Phosphoric Acid Titration Using Ph Meter Experiment 1.

[pic 1]

Faculty of Science and Agriculture

Department of Chemistry

                Practical Cover Sheet

Group members

1. Gumede B.N 20145394
2. Fakude N.C 201417442
3. Sikhakhane M.F 201526494

Aim

To titrate the polyprotic acid (H3PO4) using the strong base NaOH. That was done by using methyl orange, phenolphthalein as indicators and pH meter. The NaOH was added as a titrant.

Abstract

 The polyprotic acid (H3PO4) was titrated with NaOH. When methyl orange was used as indicator the colour changed from pink to pale yellow when 11.50 mL of titrant added. In part 2 the phenolphthalein was used as indicator the colour changed from yellow to pale pink when the 18.13 mL volume of titrant was added.  The last part of the experiment was phosphoric acid titration using the pH meter which showed the two equivalent points.  The first equivalence point at pH 4.65 and the second equivalence point at 9.19.

Introduction

Phosphoric acid H2PO4 is the triprotic acid meaning that has three hydrogen protons. It protolyze in three phases, each phase having a distinctive ionization constant and equation as shown below.

               ………[1][pic 2][pic 3][pic 4]

               ……….[2][pic 5][pic 6]

                  …………...[3][pic 7][pic 8]

Phosphoric acid is a weak acid in the first ionization step since it does not ionize completely in the aqueous solution. It is about 10-5 in weaker acid in the second and very weaker in the third ionization about 10-5.

If the strong base is used to titrate the polyprotic acid, the titration reaction takes place in steps. The most acidic group is neutralized first, followed by the second most acidic and third one. The pH in the midpoint of the titration when the titration curve is flat is equal to the pKa. It is necessary for the two equivalence points to differ by at least three orders of the size so that it can be easy to identify them. The equivalence points of  have a difference of about 10-5. The first and the second equivalence point are easily differentiated from each other. The third equivalence point is very small so it difficult to identify in aqueous solution or titration.   [pic 9]

 From equation [1]    introducing –log in both sides this equation can be ---log…………………………………….[4][pic 10][pic 11][pic 12]

p= -log………………………………………………………………………..[5]

p…………………………………………………..[6][pic 13]

Making pH subject of the formula so we transpose  to get equation 7 which is known as the Henderson-Hasselbalch equation.[pic 14]

p………………………………………………………[7][pic 15]

At precisely one half equivalence point volume = [pic 16][pic 17]

p………………………………………………………[8][pic 18]

That means p[pic 19]

The general trend of decreasing in acidic is . [pic 20]

Reagents and materials    

• Burette stand and one burette
• Pipet (10 mL)
• Magnetic stirrer bar
• pH meter
• pipetting syringe
• 0.09212M of NaOH
• Two 250 mL Erlenmeyer flask
• Two 50 mL beakers
• Distilled water
• 0.1000 M of H3PO4
• Methyl indicator
• Phenolphthalein indicator

Procedure  

Methyl Orange:

The burets were rinsed with the 5 mL NaOH. The air bubbles were removed thoroughly and each burette was filled with NaOH to 1 cm above the top graduate mark. The pipet was rinsed with small quantities of H3PO4. The 10 mL of H3PO4 was transferred to the Erlenmeyer flask using pipet. The 50 mL of distilled water was added for bulk. Three drops of the methyl orange indicator were added to the H3PO4 solution with water, the solution in the flask turned to pink from colourless. The solution was stirred using the magnetic stir bar. The solution was then titrated using NaOH from the burette until the colour changed from pink to pale yellow. The triplets for that procedure were taken. The average volume was recorded which was 11.50 mL of titrant added.

Phenolphthalein:

 The flask was rinsed first using water and then using H3PO4. The 10 mL of H3PO4 was pipetted and transferred to the flask and 50 mL of distilled water was added to the flask. Three drops of phenolphthalein were added to the flask and the solution changed from colourless to pale yellow. The solution in the flask was then titrated using NaOH under burette. The color of the solution changed from pale yellow to pale pink and the volume of the titrant added was recorded. The triplets for that procedure were done and the average of 18.13 mL of NaOH was recorded.

pH Meter Titration

The pH meter was calibrated according to the meter directions. The buret was filled to the zero mark with NaOH. The equipment was already set up by the laboratory technician. The pipet was rinsed with small volume of H3PO4. The 10 mL of H3PO4 was pipetted and transferred to beaker. The 50 mL of deionized water was added to the beaker. The electrode was inserted to the beaker just above the magnetic stirrer bar. The initial pH was recorded when 0.00 mL of NaOH was added. The base was added from buret into the flask as both 1 mL or 0.2 mL increments and pH recorded as shown in table 3.

Results