Experiment-12: Analysis of an Antacid

Back Titration

Molarity calculation

Antacid active ingredients

The human digestive system uses hydrochloric acid to help breakdown food in the stomach. The parietal cells in the stomach secrete hydrochloric acid at a concentration of about 0.155 M HCl (pH 1-2), quite concentrated! The flow of HCl increases when food enters the stomach. When you eat or drink too much, your digestive system may generate too much acid. You may develop a condition called "heartburn" or indigestion. Antacids are ingested to reduce indigestion caused by excess stomach acid. Antacids neutralize the excess acid and "relieve" but not eliminate the condition. The reaction that takes place is an acid/base reaction. A little bit of NaOH might be equally effective, but it is extremely rough on the rest of the digestive system, so antacids need to be formulated to reduce acidity while avoiding physiological side-effects. Therefore, active ingredients of commercial antacids are weak alkaline (basic) chemicals. In addition to the active ingredient (base), Antacids also contain flavors, sweeteners, binders, fillers, antifoam agents, pain relievers (aspirin), etc. The base in antacids varies with the brand as shown below:

Brand Name / Active Basic Ingredient(s)
Pepto-Bismol / BiO(HOC6H4COO)
Milk of Magnesia / Mg(OH)2
Rolaids / NaAl(OH)2CO3 (newer tablets: CaCO3)
Tums / CaCO3
Alka-Seltzer II / NaHCO3 and KHCO3
Maalox / Mg(OH)2 and Al(OH)3
Gaviscon / Al(OH)3

The active basic ingredients will neutralize excess acid as per the following equations:

BiO(HOC6H4COO) + 3 H+(aq) Bi3+(aq) + H2O(l) + HOC6H4COOH(s)

Mg(OH)2(s) + 2 H+(aq)  Mg2+ (aq) + 2 H2O(l)

Al(OH)3(s) + H+(aq)  Al(OH)2+ (aq) + H2O(l)

CaCO3(s) + H+(aq) Ca2+(aq) + HCO3- (aq)

HCO3- (aq) + H+(aq) CO2(g) + H2O(l)

In this experiment, you will analyze Milk of Magnesia, which contains magnesium hydroxide (Mg(OH)2 ) as the active portion. Your goal is use volumetric analysis to find out the amount of Mg(OH)2 present in 5.00 mL Milk of Magnesia. The side panel on the bottle indicates that each 5.00mL portion of Milk of Magnesia delivers 400 mg of Mg(OH)2 . Therefore, the percent error can be calculated at the end of your experiment. The excess acid in the stomach will be neutralized by Milk of Magnesia as per the balanced reaction: Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l)

The technique you will use today is known as Back Titration. First, a 5.00 mL portion of Milk of Magnesia will be neutralized using more than enough volume (16.0 mL) of a standard HCl(aq) (1.00M). Then, any remaining excess of HCl(aq)will be titrated against a standard NaOH(aq) solution as in the last experiment. You can calculate the mol NaOH of used in the titration, since the molarity (0.1M) and volume of NaOH(aq) is known. Because NaOH and HCl react in 1:1 molar ration, the mol NaOH is equal to the remaining excess ofmol HCl.

[Mols of standard HCl used] = mols of HCl neutralized by Milk of Magnesia+ remaining excess of mol HCl

[Molarity x Volume of standard HCl] = X + (mol NaOH)

[1.0 M x 16.0 x 10-3 L] = X + (Molarity x Volume in L of standard NaOH)

[16.0 x 10-3 mol] = X + (0.1M x Volume of NaOH used ÷ 1000)

X = [16.0 x 10-3 mol] - (0.1M x Volume of NaOH used ÷ 1000)

Mols of Mg(OH)2(s) per 5.00 mL of Milk of Magnesia = 0.5 x X mol

Convert 0.5 x X mol of Mg(OH)2 into grams = 0.5 x X x molar mass of Mg(OH)2

Procedure:

  1. Sample Preparation: Shake the Milk of Magnesia bottle well before dispensing. Using a small (10-mL capacity) DRY graduated cylinder, transfer exact 5.0 mL Milk of Magnesia into a clean Erlenmeyer flask. If inside of this Erlenmeyer flask is wet, it will not affect your results. To remove all the Milk of Magnesia, which is stuck to inside wall of small cylinder, rinse it with 10 mL distilled water and transfer into the Erlenmeyer flask. Rinse two more times with 10 mL distilled water (total water added is about 30 mL). Carefully swirl the contents of Erlenmeyer flask for 1 minute.
  2. Using a DRY graduated cylinder, carefully obtain exact 16.0 mL of 1.0 M HCl (CAUTION: HCl is CORROSIVE!) from the big supply container. Do not remove the big supply container. Slowly transfer this HCl into the Erlenmeyer flask prepared in step-1. Carefully swirl the contents of Erlenmeyer flask for 2-3 minutes. Add 3 drops of phenolphthalein. Swirl the flask for 1 more minute to mix the contents thoroughly. At this stage, there are four items in your Erlenmeyer flask (exact 5.0 mL Milk of Magnesia + about 30 mL distilled water + exact 16.0 mL of 1.0 M HCl +3 drops of phenolphthalein).

Note: A pink color indicates that more HCl is required. Consult your teacher!

  1. Buret Set-Up: Review the figure. Transfer approximately 100 mL 0.1 M NaOH into a 250 mL beaker and take it to your bench. You will use this NaOH to fill-up the buret. Do not take the big bottle of NaOH to your desk. Wash and rinse a buret with distilled water. Then rinse this buret twice with 5 mL portions of the 0.1 M NaOH, and drain into the designated waste container. Fill the buret almost to the top with 0.1 M NaOH. Clamp this buret to the iron stand. Open the stopcock/control knob slowly to drain out NaOH until the lowest level of meniscus is at 0.00 mL mark. Make sure there are no air bubbles. 0.00 mL is your initial reading.
  2. Titration: Place the Erlenmeyer flask from step-2 under the buret and start titration as demonstrated by your teacher. Remember to keep swirling the flask while adding NaOH in a drop wise and slow manner. One may place a white piece of paper under the flask to observe the end point (colorless to permanent lightest pink). Once you reach the end point add no more NaOH. Read the buret. This is your final buret reading. The difference between final and initial reading is the volume of 0.1 M NaOH used in this trail#1 (______mL)
  3. Discard the contents of Erlenmeyer flask into the designated waste container.
  4. Using a small funnel carefully refill the buret to 0.00 with 0.1 M NaOH solution.
  5. Second Trail: Repeat step-1 and 2.
  6. Repeat step-4. Volume of 0.1 M NaOH used in this trail#2 (______mL)
  7. One by one discard the contents of Erlenmeyer flask, buret and left over solutions into the designated waste container.
  8. Calculate the molar mass of Mg(OH)2 using a periodic table.

Laboratory Report#12: Analysis of an Antacid

Last Name______, first name______

Date of Experiment______Instructor’s Initials______

Data and Calculations

  1. Volume of Milk of Magnesia used in each trail= 5.00 mL
  2. Molarity of NaOH used in each trail= 0.1 M
  3. Volume of NaOH used in trail#1 (Final – Initial buret reading) = ______mL
  4. Volume of NaOH used in trail#2 (Final – Initial buret reading) = ______mL
  5. Average Volume of NaOH used = (#3 + #4) / 2 = ______mL ÷ 1000 = ______L
  6. Mols of NaOH used = Molarity on NaOH x Volume of NaOH in Liter = step-2 x step-5 = ______mol NaOH = remaining excess mol of HCl
  7. Volume of standard HCl used in each trail = 16.0 mL = (16.0 ÷ 1000) L= 0.016 L
  8. Exact Molarity of standard HCl used in each trail = 1.0 M
  9. [Mols of standard HCl used] in step-2 = step-7 x step-8 = 0.016 mol HCl
  10. mols of HCl neutralized by Milk of Magnesia = step-9 – step-6 = _____mol
  11. Mols of Mg(OH)2 per 5.00 mL of Milk of Magnesia = (0.5 x step-10) mol= ______mol
  12. grams of Mg(OH)2 per 5.00 mL of Milk of Magnesia = step-11 x molar mass of Mg(OH)2 = step-12 x 58.32 g/mol = ______g
  13. milligrams of Mg(OH)2 per 5.00 mL of Milk of Magnesia = step-12 x 1000 = ______mg
  14. Actual milligrams of Mg(OH)2 per 5.00 mL of Milk of Magnesia from the side label = ______mg
  15. % error = [(Experimental Value – Actual Value)÷ Actual Value] x 100 %

= [(step-13 – step-14)÷ step-14] x 100 % = +______%

Show all your work! Use proper units and significant figures!!

  1. Write the chemical names of the following bases:

Mg(OH)2______

CaCO3______

NaHCO3 ______

KHCO3______

Al(OH)3______

  1. Calculate the mL of 1.0 M HCl required to neutralize 800 mg of Mg(OH)2 .

1 Mg(OH)2(s) + 2 HCl(aq) MgCl2(aq) + 2H2O(l)

  1. Convert 800 mg of Mg(OH)2 into mols______
  1. Mols of HCl required = 2 x Mg(OH)2 mols ______
  1. Volume, in Liter, of HCl required

= Mols of HCl required/ Molarity

= ______L (Note: Molarity is given = 1.0 M)

= ______mL

3. Why is it necessary to shake the Milk of Magnesia bottle well before dispensing?