Acid-Base Titration



Acid-Base Titration

Neutralization of a Weak Acid with a Strong Base

Purpose

To standardize a solution of sodium hydroxide by titration with a primary standard.

To determine the equivalence point of the titration of a sample of commercial vinegar using a visual indicator.

To determine the equivalence point of the titration using a pH meter.

To determine the Ka of the acid from the plot of pH versus volume of base.

To determine the mass % of acetic acid in a commercial vinegar.

Introduction

A solution of sodium hydroxide of exact molarity cannot be made because of the tendency of the solid to absorb water and carbon dioxide from the air. Therefore, the sodium hydroxide solution must be standardized. In this experiment the standardization of the solution is done by titrating it against a pure sample of potassium hydrogen phthalate (KHC8H4O4) of known mass. This monoprotic acid is abbreviated KHP and has a molar mass of 204.2 g/mol.

After the sodium hydroxide solution is standardized, it is used to titrate acetic acid, the active ingredient in commercial vinegar. The vinegar is first titrated using phenolphthalein as an indicator. The titration is then repeated using a pH meter to monitor it.

From a graph of pH versus mL NaOH, the equivalence point and the half-equivalence point will be found. From these points, the amount of acid present in the solution and the pKa of the acid can be determined.

Procedure

Part I Preparation of 0.100 M Sodium Hydroxide Solution

1. Heat about 1 liter of distilled water to boiling in a 1 L Erlenmeyer flask to remove the dissolved carbon dioxide.

2. Cool the flask in tap water and transfer about 400 mL of the contents to a 500 mL Florence flask.

3. Add about 4.00 mL of stock solution of carbon dioxide-free sodium hydroxide (about 14.3 M) to the flask and fill the remainder of the flask with CO2 free water. Mix thoroughly. Save the rest of the water for Part II.

Part II Standardization of Sodium Hydroxide Solution

1. Weigh out a sample of KHP between 0.25 - 0.40 g and put it into a clean Erlenmeyer flask.

2. Add about 100 mL of CO2 free water and agitate until the KHP is dissolved.

3. Add 2-3 drops of phenolphthalein indicator solution and swirl gently.

4. Rinse a clean buret with about two to three 5 mL portions of the sodium hydroxide solution previously prepared. Discard washings down the drain.

5. Fill the buret with NaOH, running some out through the tip. Take an initial reading to the nearest 0.01 mL.

6. Slowly add the NaOH to the flask until a pink color persists for 30 seconds, which signals the equivalence point. Record the final buret reading to the nearest 0.01 mL.

7. Repeat two more times.

Part III Titration of Vinegar Using Phenolphthalein Indicator

1. Into a clean and dry 250 mL beaker, pour approximately 50 mL of the vinegar sample.

2. Rinse a 10 mL volumetric pipet with the vinegar by drawing about 5 mL into it . Repeat 2 more times. Discard washings into sink.

3. Carefully pipet 10.00 mL of the vinegar into a 250 mL Erlenmeyer flask.

4. Add about 40 mL of distilled water to the vinegar sample.

5. Add three drops of phenolphthalein and swirl to mix.

6. Titrate with your standardized NaOH.

7. Repeat two more times. You should have three trials that agree to within 0.05 mL.

Part IV Titration of Vinegar Monitored with pH Meter

1. Fill the buret with your standardized NaOH to the 0.00 line.

2. Carefully pipet 10.00 mL of the vinegar into a clean 250 mL beaker. Add 90 mL of distilled water and a magnetic stirring bar.

3. Place the beaker on a magnetic stirrer. Clamp the pH meter into place so that the magnet is not touching the electrode. Turn on the stirrer.

4. Set up CBL with pH probe. Use stored calibration. Under collect data, select TRIGGER/PROMPT. Follow directions on the calculator screen to allow the system to warm up (about 30 seconds), then press enter. Before adding NaOH, monitor the pH value on the CBL screen. Once the pH has stabilized, press TRIGGER on the CBL and enter “0” (the buret volume, in mL) in the calculator. You have now saved the first data pair for this experiment. Select MORE DATA to collect another data pair.

5. Begin by adding 5.00 mL increments of NaOH for the first 30 mL. Then add 1 mL increments until the pH reaches pH 6. When the pH reaches 6.0, add 0.10 mL increments of NaOH solution until the pH reaches 11.0. Once pH 11.0 has been reached, take at least five additional pH readings after additional 1.00 mL increments are added. After each increment, allow pH to stabilize, press TRIGGER and enter volume on buret.

Calculations

Part II

1. Calculate the molarity of the sodium hydroxide solution for each trial and then average the three trials. The molar mass of the standard acid, KHP, is 204.2 g/mol.

Part III

1. Calculate the moles of acetic acid in 10.00 mL of vinegar using the average volume of NaOH used in the titration.

2. Calculate the mass of acetic acid in 10.00 mL.

3. Assuming the density of vinegar is 1.008 g/mL, calculate the mass % acetic acid in the vinegar.

Part IV

1. Make a graph of pH versus the volume of NaOH by transferring your data to graphical analysis.

2. Locate the approximate equivalence point by one of the three following methods.

(a) Extend the vertical portion of the curve with a ruler. Mark the tangent points at the top and bottom of this extended line and locate the midpoint.

(b) An alternative way to determine the equivalence point is to examine the data in your calculator in the vertical portion of your graph. Find the largest increase in pH. Find the volume of NaOH required to give this largest increase. This volume represents the equivalence point.

(c) A second alternative for determining the equivalence point is to use graphical analysis to take the first derivative of the points on the vertical portion of your graph. This number represents the slope of the line at that point. The steepest point on the line (greatest slope) is the equivalence point. To find the first derivative, create a third column and under Data, choose New Column Calculations, Other Functions, and Derivative. Type in pH for dependent column and volume for independent column. Make another graph, plotting the first derivative on the y-axis and the volume of NaOH on the x-axis. The graph should have a peak at the equivalence point.

3. Calculate the mass % of acetic acid in vinegar based on this equivalence point.

4. Determine the half-equivalence point (one-half of the volume of NaOH needed for the equivalence point). Find the pH of the solution at that point.

5. Determine the Ka of the acid.

Data

Part II

| | Trial 1 | Trial 2 | Trial 3 |

|Mass of KHP | | | |

|Initial reading on buret | | | |

|Final reading on buret | | | |

|Volume NaOH used | | | |

Part III

| | Trial 1 | Trial 2 | Trial 3 |

|Initial reading on buret | | | |

|Final reading on buret | | | |

|Volume NaOH used | | | |

Part IV

|Volume NaOH | pH | |Volume NaOH | pH |

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Results

Part II

| |Trial 1 |Trial 2 |Trial 3 |Average |

|Molarity NaOH | | | | |

Part III

|Moles acetic acid | |

|Mass acetic | |

|Mass % acetic acid | |

Part IV

|1 |Approximate equivalence point | mL NaOH |

|2 |Precise equivalence point | mL NaOH |

|3 |Mass % acetic acid | |

|4 |Half-equivalence point | mL NaOH |

|4 |pH at half-equivalence point | |

|5 |Ka of acetic acid | |

Questions

1. Explain how the Ka of an acid can be determined from the titration of the acid with a strong base.

2. You followed the titration with a visual indicator and with a pH meter. Which technique was most accurate? Explain why.

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