titrationcurves.6.pdf

8/17/2019 TitrationCurves.6.pdf

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How to Interpret Titration Curves

• find the equivalence point

– it is the steepest part of the curve where the pHrises the fastest

– the equivalence point can be used to determinethe equivalent weight (molar mass) of the acid

• find the mid point

– located in the center of the buffer region

– geometrically halfway between the equivalencepoint and the beginning of the titration

– sometimes it is a little more complicated than this- see the example

– the midpoint determines the pKa of the acid


• things to do first

– graph your data as seen in the next slide

– make sure you turn on the major and minor tick

marks on both axes

• right click on the axis, choose Format Axis…, change

Major/Minor Tick Mark settings

– Is there enough precision in the tick marks?

• you should have at least 1 mL or smaller for the minor

tick mark on the x-axis

• you should have at least 0.2 pH units or smaller for the

minor tick mark on the y-axis

• Format Axis…, change Major/Minor Units

Two Different Methods

• there are two methods of analysis that will be

shown

– geometric method

• requires a ruler, a pencil, and the titration graph

– 1st derivative method

• requires a spreadsheet and some formula entries

• gives you cool graphs with the 1st derivative pointing

to the equivalence and mid points

• scores you brownie points with the instructors

• pick your method (either will work)

2

4

6

8

10

12

0 10 20 30 40 50

p H

Net Titrant Volume Added (mL)

Weak Acid Titration Curve

equivalence

point

mid point

A Typical Titration Curve


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

equivalence point

Find the Equivalence Point

(Geometric method)

1) using a ruler,

draw lines that

follow the flat,

more horizontal

part of the curve

2) draw a line that

follows the flat,

more vertical part

of the curve


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

equivalence point


(Geometric method)

3) using a ruler,

measure the distance

between the top

intersection and the

bottom intersection

4) the geometric

center of this line

segment is the

equivalence point


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2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

equivalence

point


(Geometric method)

5) draw a verticalline from the

equivalence point

to the x-axis

6) where the line

crosses the x-axis is

the volume at the

equivalence point

(28.7 mL in this case)

Weak Acid Titration Curv e

2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

mid point

Find the Mid Point (Geometric

method)

1) if there is a steep rise in the pH

at the beginning of the graph,

draw a line that follows the steep

part of the curve


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

equivalence

point


method)

2) using a ruler, measure the

distance between the far left and

right intersections

3) the geometric center

between these points is

the mid pointmid point


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

equivalence

point


method)

4) draw a horizontal line from the

mid point to the y-axis

5) where the line crosses

the y-axis is the pH at

the equivalence point

(pH = 7.2 in this case)

mid point


• find the equivalence point

– make sure you subtract the initial buret volume!

– in this case, the initial buret volume was 1.07 mL

– true equiv. pt. = 28.7 mL - 1.07 mL = 27.63 mL

– the 3 is the indicate the limit of the significantfigures

• calculate the equivalent weight (molar mass)

– equiv. wt. = (acid mass)/[(NaOH conc)(equiv. pt.)]

– equiv. wt. = (430.2 mg)/[(0.1139 M)(27.63mL)]

– equiv. wt. = 136.699 = 137 g/mol


• find the mid point

– mid pt = 7.2 = pKa of the acid


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For you Excel Aficionados

• equivalence point

– use the first derivative d pH / d Vol

– plot volume as x and 1st derivative as y in a 2nd series on graph

– the spike in the graph points to the equiv. pt.• mid point

– make a new graph and reverse the axes for the pH curve

• x axis = pH values; y-axis = Vol values

– use the first derivative d Vol / d pH

– plot pH as x and 1st derivative as y in a 2nd series on graph

– the spike in the graph points to the mid point

• use extra columns in the spreadsheet to make these calcs

– equiv. pt. deriv. ( d pH / d Vol) = (pH2 - pH1)/(Vol2 - Vol1)

– midpoint deriv. ( d Vol / d pH) = (Vol2 - Vol1)/ (pH2 - pH1)

• or just the reciprocal of the equiv. pt. derivative


2

4

6

8

10

12

0 10 20 30 40 50

Buret Volume (mL)

p H

0

1

2

3

4

5

6

7

1 s t D e r i v a t i v e ( m L - 1 )

pH

1st deriv.


(derivative method)

1) identify volume

value at the peak

1) identify volume

value at the peak

(28.5 mL in this case)


0

5

10

15

20

25

30

35

40

45

50

2 4 6 8 10 12

pH

B u r e t V o l u m e ( m L )

0

5

10

15

20

25

30

35

40

1 s t D e r i v a t i v e ( m L )mL

1st deriv.

Find the Mid Point (derivative

method)

1) identify pH value at the peak

(pH = 7.3 in this case)

titrationcurves.6.pdf

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