Class: BE210
Group: W3
Members:

Date:May, 2002

Full Text

Abstract:
One of the main objectives of this project was to improve the standardization methods involved in acid and base titrations.  In the first phase of the experiment, sodium hydroxide (NaOH), a strong base, was titrated against potassium acid phthalate, the primary standard.  The titrations were repeated twenty times to determine the concentration of NaOH to an accuracy of 0.5% and a to precision of ± 0.005 M as specified by the manufacturer: 1.000 ± 0.005 M.  Hydrochloric acid (HCl), a strong acid, was then titrated against the standardized NaOH solution to determine its concentration.  The titrations were repeated twenty times to determine the concentration of HCl to an accuracy of 0.5% and to a precision of ± 0.005 M as specified by the manufacturer: 0.995 – 1.005 M.  Burets accurate to ± 0.05 mL were used in conjunction with the indicator phenolphthalein to determine an endpoint of the titrations.  A separate mass-balance method, in which the masses of pre-titration and post-titration samples were taken and converted to volumes via density calculations, was also explored.  The mass-balance method was determined to be more accurate than the volume method and produced the following results: NaOH concentration was 1.000 ± 0.003 M and the HCl concentration was 1.000 ± 0.001 M.  The buret volume method gave the following results: NaOH concentration was 1.005 ± 0.005 M and HCl concentration was 0.998 ± 0.002 M.  Even though the mass-density method was more accurate, both methods of calculation gave results that were within the range of the specific aims.  In the second phase of the experiment, a mixed indicator was created using phenolphthalein and bromocresol green.  A universal buffer was used to create solutions ranging from pH 2-12 in one pH-unit increments.  The optimal mixed concentration in buffer contained 2.854 ? 10-5 mol/L for bromocresol green and 7.233 ? 10-9 mol/L for phenolphthalein.  The molar extinction coefficients of bromocresol green were (5 ± 0.8) ? 106 L/(mol•cm) at 442 nm and (1.43 ± 0.3) ? 107 L/(mol•cm) at 616 nm, and that of phenolphthalein was (1 ± 0.8) ? 108 L/(mol•cm) at 553 nm. The indicator's viability was tested in a titration of HCl against sodium carbonate (Na2CO3) to determine the molar weight of Na2CO3 to an accuracy of 1%, which was found to be 106.7 ± 0.7 g.