User:Monika Gasiorek/Notebook/CHEM-571 2014F/2014/09/02

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September 2, 2014

Titrant Stock Solution Preparation and Standardization

500 mL of approximately 0.150 M NaOH solution was prepared from 2.99978 g of solid NaOH and 500 mL of H2O.

  • the specified molarity was chosen with regards to a preferred amount of 20 mL of titrant per titration and approximately 200 g of the heaviest solid amino acid to limit waste and provide precision

The titrant was standardized in order to determine the actual concentration.

  • approximately 0.500 g of KHPhth in approximately 15-30 mL of H2O was titrated with the stock titrant in the presence of phenolphthalein indicator for a total of 4 trials
  • the molarity of the NaOH titrant was calculated for the 2 trials that demonstrated reproducible results

NOTE:The first trial did not result in a color change. The second trial required approx. 25 mL of NaOH for a color change to be observed. The error here could possibly be due to the failure to completely dissolve all of the solid NaOH crystals in 500 mL of water. The stock bottle was shaken properly before the 3rd and 4th trials, resulting in reproducible results.

The calculated average for the NaOH titrant stock solution was 0.1575 M.

The data from the titrant standardization is included below.

Trial KHPhth (g) NaOH added (mL) Molarity (mol/L)
1 0.49870
2 0.50399 25.00 0.09871
3 0.50230 15.80 0.1557
4 0.50264 15.45 0.1593

Hydrophobic Amino Acid Titration: Valine

A solution of valine was titrated with the 0.1575 M NaOH titrant in order to produce a characteristic titration curve for valine.

  • a calculation was done to determine the amount of solid valine that would be required for complete titration with 20 mL of 0.1575 M NaOH
  • a 0.3678 g sample of valine was dissolved in 10 mL of 21 mM HCl, as well as 10 mL of H20 [Trial 1]
  • a 0.3689 g sample of valine was dissolved in 10 mL of 21 mM HCl, as well as 10 mL of H20 [Trial 2]

A pH probe was calibrated and used to track the change in pH as additional amounts of 0.1575 M NaOH were added over time.

  • two trials were done, with respective data presented below
    • trial 1 followed a basic schematic of 5 mL increments, followed by a more thorough 2nd trial guided by the data demonstrated in trial 1
  • a pH curve was constructed from the data points in each trial

Trial NaOH added (mL) pH
1 0.00 3.317
1 5.00 9.106
1 10.00 9.631
1 15.00 10.087
1 20.00 10.945

Trial NaOH added (mL) pH
2 0.00 3.346
2 1.00 8.263
2 2.00 8.263
2 3.00 8.675
2 4.00 8.949
2 5.00 9.115
2 6.00 9.254
2 7.00 9.370
2 8.00 9.473
2 9.00 9.568
2 10.00 9.659
2 12.50 9.901
2 15.00 10.139
2 17.50 10.459
2 20.00 11.071

Valine Titration T1.png

Valine Titration T2.png