Phenol red immobilized PVA membrane for an optical pH sensor with two determination ranges and long-term stability

A novel method was developed to immobilize phenol red into poly(vinyl alcohol) (PVA) membrane for an optical pH sensor with two determination ranges and long-term stability. Phenol red was first reacted with the formaldehyde to produce hydroxymethyl groups, and then attached to PVA membrane via the hydroxymethyl groups. The dynamic response ranges of the sensor membrane became somewhat wider than those of free forms, from pH 0–3 and pH 6.5–9.5 to 5 M [H+]–pH 3.0 and pH 6.5–10.5, respectively. Experimental results indicated common cations, such as Na+ and K+ showed insignificant influence when 0.1 M of the respective chloride salts were added. Moreover, the optode could perform fast response (within 20–50 s) and long-term stability (continuous use for at least 3 months).