Determination of the Neutralizing Capacity of a Commercial Antacid Formulation Class Notes


Antacids are a type of drug used by people who have pain due to too much acid in their stomachs. When the antacid is taken, the acid undergoes a chemical reaction with the antacid. The antacid neutralizes the acid by changing it into water, a neutral liquid. The stomach pain disappears because there is no longer too much acid in the stomach.

Ex. 2HCl(stomach acid) + Mg(OH)2 (base) ---> 2 H2O + MgCl2

Not all antacids are alike. Some may neutralize more acid than others. But your choice of antacid may not be based only on its neutralizing capacity, since there may be individual tolerances of the different formulations. Some active ingredients found in various antacids are NaHCO3, CaCO3, Al(OH)3, MgCO3, and Mg(OH)2

NaHCO3 (baking soda), a commonly taken antacid, can neutralize too much acid leaving the stomach neutral. The natural state of the stomach is acidic and this lack of stomach acid due to the sodium bicarbonate leads to acid rebound, a condition in which the stomach produces excess gastric acid to a greater degree. Acid rebound can cause peptic ulcers. Another problem with sodium bicarbonate is that it contains sodium. If you take an excess of it or use it frequently, you may have swelling of the legs and feet, as well as increasing the amount of sodium in your body. This is particularly important for people with high blood pressure, heart diseases or edema.

MgCO3 is effective only as long as there is excess acid in the stomach. Once the excess acid has been neutralized, MgCO3 is no longer effective. However, it can be converted to a soluble bicarbonate which may act as a mild laxative in some people.

Other problems from antacids: the use of Al(OH)3, aluminum hydroxide, as an antacid can lead to constipation and blockage of the bowel. Long term use of antacids containing aluminum can lead to subtle poisoning, mental changes and weak bones. Calcium carbonate, CaCO3, (found in Tums, Maalox, and Mylanta) provides additional calcium but it can also result in acid rebound, constipation and nausea.

Back Titration vs Direct Titration

In direct titration you can calculate the number of moles of acid if you know the molarity of the base, the volume of base used in the titration and the volume of acid you started with.

In back titration you add an excess of acid, some of which is neutralized by the antacid. You titrate the excess acid with a strong base and calculate the number of moles of acid that you titrated.This method is called back titration. Simple subtraction of the number of moles of acid titrated from the number of moles of acid started with gives you the amount of acid neutralized by the antacid. Back titration is used because not all of the antacid is soluble. It contains binders and fillers which are insoluble.


Accuracy, Precision and Standard Deviation

Accuracy of a measurement refers to how close the measured value is to the true or accepted value.

Precision refers to how close together a group of measurements are to each other. It is determined by the reproducibility of repeated measurements done under identical conditions. You must strive to have both accuracy and precision in the lab. Precision has nothing to do with the true or accepted value of a measurement. You can be very precise but totally inaccurate. You can have precision WITHOUT accuracy.

To help you understand the difference between accuracy and presision, refer to the images below. Imagine a person shooting arrows at the target, trying to hit the bullseye. The closer the arrow is to the bullseye, the more accurate he/she is. If the person misses the bullseye with each shot, but all of the shots land close together, the shots are precise.

precision and accuracy

Standard deviation is one of the most important statistical measures of precision. The smaller the calculated standard deviation, the more precise the data.

The Procedure

First the tablet is dissolved and an excess of acid of known concentration is added to the tablet. The solution is heated briefly to insure that all the antacid reacts and that the buffer equilibrium is shifted to the left (buffer system is destroyed) which helps achieve the sharp color for the indicator end point. Finally, the remaining unreacted acid is titrated with base (NaOH) to determine the amount of acid left over. By subtracting you can find the amount of acid reacted with the tablet.


titration results

The two flasks on the left are good results. The middle flask is cloudy due to the undissolved binders and fillers. The flask on the right is the result of overshooting the endpoint and not the desired endpoint color.


Sample Problem

You add 25.00 ml of standard acid with a molarity of 0.125 M to a flask containing 0.20 g of dissolved antacid. You use 5.00 mL of base with a molarity of 0 .200 M to titrate the excess acid (reaches end point). What is the neutralizing power (mol/g) of the antacid?

Step 1

moles acid added = Macid x Vacid

moles acid added = 0.125 M x .02500 L

moles acid added = 0.00313 moles


Step 2

moles acid left over = moles of base added = Mbase x Vbase

moles acid left over = moles of base added = 0.200 M x 0.00500 L

moles of acid left over = moles of base added = 0.00100 moles


Step 3

Moles of acid used (consumed) by the antacid = moles acid added - moles acid left over

Moles of acid used = 0.00313 - 0.00100

moles of acid used = 0.00213 moles


Step 4

Neutralizing power of antacid = moles of acid used / grams of antacid

Neutralizing power of antacid = 0.00213/ 0.20 g

Neutralizing power of antacid = 0.011 moles of acid/1 gram antacid


If the tablet weighs 1.85 g what is the neutralizing power of the tablet?

Neutralizing power of tablet of antacid = 0.011 moles/g x 1.85 g/tablet

Neutralizing power of tablet of antacid = 0.020 moles of acid/tablet


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last updated: November 6, 2013