A paper-based visual indicator for detection of ammonia using Ruellia simplex

Ammonia is one of the most important chemicals for agriculture, chemical, and food industries. This condition prompts the need for a research on detecting ammonia gas with a simple and easy method in daily practices. One of the methods for detecting ammonia gas is done through the use of visual indicators. Aiming for making the visual indicators, Ruellia simplex flower extract is used in this research as an indicator, which visually detects ammonia gas. The extract functions as a smart material, which changes its colour according to the extract's pH condition. At pH 2 - pH 3 the extract colour is pink and then turn to purple at pH 6 and becoming green at pH 10 - pH 11. The results of the UV-Vis spectra for the extract solution at pH 2 - pH 3 show the absorbance peak at 535 nm wavelength, then shifted to 537 nm for pH 6 and shifted to 621 nm for pH 10 - pH 11. For developing paper-based indicators, cellulose paper (1cm × 6cm) is used for substrates. The cellulose paper's tip has been coated with extracts whose pH have been preconditioned to 2 correspondingly. Using ammonia gas with a concentration between 0% and 0.1%, a test on the visual indicator paper is performed. Colour changes that occur in the indicator paper are observed as a function of detection time and then analyzed through RGB image analyzer. The result of the experiment shows that the indicator paper undergoes a change of colour upon the detection of ammonia gas. The colour change depends on the detected gas concentration. The paper indicators with pH value of 2 has the most optimum contrast in colour change as the paper's original pink colour changes into green upon the detection of ammonia. This research may be able to be developed further for the purpose of detecting food freshness.