OBJECTIVE: to find out which of two indicators, methyl orange or phenolphthalein, gives more reliable and consistent results in the titration of H3PO4 with NaOH.
REQUIREMENT: (1). Phosphoric acid solution of unknown concentration. (2). Standard NaOH solution. (3). Methyl orange and phenolphthalein indicators. (4). 5cm^3 pipette, 10cm^3 burette, conical flasks etc.
PROCEDURE: (1). Titrate 5.0ml of the phosphoric acid solution with sodium hydroxide using 2-3 drops of methyl orange as indicator. Record the titre value. (2). AFTER the methyl orange end point, add 2-3 drops of phenolphthalein to the solution and CONTINUE THE TITRATION to the phenophthalein end point. Record this titre value. (3). Repeat the titration one more time. (4). For the third titration, add ONLY 2-3 drops of phenolphthalein to the 5.0ml phosphoric acid solution pipetted, and record the titre value required to reach the end point.
(1). Write the chemical reactions corresponding to the end points with (a) methyl orange (b) phenolphthalein
(2). Calculate the concentration of the phosphoric acid from (a) methyl orange end point (b) phenolphthalein end point
(3). Which result do you consider more reliable, and why?
2 answers