Kinetic response of pH colorimetric sensors: Role of the cationic surfactant concentration and amount and type of solvent used in the preparation of the sensing spot

Abstract

A pH colorimetric sensor array (CSA) with a kinetic response comparable to those of the pH-meter was prepared. The signal comes from the Hue coordinate (H) and RGB profiles of some pH indicators such as Bromothymol Blue (BB) and Nitrazine Yellow (NY) in polyvinylidene fluoride (PVDF)-supported organically modified silicate (OrMoSil) spots. The t95 is of the order of 50–150 s for RGB coordinates but is lowered of one order of magnitude employing the H coordinate as the detection signal. The addition of the Hexadecyltrimethylammonium p-toluenesulfonate (CTApTs) additive in the interval 0.10 < R < 0.15 gCTApTs/gprecursors, determined the minimum time to reach the basic and the acidic plateau (t95b and t95a) for both the indicators (BB: 5 and 32 s; NY: 2 and 7 s). At concentration ratio, R > 0.19 gCTApTs/gprecursors, the transition from the basic to the acid form was hindered by the cationic interface due to the surfactant. The CTApTs concentrations, leading to a t95 lower than one minute, were in the interval 0.10 < R < 0.37 gCTApTs/gprecursors for NY and 0.10 < R < 0.28 gCTApTs/gprecursors for BB, comparable to the response time of the potentiometric measurements. Indeed, the kinetic profiles did not change with the amount and type of solvent.