AcidsLesson 17

BuffersIndicators

Buffer Solutions

Buffer solutions are equilibrium systems, which maintain a relatively constant pH when small amounts of acids or bases are added.

Acid Buffer Recipe

Weak Acid + Salt (conjugate)

HF NaF

Buffer Equation Same as weak acid

HF ⇋ H+ + F-

HF ⇋ H+ + F-

[High] [low] [High]

[HF] is high and [H+] is low because HF is a weak acid.

[F-] is high due to adding the salt NaF.

This buffer solution can shift left or right and the [H+] and the pH remains fairly constant.

Base Buffer Recipe

Weak Base + Salt (conjugate)

NH3 NH4Cl

Buffer Equation Same as weak base

NH3 + H2O ⇋ NH4+ + OH-

[High] [High] [Low]

Fill in the blanks to get buffer solutions.Acid Base 

 

 

 

 

 

 

HCN NaCN

H2CO3 NaHCO3

H3PO4 NaH2PO4

CH3COOH KCH3COO

NH4Cl NH3

NaHCO3Na2CO3

NaH2PO4Na2HPO4

Buffer Animation 1

 Buffer Application

 1. Pick the two buffer solutions 

A. NH4Cl and HCl

 B. NaCl and NaOH

 C. HCN and NaCN

 D. NH4Cl and NH3

 

 

 2. Write the equations for the above two buffer solutions. Label the concentrations as high or low.

    HCN ⇋ H+ + CN-

[High] [Low] [High]

  NH3 + H2O = NH4+ + OH-

[High] [High] [Low]

3. A few drops of 0.10 M NaOH is added to a buffer solution at pH = 5.00. The new pH could be:

 A 4.98

 B. 5.02

 C. 8.05

 D. 2.11

 

 

 

 

Acid Base

Yellow Red

Shift Colour

At pH < 10.1 Left Yellow [HInd] > [Ind-]

At pH > 12.0 Right Red [HInd] < [Ind-]

At pH = 11.05 Middle Orange [HInd] = [Ind-]

HInd ⇋ H+ + Ind-Alizarin Yellow

Indicators

 

@ the transition point

[HInd] = [Ind-]

Ka = [H+] 

Ka = 10-pH

Ka = 10-11.05

= 9 x 10-12