Flow-injection electrochemical determination of citric acid using a cobalt(II)–phthalocyanine modified carbon paste electrode Raphael Fonseca do Nascimento a , Thiago Matheus Guimar ~ aes Selva b,c , Williame Farias Ribeiro d , Mˆ onica Freire Belian e , Lu ´ cio Angnes f , Valberes Bernardo do Nascimento a,n a Univesidade Federal Rural de Pernambuco, Departamento de Quı ´mica, Recife-PE 52171-030, Brazil b Universidade Federal de Pernambuco, CCEN, Departamento de Quı ´mica Fundamental, Cidade Universita ´ria, Recife-PE 50670-901, Brazil c Instituto Federal de Educac - ~ ao, Ciˆ encia e Tecnologia de Pernambuco, Cidade Universita ´ria, Recife-PE 50740-540, Brazil d Universidade Federal da Paraı ´ba, CCEN, Departamento de Quı ´mica, Jo ~ ao Pessoa-PB 58051-970, Brazil e Univesidade Federal Rural de Pernambuco, Departamento de Ciˆ encias Moleculares, Recife-PE 52171-030, Brazil f Universidade de S ~ ao Paulo, Instituto de Quı ´mica, Av. Prof. Lineu Prestes, 748, S ~ ao Paulo-SP, Brazil article info Article history: Received 30 August 2012 Received in revised form 16 October 2012 Accepted 17 October 2012 Available online 26 October 2012 Keywords: Citric acid Cobalt(II)–phthalocyanine Carbon paste electrode Flow injection analysis Amperometry Square wave voltammetry Fruit juices abstract The electrocatalytic activity of cobalt(II)–phthalocyanine immobilized in a carbon paste electrode for citric acid oxidation under a diffusion-controlled electrochemical process was employed to develop a selective flow injection amperomettric analysis method. Under optimized conditions, rapid and reliable determinations of citric acid (65 samples per hour) were achieved for analyses of commercial fruit juices with minor pretreatment of the samples. A linear response was accomplished ranging from 0.25 to 15 mmol L 1 (r 2 ¼0.9979) with an estimated detection limit of 0.117 mmol L 1 . Alternatively, a very sensitive and selective assay was developed for the micromolar range, obtaining a linear response ranging from 3.81 to 91.2 mmol L 1 (r 2 ¼0.9993) with an estimated detection limit of 2.54 mmol L 1 , by using square wave voltammetric methodology. The speed and simplicity of the proposed approaches compares advantageously to the all other citric acid determination methods. & 2012 Elsevier B.V. All rights reserved. 1. Introduction With a worldwide production close to 2 million tons [1], citric acid (CA) is a very important organic chemical. The major part (  75%) is consumed by the beverage, food and pharmaceutical industries. CA improves the flavor of soft drinks, juices and condi- ments, acting simultaneously as an antibacterial agent [2]. It is also an important constituent in a number of pharmaceutical formula- tions, acting mainly as a flavoring and buffering agent. Citrate salts of various metals are used for delivering minerals in dietary supplements [3]. CA is also utilized in a variety of processes such as photographic and electroplating [4], catalysts preparation, synth- esis of nanostructured compounds [5], polymer crosslinking [6], soil remediation and plant growing mechanisms [7]. The reference methods [8] for the determination of CA in food products are very laborious and involve large amounts of reagents. Currently there are a large number of research projects to develop practical analytical methods for citric acid determina- tion. Notable examples have been reported for the measurement of this acid in diverse samples. HPLC selective methods are preferred for use with food samples [9,10]. Alternative and less time consuming methods based on photometric [11,12] and potentiometric approaches [13], enzymatic sensors [14–16], che- miluminescence [17,18] and atomic absorption spectrometry [19] are other representative examples. Voltammetric methods have also proved to be useful for citric acid determination using modified electrodes. The high oxidation potential of CA hampers the use of conventional electrodes (Pt, Au and many forms of carbon) which has inspired a number of research efforts for the purpose of modifying electrodes [14,20,21]. Porphyrins and phthalocyanines redox mediators have been largely employed to lower the oxidation potential of important organic species [22–26], but still have not been thoroughly investigated for target analytes such as CA. In our search, only two studies were encountered. The first one explored Langmuir– Blodgett films on carbon paste electrodes containing different phthalocyanines (Eu, Gd, Lu, Pr and Co) [25], to constitute an electronic tongue device, and the study was performed with relatively elevated analyte concentrations. The other one involved Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/talanta Talanta 0039-9140/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.talanta.2012.10.055 n Corresponding author. Tel./fax: þ55 81 3320 6375. E-mail addresses: valberes@pq.cnpq.br, valberesbn@gmail.com (V.B. do Nascimento). Talanta 105 (2013) 354–359