Reaction rate of NaOCl in contact with bovine dentine: effect of activation, exposure time, concentration and pH R. G. Macedo 1 , P. R. Wesselink 1 , F. Zaccheo 2 , D. Fanali 2 & L. W. M. van der Sluis 1 1 Department of Cariology Endodontology Pedodontology, Academic Centre for Dentistry Amsterdam, Amsterdam; and 2 Visiting Researcher, Department of Cariology Endodontology Pedodontology, Academic Centre for Dentistry Amsterdam, Amsterdam, the Netherlands Abstract Macedo RG, Wesselink PR, Zaccheo F, Fanali D, van der Sluis LWM. Reaction rate of NaOCl in contact with bovine dentine: effect of activation, exposure time, concentration and pH. International Endodontic Journal. Aim To determine the influence of activation method (ultrasound or laser), concentration, pH and exposure time on the reaction rate (RR) of NaOCl when in contact with dentinal walls. Methodology The walls from standardized root canals in bovine incisors were exposed to a standard- ized volume of sodium hypochlorite (NaOCl) with different concentrations (2% and 10%), pH (5 and 12) and exposure times (1 and 4 min). Two irrigation protocols were tested: passive ultrasonic irrigation or laser activated irrigation with no activation as the control. The activation interval lasted 1 min followed by a rest interval of 3 min with no activation. The RR was determined by measuring the iodine concentration using an iodine/thiosulfate titration method. Results Exposure time, concentration and activation method influenced the reaction rate of NaOCl whereas pH did not. Conclusions Activation is a strong modulator of the reaction rate of NaOCl. During the rest interval of 3 min, the consumption of available chlorine increased significantly. This effect seems to be more pronounced after irrigant activation by laser. pH did not affect the reaction rate of 2% NaOCl. Keywords: activation, convection, irrigation, laser, reaction rate, ultrasonic. Received 21 April 2010; accepted: 23 June 2010 Introduction Sodium hypochlorite (NaOCl) solutions are commonly used in endodontics because of at least three important properties: organic tissue-dissolving capacity, bacterici- dal effect and when properly used, the absence of clinical toxicity (Moorer & Wesselink 1982). In alkaline solutions, when pure NaOCl is dissolved in water, the following reaction takes place: NaOCl ! Na þ þ OCl  OCl  þ H 2 O $ HOCl þ OH  The free available chlorine consists of hypochlorous acid (HOCl) and the hypochlorite ion (OCl ) ), which exist in an equilibrium, depending on the pH of the solution (Baker 1947). The biological activity of NaOCl can be defined as its tissue-dissolving capacity and bactericidal effect and will be influenced by this equilibrium. In alkaline solutions (pH > 7), OCl ) pre- vails, which has a powerful oxidative effect and therefore a higher tissue dissolving capacity than HOCl (Baker 1947). On the other hand, HOCl prevails in acidic solutions (3 < pH < 7) and has a powerful Correspondence: Ricardo Macedo, DMD, Academic Centre for Dentistry Amsterdam, CEP-Endodontology, Louwesweg 1, 1066 EA Amsterdam. the Netherlands (Tel.: +31 20 5188367; fax: +31 20 6692881; e-mail: r.macedo@acta.nl). doi:10.1111/j.1365-2591.2010.01785.x ª 2010 International Endodontic Journal International Endodontic Journal 1