Titration of Fumaric and Maleic Acid

Computer-Interfaced Experiments - pH Measurement
Titration of Fumaric and Maleic Acid

Objectives: Determination of the Equivalence Point Volumes and the pka Values

Peter Keusch

Datalogging and data analysis using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
aqueous fumaric acid (approx. 0.025 M)
aqueous maleic acid (approx. 0.025 M)
0.5 M NaOH

Apparatus and glass wares:
magnetic stirrer
magnetic stirring bars
stirring bar remover
2 beakers 250 mL
burette 20 mL
2 volumetric pipettes 50 mL
2 pipette bulbs
pH probe (IBK electronic + informatic)
drop counter (IBK electronic + informatic)

Hazards and safety precautions:

Maleic acid is harmful if swallowed. Corrosive - causes irritation or burns.

Safety glasses and protective gloves required.

Theoretical background

Determination of the dissociation constant of weak acids

The pk_a can be found experimentally from the data accumulate during a pH titration. pH and pka are related by the Hendersen-Hasselbach Equation.

The relationship relates

By solving for

and taking the -log of both sides the Hendersen-Hasselbach Equation is obtained:

At the point on the titration curve halfway to neutralization [ A^- ] = [ HA ]. The second term on the right side of the equation above becomes zero and pH = pka

Calibration of the pH probe:

Fig. 1: Experiment set-up

Both drop counter (2) and pH probe (1) are connected to the CHEMBOX.

From the menu of the program Chemex 'Options/Calibration/pH-Value' is selected. A calibration dialog with calibration instructions appears as follows.

Fig. 2: Calibration of the pH-electrode

First the unit "pH" is entered, then the reference points at which the probe needs to be calibrated (7.0 and 9.0 respectively).

Calibration procedure:
First reference point: The pH probe is immersed in the pH 7.0 buffer. When the displayed value is stable, then Set Ref1 is clicked.
2nd reference point: The probe rinsed with distilled water is placed into the pH 9.0 buffer. The constant value is affirmed by clicking on Set Ref2. The calibration process is complete. If a calibration point is inoptimal click on Set Ref... > again. The button returns to its unpressed position and a new reference point can be determined.

Calibration of the drop counter.

Using the titrant 0.5 M NaOH the calibration procedure is carried out as described in Redox titrations

Experimental procedure:

Aqueous solutions of the butenedioic acids are titrated with 0.5 M NaOH.

The burette is filled with the standard solution (0.5 M NaOH).100 mL of the sample solution (aqueous butenedioic acid) is pipetted into the beaker and the stirrer is started. The pH probe is immersed in the solution of the acidic solution (Fig. 1). The standard solution is added dropwise to the sample and immediately the sensing software is started.

Results:

Fig. 3: Realtime graph titration curves: fumaric acid (grey), maleic acid (red)

Determination of the equivalence point volumes and of the pka values:

The equivalence point is reached in an acid-base titration when equivalent amounts of acid and base have reacted. It is taken at the steepest point in the titration curve's inflection.

pka is the pH at the half-way point in a titration of a weak acid. The pKa values of maleic acid can be determined from the pH values in the buffer regions of the titration curve.

At first, the determination of the equivalence point volumes is accomplished. Subsequently the determination of the pka values is carried out (Fig. 3)

Determination of the equivalence point volumes:
1. Using the tool 'Line' provided by CHEMEX a line (perpendicular to the volume axis) is drawn tangent to the linear portion in the injection region of the titration curve. This procedure allows in the present case to find the second equivalence point volume in the titration curve of maleic acid.
2. The equivalence point volume can also be found using the steepest tangent to the curve. The first equivalence point in the titration curve of maleic acid is the mid point of the corresponding linear portion.

Determination of the pka values:
1. The pH value at the first half-titration volume is equal to the pka1 value. The first half titration volume can be found by dividing the first equivalence point volume by two.
2. Similarly, the pH value at the second half-titration point is equal to the pka2 value. The midpoint volume is the average volume of the first and second equivalence point volume. The second half-titration volume is exact midway between the first and the second equivalence point.

By positioning the mouse cursor at the appropriate point on the graph (Fig. 3 cursor: nw-resize) the equivalence point volumes, half titration volumes and the pKa values are displayed in the title bar on the top of the screen.

Discussion:

Fig. 4: Geometric isomerism of the butenedioic acids

Maleic acid is the cis-(Z)-2-butenedioic acid and fumaric acid is the trans-(E)-2-butenedioic acid. The geometrical isomers differ from each other in chemical and physical properties.

	Fumaric acid	Maleic acid
Melting point	287 °C	130 °C
Water solubility	difficult soluble, 0.63 g / 100 mL (25 °C)	easily soluble
pka1	3.02 (18 °C)	1.92 (25 °C)
pka2	4.44 (18 °C)	6.07 (25 °C)

The two carboxyl groups of fumaric acid are attached on opposite sides of the double bond. They are identical and do not interact - formation of an intramolecular H-bond is impossible. Unlike the fumaric acid molecules, the two carboxyl groups in maleic acid molecules are on the same side of the double bond - formation of an intramolecular H-bond is allowed (Fig. 4).

The titration curve of maleic acid shows two potential jumps. On the other hand the two pka values of fumaric acid are very close together (Fig. 5) - so close, in fact, that two distinct equivalence points can not be discerned from the titration curve. Evidently the two potential jumps are superimposed. The result is a gradual increase of the ph value up to the equivalence point (Fig. 3).

Fig. 5: Dissociation of fumaric acid

Maleic acid has pka values seperated by several orders of pH units. The cis-isomer ionizes in two steps. After the first ionization, intramolecular hydrogen bonding stabilizes the cis-monoanion (Fig.6). When 'one' of the carboxylic groups deprotonates, the other can form a strong hydrogen bond to it; overall, the effect is to favor the deprotonated state of the hydrogen-bond-accepting group, thereby lowering its pKa from ~ 4 to 1.92 and to favor the protonated state of the hydrogen-bond-donating group, raising its pKa from ~ 4 to 6.07
(a pKa of ~4 is typically for carboxylic acids). The strong intramolecular hydrogen bond in the maleate monoanion favors the formation of the maleate H⁺, and opposes the removal of the second proton from that species. As a result the first ionization is easier and the second is difficult.

Fig. 6: Dissociation of maleic acid

The maleate monoanion forms a stronger hydrogen bond than the corresponding free acid. The conjugate base is more stable than the monoanion of fumaric acid. In the trans isomer, the two carboxyl groups are always far apart, so hydrogen bonding is not observed. Therefore maleic acid is much stronger than fumaric acid. On the other hand, the monoanion of fumaric acid is the stronger acid than the analog maleate ion. In the latter case, the intramolecular hydrogen bond is reversed in the second ionization step and the formed dianion is destabilized by the electrostatic repulsion between the neighboring carboxylic acid anions. The interaction of the two adjacent negative charges in the dianion is energetically unfavorable (Fig. 6).

Also melting points and solubility of the two acids show variations attributable to geometric isomer effects. The trans-isomer fumaric acid molecule is more symmetrical in shape than that of maleic acid. Fumaric acid forms cyclic acid dimers in the solid state forming two intermolecular hydrogen bonds per molecule. On the other hand, maleic acid forms intermolecular catamers and also intramolecular hydrogen bonds, so only one intermolecular hydrogen bond per molecule is formed. As a result, there will be less carboxyl groups available for the formation of intermolecular hydrogen bonds, which determine the melting point (Fig. 7). The difference in the melting points between maleic acid (130 °C) and fumaric acid (287 °C) is indeed due to the geometry resulting in a different number of intermolecular hydrogen bonds and not to the weak van der Waals strain, presumed in maleic acid.

Fig. 7: Intermolecular hydrogen bonding of maleic acid (1) and fumaric acid (2)

Maleic acid is soluble in water whereas the solubility of fumaric acid in this solvent is very limited. Also this property of maleic acid can be explained on account of the intramolecular hydrogen bonding that takes place at the expense of intermolecular interactions. Moreover, the molecules of maleic acid have a stronger net dipole moment than fumaric acid. They are polar and are easily hydrated by the polar water molecules. Fumaric acid is non-polar and has dipole moments that cancel out each other, giving the molecule a zero net dipole moment. Also the tightly and efficiently packing of the molecules in the solid state is partly responsible for the low solubility of fumaric acid in water.

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