Experiment 4: Common-Ion Effect and Buffers

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EXPERIMENT 2: COMMON-ION EFFECT AND BUFFERS

MARVILE REA R. FERRER1
1DEPARTMENT OF MINING, METALLURGICAL & MATERIALS ENGINEERING, COLLEGE OF ENGINEERING UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES DATE SUBMITTED: DECEMBER 13, 2012
DATE PERFORMED: DECEMBER 07, 2012
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INTRODUCTION

Monitoring the pH range of a laboratory reaction or a process is very important. It is important to keep the pH almost constant even when addition of acids or bases takes place. One can easily know if a solution is an acid or a base using a visual indicator. Indicators are organic dye added to a solution that changes color base on the concentration of H3O ions in the solution. Each color change corresponds into a pH value. Common indicators used in the laboratory are methyl orange, methyl red, bromthymol blue, neutral red and phenolphthalein. In this experiment, methyl orange and phenolphthalein are used.

To keep the pH within the narrow range we used a process called buffer system. It resists changes in pH using conjugate acid-base pair. In doing this, concept of Le Chatelier’s Principle is used specifically the concept of common-ion effect. Common-ion effect is a behavior where the ionization of the weal electrolyte is prevented. There are common encountered type of solution that showed this behavior: a weak acid solution plus a soluble ionic salt of the weak acid or base. These are the solutions that are used in the experiment.

In calculating the pH of the solution the experimenters used the following equations:

Initial pH

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Buffer Solution (Henderson – Hasselbach Equation)

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Buffer Solution after the Addition of Acid or Base

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In this experiment, the experimenters prepared solutions of HOAc, NaHOAc, NH3, NH4Cl, NaOH and HCl. When the needed solutions are already prepared the experimenters used visual indicators and observed their colors. After that the experimenters used the pH meter in each solution to know the solution’s pH value. Next, buffer solutions are made and approximations of pH are made through their color then they used the pH meter and lastly they calculated the value using the equations above. In the second part of the experiment the solutions are divided into three equal parts, the experimenters didn’t add anything in the third beaker; therefore the third beaker became the controlled set-up in the experiment while the other two became the experimental set-up.

The activity aimed to understand and know more about the common-ion effect and buffers by doing calculations, observing the effects and distinguishing.

DATA, RESULTS AND DISCUSSION

In the first part of the experiment the experimenters used visual indicators and pH meter to know the pH of each solution. The experimenters used methyl orange and phenolphthalein as indicators in the experiment. The experimenters dropped the indicators in the solution and the color of the solution immediately changes. This change let us know the pH value of the solution. The methyl orange indicator is red when the solution’s pH is 1 to 3. It is slightly red slightly yellow when the solution’s pH is 3 to 4.5 and yellow when the solution’s pH is 5 or above. In the experiment solution 1’s pH is 3 to 4.5 and solution 2’s is 5 or above. Phenolphthalein indicator is colorless when the solution’s pH is 1 to 8 and 13.0 beyond. It is faint pink when the solution’s pH is 8 to 10 and bright pink when the solutions pH is 10 to 13. In the experiment solution 3’s pH is 10-13 and solution 2’s is also the same.

Table 1: pH using Visual Indicators and pH meter
|Solution |Color of Solution |pH reading | |...
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