Experiment 7: ACID-BASE TITRATION: STANDARDIZATION OF A .

Experiment 7: ACID-BASE TITRATION:STANDARDIZATION OF A SOLUTIONPurpose:Determine molarity of a solution of unknown concentration by performingacid-base titrationsPerformance Goals:Apply the concepts of titration and standardizationGain practice in the use of the analytical balance and buretUse acid-base titration to standardize a NaOH solutionCalculate molar concentration of a NaOH solutionIntroduction:Titration is an analytical technique for determining the concentration of a solution (analyte)by measuring its volume required to completely react with a standard, which could be asolid of high purity or a solution of known concentration. The concentration of the analyteis calculated based on the stoichiometry of the reaction between the analyte and thestandard. Different types of titrations such as acid-base titrations and oxidation-reductiontitrations have been utilized in chemical analyses. In this experiment an acid-base titrationwill be used to determine the molar concentration of a sodium hydroxide (NaOH) solution.Acid-base titrations are also called neutralization titrations because the acid reacts with thebase to produce salt and water.During an acid-base titration, there is a point when the number of moles of acid (H ions)equals the number of moles of base (OH– ions). This is known as the equivalence point. Forexample, in the reaction between HCl and NaOH (Equation 1), the number of moles of H will be same as the number of moles OH– at the equivalence point since the molar ratiobetween HCl and NaOH is one-to-one. For a reaction between a diprotic acid such assulfuric acid (H2SO4) that contains two moles of H ions per mole of H2SO4 and a base suchas NaOH that contains one mole of OH– ions per mole of NaOH, the moles of H and OH–will still be same at the equivalence point, but the moles of acid (H2SO4) and base (NaOH)will not be same. There will be twice as many moles of NaOH because every mole ofH2SO4 generates two moles of H ions, requiring two moles of NaOH to neutralize them(Equation 2).HCl(aq) NaOH(aq)H2SO4(aq) 2NaOH(aq)NaCl(aq) H2O(l)Na2SO4(aq) 2H2O(l)Equation 1Equation 2A chemical substance called an indicator is used to determine the equivalence point. Anindicator is a solution that changes color based on the nature (acidity or basicity) of thesolution. For acid-base titrations the indicators used are weak acids or bases. They haveone color in acidic solutions and another color in basic solutions because they undertakedifferent forms in different solutions. An acid-base indicator in acid form undergoes achange when put in water as illustrated by Equation 3.HIn(aq) H2O(l)In–(aq) H3O (aq)Equation 3acid formbase form87

88EXPERIMENT 7: ACID-BASE TITRATION: STANDARDIZATIONThe indicator, phenolphthalein, is often utilized when strong acids and/or bases are used in atitration. Phenolphthalein is colorless in its acid form, but is pink in the presence of excessbase. When phenolphthalein changes from colorless to a pale pink color as a base is added toan acid solution, it is an indication that the mixture has just passed the equivalence point.This observable instant is referred to as an endpoint. An indicator is chosen such that theobservable endpoint occurs at or very close to the stoichiometric equivalence point. If thecolor is a bright pink, it is an indication that too much base has been added and we no longerhave the equivalence point.Solutions of strong bases, such as NaOH, are usually made by dissolving the solid in water.Presumably its concentration can be calculated from the mass of the solid and the volume ofthe solution produce. However, the solid NaOH is hygroscopic (easily absorb moisture fromair.) In addition, the carbon dioxide that is naturally dissolved in water further changes theacidity of the solution. Thus, the actual concentration of the NaOH solution has to bedetermined experimentally using a process called standardization. To standardize a basesolution such as NaOH, an acid whose amount can be determined to a high degree ofaccuracy (called a primary standard) is needed. Potassium hydrogen phthalate (KHP), amonoprotic acid, is often used as a primary standard for titrating bases. KHP can be dried inan oven to remove traces of water. The reaction between sodium hydroxide and KHPsolutions proceeds according to the following reaction (Equation 4).OOCCCOHOK NaOHOCONaOKEquation 4 H2OOKHP (MM 204.2 g/mol)Note the stoichiometry: one mole of KHP reacts with one mole of NaOH.Alternatively, solutions of HCl with known concentration can be purchased commerciallyand used to standardize basic solutions. Such solutions are referred to as standard solutions.The reaction between solutions of HCl and NaOH is illustrated by Equation 1.In this experiment, standardization of a NaOH solution will be carried out either using KHPas the primary standard or by using a standard HCl solution of known concentration.Equipment/Materials:Using KHP as standard: Buret, 250-mL Erlenmeyer flasks (three), ring stand, clamp,phenolphthalein, NaOH solution of unknown concentration, solid potassiumhydrogen phthalate (KHP), electronic balance (magnetic stir bar and stir plate maybesupplied)Using the HCl as standard: Burets (two), 250-mL Erlenmeyer flasks (three), ringstand, clamp, phenolphthalein, NaOH solution of unknown concentration, standard0.1000 M HCl solution, (magnetic stir bar and stir plate may be supplied)

EXPERIMENT 7: ACID-BASE TITRATION: STANDARDIZATION89How to Record Buret Readings1. The curved surface of a liquid is called a meniscus. Water has a meniscus that curves down. Itis important that you read the buret with the meniscus at eye level to avoid the problems ofparallax. You know you are not at eye level if the markings on the buret on the backside arevisible, as in Fig.1. If you are at eye level, the markings should be as in Fig. 2.Fig. 1Fig. 22. As usual, you record to 1/10 of the smallest increment of your measuring device. In the case ofburets (see sketches below), what is the smallest division shown? How many decimal placesshould you record?Buret ABuret Readings: A 41.26 mLBuret BB Buret CC Buret DD If the meniscus is “exactly” on 40 mL, how should you record it? Ans.Examine the buret reading for A and then write down your readings for B through D. Yourinstructor will go over what those readings should be in the Pre-Lab discussion.

90EXPERIMENT 7: ACID-BASE TITRATION: STANDARDIZATIONProcedure:Cleaning the Buret: Having a clean buret plays an important role in thestandardization process. Therefore, it is necessary to make sure that the buret is clean.Aqueous solutions and water in the buret should form a meniscus and should not formbeads on the walls of the buret.Your instructor or lab technician will indicate whether the burets supplied to you areclean. If they are not, follow the instructions below on how to clean them.In order to clean a buret the stopcock of the buret should be turned perpendicular to theburet. Deionized or distilled water should be run down the sides of the buret using awash bottle. Water should be poured out of the buret, rotating it so that water runs alongthe entire inner surface. Caution must be exercised not to hit the buret on the sink or anyother objects. The above process should be repeated two more times. The buret shouldbe filled with water. If a meniscus forms and water does not form beads on the sides, theburet is clean. If the buret is not clean, then the buret should be cleaned with soap andwater followed by washing with deionized water three times. Make sure that no residueof soap remains inside the buret. The stopcock and the tip need to be cleaned byopening the stopcock and letting water flow through the stopcock and tip into a beaker.Setting up the buret: Setting up the buret correctly is important for a successfulacid-base titration. Figure 7.1 illustrates correct set up of a buret.magneticstir plate(if available)Figure 7.1. Correct setup of a buretCheck that the buret is vertical. If it is even slightly tilted, it means it has not been properlyfastened to the buret clamp, and there is a danger of the buret popping off the clamp andsplattering its content on you.If a standard HCl solution is used, there would be a second buret fastened to the other sideof the buret clamp.

EXPERIMENT 7: ACID-BASE TITRATION: STANDARDIZATION91Standardizing the NaOH SolutionIn the lab notebook, set up a data table similar to the one given at the end of this exercise.Record all data directly into the data table in your lab notebook.Your instructor will indicate which procedure you are to follow (Part I or Part II).Some labs are equipped with magnetic stir bars and stir plates. If none are available,your instructor will demonstrate how to swirl the contents of the Erlenmeyer flask.Part I: Using KHP for Standardization1. Label a 250-mL beaker as “NaOH” and a 400-mL beaker as “Waste.”2. Set up a ring stand with a buret clamp and place a clean buret in the buret clamp.Position a magnetic stir plate in such a manner that the buret tip is directly overthe center of the stir plate as shown in Figure 7.1.3. Place the waste beaker under the buret and close the stopcock.4. Pour about 100 mL of the NaOH solution into the NaOH beaker, Rinse the buretwith the NaOH solution twice with approximately 10-mL portions. The rinsesolution should go into the waste beaker. Fill the buret to the very top. Nextdrain about 5 mL of the solution through the tip to rinse and fill the tip with thesolution. (Hint: If you tilt the buret slightly, it will make pouring much easier.The use of a funnel is not recommended, but if you decide to use one, it must beabsolutely clean and dry, and you must remember to remove it from the buretbefore you start your titration.) Your instructor will demonstrate how to rinse aburet properly.5. At the end of rinsing, check to make sure there are no air bubbles at the tip ofeach buret. Next, carefully drain enough NaOH solution from the buret so thatthe meniscus is on the zero calibration mark or slightly below.6. Record the actual volume reading on the buret. This will be called the initialreading of the buret. It does not have to be at zero. Remember that the volumemarkings on burets are read from top to bottom and volumes are recorded to twodecimal places. The first decimal place can be read accurately and the seconddecimal place needs to be estimated.7. Label three clean 250-mL Erlenmeyer flasks (labeled 1, 2, and 3). They do notneed to be dry.8. Tare a plastic weighing boat to zero. Add about 0.3–0.4 g of KHP and record theexact weight. Transfer the KHP sample carefully into Erlenmeyer flask #1.Repeat for flasks #2 and 3, being careful not to mix up the flasks.9. Add about 50 mL of distilled water to each flask (use graduations on the flask)and 3 drops of phenolphthalein indicator to each flask. It may be necessary towarm the flasks slightly on a hot plate in order to dissolve the KHP. However,the solutions must cool back to room temperature before you begin the titration.10. Slide a clean magnetic stir bar into the flask, taking care not to let any solutionsplash out.11. Check the initial buret reading. If it has changed, your buret has a leak.12. Turn on the magnetic stirrer and titrate the samples with NaOH solution from theburet until the first shade of pink persists for 30 seconds. As you near theendpoint, use the wash bottle of deionized water to wash down any splatter thatmay have collected on the inside surface of the flask.