Ideas came from when bacteria decomposed their foods such as glucose, they will generate gas and ion which could change the pH of water or sample. The way to measure the changing of pH is indicator. The indicator that we focused is red onion which is natural indicator because it’s traditional vegetable. The changing of color in red onion came from its chemical pigment 'anthocyanidin' which can be effected in pH. From theory, the purple color of red onion water changes to red in acidic solution (pH < 7) and green in basic solution (pH > 7) in figure1.
Figure1. Various types of indicator and their color changes in solution.
After testing with nitrite (NO2-) standard solution (acid, pH = 5) and potassium hydroxide (KOH) solution (base, pH = 10), results (figure2) showed that the solution changed from purple to green color in KOH solution (No.1) and the fading of purple color founded in NO2- solution (No.3). These results supposed to be same as theory.
Figure2. Color changing of red onion indicator.