Basic Techniques of Microbiology

Dominic Held

Micro Scale Titration

Overview:

In this experiment, the method of titration will be used to determine the gram molecular weight (GMW) of citric acid by titration with NaOH. Using the experimental GMW, the normality of citric acid will be calculated. Phenolphthalein will be used to indicate when the titration is complete. With just 1 to 2 drops of phenolphthalein, the color of the solution will be colorless when it is acidic, and become pink when the solution is neutralized. This titration will be performed on a micro scale. Instead of measuring the volume of acid and bases used, drops from a transfer pipette will be counted.

Objectives:

Students will prepare a 0.2 molar solution from 1.0 molar solution of NaOH that will be supplied to them. Students will gain experience with precision when working with the small amount of solution that is used. They will also be introduced to the method of titrations. Students will then calculate the gram molecular weight of citric acid using their data.

Materials:

24 well reaction plates Coffee stirrers Transfer pipettes Phenolphthalein 0.2 M NaOH

Procedure:

Groups of 2 will be assigned.

1. Students must wear safety goggles at all times. 2. Using the 1.0 M NaOH solution that is given, make a small sample of a 0.2 M solution of NaOH. 3. Using your transfer pipette place seven drops of citric acid in five wells. 4. Add one drop of phenolphthalein in each of those five wells. 5. Counting each drop, begin adding 0.2 M NaOH stirring the solution as you go. 6. When the solution stays pink despite stirring, the solution has been neutralized. Record the number of drops that were added. 7. Use this data to calculate the experimental value of the gram molecular weight of the citric acid. Dominic Held

Calculations:

We diluted solutions to lower Molarity in previous labs. Give your calculations for diluting a 1.0 M NaOH solution into a 0.2 M NaOH solution.

Trial # drops # drops # drops moles acid C6H8O7 indicator 0.2M NaOH #1 #2 #3 #4 Average GMW Molarity of citric acid

Citric acid is a 100 g/L solution. NaOH has a normality of 1.

[Citric acid] = [NaOH] x (drops of NaOH/drops of citric acid)

GMW = Concentration (in g/L) of citric acid in solution x 3 equivalents Normality of citric acid

Molarity of citric acid = 1 / (GMW x 1L/100g)

Normality of citric acid = ([NaOH] x drops NaOH)/ ([citric acid] x drops citric acid)

Follow-up:

1. What is the molecular weight of citric acid?

2. What is the molecular weight of NaOH?

3. How many OH- ions can one molecule of NaOH produce?

4. How many H+ ions can citric acid produce?

5. How does this affect the titration of citric acid with NaOH?

6. What was your experimental normality of the citric acid? Dominic Held

Assessment:

Participation- Student is on task, working well with group. Instructor should not have to settle disputes between group members on how and who will complete the objectives of the exercise.

Lab Report- This includes data and calculations.

Essay questions: Each student will submit their responses to the essay questions. Responses will be graded on completeness, thoughtfulness, grammar, punctuation, and spelling.

Lab Report and essay questions will be submitted on time. No late work will be accepted without a written excuse from a parent or doctor.

Participation in Data Collection ------25 pts Lab Report ------50 pts Essay Questions------25 pts

Total------100 pts Dominic Held

Sample Data

Trial C6H8O7 Indicator 0.2M NaOH moles acid 1 7 1 56 1.60 2 7 1 55 1.51 3 7 1 53 1.51 4 7 1 54 1.54 Average 7 1 54.5 1.56 GMW 192.3 Molarity of 0.521 citric acid

(0.2 x 54.5) / (0.521 x 7) = 2.99