Bromothymol Blue (BTB) is added during a titration to determine the quantity of Sodium or Potassium Hydroxide required to neutralise residual free fatty acids in solution with recovered organic oils before processing into methyl esters (biodiesel). Unlike Phenolphthalein, which exceeds pH neutral by providing a visual pink indication at pH 8.2, BTB reacts to acidic (less than pH 7), neutral (pH 7.0 to 7.4), and alkaline (pH 7.5 and above) solutions by displaying a yellow, green then blue respective visual colour change. 1L of BTB solution is sufficient for over 2000 titrations.