CO₂ measurement in aquaria using a plastic film colourimetric indicator

The preparation of a colourimetric, plastic film indicator is described, based on the extrusion of low-density polyethylene, LDPE, mixed in with a small amount (5 wt%) of a CO₂ sensitive pigment, which is blue and yellow in the absence and presence of CO₂, respectively. The pigment comprises a mixture of a pH-indicator dye, Xylenol Blue, XB, with a base, tetrabutyl ammonium hydroxide, coated onto nanoparticles of silica. The extruded XB-LDPE film exhibits a reversible sensitivity towards dissolved CO₂, units: %CO₂, and a striking colour change over the key operating range, 1–2 %CO₂, that needs to be maintained to keep both the fish and any plant-life healthy in an aquarium at 25 °C. The CO₂ sensitivity of the XB-LDPE indicator is independent of the pH and ionic strength of the water under test and so can be used to measure %CO₂ in sea- and fresh-water aquaria. Digital photography coupled with digital colourimetric analysis, DCA, is used to quantify the response of the XB-LDPE indicator in aquarium sea- and freshwater, and a user-friendly macro is provided to make this process simple to effect. The XB-LDPE indicator offers an attractive, faster, more stable, and much more informative alternative to the popular current, qualitative, drop-checker method for measuring %CO₂. The possible use of the XB-LDPE indicator to measure %CO₂ in natural waters is discussed.