Titrations. Identify unknown. carboxylic acid

Transcription

1 sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total insoluble impurities dissolve in water, filter aq. sol n of unk. sodium salt + sol. impurities account for all masses; calculate %recovery CH361/361H Week 2 Lecture Identify unknown carboxylic acid End-point decolorize, & filter Potentiometric colored dye(s) mass: ~ 20 mg Titrations aq. sol n of unk. sodium salt + sol. impurities conc. H; filter mp, MW, pk a, derivative mp water-soluble imp. crude unk. carboxylic acid recrystallize pure unk. carboxylic acid melting point ( mp )

2 sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total dissolve in water, filter account for all masses; calculate %recovery insoluble impurities aq. sol n of unk. sodium salt + sol. impurities decolorize, filter Identify unknown carboxylic acid colored dye(s) mass: ~ 20 mg aq. sol n of unk. sodium salt + sol. impurities conc. H; filter mp, MW, pk a, derivative mp water-soluble imp. crude unk. carboxylic acid recrystallize pure unk. carboxylic acid melting point ( mp )

3 sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total dissolve in water, filter account for all masses; calculate %recovery insoluble impurities H Br aq. sol n of unk. sodium salt + sol. impurities decolorize, filter H Identify unknown carboxylic acid mp = 148 C mp = C colored dye(s) mass: ~ 20 mg C 7 H 5 Br 2 mol. wt. = water-soluble imp. aq. sol n of unk. sodium salt + sol. impurities C 7 H mol. wt. = conc. H; filter pk a = 2.85 pk a = 1.82 crude unk. carboxylic acid recrystallize mp, MW, pk a, derivative mp pure unk. carboxylic acid melting point ( mp )

4 Titration Method to determine the concentration of an analyte, through the slow, incremental addition of a known concentration of another reagent Acid-Base Titration: NaH unknown acid (weak) hydroxide (strong base) conjugate base conjugate acid ArCH Determining the Equivalence Point: ph vs. vol. NaH added (potentiometric) Color change of ph indicator (end-point) Figures from Principles of Chemistry: A Molecular Approach, 2 nd Edition by N. Tro (Pearson)

5 End-Point Titration data allows calculation of equivalent weight of unknown acid (for a monoprotic acid, equivalent weight = molecular weight) Phenolphthalein is the end-point indicator you will use ** unless you are colorblind to red ** (if so, inquire about alternative indicators) H ph Indicators are H weak acids that ionize in water: (phenolphthalein) colorless H pink C 2 phenolphthalein colorless: ph 0 to 8.2 phenolphthalein: dianion form pink: ph Figure from Principles of Chemistry: A Molecular Approach, 2 nd Edition by N. Tro (Pearson)

6 End-Point Titration: Practical Considerations Titrations must be carried out carefully! obtain accurate mass of unknown acid must be completely dissolved (~0.3 g/50 ml; can add some ethanol if needed) H - proper buret reading may need to add fraction of a drop of NaH solution must obtain 2 measurements within 0.5% of each other ArCH do not overshoot the end-point make all measurements to 4 significant figures Figures from Principles of Chemistry: A Molecular Approach, 2 nd Edition by N. Tro (Pearson)

7 End-Point Titration: Practical Considerations Titrations must be carried out carefully! Example calculation and consequences of errors obtain accurate mass of unknown mg of dry purified acid titrated with N aq. NaH acid must be completely dissolved (~0.3 g/50 ml; can add some ethanol if needed) end-point reached upon addn. of ml of base proper buret reading # mmols of base = mmol ml 1 x ml may need to add fraction of a drop of NaH solution = mmol must obtain 2 measurements within 0.5% of each other mol. wt. of acid = mg / mmol do not overshoot the end-point = g mol 1 H Br H - ArCH H mp = 148 C mp = C C 7 H 5 Br 2 mol. wt. = Figures from Principles of Chemistry: A Molecular Approach, 2 nd Edition by N. Tro (Pearson) C 7 H mol. wt. =

8 End-Point Titration: Practical Considerations Titrations must be carried out carefully! Example calculation and consequences of errors obtain accurate mass of unknown mg of dry purified acid titrated with N aq. NaH acid must be completely dissolved (~0.3 g/50 ml; can add some ethanol if needed) end-point reached upon addn. of ml of base proper buret reading # mmols of base = mmol ml 1 x ml may need to add fraction of a drop of NaH solution = mmol must obtain 2 measurements within 0.5% of each other mol. wt. of acid = mg / mmol do not overshoot the end-point = g mol 1 H - ArCH BUT, what if acid was not dry? e.g., only 95 wt.% pure Br H H true mol. wt. of acid = mg / mmol = g mol 1 Figures from Principles of Chemistry: A Molecular Approach, 2 nd Edition by N. Tro (Pearson) mp = 148 C mp = C C 7 H 5 Br 2 mol. wt. = C 7 H mol. wt. =

9 End-Point Titration: Practical Considerations Titrations must be carried out carefully! Example calculation and consequences of errors obtain accurate mass of unknown mg of dry purified acid titrated with N aq. NaH acid must be completely dissolved (~0.3 g/50 ml; can add some ethanol if needed) end-point reached upon addn. of ml of base proper buret reading # mmols of base = mmol ml 1 x ml may need to add fraction of a drop of NaH solution = mmol must obtain 2 measurements within 0.5% of each other mol. wt. of acid = mg / mmol do not overshoot the end-point = g mol 1 H - ArCH BUT, what if we over shot end-point? e.g., true value ml Br H H true mol. wt. of acid = mg / mmol = g mol 1 Figures from Principles of Chemistry: A Molecular Approach, 2 nd Edition by N. Tro (Pearson) mp = 148 C mp = C C 7 H 5 Br 2 mol. wt. = C 7 H mol. wt. =

10 Potentiometric Titration data allows determination of pk a of unknown acid K a = ArC [H 3 + ] [ArCH] pk a = log(k a ) as hydroxide is added, a buffer system develops ph = pk a + log ArC ArCH when [ArC - ] = [ArCH], ph is equal to the pk a of the unknown acid

11 Potentiometric Titration Figure downloaded from UC Davis ChemWiki website

12 Potentiometric Titration: Practical Considerations prepare a solution of ~100 mg in ml H - acid must be completely dissolved (ethanol will obscure results use only if needed) heat solution to aid dissolution do not begin titration until solution has cooled to 25 o C ArCH interpret pk a data with caution H H Br mp = 148 C mp = C C 7 H 5 Br 2 mol. wt. = C 7 H mol. wt. = pk a = 2.85 pk a = 1.82