Expt 5-Common Ion Effect

Common Ion Effect

Objectives

To determine the effect of the presence of a common ion on the extent f ionization of an electrolyte

To determine the type of substance which comprise buffer solutions

To identify buffer solutions To determine the effect of a common

ion on the solubility of a slightly soluble substance

Introduction

An acid-base buffer is a solution that lessens the impact on pH from the addition of acid or base.

A buffer must contain an acidic component that can react with the added OH- ion and a basic component that can react with added H3O+, to withstand the addition of strong acid or strong base without significantly changing its pH.

Introduction

However, these buffer components cannot be just any aid and base because they would neutralize each other.

Most commonly, the components of a buffer are the conjugate acid-base pair of a weak acid.

Buffers work through a phenomenon known as the common-ion effect.

Introduction

Buffer Capacity Buffer capacity is a quantitative measure

of the resistance of a buffer solution to pH change on addition of hydroxide ions.

where Kw is the self-ionization constant of water and CA is the analytical concentration of the acid, equal to [HA]+[A-].

Introduction

Buffering Capacity Buffer capacity of a weak acid reaches

its maximum value when pH = pKa

At pH = pKa ± 1 the buffer capacity falls to 33% of the maximum value. This is the approximate range within which buffering by a weak acid is effective.

Buffer capacity is directly proportional to the analytical concentration of the acid.

Rationale

Knowledge on buffers and the common-ion effect is significant because: Buffer solutions are necessary to keep the

correct pH for enzymes in many organisms to work. Many enzymes work only under very precise conditions; if the pH strays too far out of the margin, the enzymes slow or stop working and can denature, thus permanently disabling its catalytic activity.

Industrially, buffer solutions are used in fermentation processes and in setting the correct conditions for dyes used in coloring fabrics. They are also used in chemical analysis and calibration of pH meters.

Methodology

Part A. Effect on the Ionization of Acids/Bases

Into 6 separate test tubes, prepare:

Methodology

Part B. Buffering Effect

Methodology

Part B. Buffering Effect (continued)

1 drop 6M

NaOH

1 drop 6M

NaOH

1 drop 6M

NaOH

1 drop 6M

NaOH

1 drop 6M

NaOH

1 drop 6M HCl

1 drop 6M

NaOH

1 drop 6M

NaOH

1 drop 6M

NaOH

1 drop 6M HCl

Methodology

Part C. Effect of Common Ion on the Solubility of slightly Soluble Salts

Results and Discussion


Reagents pH10 mL 0.1M HCl + 2mL H2O 1.4

10 mL 0.1M HCl + 2 mL 0.1M NaCl

1.39

10 mL 0.1M HOAc + 2mL H2O 2.96

10 mL 0.1M HOAc + 2mL 0.1M NaOAc

3.43

10 mL 0.1M NaOH + 2mL H2O 12.23

10 mL 0.1M NaOH + 2mL 0.1M NaCl

12.19

Table 1. Experimental pH for the effect on the Ionization of Acids/Bases



The common ion effect is an application of LeChatelier’s Principle. Whenever a strong electrolyte is added to a weak acid or a base which has an ion in common with it, the dissociation decreases. The equilibrium will respond so as to undo the stress of added common ions. This means that the equilibrium will shift to reduce the effect of the common ion which is to the left side of the equation (backward) thus reducing the solubility of the slightly soluble system.


Part A. Effect on the Ionization of Acids/Bases For the first solution, a low pH is expected

due to the presence of a strong acid, HCl. The pH is determined by the concentration of the H+ in the solution by using the M1V1=M2V2 formula.

(10mL)(0.1M)=(12mL)(M2)

M2 = 0.0833 M

pH = -log[0.0833]

pH = 1.08


Part A. Effect on the Ionization of Acids/Bases For the 2nd solution, the pH is expected to be

similar as the first solution because the addition of extra Cl- ions does not affect the concentration of of H+.

The pH of the 4th solution is expected to be higher than the pH of the 3rd one because it is a weak acid and it dissociates to a lesser extent. The addition of the acetate ion from NaOAc, which is a strong electrolyte, will cause the reaction to shift towards the opposite of dissociation, thus it will cause the pH to increase.


Part A. Effect on the Ionization of Acids/Bases The pH’s of the 5th and 6th solutions are

expected to be high because of the presence of the strong base, NaOH. The pH of both are close to each other since NaOH dissociates completely and it is not affected by the presence of any other ion.


Part B. Buffering Effect

Solution pH of origina

l

pH theoretic

al

pH after HCl pH after NaOH Theoretical Conclusion

Distilled H2O 7 2.6 10.7

10 mL 0.5M HOAc + 10mL 0.5M

NaOAc 4.13 4.76 3.81 4.31 Buffer

10 mL 0.5M HCl + 10mL 0.5M NaCl 1.23 0.6 1.11 1.22 Not Buffer

10 mL 0.5M HNO3 + 10mL 0.5M

NaNO31.32 0.6 1.25 1.69 Not Buffer

10 mL 0.5M NaH2PO4 + 10mL

0.5M H2PO46.77 7.2 6.58 6.95 Buffer

10 mL 0.5M NH4OH + 10mL 0.5M

NH4Cl 9.04 9.3 8.66 8.87 Buffer

Table 2. Experimental pH variations on Buffering effects


Part B. Buffering Effect As said earlier, a buffer consists of an

acidic and a basic component which does not consume each other in a neutralization reaction.

Their behavior is based on establishing excesses of both the original acid or base, and the conjugate (generally obtained by adding a salt).


Part B. Buffering Effect For the hypothetical pair HA (a weak acid)

and NaA (a salt containing the ion A-, the conjugate base of HA) this system of reactions is relevant in aqueous solution:

HA(aq) + H2O(ℓ) H3O+(aq) + A-(aq) NaA(s) → Na+(aq) + A-(aq)

The presence of excess A- (the "common ion") causes a shift in the equilibrium of the first reaction and sets up the required condition for buffering behavior.


Part B. Buffering Effect Given the following reaction:

HXH+ + X-

where Ka = [H+][X-] ____________

[HX] For this reaction, the pH depends on the

acid dissociation constant and the ratio of the concentrations of the conjugate acid-base pair.


Part B. Buffering Effect Upon addition of H+, X- ions are consumed

and the concentration of HX increases slightly. On the other hand, adding OH- ions to the buffer will cause the HX to dissociate more and form X- and water, thus the concentration of X- is increased.

As long as the amounts of HX and X- in the buffer are larger compared to the amount of OH- or H+ added, the ratio [HX]/[X-] does not change that much.


Part B. Buffering Effect Consequently, it can be observed that when

the concentration of HX and X- ions are equal the pH becomes equal to the pKa of the solution. Thus buffers whose acid form has a pKa that is close to the desired pH is often used.

When strong acids are added to the buffer solution, the strong acid will react and be neutralized by the base present in the buffer. When strong bases are added to the buffered solution, the strong base will react and be neutralized with the weak acid present in the buffer.

Results

Part C. Effect of Common Ion on the Solubility of slightly soluble salts

Volume of 0.01 M NaOH used----------------37mL

Solubility of benzoic acid----------------0.0144 M

(from previous experiment)Solubility of benzoic acid in sodium benzoate

solution---------------------------- 1.478 x 10-3 M


Part C. Effect of Common Ion on the Solubility of slightly soluble salts The presence of C6H5COO- in the solution

theoretically reduces the solubility of C6H5COOH, shifting the solubility equilibrium to the left.

This reduction in solubility is also due to the common ion effect. Generally speaking, the presence of a second solute that gives a common ion decreases the solubilty of a slightly soluble salt.


Part C. Calculations – Experimental Ksp = [C6H5COO-][H+]

C6H5COOH C6H5COO- + H+

0.5 g NaOAc x 1 mole NaOAc/132 g NaOAc = 0.003788 moles

Initial - 0.07576 0

Change - +x +x

Equilibrium -0.07576

+ xx


Part C. Calculations – Experimental Molarity = 0.003788 moles/0.050 L

= 0.07576 M

Ksp = 1.12 x 10-4 = [C6H5COO-][H+] = (0.07576+x)(x)

*we assume that 0.07576 + x = 0.07576 1.12 x 10-4 = (0.07576)(x) x = 1.478 x 10-3


Part C. Calculations – Theoretical Ksp = [C6H5COO-][H+] = 2.25 x 10-4 M

C6H5COOH C6H5COO- + H+

0.5 g NaOAc x 1 mole NaOAc/132 g NaOAc = 0.003788 moles

Initial - 0.07576 0

Change - +x +x

Equilibrium -0.07576

+ xx

Table 3 . Ice Method


Part C. Calculations – Experimental Molarity = 0.003788 moles/0.050 L

= 0.07576 M

Ksp = 2.25 x 10-4 M = [C6H5COO-][H+] = (0.07576+x)(x)

*we assume that 0.07576 + x = 0.07576 2.25 x 10-4 M = (0.07576)(x) x = 2.97 x 10-3


Part C.

Experimental - 1.478 x 10-3

Theoretical - 2.97 x 10-3

Percent yield = 49.76%


To summarize the effects of the common ion: The presence of a common ion suppresses

the ionization of a weak acid or a weak base. When adding common cations, the

concentration of the salt increases slightly bcause the anions are consumed, consequently when you add common anions to the buffer solution will cause the salt to dissociate more and thus the concentration of the anion increases.

Addition of common ions decreases the solubility of slighlty soluble salts.

Recommendations

The experimental values we got for part B were very bad in the sense that it was not able to show us what solutions were the buffers and the ones that are not. Therefore, we recommend to avoid the following: Contamination of reagent bottles, whether it

may be seen clearly or not Deviating from the procedure written in the lab

manual, specially the measurements and the equipment needed to measure the amounts.

Recommendations

We also recommend that only one type of dropper/Pasteur pipette be used for the reagents. This is because different droppers and pipettes may have different sizes of drops, thus the measurements of the reagents are not totally valid.

We also recommend that a pH meter should be provided for each group, because a great deal of time is consumed for waiting for another group to finish using it.

We also recommend that the pH meter bulb should be washed thoroughly to avoid contamination and to have accurate readings.

References

Chang, R. Chemistry, 5th Ed. 2000

Silberberg, M.S. Chemistry. 4th Ed. New York: McGraw-Hill Companies Inc. 2006.

pH calculator <http://www.sensorex.com/support/education/pH_calculator.html>

Documents

Expt 5-Common Ion Effect