Name:Partner:Formal Laboratory ReportC125 Experimental Chemistry ISection 5975IUPUIJuly 21, 2005Determination of Purity Using TitrationINTRODUCTIONA common problem in chemistry is determining whether or not a substance isimpure and to what extent it is impure. Chemicals used in experiments or for commercialpurposes often need to be pure to ensure safety and efficiency during reactions, butdetermining whether or not a given substance is pure can be difficult. If the chemical isan acid or a base, titration with a standardized solution can be used to assess its purity.In a generic titration, “one solution of known concentration is used to determinethe concentration of another solution through a monitored reaction.”1An acid-basetitration works on the principle that the acidic solution will combine the basic solution toneutralize, altering the pH of the solution.H3O+(aq) + OH-(aq)2H2O(l)(1)Eq. 1 shows that when equal moles of the H3O+ion and the OH-ion are present, thesolution is completely neutralized.2When this dynamic equilibrium occurs in a titration,it is called the equivalence point, and at that point a given reaction has a characteristicpH. Thus, indicators, substances that can change color depending on the pH, are valuabletools for monitoring the progress of titration reactions. A few drops of indicator are addedto one of the solutions and the other solution is added dropwise. When the moles of acidand base are equivalent, the equivalence point is reached and adding one more drop of thetitrant will cause the indicator to change color, signaling the end point of the reaction.The end point occurs just after the equivalence point, but the extra titrant required toreach the end point is negligible in calculations.The titration method is useful in determining purity only if a standardizedsolution, one with a known molarity, is available for the titration. If a known volume ofstandardized solution is used in a titration, then the moles of both acid and base can be
determined. From the moles, the mass of the pure substance can be determined andcompared to the mass of the impure substance to find the percent purity.