Antacid titration v2004 Page 1 4/14/202011
Objectives:
 Make a 0.225 M solution of HCl and standardize it. Make a 0.1 M solution of
NaOH and standardize it (option). Perform a back titration.
 Determine the moles of HCl which can be neutralized by an over-the-counter antacid.
 Be amazed at how much different antacids do/don’t differ.
 GO back to the CH 103 web site and get the “Protocol for Making 0.1 M
NaOH” from the WEEK 6 lab
( http://academics.smcvt.edu/chemistry/CHEM%20103/CHEM_103_Labs/Aspirin/Aspirin_Lab_Week
1.htm
)
 Get the “Protocol for Standardizing NaOH by titration with KHP” From the WEEK 8 Lab;
( http://academics.smcvt.edu/chemistry/CHEM%20103/CHEM_103_Labs/Aspirin/Aspirin_Lab_Week2.htm
)
 Calculate how many mL of 12 M HCl (conc. HCl) you will need to make 1 L of 0.25 M HCl. This is another “quick and dirty” procedure.
Write a protocol describing how you will make the solution. What glassware and other lab equipment will you use? What kind of water will you use? Will you need the balance?
All over-the-counter (OTC) antacids “neutralize excess stomach acid”. Stomach acid is a dilute solution of HCl, produced by parietal cells lining the stomach. Your stomach produces approximately 1-2 L of 0.155 M HCl a day.
Anyone who has ever watched commercial TV knows that many dollars are spent by pharmaceutical companies who wish to convince you that their antacid is the one to buy. We will take some of the “fast-acting” antacids such as Tums ®, Rolaids ®,
Maalox ®, and grocery store house brands to the lab and put some numbers on the claims. We
WON’T look at the newer generation of antacids such as Pepsid AC ®, as they work by a different mechanism.

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The following information has been taken from package inserts of some common OTC antacids:
Tums antacid ®
Each tablet contains: Calcium carbonate Ph Eur 500mg.
Also contains: Sugar, glucose, starch, talc, magnesium stearate.
Citric acid flavours: Blackcurrant, lemon and orange.
Colours: Violet (E124, E121), quinoline yellow (E104), sunset yellow (E110).
THE CHEMISTRY:
CaCO
3(s)
+ 2HCl
(aq)
 CaCl
2(aq)
+ H
2
CO
3(aq)
 H
2
O + CO
2
Before you come to lab: Write the balanced net ionic equation for this reaction.
Rolaids ®:
ACTIVE INGREDIENTS
: All Flavors: Calcium Carbonate 550 mg and Magnesium Hydroxide 110 mg per tablet.
INACTIVE INGREDIENTS : Original & Spearmint Flavors: Dextrose, Flavoring, Magnesium Stearate,
Polyethylene Glycol, Pregelatinized Starch, and Sucrose. Cherry: Dextrose, Flavoring, Magnesium
Stearate, Polyethylene Glycol, Pregelatinized Starch, D&C Red No. 27 Aluminum Lake, and Sucrose.
THE CHEMISTRY:
CaCO
3(s)
+ 2HCl
(aq)
 CaCl
2(aq)
+ H
2
CO
3(aq)
 H
2
O + CO
2
Mg(OH)
2(s) +
2HCl
(aq)
 MgCl
2(aq)
+ 2H
2
O
(l)
Before you come to lab: Write the balanced net ionic equation for these reactions.
Alka-Seltzer
®
Original
Contains:
Aspirin 325 mg, heat treated sodium bicarbonate 1700 mg, citric acid 1000 mg. Alka-Seltzer
®
in water contains principally the antacid sodium citrate and the analgesic sodium acetylsalicylate. Also In
Cherry and Lemon Lime Flavors
THE CHEMISTRY:
NaHCO
3(aq)
+ HCl
(aq)
 NaCl
(aq)
+ H
2
CO
3(aq)
 H
2
O + CO
2
Na
3
C
6
H
5
O
7(aq)
+ 3HCl
(aq)
 C
6
H
5
O
7
H
3(aq)
+ 3NaCl
(aq)
(sodium citrate)
Maalox ®
Tablets: Each round, flat-faced, beveled, white (mint-flavored) or pink (cherry-flavored), chewable tablet, engraved "Maalox" on one side and on the other side, contains: magnesium hydroxide 200 mg and aluminum hydroxide dried gel USP 200 mg (equivalent to 153 mg aluminum hydroxide).

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Nonmedicinal ingredients: flavors, glycerin, magnesium stearate, mannitol, sodium saccharin, sorbitol and talc.
THE CHEMISTRY:
Al(OH)
3(s)
+ 3HCl
(aq)
 AlCl
3(aq)
+ 3H
2
O
(l)
Mg(OH)
2(s) +
2HCl
(aq)
 MgCl
2(aq)
+ 2H
2
O
(l)
Before you come to lab: Write the balanced net ionic equation for these reactions.
Your mission today: Determine how much stomach acid an antacid can neutralize.
In this lab, standardized 0.1 M NaOH will be provided for you. Consult the lab instructor to find out the
[NaOH]. You will make and standardize 0.225 M HCl. You will then use the HCl and NaOH to analyze an antacid tablet.
Before you come to lab: G et the files for “
Protocol for Making 0.225 M HCl
”, and the file for
“S tandardizing Your HCl
” off the website: http://academics.smcvt.edu/chemistry/CHEM 107/107_lab_stuff/Tums/antacid_analysis/antacid_analysis.htm
THE EXPERIMENT:
You will need:
To Make HCl:
1 L bottle and stopper 1
50 mL graduated cylinder 1
******************************
12 M (Conc) HCl in stoppered ehrlenmeyers in the hood
To Standardize HCl:
DEMO SET UP:
2 drops of methyl orange in pH 4 buffer, in Erlenmeyer, stoppered. 1 per riser.
250 mL Erlenmeyer flasks 4
Magnetic stirrer and spin 1 vane
50 mL buret
Buret reading card
1
1
Primary standard Na
2
CO
3
2 g/student pair
1% methyl orange in water 2 dropper bottles/riser

Antacid titration v2004
To Analyze the Antacid:
250 mL Erlenmeyer flasks 4
Magnetic stirrer and spin vane
1
Page 4
Hot plates
50 mL buret
Buret reading card
25.00 mL analytical pipet and blue bulb
Mortar and pestle (if necessary)
1
1
1
1
1/bench
Commercial antacid tablets
(Tums, Rolaids and the ilk) The package
4-6 /student pair
info and the price should be available.
0.2 M NaOH, standardized by TA 200 mL/student pair
0.225 M HCl, standardized by the student 200 mL/student pair
2% phenolphthalein indicator in ethanol 2 dropper bottles/riser
4/14/202044
Caution: NaOH and HCl are caustic and corrosive. Protect your eyes with safety glasses.
NEVER pipet by mouth!
THE PROCEDURE:
Before you come to lab: Make an Excel Template of the data chart on page
8. PUT IT ON A FLOPPY DISK & BRING IT TO LAB.
BRING YOUR KNIGHTCARD TO LAB!!!!
1.
Make up your 0.225 M HCl solution per the quick and dirty protocol in the handout “ Protocol For
Making 0.225 M HCl
”.
2.
Standardize your 0.225 M HCl according to the protocol outlined in the handout “Standardizing Your
HCl”.
3.
CALCULATE AND WRITE DOWN the average [HCl] you determined in part 2 in your notebook!

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Now you have the tools in hand to analyze your antacid.
4.
Obtain 4 commercial antacid tablets from the TA, or use your own. BEWARE! If the antacids come in fruit flavors with red or yellow coloring agent it makes it almost impossible to spot the endpoint color change. White tablets work best. Whatever option you chose, write down in your notebook:
 the brand name,
 the pharmaceutical company,
 the ingredients information,
 the price of the package,
 the number of tablets in the package,

The color of the tablets.
What we are going to do in this experiment is a “Back Titration”. That is, we are going to react the antacid with a known amount of excess HCl, and then titrate the left over acid with a known solution of NaOH.
That will let us calculate how much acid did not react with the antacid tablet. Since we know how much acid we started with, and how much acid is left over, Acid start
– Acid left over
= Acid neutralized by the antacid.
[Why bother with a back titration? Why not just titrate the antacid directly with the HCl?
You could do it that way, but the solid antacid tends to dissolve slowly and the titration takes a long time. Also, the endpoint is hard to spot, again, because the reaction is so slow. In this case, a back titration is faster and more precise.]
5.
Rinse out your Erlenmeyer flasks from the HCl standardization with distilled water. Don’t bother to dry them.
6.
Weigh (to the nearest 0.1 mg) each of your 4 antacid tablets.
Write this down in your notebook!
Put 1 weighed tablet into each of your 4 labeled Erlenmeyer flasks.
7.
Get your bottle of standardized HCl and shake it up. With an analytical pipet, add 75.00 mL of your
HCl to each flask and tablet . Do you see bubbling and fizzing? If so, what is the gas being evolved?
Calculate how many moles of HCl you are adding to each tablet.
8.
Carbon dioxide, which is evolved by many antacids when they react with HCl, is itself an acid:
CO
2
+ H
2
O

H
2
CO
3

H + + HCO
3
-
To get rid of CO
2
, heat each antacid/HCL solution to boiling, and let it boil for 3-5 minutes. Let the flasks cool to room temperature. Since there is a lot of insoluble filler in most antacids, you will probably see some crudlys swirling around even after this step. Describe the solution and move on.
9.
While the heating and cooling step is going on, you can obtain some standard 0.2 M NaOH from the
TA, and WRITE DOWN the concentration.
10.
Clean your 50.00 mL buret and ring stand. Drain it, and rinse it several times with distilled water.
Each time you rinse it, make sure you open the stopcock and run distilled water through the tip. When the buret is clean, it will have some water in it. You are going to put your NaOH into the buret, so that you can determine what volume of NaOH is required to react with the left over HCl. You don’t want to dilute your NAOH solution, since the TA has carefully standardized it, so now;
11.
Rinse your buret with your NaOH solution. Carefully pour about 5 mL of your well-mixed NaOH solution into your buret (Make sure the stopcock is closed!). At the sink, open the stopcock and drain a little solution through the tip. Close the stopcock. Now, tilt your buret down to near horizontal, twirl

Antacid titration v2004 Page 6 4/14/202066 it to coat the walls with solution, and then discard the rinse solution. Repeat this process 3 more times.
Now, return to your bench and;
12.
Fill your buret with your NaOH solution. Fill the buret to close to the top, but DO NOT fill to the 0.00 line. This introduces error! Your eye tends to tell your brain what your brain wants…so your reading at 0.00 will be your least accurate reading! If your brain doesn’t know what the reading is supposed to be, it will tend to pay more attention to what your eye is telling it. Despite this well-known fact, the rare student will persist in trying to get the bottom of the meniscus exactly on the 0.00 mL line. If you do this, you should know that this is not only a waste of your time, limits your accuracy, and will cost you 5% off on your lab report.
13.
Read your buret volume & record it. Read the volume on your buret (+/- 0.02 mL). Remember to read the bottom of the meniscus, with your sight-line level with the meniscus. Use a buret reading card. Record the volume in your notebook.
You are almost ready to begin titrating.
In this piece of the lab, the titrant is your standardized NaOH, & the analyte you will standardize is the left over
HCl that didn’t react with the antacid. The reaction is of course:
HCl
(aq)
+ NaOH
(aq)

NaCl
(aq)
+ H
2
O
(l)
HC
HC
1 mole of HCl will react with 1 mole of NaOH. Since you know the [NaOH] from your standardization (use the average [NaOH]), you can easily calculate the number of moles of HCl left over in each flask. If you can add just enough NaOH to react with the HCl, no less and no more, you will know how many moles of HCl you have left. If you also know the number of moles of HCl you started with, determining the moles of HCl sopped up by the antacid tablet is child’s play. Subtraction is easy.
How do you know when you’ve added “just enough NaOH to react with the HCl, no less and no more”, or, as
H
C
C
H
CH
C
CH
HC
C
H
C
C
H
C
CH
OH
HC
HC
H
C
C
H
CH
C
C
HC
C
H
C
C
H
C
CH
O one says in the trade, a “stoichiometric amount”? This is easy. You’ll add an indicator which changes color when the reaction is done. You’ll use 2 drops of 2% phenolphthalein in ethanol. Phenolphthalein is an
OH
O
-
acid-base indicator.
Initially, phenolphthalein will be in an acidic
HC
HC
C
C
C
O
HC
HC
C
C
C
O environment, the HCl solution. As you add NaOH, it will react with the HCl and the solution will remain
C
H
CH
C
H
CH colorless. But when the last of the HCl is reacted, the very next drop of NaOH you add from your buret will
Phenolphthalein in acid...colourless
Phenolphthalein in excess OH
fluorescent pink!
turn the solution in your flask a pale pink color. That is the endpoint of your titration.
So the phenolphthalein will tell you when you’ve added a stoichiometric amount of NaOH to the HCl.
You know that you’ve added as many moles of NaOH as you had HCl to begin with. You need to determine how many moles of NaOH are in a liter of solution. The buret is the instrument designed to tell you what volume of NaOH you added. Get a buret reading at the beginning of your titration. Read the buret again at the endpoint.

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DOING THE TITRATION:
14.
Get your first labeled flask with the known and recorded mass of an antacid tablet. Add 2 drops of phenolphthalein indicator to it. Set your magnetic stirrer up under the buret & make sure the buret tip has no drops dangling from the tip. If it does, rinse the tip with distilled water from your wash bottle into a waste beaker. Put your flask on the stirrer, and add a clean washed magnetic spin vane (it need not be dry) to the flask.
Slowly turn on the stirrer until you get a nice gentle vortex. Once again, stirring is key!
15.
Have you recorded your buret volume? If not do it now, to the nearest 0.02 mL.
16.
Start adding NaOH. Slowly open the stopcock and start running
NaOH into your flask. Rinse down the sides of the flask from time to time with your wash bottle. Soon, you will see pink color
Wash down the sides of your flask while titrating. forming around the drops of NaOH as they hit the solution. The pink color will rapidly disappear.
17.
Slow down! As you move toward the endpoint, the pink color will fade more slowly. When this happens, slow down the rate at which you add NaOH. Add it one drop at a time. Rinse the sides and tip of the buret with distilled water from your wash bottle. As you go on, add NaOH slower. When the faint pink blush of phenolphthalein lasts for > 30 secs, you are done. You should be able to hit this endpoint +/- half a drop. Save your flask so you can match the faint pink color with your next endpoint.
18.
Record your buret reading +/- 0.02 mL.
19.
Repeat steps 1-6 with your other 3 flasks of previously massed and recorded HCl.
That persistent faint pink blush of a good endpoint.

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Calculations:
A.
Calculate how many mL of NaOH you dispensed;
B.
Calculate how many L of NaOH this is;
C.
Calculate how many moles of NaOH you dispensed. You will use the average moles/L NaOH you calculated from your standardization. REM: L NaOH x (moles NaOH/L) = moles NaOH;
D.
Calculate how many moles of HCl were left over . Use the balanced equation: HCl + NaOH

NaCl +
H
2
O
E.
Calculate how many moles of HCl you started with.
F.
Calculate how many moles of HCl absorbed by antacid
G.
Calculate moles of HCl absorbed/ tablet.
H.
Calculate moles of HCl absorbed/gram antacid .
I.
Convert moles of HCl to grams (MW HCl = 36.461 g/mol) and calculate g HCl absorbed/gram antacid.
J.
Calculate g HCl absorbed /1 cent.
Sample calculation: NOTE THAT THIS (*)DATA IS DIFFERENT THAN YOURS! USE YOUR OWN!
Data: *[NaOH]: 0.1040 M
*[HCl]: 0.2537 M
Volume HCl: 75.00 mL
*NaOH start
: 2.45 mL
*NaOH end
: 27.32 mL
*Mass Rolaids tablet 1: 1.3450 g
*Cost/100 Rolaids at Underhill Center Market: $4.98
A.
mL of NaOH: 27.32 mL - 2.45 mL = 24.87 mL ;
B. L of NaOH: 24.87 mL (1 L/1000 mL ) = 0.02487 L
C.
Moles of NaOH: 0.02487 L (0.1040 mole NaOH/L) = 2.586 x 10 -3 moles NaOH
D.
Moles of HCl left over: 2.586 x 10
-3 moles NaOH (1 HCl/1 NaOH) = 2.586 x 10 -3 moles HCl
E.
Moles HCl started with: 75.00
mL (1 L/1000 mL)(0.2537 mol HCl/L) = 1.903 x 10 -2 moles HCl
F.
Moles HCl absorbed by antacid: 1.903 x 10
-2 moles HCl - 2.586 x 10
-3 moles HCl =1.644 x 10 -2 moles
HCl
G.
Moles HCl absorbed/tablet: 1.644 x 10 -2 moles HCl/tablet
H.
Moles HCl absorbed/gram antacid : 1.644 x 10
-2
moles HCl/tablet (1 tablet/1.3450 g) = 1.222 x 10 -2
moles HCl/g.
I.
g HCl absorbed/gram antacid : 1.222 x 10
-2
moles HCl/g antacid (36.461 g/mol HCl) = 0.4457 g
HCl/g Rolaids.
J.
g HCl absorbed/1 ¢
: 1.644 x 10
-2
moles HCl/tablet(36.461 g/mol HCl)(498 ¢/100 tablets) = 2.985 g
HCl/¢
20.
CLEAN UP! Rinse and refill burets with distilled water. Be sure to rinse through the tips!
Return spin vanes! Rinse all glassware with distilled water and return them!
ALL WASTE FROM THIS EXPERIMENT GOES IN THE JUG LABELLED “ACIDS AND
BASES” IN THE HOOD.

Antacid titration v2004
SAMPLE EXCEL DATA TABLE:
Page 9 4/14/202099
LAB WRITE UP:
You should have all your data from your HCl standardization and Antacid analysis in Excel.
Include all pertinent information about the antacid you used.
Include a summary of your data and an analysis of your results.