lethylamine, DA) (see Fig. 1 below) belongs to this chemical group, which is sensitive to air and light because the catechol group that is part of the structure is readily oxidisable to a quinoid form [1]. Examination of the DA structure allows dis- cernment of three acidity constants associated to deprotonation of the phenol and amine groups present in the molecule. Calculation of the said constants is not an easy task, as reported by Martin [2], because the acidity of each group is comparable to the other groups that also partici- pate in the structure. Hence their respective deprotonations occur in the same pH region, thus making calculations rather difficult. * Corresponding author. Tel.:  /52-555-804-4670; fax:  /52- 555-804-4666. E-mail address: mtrs218@xanum.uam.mx (M.T. Ramı ´rez- Silva). Spectrochimica Acta Part A 59 (2003) 3193  /3203 www.elsevier.com/locate/saa 1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(03)00138-0