ABT: Acid-Base Titration
Purpose: Titration is an important quantitative analysis technique where a solution of accurately known concentration (the titrant) is added incrementally (usually by a buret) to another solution of unknown concentration (the analyte) until the chemical reaction between the two solutions is complete (i.e. no more analyte remains). This is known as the equivalence point. The titrant concentration and volume used to reach the equivalence point, along with the volume of the analyte titrated are used to calculate the concentration of the analyte. This technique is commonly used in biomedical diagnostic labs. Since there is often no visible change at eh equivalence point, an indicator must be added. Indicators may involve a change of color near the equivalence point, pH (if the reaction is an acid/base), or some other physical property. This lab will familiarize you with the following:
Background reading:
Cooperative Chemistry Laboratory Manual
1. Page 45-46 Containers
2. Pages 47-48 Measuring Liquids by Volume
3. Pages 67-69 Titration
4. Page 91 pH Meter- Its Care and Use
Chemistry & Chemical Reactivity
5. Section 5.3 Acids and Bases (pp 185-190)
6. Sections 5.4 Reactions of Acids and Bases (pp191-193)
7. Sections 5.8 Measuring Concentration of Compounds in Solution (pp 205-208)
8. Section 5.9 pH, a Concentration Scale for Acids and Bases (pp 212-214)
9. Section 5.10 Stoichiometry of Reactions in Aqueous Solution (pp 214-217)
10. Section 1.8: Mathematics of Chemistry Section on Graphing only (pp 43-44)
Equipment and Materials:
You will work in-groups of two for this experiment. Each group will need the following:
Notes on titrating and using burets:
Chemicals:
Dilute hydrochloric acid solution
Dilute sodium hydroxide solution
Phenolphthalein indicator
Distilled water
Buffer solution
Safety Precautions:
Sodium hydroxide and hydrochloric acid solutions are corrosive chemicals and skin-irritants. If you splash either on your skin, rinse the area with lots of running water. You must wear your safety goggles at all times. Remember: even though you may be finished in lab does not mean everyone else is finished and hence an accident could still happen. Any spills must be cleaned up immediately. Wash everything you used that contained sodium hydroxide and/or hydrochloric acid with water before putting it away. Remember, chemical residues may linger for weeks.
Overview:
The purpose of this lab will be to determine the concentration of a dilute hydrochloric acid solution. In order to do this, you will use an experimental technique called titration. A titration involves using two solutions: first the "titrant" which in this case will be a solution of sodium hydroxide of accurately known concentration, and the second solution is refereed to as the "unknown" which is a solution of hydrochloric acid of unknown concentration. The sodium hydroxide solution of accurately known concentration is added incrementally to the unknown solution until the chemical reaction between the two solutions is complete. The overall reaction is: NaOH + HCl ® NaCl + H2O. Note that in this balanced reaction, one mole of NaOH reacts with one mole of HCl, and therefore the amounts of each reactant are related. Suppose we have an unknown number of moles of HCl. In the titration, we add a known number of moles of base (volume x concentration) to a known volume of the unknown acid solution until the reaction is complete. At the completion point, the moles of base added to the unknown acid is numerically equal to the number of moles of acid in the unknown acid solution (1 mol base = 1 mol acid). By knowing the volume of the unknown acid solution used in the titration, the concentration of the unknown acid solution may be calculated (M = moles acid/volume acid). Now, all we need is a method to determine when the reaction is complete. All reactants and products in the above reaction are clear solutions. Two techniques can be used to monitor the reaction. First, an acid/base indicator can by used to visually monitor the reaction. Phenolphthalein is an indicator that is colorless in acidic solutions and pink in basic solutions. Therefore, you will monitor the reaction between NaOH and HCl by monitoring the color change that takes place (as the reaction changes from an acidic solution to a basic solution). Secondly, a pH meter can be used to monitor the reaction. As you add the NaOH to the HCl, you will monitor the reaction for changes in pH. You will record the volume of titrant added and the pH of the unknown solution. Once sufficient data has been collected, you will use Excel to generate a titration curve and then you will be able to determine the concentration of the unknown acid.
Experimental Procedure:
Part A: Acid-Base Titration Using an Indicator.
In this experiment, you will be using a pH meter. A pH meter must be calibrated before it can be used to collect data. Be sure your pH meter is calibrated correctly before collecting data.
Calibration of a pH Meter.
Part C: Acid-Base Titration using a pH meter.
Data Analysis:
A. Calculation of the Molarity of Unknown Acid
Example:
Five mL of HCl (aq) was titrated to the endpoint with 23.78 mL of a 0.1000-M solution of NaOH. Calculate the molarity of the HCl solution.
Solution: Start with the balanced equation for our reaction:
HCl(aq) + |
NaOH(aq) ® |
NaCl(aq) + |
H2O(l) |
Unknown Concentration |
Known Concentration |
|
|
Known volume |
Known volume |
|
|
Since the acid was titrated to the endpoint (the point at which the unknown acid has been completely consumed) with the base and there is a one to one mole ratio between HCl and NaOH in the balanced equation, it follows that the moles of HCl that reacted equals the moles of NaOH added from the buret.
1. Calculate the number of moles of NaOH added to the HCl |
2. Use the balanced equation to determine the number of moles of HCl that was titrated. |
3. Calculate the concentration of HCl in moles/L. |
1. (0.1000 mol NaOH/L)(0.02378 L NaOH) = 0.002378 mol NaOH
2. (0.02378 mol NaOH)(1 mol HCl/1 mol NaOH) = 0.002378 mol HCl
3. MHCl = 0.002378 mol HCl / 0.00500 L HCl) = 0.4756 M
*To summarize, if we need to find the concentration of an unknown acid, titrate a known volume of the unknown acid with a known concentration of base. We measure the volume of base required to get to the endpoint. We know when we are at the endpoint by using an indicator or a pH meter.
Set up a spreadsheet with your data from Sample 2 only as follows:
|
A |
B |
1 |
mL NaOH added |
pH |
(The endpoint is volume of base added where the slope of the pH curve is greatest.)
*No data sheet for Part B.