C126 Experimental Chemistry II
October 12, 2012
Titration of Acids and Bases
According to Arrhenius definition, acid is substance that produces H3O+ ion while base is substance that produces OH- ions. The reaction between acid and base often yields the products of salt and water. The formation of water in this neutralization reaction is caused by the combination of H3O+ and OH- ions.
In order to determine the concentration of an unknown acid and base, a method called acid-base titration is used. The end of the titration has been reached when the moles of acid equals the moles of base. This is called the equivalent point. However, end point os the actual point that is reached in neutralization reaction. End point is signalled by the change in color of the solution because of the presence of pH indicator. The common used indicator is phenolphthalein; it is colorless in acidic solution and pink in basic solution. In this lab, the concentration of sodium hydroxide (NaOH) was determined by using a known amount of Potassium Hydrogen phthalate (KHP). The balanced chemical equation for the reaction is: KHC8H4O4(aq) + NaOH(aq) => H2O(l) + KNaC8H4O4(aq) (1)
KHP was chosen as a good acid to standardize NaOH because it has high molecular weight and stable on drying. The standardized NaOH solution was then used to determined the percent composition of KHP in an unknown substance #47.
Part 1: Standardization of NaOH
NaOH solution was prepared by taking 75mL. NaOH from stock solution and then diluted to 750 mL with DI water in Nalgene bottle. Three samples of KHP were weighed on analytical balance model BP2505, and placed in 250 mL Erlenmeyer flask. The samples of KHP were warmed up on hot plate to dissolve in Erlenmeyer flasks with 100 mL of DI water added. Two drops of indicator phenolphathalein were added to each flask after KHP completely dissolved.