In this experiment you will measure the amount of stomach acid consumed (or neutralized) by various antacid tablets (Maalox, Tums, Rolaids: no Pepcid or Tagamet!). If you have a favorite one, bring a package to the lab (one color only). Inside your stomach, excess hydrochloric acid is neutralized by the antacid. Different antacids use different metal hydroxides, such as Al(OH)3 or Mg(OH)2. The general formula for this reaction is: M(OH)y + Y HCl [pic] Y H2O + M+y + Y Cl-
Additionally, some brands of antacids use calcium carbonate as a neutralizing reagent. CaCO3 + 2 HCl (aq) [pic] H2CO3 (aq) + Ca2+ (aq) + 2 Cl-(aq) The carbonic acid formed in this reaction may undergo further reaction: H2CO3 (aq) [pic] H2O (l) + CO2 (g)
This experiment will involve several steps. The first step will involve a simple reaction to determine the concentration of the NaOH that will be used in this experiment using a solid acid standard. Using the standardized NaOH, the next step of the lab will involve determining the concentration of the HCl. Once we have the concentration of the standardized HCl, we will conclude the lab by determining the amount of acid that is consumed when you use an antacid tablet. First the tablet is dissolved and an excess of acid of known concentration is added to the tablet. The solution is then briefly heated to insure that all of the antacid reacts. Finally, the remaining unreacted acid is titrated with base (NaOH) to determine the amount left over and hence, the amount that reacted with the tablet. The chemical reaction which occurs is: NaOH(aq) + HCl(aq) [pic] NaCl(aq) + H2O(l)
A detailed description of the individual steps in the analysis follows. Before we get started, there are a few points on using a buret that need to be covered. Adding a solution from a buret can be a little tedious, but the errors that are technique dependant will cause errors that will propagate themselves throughout the experiment. Add material to the titration from the buret slowly and be sure to swirl the flask after adding. As you get near the endpoint of the titration, it is hard to add complete drops of solution from the buret and not to overshoot the titration endpoint. A solution for this problem is to wash the tip of the buret with a few drops of deionized (DI) water since this will add “fractional drops” of material into the titration flask. Your TA will provide some additional suggestions for proper buret use during your recitation.
Step 1: Digestion of the antacid tablets
Weigh an antacid tablet and transfer it to a 250-mL Erlenmeyer flask. Label the flask antacid 1. Record the weight of the tablets you are going to analyze to the nearest mg in your lab notebook. Add 50.00 mL of hydrochloric acid solution with a burette to the flask containing the tablet and record the exact concentration used. Record into your lab notebook the exact volume of acid used. If the tablet does not readily dissolve, cover the flask with a watch glass and boil gently for 5-10 minutes on a hotplate. Set it aside to cool. While the flask is cooling, weigh out another tablet of the same brand and repeat the dissolving procedure three more times. Label the flasks/samples ANTACID 2, ANTACID 3 and ANTACID 4. Step 2: Standardization of HCl
For this step we will use a solution of HCl. Using another burette filled with HCl, you will place between 20 and 25 mL of HCl each into three separate flasks labeled HCl 1, HCl 2 and HCl 3. It is very important to record the EXACT VOLUMES of HCl placed into the flasks since the unknown variable in this part of the experiment will be the concentration of the HCl. Add 3-5 drops of phenolphthalein to each of the acid solutions and titrate each acid solution with NaOH. Be very careful not to take the titration to a raspberry red/pink color since going to the color indicates that you may have overshot the endpoint and...