Acid-Base Titration with Antacids – Two Methods

 Objective:  This first experiment is designed to find the mass of an HCl solution consumed per gram by an antacid tablet.  The second titration method uses microscale techniques to quickly test various brands.  Then we will determine the best buy among the different brands tested.

 Safety Concerns:  All normal safety rules apply to this lab.  Special concerns deal with handling the acid and base solutions as well as glassware and the use of the Bunsen burners.  A review of these will be done in the pre-lab discussion.

 Macroscale titration with buret procedure:

 1.  Since there is often some difference in each tablet you are to grind up three tablets of your given brand with the mortar and pestle.  Divide the powder into two approximate halves and mass/record each portion.  Disregard any powder lost in the mortar.  One of the portions should be labeled “Trial 1” and the other “Trial 2”.  Trial 2 will follow a duplicate procedure of Trial 1.  Add the powder of Trial 1 into a clean, dry Erlenmeyer flask.  Obtain exactly 100.0 mL of the 0.100 M HCl from the supply bottles.  Add this to the Erlenmeyer with the antacid powder.

 2.  Stir with the rubber end of the glass stirring rod and heat over the burner till it has gently boiled for 1 to 2 minutes to allow the reaction to run to completion.  Turn off the burner and continue stirring the mixture for another minute or so.  Using the hot gloves filter the solution through a coffee filter (in a  funnel) into another Erlenmeyer flask.  Any powder left in the coffee filter is a binder and not important to our experiment.

 3.  Add 2 drops of phenolphthalein from the dropping bottle to the flask with the filtered solution.  Using any available buret titrate with 0.100 M NaOH till endpoint.  Remember to keep track of the exact amount of NaOH added to the flask.  You will need to go slowly near endpoint (one drop at a time with swirling).  The solution should stay pink for at least 15-30 seconds of swirling.

 4.  You will need to clean the flasks and allow them to drain briefly before repeating the experiment with the Trial 2 portion of the antacid powder.  Clean and drain all equipment once you have finished this experiment before going on to the second experiment.  All calculations will be done after the second experiment is completed.

 Microscale titration with spot plates:

 1.  Obtain one dose of an antacid brand selected for your group from Mr. Jones.  It is not necessary to mass the tablet(s).  Grind the tablet(s) as before and add the powder to a clean beaker.  Obtain 75.0 mL of the microscale acid solution (molarity not given to you).  Use some of the acid solution to rinse the mortar, adding all the liquid solution to the powder in the beaker.  Obtain 25.0 mL of distilled water in your graduated cylinder and use that water to rinse the mortar one final time.  Stir and swirl the solution using only hand warmth to heat the beaker.  Filter the solution through a coffee filter and place the final solution in the container marked for your antacid brand. 

 2.  Obtain approximately 6.0 mL of the microscale acid solution in your 10.0 mL graduated cylinder.  Add 15 drops of this solution to the large well on the spot plate using your transfer pipet.  Make certain you use proper technique when using the pipet (clean, held vertically, same volume drops, no air bubbles, etc.)  Add 1 drop of phenolphthalein to the acid solution and then titrate with the NaOH in the black dropping bottles till endpoint.  Record the number of drops needed.  Be certain you are stirring with the clean glass end of the stirring rod.  It must be cleaned after each use.

3.   Obtain approximately 6.0 mL of any antacid solution.  Add 15 drops into each of 4 wells on the spot plate.  Add one drop of phenolphthalein into each well.  Using the NaOH solution in the dropping bottles and a clean stirring rod, titrate till endpoint in each well.  Record the number of drops of NaOH needed to reach end point in each well.

 3.  Repeat with as many other brands of antacids as possible till clean-up time is called.  At that point you must clean all equipment for the next class.

 Calculations will be completed during class the next class period.  Data from the microscale section of the experiment can be typed into the computer at any time during the period.  A copy of the final spreadsheet will be given to everyone so that the entire class can have that data to use in their report.

Macroscale Calculations:  Keep Trial 1 and Trial 2 data separate till Step 5.

Conversions needed:      0.100 M = 0.100 moles/L  =  0.100 mmol/mL

100.0 mL of HCl  x  0.100 mmol/mL =  10.0 mmol of HCl started with by everyone

sample data:  1.36 g of antacid powder used and 30.45 mL of NaOH needed to titrate

 1.  Calculate the number of mmol HCl consumed by the tablet:

 sample calculation:        *  30.45 mL of 0.100 M NaOH used to titrate leftover acid

                                    *  30.45 mL x 0.100 mmol/mL = 3.04 mmol of NaOH used in titration so

                                        3.04 mmol of HCl was left over after adding acid

                                    *  10.0 mmol of HCl – 3.04 mmol HCl left = 7.0 mmol HCl consumed by tablet.

 2.  To convert mmol to volume just use the expression for concentration in terms of mmol and mL:

                                     Molarity = mmoles of solute/mL of solution, so rearranging the equation gives:

                                    mL of solution = mmol of solute/molarity

 sample calculation:        *  7.0 mmol / 0.100 M = 70. mL of HCl consumed by tablet

  3.  Since the density of the solution is given as 1.00 g/mL,  then  mL of solution = gram of solution…

4.  To get the mass of HCl solution consumed per gram of antacid tablet, just divide the mass of solution
 (70. grams) by the antacid tablet mass

sample calculation:        *  70. g HCl solution / 1.36 g antacid tablet = 51.8 g HCl/g antacid

5.  Average your two Trial sets of data.

Microscale Calculations:

1.  The spreadsheet will calculate averages for you.  You must choose (and indicate in your report) whether or not you used the regular average or the compensation average for each brand.
2.  Give the cost of the antacid package and the number of doses per package, then calculate the cost per dose. 
3.   Develop an effectiveness rating combining the cost and efficiency at neutralizing the acid solution.  This mathematical rating requires creative thinking as well as sound mathematical reasoning.  Realize that this rating is the most important and the most difficult to develop.  You must help the consumer decide what your data shows (i.e., which antacid works the best, is the best per cost, overall, etc.)  Look at other methods to evaluate the effectiveness of antacids.  You might include referenced evaluations of other methods (found on the Internet or in journals/periodicals.) Sample calculations will be discussed in class.

Results:

Macroscale Titration:   This result will be the average of the two trials (in g HCl/g antacid).  We will compare your result to a class average for that brand (if available).

 Microscale Titration:  The spreadsheet printout will have your results.  You should identify your data in some fashion.  You will also need the cost per dose results and your effectiveness rating for each brand.

Make at least three graphs/chart combinations: (with complete explanations of each in text accompanying them)

a)    antacid brand vs # of drops needed to read endpoint

b)   antacid brand vs cost per dose

c)   effectiveness rating (from best to worst)

Conclusions:

Here you would compare your macroscale result with the class result (since there is no actual correct known answer).  You could also compare the results of your effectiveness scale with the results from the rest of the spreadsheet data (was your data included in the class averages – you can use the spreadsheet data and eliminate your data if you are willing).  Of course you will be answering all the questions given in the lab report format directions for the conclusion section as well.

Discussion:

Answer all questions given in the lab report format directions.  The only extra question to be answered here would be either a short report on how antacids work (chemically) or a short report on the antacid industry (trends, concerns, etc.)

General notes:  Strong soluble bases likes NaOH would certainly neutralize the acid in our stomach but it would also destroy the lining of the mouth and esophagus on the way down.  There are several dangers in taking too much antacid into your system.  One danger is described as the “acid rebound effect”.  This is when the stomach acid is neutralized the acid-secreting cells of the stomach will produce more acid.  A second danger lies in those antacids which contain magnesium ions.  Magnesium ions are strong purgatives and abuse leads to chronic diarrhea and dehydration (yuck).