Lab Activity 1:Total Acidity

Acidity depends on:

• Acid strength in solution (measured by the degree of hydrogen ion (H+) dissociation1. Strong acids: almost totally dissociates, releasing their hydrogen ions for bonding to other ions (HCl)2. Weak acids: Have their H ions strong bonded (only few released) CH3COOH

• Concentration of acidic compounds in solution. (expressed in:

• %w/w, • %v/v, • ppm, • M (moles/l), • N (Equivalent Weight (EW/Volume)

Total Acidity

• A measure of all the H+ of both dissociated and undissociated acids in solution.

• Titratable acidity is used to approximate total acidity.

• Titratable acidity can be measured by:1- reacting the acid present with a base such as NaOH to the end point (equivalence point or neutrality point( at which H+ are equal to OH- in solution) indicated by a color change of an indicator.

Titrations

• SA/SB –equivalence point is neutral.

• WA/SB –equivalence point is basic.

• WB/SA –equivalence point is acidic.

Titration

• Titration– A standard solution with a

known [ ] (the titrant) is used to determine the [ ] of an unknown solution.

– The reaction that occurs is a neutralization reaction.

standard solution (in buret)

unknown solution

• Equivalence point– Point in the rxn which equal amounts of H3O+

and OH- exist.• dramatic change in pH

• End Point– Point at which a color

change is observed.• The equivalence point and

end point are not necessarily concurrent.

Titration

equivalence point

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.00.0 10.0 20.0 30.0 40.0

pH

Volume of 0.100 M NaOH added(mL)

Titration of a Strong Acid With a Strong Base

Solutionof NaOHSolutionof NaOH

Solutionof HCl H+

H+ H+

H+

Cl

Cl-

Cl-

Cl-

Na+

Na+

Na+

Na+

OH-

OH-OH-

OH-

Titration Curve

Features of the Strong Acid-Strong Base Titration Curve

1. The pH starts out low, reflecting the high [H3O+] of the strong acid and increases gradually as acid is neutralized by the added base.

2. Suddenly the pH rises steeply. This occurs in the immediate vicinity of the equivalence point. For this type of titration the pH is 7.0 at the equivalence point.

3. Beyond this steep portion, the pH increases slowly as more base is added.

The four Major Differences Between a Strong Acid-Strong Base Titration Curve and a Weak Acid-Strong Base Titration

Curve

1. The initial pH is higher.

2. A gradually rising portion of the curve, called the buffer region, appears before the steep rise to the equivalence point.

3. The pH at the equivalence point is greater than 7.00.

4. The steep rise interval is less pronounced.

Features of the Titration of a Polyprotic Acid with a Strong Base

1. The loss of each mole of H+ shows up as separate equivalence point (but only if the two pKas are separated by more than 3 pK units).

2. The pH at the midpoint of the buffer region is equal to the pKa of that acid species.

3. The same volume of added base is required to remove each mole of H+.

For Experimentsfollow the lab manual !!