Lab Standardization of Sodium Hydroxide Solution

CHE122 - Laboratory Report

Title:  Lab Standardization of Sodium Hydroxide Solution

Purpose/Background:

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Figure 1. Coral in an Aquarium

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Background

Sometimes when preparing a solution, only the approximate final concentration is known. However, in some cases the exact concentration of a solution is needed to perform quantitative chemical analyses. For example, solid sodium hydroxide (NaOH) is very hygroscopic, meaning it absorbs moisture from the atmosphere, and therefore it is difficult to weigh accurately.

Depending on the storage conditions, the mass percent of water can vary significantly and there will be less NaOH in a given mass of the solid than expected.

To prepare an NaOH solution with an exact molar concentration, it must be standardized with an acid that is a primary standard. For a substance to be a primary standard, the following criteria should be met, or at least approached:

• It must be available in very pure form.
• It must be reasonably soluble.
• It must be stable in the pure form and in solution.
• It must be non-hygroscopic and easily dried.
• It must be a compound with a reasonably high molar mass to minimize weighing errors.
• It must react rapidly with the substance being standardized with a well-known stoichiometry.

Benzoic acid is a good primary standard because of its high purity, relatively large molar mass, and because it is not hygroscopic. The balanced acid–base reaction with sodium hydroxide is shown below.

C₇H₆O₂(aq) + NaOH(aq) → NaC₇H₅O₂(aq) + H₂O(l)

Standardization Titrations

Titration is a volumetric analysis technique used to determine the unknown concentration of a solution by using the known concentration of another. Solutions are often standardized using titration. In an acid–base standardization titration, either a solution of a base of known concentration (titrant) is used to determine the exact concentration of a solution of acid (analyte) or vice versa. However, some standardization titrations use a titrant solution of approximate concentration to titrate a known amount of primary standard.

A color indicator or a pH meter can be used to determine the end point of the titration. The end point of a titration occurs when the indicator changes color, typically when a small excess of the titrant has been added. If the indicator is chosen well, the end point will correspond closely to the point when all the analyte has been neutralized by the titrant (equivalence point). Phenolphthalein is a commonly used acid–base indicator that is colorless in acidic solutions, but becomes pink when the solution turns basic. A sample titration set up is shown in Figure 2.

Figure 2. Acid–Base Titration Set-Up
An acid sample is titrated with a solution of NaOH using phenolphthalein as an indicator. (Image source: Hayden McNeil, LLC)

Molarity Calculations

The number of moles of primary standard used is calculated using the formula below.

n_analyte = m ÷ MM

where n_analyte is the number of moles of standard (analyte), m is the mass of standard used, and MM is its molar mass. The molar mass of benzoic acid is 122.12 g/mol.

The number of moles of titrant can then be calculated from the reaction stoichiometry. For example, if the titrant reacts with the analyte in a 1:1 ratio, the number of moles will be equal. 

n_titrant = n_analyte

where n_titrant is the number of moles of titrant and n_analyte is the number of moles of analyte.

For reactions with a different stoichiometry, the molar ratio needs to be included as shown below.

n_titrant ÷ a_titrant = n_analyte ÷ a_analyte

where a_titrant is the coefficient of the titrant and a_analyte is the coefficient of the analyte in the balanced chemical equation.

The volume of titrant dispensed during the titration is used to determine the molarity of the titrant solution.

M_titrant  = n_titrant ÷ V_titrant

where M_titrant is the molarity of the titrant solution in mol/L, n_titrant is the number of moles of titrant, and V_titrant is the volume dispensed during used in the titration in L.

For acid–base reactions with a 1:1 stoichiometry, the following one-step formula can be used.

M_titrant × V_titrant = M_analyte × V_analyte

where M_titrant and V_titrant are the concentration and volume of the titrant, and M_analyte  and V_analyte are the concentration and volume of the analyte.

About This Lab

In this lab, you will use dry benzoic acid as the primary acid standard to determine the exact molar concentration of a sodium hydroxide solution. To do this, you will titrate an accurately measured mass of dry benzoic acid with a prepared NaOH solution of approximate concentration. You will use phenolphthalein as an indicator. You will then use the standardized NaOH solution to titrate a sample of acetic acid of unknown concentration.

Procedure: 

Experiment 1: Standardize an NaOH Solution Using Benzoic Acid as Primary Standard

Part 1: Prepare the NaOH Solution

1. Take a 250 mL beaker from the Containers shelf and place it on the workbench.
2. Take sodium hydroxide from the Materials shelf and add 0.800 g to the beaker.
3. Take water from the Materials shelf and add 200 mL to the beaker.
4. Calculate the expected concentration of the sodium hydroxide solution. The molar mass of NaOH is 39.997 g/mol.
5. Calculate and record the expected mass of benzoic acid required to react with 20.00 mL of a 0.100 M sodium hydroxide solution. Record the mass of benzoic acid using three significant digits.

Part 2: Perform a Coarse Titration

1. Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench.
2. Take water from the Materials shelf and add 100 mL to the Erlenmeyer flask.
3. Add the required mass of benzoic acid calculated in step 5 of part 1 to the Erlenmeyer flask. The 100 mL of water should be enough to dissolve the solid benzoic acid.
4. Take phenolphthalein from the Materials shelf and add 2 drops to the Erlenmeyer flask.
5. Take a pH meter from the Instruments shelf and place it into the flask. Record the initial pH of the solution.
6. Take a burette from the Containers shelf and place it on the workbench.
7. Take the prepared sodium hydroxide solution from the workbench and add 50 mL to the burette. Pass the mouse cursor over the burette and a gray tool tip will briefly display the total volume in the burette as well as the volume dispensed. Record these initial volumes. You can also double click on the burette and select show closeup in the Item Properties window to view the volume of titrant in the burette.
8. Move the Erlenmeyer flask onto the base of the burette.
9. Perform a coarse titration, adding large increments of titrant (~ 2 mL) by pressing and holding the stopcock at the bottom of the burette for a few seconds. Pause after each dispensation. Record the volume dispensed.
10. Check if the end point has passed. When the reaction reaches the end point, the solution changes color. Also, as sodium hydroxide is added to the benzoic acid solution, the pH increases.
11. Stop once you reach the end point. Record the volume dispensed and pH at which this occurs. Record both the last dispensed volume where the solution was colorless (right before the end point) and the first dispensed volume where the solution changed color. You will use the volume dispensed before the end point in the fine titration.
12. Discard the Erlenmeyer flask by dragging it to the recycling bin beneath the workbench.

Part 3: Perform Fine Titrations

1. Prepare an Erlenmeyer flask as described in steps 1 - 5 in part 2 of experiment 1.
2. Take sodium hydroxide solution from the workbench and refill the burette to a total volume of 50 mL. Record this initial volume.
3. Move the Erlenmeyer flask onto the base of the burette.
4. Click and hold the stopcock of the burette to quickly add the before the end point volume of sodium hydroxide determined in the coarse titration.
5. Add titrant in small increments, down to one drop at a time. This can be tedious, but if you click and hold you might miss the exact end point of the titration.
6. When the solution changes color, stop adding titrant. Record the pH and volume dispensed.
7. Clear your workstation by dragging the Erlenmeyer flask to the recycling bin.
8. Repeat the fine titration two more times, and record the results.
9. Calculate the concentration of the sodium hydroxide solution.The rest of the sodium hydroxide solution can now be used in further lab work as a secondary standard with a reliably known concentration equal to the average of the three titrations.

Experiment 2: Use the Standardized NaOH Solution to Determine the Concentration of an Acid

Part 1: Perform a Coarse Titration

1. Take an Erlenmeyer flask from the Containers shelf and place it on the workbench.
2. Take acetic acid #1 (unknown concentration) from the Materials shelf and add 25 mL to the flask.
3. Take phenolphthalein from the Materials shelf and add two drops to the flask.
4. Take a pH meter from the Instruments shelf and place it into the Erlenmeyer flask. Record the pH.
5. Take the NaOH solution from the workbench and refill the burette to a volume of 50 mL. Pass the mouse cursor over the burette and a gray tool tip will briefly display the total volume in the burette as well as the volume dispensed. Record these volumes.
6. Move Erlenmeyer flask onto the base of the burette.
7. Perform a coarse titration, adding large increments of titrant (~ 2 mL) from the burette by pressing and holding the stopcock at the bottom of the burette. Pause after each dispensation. Record the volume dispensed.
8. Check if the end point has passed. When the reaction reaches the end point, the solution changes color. Also, as more sodium hydroxide is added to the acetic acid solution, the pH increases. 
9. Stop once you reach the end point. Record the dispensed volume and pH at which this occurs. Record both the last dispensed volume where the solution was colorless (right before the end point) and the first dispensed volume where the solution changed color. You will use the volume dispensed right before the end point in the fine titration.
10. Clear your station by dragging the Erlenmeyer flask to the recycling bin.