Lab Overview Investigation #21: "Some Commercial Antacids"

Concepts of the Experiment

CaCO₃ + H⁺ + Cl^- ® Ca²⁺ + HCO₃^- + Cl^-

CaCO₃ + 2H⁺ + 2Cl^- ® Ca²⁺ + 2Cl^- + H₂O + CO₂

NaHCO₃ + H⁺ + Cl^- ® Na⁺ Cl^- + H₂O + CO₂

Mg(OH)₂ + 2H⁺ + 2Cl^- ® Mg²⁺ + 2Cl^- + 2H₂O

The above reactions are all common reactions between an antacid and stomach acid, HCl. Notice that the main ingredients are: CaCO₃, NaHCO₃, Mg(OH)₂ and of course HCl. H₂O is pretty much a main ingredient of everything. Organic chemists hate that.

In all of these reactions the H⁺ ion is converted to either water or hydrogen carbonate. So when your stomach is "upset" adding antacid neutralizes excess hydronium ions. The "acid-neutralizing capacity" of an antacid is defined as the moles of HCl neutralized by one gram of the antacid.

You will determine the "acid-neutralizing capacity" by reacting a known mass of antacid in excess HCl. You will know the concentration of the HCl as well as the volume. After all of the antacid has finished reacting with the HCl, you will titrate the remaining HCl with NaOH till the endpoint is reached. Again, you will know the concentration as well as the volume of NaOH needed to neutralize the HCl.

Since you know the concentration as well as the volume of HCl, you can calculate the number of moles of HCl. Remember that M (mol/L) * L = moles. You can do the same thing with the NaOH. Also remember that the balanced equation for the reaction between HCl and NaOH is:

So by inspection, the number of moles of NaOH reacting with HCl is equal to the number of moles of HCl in the reaction. So if you take the number of moles of HCl total in the beaker you can subtract the number of moles of HCl that reacted with the NaOH and you will get the number of moles of HCl that reacted only with the antacid.

Now that you have moles of HCl, which reacted with the antacid tablet, divide this by the grams of antacid to get the "acid-neutralizing capacity."

In this experiment you will be using bromophenol-blue which changes from yellow to blue at pH = 4.6. Why would you use an indicator that changes colors while still acidic? Why not use a different indicator?

If we use an indicator which changes colors closer to pH 7.0 then we start to have problems with some of the side ingredients in the antacid tablets. Mg and hydrogen carbonate both react with excess OH^-. This would cause an error in our calculation of "acid-neutralizing capacity" as well as frustrate the heck out of all you folks who just can't wait to see that color change. We can't keep you waiting.

Guess what? Before you do all of this stuff though, you get to standardize your acids and bases. To do this, you will first make all of our solutions. Remember that M₁V₁ = M₂V₂ for calculations which involve diluting a more concentrated solution to a less concentrated solution. After you have done this, you will titrate the NaOH with a standardized acid, KHP. The reaction is one to one meaning that for every mole of NaOH that reacts one mole of KHP reacts. From this you should be able to get the standardized concentration of NaOH. But since your titration skills may be rusty you will have to do this again and again. Then you will calculate the average concentration of NaOH and use that value from now on. Finally, you will use your NaOH with a known concentration to titrate and determine the concentration of the HCl you made. You should also repeat this twice more and average the final values.
 
 

In the beginning, you will standardize the NaOH with KHP. Determine the number of moles of KHP. At the endpoint of the titration, the number of moles of KHP will equal the number of moles of NaOH. Now that you know the number of moles of NaOH, divide by the volume of NaOH titrated to get the standardized concentration of NaOH..

Secondly, you will standardize HCl with the now standardized NaOH. You know both the concentration of the NaOH and the volume so from this you should be able to find the number of moles titrated. At the endpoint, the number of moles of NaOH will equal the number of moles of HCl. And again, now that you know the number of moles of HCl, divide by the volume of HCl.

To determine the acid neutralizing capacity of your antacid, you took a sample of the antacid and placed it in a measured volume of HCl solution. You then made sure that you had an excess of HCl by adding measured volumes until the indicator returned to its yellow color. At this point, all of the base in the antacid has reacted and the remaining HCl is an excess. To find the total number of moles of HCl, just calculate from the total volume of HCl placed in the beaker and the concentration. The number of moles of excess HCl is titrated and determined. Therefore, the number of moles of HCl that reacted with the antacid should be equal to the total number of moles of HCl minus the number of moles of excess HCl. Take this amount and divide by the mass of the sample and you have your acid neutralizing capacity.