Vinegar Titration

Hope College: General Chemistry Laboratories
Fall: Chem 127 & Chem 132 - Spring: Chem 128

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INTRODUCTION

In this experiment, you will determine the molarity, M_a, of a CH₃CO₂H (aq) solution, using two different types of methods. The first method is the indicator (equivalence point) method. In this method, you will manually titrate CH₃CO₂H(aq) with some amount of NaOH (aq) to find out the concentration of the given vinegar solution. The second method is the pH sensor method. Like the first method, you will be titrating the strong base and the weak acid to determine concentration of vinegar. However, this time, you will be collecting data using MicroLab Interface using a pH sensor.

 

Method 1: Standardization of Unknown CH₃CO₂H(aq) Solution, Indicator Equivalence Point

 

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Step 1.1

You will be given some concentration of acetic acid solution. 250 mL Erlenmeyer flask with a labeled, clean and dry it. Then obtain some amount of C₂H₄O₂(aq) - or CH₃CO₂H(aq) - of unknown concentration. Record your unknown later and always save your acid.

 

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Step 1.2

Obtain a buret and 25 mL transfer pipet from the shared buret cabinets and the stock room and clean them once with RO water (See Appendix III, section D). The buret will be washed with RO water and a small amount of NaOH (aq) and the 25 mL transfer pipetwill be washed with RO water and a small amount of C₂H₄O₂(aq) in later step.

 

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Step 1.3

Pour indicated amount of the unknown acid into a clean, dry 50 mL beaker.
Rinse the 25 mL transfer pipet three timeswith a few mL of the acid discarding the rinse down the sink (or waste beaker) each time (See Appendix III, section D). After rinsing is complete, discard C₂H₄O₂(aq) remaining in the beaker.

 

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Step 1.4

Pour some of the unknown C₂H₄O₂(aq) into the beaker and pipet indicated amount of the acid into a clean, not necessarily dry, 125 mL Erlenmeyer flask.

 

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Step 1.5

Add a couple of drops of phenolphthalein, the acid-base indicator to the 125 mL Erlenmeyer flask which contains 25.00 mL of C₂H₄O₂(aq).

 

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Step 1.6

Obtain some amount of the standard NaOH(aq) in a 50 mL beaker. Rinse the clean buret three times with approximately 3mL portions of the NaOH (aq) solution (see Appendix III, section D) . Drain the rinse into the sink through the stopcock.

Obtain more standard NaOH (aq) in the 50mL beaker and almost fill the buret. You may use a funnel to fill the buret, however be sure that is cleaned and rinsed as well.

 

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Step 1.7

Expel all air bubbles in the stopcock and buret tip, allowing the liquid level to move just below the 0.00 mL mark. Record the initial buret reading in your laboratory notebook. You do not need to set the initial buret reading exactly to 0.00 mL.

 

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Step 1.8

Titrate some amount of C₂H₄O₂(aq) and NaOH(aq) against a white background to the permanent indicator color change of light pink.

C₂H₄O₂(aq) + NaOH (aq)→NaC₂H₃O₂(aq) + H₂O(l)

 

Remember that all buret readings must be recorded to the nearest 0.01 mL. Use meniscus enhancer (white card with black strip on it) when reading the buret. All buret reading must be recorded.

 

Method 2: Standardization of Unknown CH₃CO₂H(aq)

 

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Step 2.1

You should still have your unknown concentration of C₂H₄O₂(aq). If you need more of it, talk to your instructor.

Clean and rinse a buret and 25 mL transfer pipet. This equipment can be obtained from the shared buret cabinets and the stockroom (See Appendix III, section D).The buret will be washed with some NaOH(aq) and the 25 mL transfer pipet will be washed with some C₂H₄O₂(aq)in later step.

 

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Step 2.2

Pour some amount of the unknown acid into a clean, dry 50 mL beaker.

Rinse the 25 mL transfer pipet three timeswith a few mL of the acid discarding the rinse down the sink each time (See Appendix III, section D). After rinsing is complete, discard C₂H₄O₂(aq) remaining in the beaker.

 

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Step 2.3

Once you obtain 125 mL Erlenmeyer flaskwith a labeled, clean and dry it. Set up the titration apparatus just like you did in the previous experiment. The only difference here is that you will need to perform calibration of the pH electrode since you will be using it to collect data on pH and volume.

 

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Step 2.4

Titrate some amount of C₂H₄O₂(aq) and NaOH(aq). The solution to be titrated is the former and the titrant solution is the latter.

C₂H₄O₂(aq)+ NaOH(aq)→NaC₂H₃O₂(aq)+ H₂O(l)

Again, follow the procedure outline in the Appendix III, section E, to make sure all the setting is correct. Once you finish collecting data, make sure you save the file with an appropriate name and write down the file name and its location in your laboratory notebook.