ACID-BASE TITRATIONS WITHOUT BURETS
In addition to showing students the acidic and basic nature of several household materials, this experiment allows students to determine the amount of acid or base present in a sample by titration.
This experiment is appropriate for a general or first-year college-prep course. The experiment allows students to gain experience in the process of titration without the use of costly burets. Measuring mass rather than volume provides an easy means of obtaining percent composition data that are readily compared to data supplied by some manufacturers of commercial products.
Plastic squeeze bottles designed for carrying liquid cosmetics or shampoo while traveling can be used to conduct acid-base titrations using household substances. The percentage of acid or base present in a product or the amount of acid neutralized by an antacid can be easily and accurately determined.
One to three lab periods depending on how many samples are titrated.
Avoid skin contact with solutions of HCl, NaOH, and/or household solutions; concentrations used in this experiment may cause irritation. Goggles must be worn throughout the experiment.
- phenolphthalein solution (1% in ethanol)*
- 5.00% NaOH solution (50 0 g NaOH dissolved in 950 mL distilled or deionized water)*
- 7-9% HCl solution (170-220 mL concentrated HCl solution diluted to 1.00 liter with distilled or deionized water)*
- one or more of the following household products--clear ammonia, white vinegar, Lysol Toilet Bowl Cleaner, or antacid
- 2 or 4-oz (65 mL or 120 mL) plastic squeeze bottles that will deliver single drops
- centigram balance
- 250-mL Erlenmeyer flasks*
- mortar and pestle
- Bunsen burner, tripod, and wire gauze or hot plate
Determination of the concentration of an acid solution
- Phenolphthalein indicator may be prepared by dissolving crushed Ex-Lax tablets from a drugstore in ethanol and filtering. Ethanol is available as rubbing alcohol, but isopropyl alcohol is also sold as rubbing alcohol.
- NaOH is available in grocery stores as lye.
- HCl solution is available from a hardware store as muriatic acid, 28% HCl; dilute this solution with 3 parts distilled or deionized water to 1 part of muriatic acid to prepare stock solution.
- Beakers or glass bottles with a quarter-sized mouth may be used in place of Erlenmeyer flasks if necessary.
Determination of the % acid or base in a household solution
- Fill one "buret" (plastic bottle) with the 5.00% NaOH solution. Determine the mass of the filled "buret" and contents and record.
- Fill the other "buret" with the HCl stock solution. Determine the mass and record.
- Transfer 10-20 g of HCl solution (about 1/3 of the contents of a 2-oz "buret") to a flask. Add 3 drops phenolphthalein solution. Gradually add the NaOH solution, with swirling, until the pink phenolphthalein color begins to persist. When near the endpoint, add the NaOH solution dropwise until one drop causes a pink color that persists for 30 seconds. If the endpoint is over-run, more acid may be added and the titration continued.
- Remass both burets.
- Repeat steps 1-4 two more times.
- From the mass of NaOH solution used in each trial, calculate the mass of HCl in the sample.
- Calculate the percent of HCl in the solution used in each trial.
- Calculate the average percent of HCl in the solution.
Determination of the amount of HCl neutralized by an antacid
- Carry out the above procedures using NaOH solution to titrate white vinegar or Lysol Toilet Bowl Cleaner. With white vinegar, phenolphthalein changes sharply from colorless to pink. When phenolphthalein is used with Lysol Toilet Bowl Cleaner, which is pale aqua-blue, the color changes sharply from aqua to light purple.
- Carry out the same procedures as above using HCl solution to titrate a solution of ammonia. With phenolphthalein as the indicator, the color change will be from pink to colorless.
Titrated solutions may be flushed down the drain with water.
These titrations involve standard acid-base neutralization reactions. The reactions are:
- Crush three antacid tablets and mix thoroughly. Mass three flasks and transfer 1-2 g of antacid to each flask and mass each again.
- Add 10-20 mL of distilled water to each flask and swirl to dissolve the antacid. Add 15-20 mL of HCl solution and 4-5 drops of phenolphthalein solution. If the solution is pink, add more acid until the pink color disappears.
- Heat each just to boiling and then cool.
- Titrate the excess acid (acid not neutralized by the antacid) with NaOH solution to a phenolphthalein endpoint that persists for 30 seconds.
- From the known concentration of the HCl solution, calculate the number of moles of HCl added to each sample.
- From the concentration of the NaOH solution, calculate the number of moles of NaOH needed to neutralize the acid not neutralized by the antacid.
- Calculate the number of moles of HCl neutralized by the antacid.
- Calculate the number of moles of HCl neutralized per gram of antacid.
The technique of back-titration is used in analyzing the antacid tablets to avoid the complex stoichiometry involved with some of the ingredients in antacids. This also makes the reaction a neutralization of a strong acid by a strong base with an endpoint at pH 7 which simplifies the choice of indicator and gives a sharper endpoint.
- H+(aq) + Cl-(aq) + Na+(aq) + OH- (aq) H2O(l) + Na+(aq) + Cl-(aq)
- CH3COOH(aq) [vinegar solution] + Na+(aq) + OH-(aq) H2O(l) + Na+(aq) + CH3COO-(aq)
- Lysol Bowl Cleaner contains HCl (see 1st equation above)
- H+(aq) + Cl-(aq) + NH3(aq) [ammonia solution] NH4+(aq) + Cl -(aq)
- H+(aq) + Cl-(aq) [excess HCl] + Ca2+(aq) + C032-(aq) [CaCO3 = antacid] H2O(l) + CO2(g) + Ca2+(aq) + 2 Cl-(aq) [H+(aq) + Cl-(aq) excess which is subsequently neutralized by NaOH]
Huhey, J., Interdisciplinary Approaches to Chemistry Diversity and Periodicity, Harper and Row Publishers, New York, 1978, p. 61. A similar experiment using glass burets is described.
- Liquid-Plumr, a drain cleaner containing NaOH, cannot be used in this experiment because it also contains NaOCl, sodium hypochlorite. When acid is added to a solution of NaOCl, chlorine is formed which destroys the indicator. Any commercial solution containing NaOCl should not be used in this experiment.
- Sani-Flush, a toilet bowl cleaner containing NaHSO4, is a granular solid that contains at least three visibly identifiable components; reproducible samples of the material are difficult to obtain. If this material is to be analyzed, a large sample of the Sani-Flush should be dissolved in water to form a solution of known concentration of about 1 g per 10 mL. Aliquot portions of this solution can then be titrated.
- It is recommended that three antacid tablets be ground together and then divided into three samples because the amount of acid neutralized varies from one tablet to another of the same brand.
- Antacids containing compounds of aluminum are not recommended for use in this experiment. Because of the amphoteric nature of aluminum, the titration does not have a sharp endpoint and results are not easily reproduced.
Submitted by John Brodemus, Elna Clevenger, Joan D'Agostino, John Davik, and Larry Ferguson
Woodrow Wilson Leadership Program in Chemistry
The Woodrow Wilson National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281