Fresenius' Joumal of Fresenius J Anal Chem (1994) 349 : 703-707 @ Springer-Verlag 1994 Potentiometric determination of the protonation constants of some phenols in 1.0 mol/L NaCl at 25°C Gorka Arana, Nestor Etxebarria, Luis Angel Fernandez Kimika Analitikoaren Saila, Euskal-Herfiko Unibertsitatea (UPV/EHU), P. K. 644, E-48080 Bilbao, Spain Received: 21 December 1993 Abstract. The potentiometric determination of the proto- nation constants of phenol, 2-chlorophenol, 2-nitrophenol, 2,4-dichlorophenol and 2-methylphenol in 1.0 mot/L NaC1 at 25 °C is presented. An automated system has been used in the experiments. The determination of the constants has been carried out using both graphical and numerical meth- ods. Introduction Phenols may be found in the aquatic environment due to different sources. Some of them have a natural origin, others are related to different manufacturing industries which produce them as by-products, e.g. in coke produc- tion, paper and pulp processing, oil refineries, coal gas liq- uefaction, etc. Phenols are also produced for use in fungi- cides, antimicrobials, wood preservatives and as reactants or intermediates for many synthetic products such as plas- tics, epoxy resins, drugs, dyes, etc. [4, 9, 11, 15]. The inadequate disposal or misuse of these compounds brings them into the aquatic environment in concentrations up to more than 20 g/L [9]. Phenols can be toxic even in low concentrations (above 2 rag/L), their toxicity increas- ing with their acid strength; they have a relatively high oxygen demand (about 2 mg OJmg phenol) [15]. They also cause bad taste of chlorinated water because of the forma- tion of chlorinated phenols, some of them suspected to be carcinogenic. For all these reasons, the separation and/or elimination of these compounds from the waste streams is necessary. There are several methods for treating phenolic wastes; the choice depends on the concentration levels of the phe- nols. For high concentrations, recovery and reuse or dis- posal by incineration is possible. Intermediate concentra- tions can be treated biologically or by adsorption on active surfaces. In more dilute wastes chemical oxidation can be Correspondence to : G. Arana used [15]. Apart from these methods, liquid membrane pro- cesses are available. Their first use in phenolic waste treat- ment was described by Cahn and Li [3] in 1974 and needs only one step to accomplish the extraction and strip steps normally found in extraction processes. These liquid mem- brane processes can be carried out in different ways: using emulsions, supported in solid surfaces, etc. Since this method is based on the extraction of the phenol into an organic phase and the driving force of the transport is the concentration gradient [17], it is necessary to know the acid base equilibria of the species involved in order to design selective liquid membrane separation processes. As a first step in the development of a method for the separation of phenols using liquid membranes, the aim of this work is to study the acid base equilibria of some of them. The phenols chosen are: phenol, 2-nitrophenol, 2-chlorophenol, 2,4-dichlorophenol and 2-methylphenol (o-cresol), the first four being considered by the US Envi- ronmental Protection Agency as priority pollutant phenols. The study has been carried out in 1.0 mol/L NaC1 medium using the potentiometric technique. NaC1 was chosen since it is reported as a common medium in the waste water of epoxy resin manufacturing [9]. Experimental Chemicals and solutions Phenol (Merck, p.a.), 2-chlorophenol (Aldrich, +99%), 2-methylphenol (Fluka, puriss.), 2-nitrophenol (Fluka, p.a.), 2,4-dichlorophenol (Aldrich, 99%), sodium chloride (Merck, p.a.), sodium hydroxide (Merck, p.a.) and hydro- chloric acid (Fluka, p.a.) were used without further purifica- tion. All solutions were prepared using MilliQ water. A solu- tion with concentrations of 0.3 mol/L in NaOH and 0.7 mol/L in NaC1 was prepared in N2 atmosphere and stan- dardized against potassium phthalate with phenolphthalein as indicator [13]. This solution was used as titrant. Different solutions of the phenols were prepared with the following general composition: B mmol/L + 10 mmol/L H + + (990-B) mmol/L NaC1