I want to add phenolphtalein in a solution of sodium hydroxide that I will use to measure methane by displacement. The solution will be 5% sodium hydroxide to dissolve CO2 and only measure methane by displacement. Its supposed that when the solution gets saturated by CO2 and get acid the phenolphtalein will change color. How much phnolphtalein i need to add to have this effect?
You can know when CO2 will be completed neutralized by NaOH based on the following reactions:
NaOH + CO2 = NaHCO3, NaHCO3 + NaOH = Na2CO3 + H2O
Phenolphtalein is only a pH indicator and its added volume will be reflected to the pink color intensity in moderate alkaline environment, but it can not influence the neutralization point. The change of color (from pink to colorless) occurs at pH of 8 - 8.2. The character of the Na2CO3 solution will be slightly alkaline due to the strong NaOH, this means that the completely neutralized NaOH solution will turn to colorless at pH around 8.2. According to the two stoichimetric reactions, 50 g NaOH that are dissolved in 1 liter solution (5%) will react with 14 liters CO2. More CO2 to be disolved will make the indicator become colorless. Only few ml of phenolphtalein (aqueous solution) are required for 1 liter NaOH sol. 5% to get a moderate pink color that will easily allow the operator to notice the color change at pH around 8.2. For the same purpose I use few ml of bromothymol blue that turns from blue to yellow at pH around 7. More or less indicator is reflected to the color intensity.
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