Experiment 20 pH Titration of Cola Drinks Qianyi Qu Alexis Wilkinson Reagan Weigel March 10, 2016 CHEM 1212k Section J09 TA: Bocheng Wu Honor Pledge – Freshman Chemistry Laboratories I did not copy any portion of this work from the work of other students, old lab reports, or unreferenced materials. I understand that such activity is a violation of the Georgia Tech Honor Code. ______________________________________ Signature
Introduction The concentration of an acidic solution (analyte) can be determined by titrating it with a volume of a standard base solution (titrant) of known concentration required to neutralize it. The purpose of the titration is the detection of the equivalence point, the point at which chemically equivalent amounts of the reactants have been mixed. The amount of reactants that have been mixed at the equivalence point depends on the stoichiometry of the reaction. Once the equivalence point is observed, the concentration of the known substance can be used to calculate the molarity of the unknown substance. In this experiment, phosphoric acid, a triprotic acid, is neutralized by the strong base NaOH with known concentration and volume. The acidity constants and concentration of phosphoric acid will determined by monitoring the changing pH of the solution throughout titration. Phosphoric acid have more than one dissociating proton per molecule. For each time deprotonation occurred, a Ka value is assigned to the reaction. The first deprotonation has a Ka value greater than the following reaction because it is easier to remove proton from neutral molecule than negatively charged ions. Thus, Ka 1 , Ka 2 , and Ka 3 are separated from each other by several orders of magnitude, and the dissociation may be regarded as taking place in a stepwise fashion as shown on the titration curve. The equivalence point is a location on the curve where the base has neutralized a single acid species and causes the pH to increase drastically. The halfway to the equivalence point is where half of the acid originally present has been neutralized by the base; therefore, titration analysis provides a visual way to estimate Ka by reading the pH at halfway to any equivalence point. The systems studied in this experiment include the standardization of NaOH and titration of phosphoric acid in cola drinks. The actual concentration of NaOH was determined by the neutralization reaction of NaOH with KHC 8 H 4 O 4 (also known as KHP). Since NaOH is a strong base, it will dissociate when react with KHP, given the balanced equation: KH C 8 H 4 O 4 ( aq ) + NaOH ( aq ) ↔H 2 O ( l ) + KNaC 8 H 4 O 4 ( aq ) The degassed cola was titrated with standardized NaOH solution. Since phosphoric acid in cola is a polyprotic acid, multiple Ka values, there are three chemical equations for the dissociation of the phosphoric acid. Each time the acid deprotonated once and gives one proton to water: − ¿ + ¿ + H 2 PO 4 ¿ H 3 PO 4 + H 2 O ( l ) ↔H 3 O ¿ 2 − ¿ + ¿ + HPO 4 ¿ − ¿ + H 2 O ( l ) ↔ H 3 O ¿ H 2 PO 4 ¿ 3 − ¿ + ¿ + PO 4 ¿ 2 − ¿ + H 2 O ( l ) ↔H 3 O ¿ HPO 4 ¿
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- pH, Equivalence point, KHP