Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

Titration Lab

Titration Lab

Introduction

A titration is an analytical technique in which a reaction is run under specifically controlled conditions. During the titration we measure the stoichiometric volume of one reactant of known concentration, the titrant, that is required to react with another reactant of unknown concentration, the analyte. The concentration of the analyte is determined from the concentration and volume of titrant and the stoichiometry of the reaction between them.

Note: In this lab we know the acid concentration (TITRANT) and we do not know the base concentration (ANALYTE). This is not always the case. 

Since volumes are measured in titrations, this process is also called VOLUMETRIC ANALYSIS.

The most common and used titrations are:  

    • ACID –BASE TITRATION: Uses the neutralization process. We use a base to “titrate” an acid or an acid to “titrate” a base. In this lab, we will titrate Chlorhydric acid using a known concentration of a base (Sodium Hydroxide)  
    • REDOX TITRATION: Uses a redox reaction in the titration process.

Below you can see the step by step calculation needed after the volume of the titrant used is aquired through the titration process. 

Materials:

  • 1 – Burette
  • 1 – 25 mL Volumetric Pipette (SOME TIMES WE USE 10 mL)
  • 1 – Pipette filler
  • 2 – 250 mL Beakers (you will receive one extra)
  • 1 – Conical (Erlenmeyer) Flask
  • 1 – 100 mL Beaker (for rinsing the burette)
  • Sharpie or wax pencils for labelling the beakers
  • 1 – Ring Stand
  • 1 – Burette Clamp
  • 1 – Funnel
  • 1 – Disposable Pipette
  • 1 – Wash bottle with distilled water Paper towel, soap, brushes (for cleaning up)
  • Paper towels

Getting the glassware ready

(do not include these instructions in your lab report)

  1. Wash the four beakers. Rinse them with tap water twice and then with distilled water twice. 
  2. Label one of the two big beakers as “ACID” and another as “BASE”. The third beaker will be used for the titration.
  3. Obtain around 100 cm3 of the acid and the base from the teacher’s container. Be sure to place the acid and the base in the correct beaker.
  4. Wash the burette and the pipette with mild soap water and rinse it twice with tap water.
  5. Rinse them twice with distilled water. Spin the burette and the pipette before releasing the water so they get thoroughly rinsed. Be sure that the valve of the burette is open so the tip of the burette gets rinsed as well.
  6. Close the burette’s valve.
  7. Rinse the burette as follows: Place around 10 cm3 of the acid in the burette. Tilt it and spin it so the acid “washes out any distilled water. The burette is ready to use. Hold it in place using the burette clamp. 
    Be sure its valve is closed before continuting.
  8. Fill the burette with acid until passing the zero level.
  9. Release some liquid so the tip gets filled with acid and THERE ARE NO BUBBLES.
  10. Refill the burette now to near the zero LEVEL.
  11. Pass 10 cm3 of the base into the small unlabeled beaker.
  12. Rinse the pipette with the base in the small beaker. Do not use the base in the labeled beaker.
  13. Release the base inside the small beaker again.
  14. Fill the 25 mL volumetric pipette with the solution contained in the beaker labeled as “base”.
  15. Pass this liquid into the flask where you will perform the titration.
    Be sure you do not drop liquid in the process.
  16. Add 4 or 5 drops of indicator (Phenolphthalein) to the beaker. It will turn hot pink.

You are now ready to begin your titration. Your apparatus should look like the one on the right:

Procedures:

READING AND RECORDING VOLUMES

When reading the burette you should record your values with 2 decimal places since the smallest division in the burette corresponds to 0.10 cm3 (see picture). In the 

 graph the exact value is between 21.10 and 21.20 cm3. You have to ESTIMATE the last digit, so the correct value in this case will be  (21.15  ± 0.05) cm3

Your CAMBRIDGE UNIVERSIRY SYLLABUS specify that the ESTIMATED digit will be only ZERO or FIVE depending on the position of the meniscus. 

If you go to an American University, they ask you to estimate the last digit to any between zero and nine. 

Before performing the titration you will build up a table where you will record the initial, final and Δ (difference) in volume. USE RULER TO BUILD THE TABLE. Include units in your table. Show this data table to your teacher for her approval before beginning.

Sample of Data Tables for Titration.

I will not accept data tables that do not follow the following format and units. Please be sure to fill the tables with measurements that contain 2 decimals. the last decimal is always zero or five.  Tables will be done in graph paper USING RULER and with the correct headings. Refer to this page for any other titration that we will perform. 

 

1) Rough titration

(do not include this value in your calculations of average volume)

Rough
Titration

Initial
Reading of
Titrant / cm3

Final
Reading of 

Titrant / cm3

Volume of 
Titrant
used / cm3

    

2) Real titration

If you get two values that are 0.10 cm3 in difference, you should not perform further titrations. You should use these two values to calculate the average volume.

Titration #

Initial Volume of 
Titrant / cm3

Final Volume of 
Titrant / cm3

Volume of 
Titrant used / cm3

Average volume of titrant used
(cm3)

1    
2  *
3  *
4   
5   

#1 – Rough Titration

Release 10 cm3 of acid on the base and move the beaker in circles without stopping. Then release 1 cm3 at a time and be sure that there is NO permanent change in color each time you close the valve. Wait between each addition of 1 cm3 volumes but continue moving the beaker.

When the color disappears, measure the volume of acid used. This is MORE than you need for your titrations. The exact value should be between the last two values measured. (example: between 24.00 cm3 and 25.00 cm3

2# – Titration -The real thing
(These instructions must be included in your report) 

  1. Record the initial reading in the burette under INITIAL VOLUME OF TITRANT.
  2. Fill the pipette with the solution contained in the beaker labeled as “base”.
  3. Pass this liquid into the flask where you will perform the titration. Be sure you do not drop liquid in the process.
  4. Add 4 or 5 drops of indicator (Phenolphthalein) to the flask. It will turn hot pink.
  5. Repeat the procedures in rough titration until before it changed color. (example: if your cange was produced between 24 and 25 cm3 , stop at 24.00 cm3) and then CAREFULLY add drop by drop, slowly  (do not forget swirling the flask at all times.
  6. Watch out for the change in color.
  7. When the color fades almost completely, record the FINAL reading of the burette in your table as FINAL VOLUME OF TITRANT.
  8. Calculate the volume used. 
  9. If you have more than the volume used still in the burette, you DO NOT NEED TO FILL OUT THE BURETTE AGAIN. You can use the 

In your data table , mark the two or three values you decide to use to calculate the average volume with an asterisk so I know what values you used for your calculations.
 Example: if you decide to use volume 2 and 3  it should show an asterisk “*“in those and the calculated average should how in the last column or you can cross out the values in the volumes NOT USED 

Marks you can get so far: 

The following picture gives you an idea of the amount of marks you can get during the practical and how to present your data properly

Calculations

In acid-base titrations the reaction that takes place in the beaker is NEUTRALIZATION. The OHin the base will react with the Hin the acid to form water.

H+ OH– H2O

Knowing how many moles of H+ we have in the acid, we can calculate the concentration of the base contained in the volume used.

Once the titration is finished and the average volume is determined, we can use the stoichiometric values to calculate the concentration of the analyte.

The following steps must be follow. In your exam you NEED to know how to perform these steps.

  1. calculate the number of moles of titrant used, using the volume measured in the titration process and the concentration given.
  2. calculate the number of moles of analyte usin regular stoichiometry. Remember that the amount of protons and hydroxide ions must match unless something else is specified.
  3. with the number of moles calculated in 2, and the volume used in the flask, you can calculate the concentration of the analyte (base in this case)

Analysis questions:

Resolve the following questions in the notebook. Copy the questions and answer just below. SHOW YOUR WORK IN EVERY STEP. USE UNITS!!!

ANALYSIS QUESTIONS

    1. It takes 80 mL of a 0.25 M NaOH solution to neutralize 240 mL of an HCl solution. What is the concentration of the HCl solution? (3 MARKS)

    2. You are titrating an acid into a base to determine the concentration of the base. The endpoint of the neutralization is reached but the stopcock on the buret sticks slightly and allows a few more drops of acid to fall into the solution. How will this affect your calculations for the concentration of the base? EXPLAIN THOROUGHLY (2 MARKS) 

    3. It takes 35 mL of 0.75 M NaOH solution to completely neutralize 125 mL of a sulfuric acid solution (H2SO4). What is the concentration of the H2SO4 solution? (3 MARKS)

    4. A few small drops of water are left in a buret that is then used to titrate a base into an acid solution to determine the concentration of the acid. Will this small amount of water have any effect on the determined value for the concentration of the acid? If so, how is it affected. EXPLAIN THOROUGHLY (2MARKS)

    5. It takes 12.5 mL of a 0.30 M HCl solution to neutralize 285 mL of NaOH solution. What is the concentration of the NaOH solution? (3 MARKS)

    6. A student carefully pipets 25.0 mL of 0.525 M NaOH into a test tube. She places the test tube into a small beaker to keep it from spilling and then pipets 75.0 mL of 0.355 M HCl into another test tube. When she reaches to put this test tube of acid into the beaker along with test tube of base she accidentally knocks the test tubes together hard enough to break them and their respective contents combine in the bottom of the beaker. Is the solution formed from the contents of the two test tubes acidic or basic? (2 MARKS)