Adaptation of on-line reactions developed for use with flow injection with amperometric detection for use in disposable sensor devices: reductive determination of phosphate as pre-formed 12-molybdophosphate in a capillary-fill device

Preliminary experiments have been carried out to show that on-line reactions developed for use with flow injection can be adapted for use in capillary-fill devices. The reversibility of the oxidation of a 1 × 10–3M solution of potassium hexacyanoferrate(II) in 1 M HCl solution at a screen-printed carbon electrode in a capillary-fill device (CFD) incorporating a screen-printed silver reference electrode was found to be only slightly less than at a smoothly operating glassy carbon electrode (GCE)[Epa–Epc= 95 mV (CFD), 65 mV (GCE)]. For the determination of phosphate as 12-molybdophosphate by reduction at a screen-printed carbon electrode in a CFD, a device with a screen-printed unchloridised silver reference/counter electrode was preferred to a chloridised silver electrode. A stable measurement potential was obtained when the measured solution was made 0.1 M in potassium chloride. The presence of potassium ion was found to affect markedly the linear sweep voltammograms obtained for the reduction of 12-molybdophosphate particularly at a GCE but also to a lesser extent at a screen-printed carbon electrode; this effect was controlled in further work with the CFD by the inclusion of potassium chloride in the measured solution. A rectilinear response was obtained for 5 × 10–6M(the detection limit)–2 × 10–3M pre-formed 12-molybdophosphate solutions at an applied potential of 0.16 V. The acidity required to carry out the reaction of phosphate can be provided by the addition of potassium hydrogen sulphate. It should be possible, therefore, to produce a satisfactory CFD for the determination of phosphate by screen-printing a mixture of sodium molybdate, potassium hydrogen sulphate and potassium chloride on to the glass face opposite to the screen-printed electrodes.