Design of a chemical sensor for the detection and determination of citric acid using indicator displacement assay method

In this study, a colorimetric sensor was introduced to determine citric acid by the indicator displacement assay(IDA) method. The sensor was made from the complex formation reaction between the Chromeazurol S and the gadolinium ions in aqueous solutions. At optimum pH(pH =7.00), Chromeazurol S (yellow) was reacted with gadolinium (III) in a 1: 1 ratio, and the Chromeazurol S-gadolinium color complex solution changed to purple. With the addition of citric acid to the designed sensor, citric acid replaces Chromeazurol S and reacts with gadolinium ion in a 1: 1 ratio. The reason for the displacement of the indicator by the analyte in combination with the gadolinium ion acceptor is that it has a larger citric acid-gadolinium complex formation constant (logKF = 6.86) than the Chromeazurol S-gadolinium formation constant (logKF = 5.67). The linear range of this measurement and detection limit of this sensor (S/N=3) were determined 2.27×10-6 to 112.90×10-6 M and 2.08×10-6, respectively. To determine the selectivity of the designed sensor, the effect of different anions such as pyrophosphate, salicylic acid, periodic acid, benzoic acid, and other compounds on the sensor response under similar conditions was investigated, and no significant color change was observed. It was indicating that the sensor has a high selectivity to measure this analyte. The efficiency of the method was evaluated by measuring citric acid in the orange juice sample by the standard addition method.