Buffers and Acid/Base Titration

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Buffers and Acid/Base Titration

Reaction of Weak Bases with Water
The base reacts with water, producing its conjugate acid and hydroxide ion: CH3NH2 + H2O  CH3NH3+ + OH- Kb = 4.38 x 10-4

Kb for Some Common Weak Bases
Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you? Base Formula Conjugate Acid Kb Ammonia   NH3  NH4+  1.8 x 10-5   Methylamine  CH3NH2  CH3NH3+  4.38 x 10-4   Ethylamine  C2H5NH2  C2H5NH3+  5.6 x 10-4   Diethylamine  (C2H5)2NH  (C2H5)2NH2+  1.3 x 10-3   Triethylamine   (C2H5)3N   (C2H5)3NH+  4.0 x 10-4   Hydroxylamine  HONH2   HONH3+    1.1 x 10-8   Hydrazine H2NNH2  H2NNH3+    3.0 x 10-6   Aniline  C6H5NH2   C6H5NH3+    3.8 x 10-10   Pyridine  C5H5N   C5H5NH+    1.7 x 10-9

Reaction of Weak Bases with Water
The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H2O  BH+ + OH- (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

Buffered Solutions A solution that resists a change in pH when either hydroxide ions or protons are added. Buffered solutions contain either: A weak acid and its salt A weak base and its salt

Acid/Salt Buffering Pairs
The salt will contain the anion of the acid, and the cation of a strong base (NaOH, KOH) Weak Acid Formula of the acid Example of a salt of the weak acid  Hydrofluoric  HF   KF – Potassium fluoride   Formic   HCOOH   KHCOO – Potassium formate   Benzoic   C6H5COOH   NaC6H5COO – Sodium benzoate  Acetic   CH3COOH   NaH3COO – Sodium acetate   Carbonic   H2CO3   NaHCO3 - Sodium bicarbonate  Propanoic   HC3H5O2    NaC3H5O2  - Sodium propanoate  Hydrocyanic   HCN   KCN - potassium cyanide

Base/Salt Buffering Pairs
The salt will contain the cation of the base, and the anion of a strong acid (HCl, HNO3) Base Formula of the base Example of a salt of the weak acid Ammonia   NH3  NH4Cl - ammonium chloride  Methylamine  CH3NH2  CH3NH2Cl – methylammonium chloride  Ethylamine  C2H5NH2  C2H5NH3NO3 -  ethylammonium nitrate  Aniline  C6H5NH2  C6H5NH3Cl – aniline hydrochloride  Pyridine  C5H5N    C5H5NHCl – pyridine hydrochloride

Titration of an Unbuffered Solution
A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

Titration of a Buffered Solution
A solution that is 0.10 M CH3COOH and 0.10 M NaCH3COO is titrated with 0.10 M NaOH

Comparing Results Buffered Unbuffered

Comparing Results Unbuffered Buffered
In what ways are the graphs different? In what ways are the graphs similar?

Henderson-Hasselbalch Equation
This is an exceptionally powerful tool, and it’s use will be emphasized in our problem solving.

Weak Acid/Strong Base Titration
Endpoint is above pH 7 A solution that is 0.10 M CH3COOH is titrated with 0.10 M NaOH

Strong Acid/Strong Base Titration
Endpoint is at pH 7 A solution that is 0.10 M HCl is titrated with 0.10 M NaOH

Strong Acid/Strong Base Titration
A solution that is 0.10 M NaOH is titrated with 0.10 M HCl Endpoint is at pH 7 It is important to recognize that titration curves are not always increasing from left to right.

Strong Acid/Weak Base Titration
A solution that is 0.10 M HCl is titrated with 0.10 M NH3 Endpoint is below pH 7

Selection of Indicators

Indicator Transitions
Low pH color Transition pH range High pH color Gentian violet (Methyl violet 10B) yellow 0.0–2.0 blue-violet Leucomalachite green (first transition) green Leucomalachite green (second transition) 11.6–14 colorless Thymol blue (first transition) red 1.2–2.8 Thymol blue (second transition) 8.0–9.6 blue Methyl yellow 2.9–4.0 Bromophenol blue 3.0–4.6 purple Congo red 3.0–5.0 Methyl orange 3.1–4.4 orange Bromocresol green 3.8–5.4 Methyl red 4.4–6.2 4.5–5.2 Azolitmin 4.5–8.3 Bromocresol purple 5.2–6.8 Bromothymol blue 6.0–7.6 Phenol red 6.8–8.4 Neutral red 6.8–8.0 Naphtholphthalein colorless to reddish 7.3–8.7 greenish to blue Cresol Red 7.2–8.8 reddish-purple Phenolphthalein 8.3–10.0 fuchsia Thymolphthalein 9.3–10.5 Alizarine Yellow R 10.2–12.0 Litmus Source: Wikipedia