Antacids Part 2

About this tutorial

     Selecting the link below will start a multimedia tutorial description of the Antacids laboratory procedure.  Pictures, step-animations, and optional video clips of the lab procedure are included with text. You will probably want to have your printed lab procedure with you when you view this material so you can take notes for future use in lab (Note:  The tutorial text goes into greater detail than the lab handout).  At the end of the tutorial you must complete a five-question, multiple choice quiz on the details of the lab procedure.  You must get a score of 100% on this quiz before you can start the lab. You may, however, take the quiz as often as you like. The main point of the quiz is to check your preparation for the upcoming lab.

      One of the most common methods of analyzing the amount of a substance in a sample is by titration.  A titrationis the gradual addition of one reactant to another until the reaction is complete.  The reactant that is gradually added is called the titrant; the reactant to which the titrant is added is in the process of being titrated. Both reactants in a titration are typically present as aqueous solutions.  By observing how much volume of one reactant solution with a known concentration is required to react completely with a given volume of a reactant solution with an unknown concentration, one can then determine the unknown concentration through a simple stoichiometry calculation.  The solution that has the known concentration is called a standard solution.

     The point at which just enough titrant has been added to complete the reaction is called the equivalence point, i.e. the point at which a stoichiometrically equivalent amount of titrant has been added to the other reactant.  The point at which the titration is stopped is called the endpoint. Ideally the equivalence point and the endpoint are identical; in practice they rarely are.  This difference in the practical values of the two points introduces error into the titration experiment. Hence good experimental technique strives to minimize the difference between these two points as much as possible.

     But how does one know when the reaction is complete?  This can be easily done if one of the reactants has a distinctive color.  The sudden appearance of that color, if the reactant is the titrant, or the sudden disappearance of that color, if the substance is being titrated, signals that the reaction has been completed.  Consider, for example, the reaction between the permanganate (MnO₄^-) and iron(II) ions according to the following chemical equation:

The permanganate ion has a distinctive dark purple color while all other species in the reaction are essentially colorless at low concentration.  The permanganate ion is the titrant in this reaction.  As it is added, the solution being titrated will remain colorless as the added permanganate reacts quickly with the iron(II).  At the equivalence point all of the iron(II) has been used up, but the solution is still colorless since no permanganate ion is present.  Now addition of a small amount of permanganate solution in excess of the equivalence point turns the solution being titrated a purple color since no iron(II) is left to react with the permangante.  This signals that the reaction is complete and the titration should be stopped.  Notice that the equivalence point and endpoint are not the same here.  A small excess of titrant must be added to observe that the reaction is complete.  In this titration experiment one would minimize the difference between these two points by adding just enough excess permanganate to turn the solution a very faint purple color.

     But in many reactions all reactants and products are colorless.  How is the completion of the reaction detected now?  In this case it is common to add a small amount of another substance that has distinctly different colors in the two reactants to the solution being titrated.  While the reactant being titratated is still present, this substance has one color.  At the equivalance point, addition of a small excess of the titrant causes this substance to change to a different color, thus indicating that the reaction has been completed.  This substance that indicates completion of the reaction is called--surprise, surprise--an indicator. 

     In an acid-base titration is simply a titration in which the two reactants are an acid and a base.  One is gradually added to the other until they have completely neutralized each other.  Since acids and bases are usually colorless (as are the water and salt produced in the neutralization reaction), an acid-base indicator is added to detect the endpoint of the titration.  Phenolphthalein is a very common indicator (and is the one we will use in lab) that has the property of being colorless in acidic solution and a bright pink color in basic solution.  Since a color change from colorless to pink is easier to observe than a change from pink to colorless, the base is used as the titrant when phenolphthalein is the indicator.  In this situation the acidic solution being titrated remains colorless as the titrant (base) is added.  At the equivalence point the solution being titrated is still colorless, but addition of a small amount of excess base now makes the solution basic and turns the indicator pink.  To make sure that the endpoint of the titration is as close as possible to the the equivalence point, just enough excess base is added to turn the solution the faintest detectable shade of pink.

      This lab will run over two lab periods and the lab report for the entire lab is due the week following the second lab period, i.e. don't hand in a lab report after the first lab period, save it all for the final lab report.  In the first period we we will prepare solutions of hydrochloric acid and sodium hydroxide for use in the second period's experiment.  Before we can use these solutions, however, we must first determine their concentrations by titrating them against substance of high purity called a primary standard.  In this lab we will use potassium hydrogen phthalate (abbreviated KHP) as the primary standard.  KHP is an acidic substance of high purity that is also a solid.  This means we can conveniently weigh out a known amount of KHP on the balance and know precisely how many moles of acid are present.  Reacting that known number of moles of KHP with your sodium hydroxide solution in a titration experiment allows you to calculate the sodium hydroxide concentration.  The sodium hydroxide solution will now be a standard solution that you can use to determine the concentration of the hydrochloric acid solution in a second titration experiment.  By the end of the lab you will have standard solutions of both sodium hydroxide and hydrochloric acid that you can use in the second period experiment.  This on-line pre-lab tutorial will cover only the procedure for the first period's experiment.