Dipicrylamine-modified triacetylcellulose membrane for optical pH and potassium ion measurement

2,4,6,2′,4′,6′-Hexanitrodiphenylamine (dipicrylamine) is covalently bonded to a transparent triacetylcellulose film. Activation and modification of the cellulose support is simple and inexpensive. An acid pretreatment step for improving the hydrophilicity of the membranes is also introduced. The membranes can be used for repetitive and reversible pH measurement in the pH range 0.0–3.2. The relative standard deviation is about 0.2% for five times measurements of the maximum changes at 450 nm from the acidic form of the indicator to its basic form. The use of the membranes for potassium ion and pH measurement in highly acidic media is also studied. In highly acidic media (> 1 M), bleaching of the indicator and low sensitivity are the main problems. The membranes respond to potassium ion concentration changes at a constant pH reversibly in the range 1.0 × 10−4–7 × 10−3M. A selectivity factor of 1.9 over sodium ion was obtained.