The purpose of this experiment is to determine the concentration of acetic acid in vinegar by volumetric titration with phenolphthalein indicator.
ACS grade KHP, phenolphthalein 1% v/v, 0.1M NaOH, sample vinegar with unknown acetic acid, reagent water
Phenolphthalein 1% solution
Hazardous to skin, eye contact, ingestion
Class 3, flammable
In water, methanol, diethyl ether
Very hazardous in case of skin contact, eye contact, ingestion, inhalation
Class 8, corrosive
Soluble in water
Potassium hydrogen phthalate (KHP)
slightly hazardous skin contact, eye contact, ingestion inhalation
Combustible at high temps
Not DOT controlled
(Method: We will use NaOH as the titrant. However, we must first standardize NaOH with KHP because NaOH usually contains contaminants when exposed to air. Then, use the standard NaOH to titrate a diluted sample of vinegar of unknown acetic acid concentration (acid-base reaction). The end point (when there enough NaOH has been added to acetic acid for stoichiometric reaction) can be observed by looking at the color change of phenophthalein. Because phenolphthalein changes color based on the pH we can determine when all the acetic acid has reacted with the sodium hydroxide. Therefore, the concentration of acetic acid in w/v % can be calculated)
Note: We are standardizing sodium hydroxide (i.e. making sodium hydroxide a standard) because the NaOH we buy are not pure. (even ones that are labeled 99% or higher assay). This is because NaOH reacts with carbon dioxide to form carbonate, and adsorbs to water in the air. This is the same case as potassium hydroxide (KOH).
The good news is that potassium hydrogen phthalate (KHP) can be bought in an almost pure form to be a primary standard to standardize NaOH.
- Prepare standard KHP by drying in oven at 100-110oC for approx. 1 hour
- After heating the KHP, place it in the desiccator for storage until use
- Make approx. 0.1M NaOH by adding 2g (96-100% assay) NaOH in 500mL of reagent water
- Stir the NaOH solution until thoroughly dissolved
- Using dehydrated KHP weigh out 4 samples of approx. 0.8 grams, and put them in separate 250 mL Erlenmeyer flasks (record the exact weight of each one)
- Dissolve each KHP in 50mL reagent water and stir until complete dissolution
- Add 1 drop of phenolphthalein indicator solution
- Titrate with NaOH solution until the end-point has been reached (a faint pink color has appeared)
- Record the amount of NaOH required to titrate to end-point with 4 significant figures
- Using the amount (in grams) of KHP and amount (in milliliters) of titrant NaOH titrated, calculate the molar concentration of NaOH
- Pipette 25mL of vinegar into a clean 500 mL beaker or flask and dilute to 500mL with reagent water
- Pipette four 50mL aliquots of this solution into a clean250mL erlengmeyer flask, add 1 drop of phenolphthalein indicator and approx. 25mL of reagent water to each flask
- Titrate with the standardized NaOH
- Record the exact amount of NaOH required to titrate to end-point with 4 significant figures
- Calculate the w/v % of acetic acid in the sample vinegar
Note: If you decide to keep your excess NaOH for other acid-base titrations. It is recommended to store the solution in closed polyethylene bottles (not in glass containers). This solution will be ok for a couple of weeks. Strong bases attack glass. Therefore, immediately wash your glasswares after use.
Data and Calculations:
Error may have been introduced from the uncertainties in the purity of the KHP, burette reading, analytical balance measurements and dilutions. The experimental value appears to be close to the literature value found on the container label; therefore, the method appears to be accurate and reliable for a rough estimate.