Chemical Analysis by Acid-Base Titration

 Introduction:

Titration is a common method of determining the amount or concentration of an unknown substance. The method is easy to use if the quantitative relationship between two reacting solutions is known. The method is particularly well-suited to acid-base and oxidation-reduction reactions. Titrations are routinely used in industry to analyze products to be sold. Many manufacturers are under strict standards of quality control because their products are sold for public consumption. In this experiment, we will analyze a number of commercial products and, in some cases, test the validity of the information given on their labels and/or the claims made in television commercials. The products to be tested include antacid tablets, vinegar, fruit juice, and household ammonia.

 

Outline:

I. Standardization of a base (NaOH) using a primary standard (KHP)

II. Standardization of an acid (HCl) with the standard base

III. Titration analysis of unknown acids and bases:

A. Antacid tablets (Tums vs. Rolaids vs. Maalox)

B. Vinegar

C. Fruit juice (apple or grape)

D. Household ammonia

 

I. Standardization of NaOH

Titrations permit the concentrations of unknown acids/bases to be determined with a high degree of accuracy. In order to analyze unknown acids/bases, we must have a "standard" solution to react with the unknowns. A standard solution is one in which the concentration is known accurately. We will first prepare a standard solution of NaOH. One way to prepare a standard solution is to dissolve an accurately massed amount of the substance and dilute it to a measured volume (like we did with the MnO4- solution in Expt. #1.6). In this way, the concentration can be calculated exactly. However, it is usually impossible to obtain NaOH of sufficient purity to use it as a primary standard. An indirect method is more practical for obtaining a standard solution of NaOH. We will prepare a solution of a approximate molarity and standardize it against a primary standard of known purity.

 

A primary standard should:

* be of high purity

* remain unchanged in air during massing and remain stable during storage

* have a high molar mass to reduce massing errors

* react with the solution to be standardized in a direct, well-defined reaction

 

Potassium acid phthalate will serve as our primary standard. This is a large molecule (KHC8H4O4) with a molar mass of 204.2 g/mol. Instead of writing the whole formula, we abbreviate it as KHP, where "P" stands for the phthalate ion, C8H4O42-, not for phosphorus. KHP is an acidic substance, with the ionizing hydrogen being set forward in the formula for emphasis. Therefore, KHP is monoprotic and will react with NaOH in a simple 1 to 1 relationship according to the following equation:

 

NaOH(aq) + KHC8H4O4(aq) ---> KNaC8H4O4(aq) + H2O(l)

 

If we write this reaction showing the complete structure for KHP and indicating the ionization which occurs, we have:

 

 

 

 

 

 

 

 

 

After we standardize our base with KHP, we will use this standard base to standardize our acid so that we will know the exact concentrations of both solutions and can then easily titrate and analyze the unknowns.

 

Procedure:

1. Dilute enough 1 M NaOH to make 1 L of 0.1xx M NaOH. This will be your titrant. Rinse a buret with water and then with a small amount of the NaOH solution. Fill the buret with NaOH solution. Fill the buret tip by momentarily opening the stopcock. Label this buret BASE. Read the initial volume.

2. Accurately mass approx. 0.8xx g of KHP into a 250 mL Erlenmeyer flask. Add about 100 mL of water and swirl the flask until the sample is dissolved. Add 3 drops of phenolphthalein indicator (colorless in acidic solution; pink in basic solution).

 

3. Titrate the KHP solution with the base solution to be standardized. Titration should proceed until the faintest pink persists for 30 sec. after swirling. The color will fade upon exposure to the air (WHY?). After completing a trial, breathe into the flask and swirl. What has happened?

4. Make duplicate determinations and calculate the average molarity of the NaOH. For excellent work, the molarities need to be within 1% of one another.

 

Data (for each trial):

* mass of KHP in sample flask

* initial buret reading

* final buret reading

* volume of NaOH required to neutralize the KHP

 

Calculations:

* molarity of NaOH for each trial

* avg. concentration

 

Question:

1. Explain why the pink color of a phenolpthalein endpoint will disappear after exposure to atmospheric air (or the air we exhale). Include 2 balanced equations.

2. Explain why the high molar mass of KHP is an advantage to its being used as a primary standard. Make up a scenario with numbers and % errors.

 

 

Now you are ready to standardize your acid with your known concentration of base.

 

II. Standardization of HCl Solution

Procedure:

1. Obtain about 50 mL of 0.1xx M HCl solution in a beaker. Rinse a second buret with water and then a small amount of HCl. Fill the buret with HCl solution. Fill the buret tip by momentarily opening the stopcock. Label this buret ACID. Record the initial volume and then deliver approx. 10.xx mL (as long as you know accurately how much you used) into a clean 250 mL Erlenmeyer flask. Add about 100 mL of water and 3 drops of phenolphtalein indicator to this sample flask.

2. Read the initial level in the standard NaOH buret. Titrate the acid with the NaOH standard to the faint pink endpoint.

3. Repeat the titration with a second sample of HCl. The calculated molarities of HCl should be within 1% of one another.

 

Data (for each trial):

* volume of acid sample

* initial buret reading

* final buret reading

* volume of NaOH used

* molarity of standard NaOH

Calculations:

* molarity of HCl for each trial

* avg. concentration of standardized HCl

 

We are now ready to titrate any and every unknown acid and base with our known concentrations of acid and base!

 

III. Titration Analysis of Unknown Acids and Bases

 

A. Analysis of Antacid Tablets

*You will be sign up for either Rolaids, Tums, or Maalox

 

Procedure:

1. Obtain an antacid tablet and mass the whole tablet. Grind the tablet with mortar and pestle. Mass two portions of approx. 0.6xx-0.7xx g each.

2. Add the samples to two 250 mL Erlenmeyer flasks. From your acid buret, add approx. 60.xx mL (must be known accurately) of the standard HCl to the antacid powder. Swirl the flask to dissolve the solid. All of it may not dissolve, since there may be some insoluble components.

3. Add 4-5 drops of methyl orange indicator to the sample flask. The solution should turn pink-red since the HCl was added in excess. If the solution is yellow, add approx. 10.xx mL more acid, measuring the volume carefully.

4. The antacid has neutralized a portion of the acid. Our job is to figure out exactly how much HCl was neutralized by titrating the acid left over with our standard base. "Back-titrate" the excess acid in the flask with the standard NaOH. As you approach the endpoint, add the base drop by drop. The endpoint of the titration occurs when the solution turns from red to pale orange.

5. Repeat with the other antacid sample.

 

Data (for each trial):

* mass of whole antacid tablet

* mass of antacid sample

* volume of HCl added

* concentration of standard HCl

* initial buret reading of NaOH

* final buret reading of NaOH

* volume of NaOH required

* concentration of standard NaOH solution

 

Calculations (for each trial):

* mmol of HCl added

* mmol of NaOH added

* mmol of acid neutralized by the antacid

* mmol of acid neutralized per gram antacid

* avg. mmol acid neutralized/g antacid

 

Data and Calculations (based on class data):

* class avg. mass of tablet for each brand

* class avg. mmol acid neutralized per gram antacid for each brand

* cost of bottle for each brand

* cost of antacid per gram (cents/g) for each brand

* best buy (cents/mmol acid neutralized)

 

Questions:

1. Rolaids once ran a national advertising campaign in which it was claimed that Rolaids consumes "forty-seven times its own weight of excess stomach acid". Can this be true?!!

*Assume that stomach acid is 0.100 M HCl and that its density is 1.00 g/mL.

2. Tums once ran a national advertising campaign in which it touted that its product "neutralizes one-third more acid than Rolaids". Are they lying?!

 

 

B. Analysis of Vinegar

Percent acidity is the basis for determining the legality of vinegar. The percent acid in a sample is calculated as g/mL x 100. Vinegar is regulated by the Food and Drug Administration (FDA) and must have a minimum of 5% acidity. The acid involved is acetic acid.

 

Procedure:

1. Pipette 5.00 mL of an unknown vinegar sample into a 250 mL Erlenmeyer flask. Add about 100 mL of water and 3 drops of phenolphthalein indicator. Titrate with your standard base. Complete a duplicate determination.

 

Data (for each trial):

* volume of vinegar sample

* initial buret reading

* final buret reading

* volume of base required

* avg. volume of base

 

Calculations:

 

* the acidity of the vinegar expressed as molarity

* the acidity of the vinegar expressed as a percent (is the vinegar legal?)

 

C. Analysis of Fruit Juice (Apple or Grape Juice)

Procedure:

1. Pipette 10.00 mL of fruit juice into a 250 mL Erlenmeyer flask. Add about 100 mL of water and 3 drops of phenolphthalein. Titrate with your standard base. Due to the yellow color of the juice, the endpoint will appear peach instead of pink. Complete a duplicate determination.

 

Data (for each trial):

* volume of fruit juice sample

* initial buret reading

* final buret reading

* volume of base required

* avg. volume of base

* concentration of standard base

 

Calculations:

1. The acidity of the juice expressed as molarity

2. The acidity of the juice expressed as a percent

3. The juices are fortified with vitamin C. Determine the percent acidity from ascorbic acid versus the native acid.

Extension:

build the carboxylic acids (acetic, malic, tartaric, and ascorbic) in ChemSite (see separate instructions).

 

D. Analysis of Household Ammonia

 

Procedure:

1. Pipette 5.00 mL of the ammonia solution into a 250 mL Erlenmeyer flask. Add about 100 mL of water and 3 drops of methyl orange indicator. The solution will turn yellow. Titrate the sample with your standard HCl solution. The endpoint is orange (if you go too far, the solution will become red). Complete a duplicate determination.

 

Data (for each trial):

 

* volume of ammonia sample

* initial buret reading

* final buret reading

* volume of acid required

* avg. volume of acid required

* concentration of standard acid

 

Calculations:

1. The molarity of ammonia

 

Lab Report #1.8

* title page

* abstract

* procedure sheets

* data, calculations, and questions by section (net ionic equations under each data table).

* results and discussion - separate paragraphs for each part. Read about

R + D on lab report format handout and consider the following: variation in duplicates, comparisons to class means and standard deviations, and comparison to labels, claims, or legal standards? Remember that you have data for the accuracy and precision of your pipetting from a previous lab.

 


Questions? Comments?

W.G. Rushin , Cary Academy