Volumetric Analysis

Volumetric Analysis INTRODUCTION Volumetric Analysis INTRODUCTION In Part I of this experiment you will prepare a solution of sodium hydroxide and standardize it against very pure potassium acid phthalate in a series of replicate titrations. Potassium acid phthalate (KHP) is a monoprotic acid with the formula KHC8H4O4. You will then use the standardized sodium hydroxide solution to determine the acetic acid content of a dilute unknown solution. A standard solution is one whose solute concentration is accurately known. If a solute can be obtained in a very pure, stable, weighable form, a primary standard solution of it can be prepared directly. This is done by dissolving an accurately determined amount of the solute in the desired solvent and bringing the total volume of the solution to an accurately known final value. Care must be taken to insure that the solution is homogeneous and that it is at ambient temperature when the final adjustment of its volume is made. If the desired reagent cannot be obtained in primary standard form, one can only prepare a secondary standard solution of it. A solution of the reagent having approximately the desired concentration is prepared which is then standardized (1) by titration against a measured mass of a suitable primary standard substance; or (2) by titration against another reliably known secondary standard solution; or (3) by direct analysis for the reagent in question by some suitable non-titrimetric method such as spectroscopic analysis. We shall use standardization method (1) in Part I of this experiment utilizing potassium hydrogen phthalate as the primary standard to prepare a secondary standard solution of sodium hydroxide. In Part II of this experiment you will prepare a solution of hydrochloric acid and standardize it against the sodium hydroxide solution you prepared in Part I. The standardized hydrochloric acid solution is referred to as a tertiary standard. You will use it to determine the neutralization capacity of commercial Antacid tablets. Antacids work by neutralizing excess acid in your stomach. The active ingredient is usually carbonate ion (CO32-) or hydroxide ion (OH-). In this experiment, you will determine the mass and percentage of hydroxide or "equivalent hydroxide" that is present in the tablet, where one mole of carbonate is equivalent to the neutralizing capacity of two moles of hydroxide. You will accomplish this by first dissolving the antacid sample in excess HCl (because many antacids do not dissolve in water alone). Some, but not all, of the added HCl will react with the base present in the tablet. You will then titrate the solution with NaOH to determine how much HCl is remaining or is in excess. By difference you will then be able to calculate the amount of HCl that reacted with the antacid and thus the neutralization capacity of the antacid tablet. Using the calculated amount of HCl that reacted, you may also determine the mass and percentage of hydroxide or "equivalent hydroxide" in the tablet. One lesson you will learn from this part of the Volumetric Analysis PART I. PROCEDURE experiment is that the analyses of "real life" unknowns are often complex and more difficult. Safety. Wear goggles throughout the entire experiment, especially when reading a buret! Be especially careful with acids and bases. All waste from this experiment can be poured down the drain with copious amounts of water. Work in pairs on this 2-day experiment. Stock Chemicals Used Chemical Maximum Amount Used 50% (w/w) Sodium Hydroxide According to calculation 6M Hydrochloric Acid According to calculation <10 mL 3g <25 mL Drops Potassium Hydrogen Phthalate 1% Phenolphthalein Indicator (KHP) Methyl Orange Xylene Cyanol FF Acetic Acid Unknowns A, B, C, D (M.O.X.C) Indicator (2nd week) Drops ~ 20 mL PART I. PROCEDURE Analysis of an Unknown Acetic Acid Solution A. Preparing the Sodium Hydroxide Solution In this step of the experiment you will prepare 500 mL of approximately 0.15 M sodium hydroxide solution by diluting a commercial stock solution that is 50% NaOH by mass and has a density of 1.525 g/mL (grams of solution per mL of solution). 1. Begin by pouring about 200 mL of deionized water into your (clean) plastic bottle. Calculate the appropriate volume of stock solution, use a polyethylene dropper to dispense it into a 25 mL graduated cylinder, and then pour the contents of the graduate into the partially filled plastic bottle. Rinse the graduate out with fresh water at least twice, adding the rinsings to the contents of the plastic bottle. Screw the cap on the plastic bottle and mix the contents thoroughly by inverting the bottle and swirling it repeatedly. Then add the remaining 300 mL of water in three 100mL batches, mixing the bottle contents thoroughly each time. The bottle should be shaken at least 30 times after the last addition. B. Standardizing the Base Against Potassium Acid Phthalate In this step of the experiment you will standardize your sodium hydroxide solution against the primary standard, potassium acid phthalate. You will also use a technique called weighing by difference. This is a very important technique to use because it eliminates systematic errors from the balance. Weighing by difference is quite simple. First, mass the container and the material from which you are going to draw your sample. Then remove some of the material and place it in a separate container. Remeasure the mass of Volumetric Analysis Part I. Procedure the original container and the remaining material. Calculate the mass removed, and repeat the process until you have removed the mass desired. Question A: Explain how weighing by difference eliminates systematic balance errors. 2. In a previous laboratory period you will have obtained about 3.0 grams of primarystandard grade potassium acid phthalate (KHP) in a vial, dried it in an oven at 110C for 2 hours, and stored it in a desiccator for use today. 3. In what follows, use a folded paper strip to handle the vial; this will keep oils from your hands from changing the mass of the vial. Accurately weigh a 0.5-0.6 gram sample of dry KHP on to a weighing boat. Using the appearance of this sample as a guide, accurately weigh two or three more such samples by difference into clean 125 or 250 mL Erlenmeyer flasks. Quantitatively transfer the first sample from the weighing boats into a 500 mL Erlenmeyer flask with the help of a small stream of water from your wash bottle, and then add about 35 mL of water to each of the three flasks and swirl them gently until the solids dissolve. Be careful not to leave drops of the solution on the side walls of the flask. 4. With the technique described in the Common Laboratory Procedures section of this manual, rinse and fill a 25 mL buret with the base solution you wish to standardize. Do not waste time trying to hit 0.00 with the meniscus. Rather, go a few drops below the zero mark and read and record the actual starting point to the nearest 0.02 mL. Be sure always to wipe off the tip of a buret before you begin a titration. Use a laboratory tissue and make one quick stroke downward beginning at the stopcock and ending in the air beyond the buret tip. 5. Add three drops of phenolphthalein indicator to the first KHP solution. Place the flask under the buret and lower the buret tip well into it. The first titration should be a cursory titration to find the approximate volume of the end point. Place a piece of white notebook paper under the flask and add about 2 mL at a time until the solution turns a permanent pink. The solution should be stirred continuously. Note the volume needed to reach the end point. Refill your buret and record the initial buret reading. Now, perform the second titration quickly but more carefully. Open the stopcock somewhat and allow a gentle but steady stream of base to flow (without splashing) into the pooled acid solution. You can add the base fairly rapidly at first, but pay attention to the region where the two solutions mix in the flask, and as the pink color which forms there begins to tail out into the solution as you stir the contents of the flask. Stop rapidly adding the base when you are within 2 mL of the approximate endpoint. Gently wash down the walls of the flask with water from your wash bottle, and then resume adding base from the buret but now dropwise. As you approach the endpoint, the pink color will increasingly linger. You should frequently wash down the interior sides of the flask to recover any reagent drops from clinging to the sides. Swirl the flask after the addition of each drop or ½ drop. A ½ drop may be added by slowly opening the stopcock until a ½ drop forms at the tip. Wash this ½ drop into the flask. Stop adding base when the entire flask has a faint pink color that persists. You may wish to record the buret Volumetric Analysis volume of several successive drops as you approach the endpoint in case you discover that you have overshot the endpoint. Record the final buret reading to the nearest 0.02 mL. Refill the buret and similarly titrate the remaining KHP samples. 6. It should be clear to you that the ratio of the NaOH titration volume to the mass of KHP being titrated should be a constant. Calculate this ratio for your three titrations and determine if one of them fails the Q-test. If it does, run another sample. When you have three samples that can be retained, calculate and report the average molarity of the NaOH solution, the 90% confidence interval for the reported molarity, and the relative average deviation of the your measurements in parts per thousand. NOTE: KHP has the actual chemical formula KHC8H4O4, formula mass = 204.23. Question B: Why doesn't it matter how much water you add when dissolving the acid or when carrying out the titration? C. Analysis of the Unknown Acetic Acid Solution 7. Obtain approximately 20 mL of an acetic acid unknown in a clean 50-mL Erlenmeyer flask. Record the unknown number in your laboratory notebook. 8. Pipet 5.00 mL samples of acetic acid unknown into each of three clean 125, 250 or 500 mL Erlenmeyer flasks and wash down the sides of the flasks with another 25 mL or so of water from your wash bottle. 9. One after the other, add three drops of phenolphthalein indicator to each of the titration vessels and titrate their contents with your standardized NaOH solution. A titration thief would probably be advisable for at least the first titration. Ideally, the range of the replicate titration volumes should be only one or two drops. One drop from these burets is about 0.05 mL, so two "5 mL" titration volumes that differ by 2 drops disagree with each other by 1%. 10. Calculate and report the average titratable acid concentration, [HC2H3O2], in the original unknown solution and the 90% confidence interval for this result. Also calculate and report your average value for the mass percentage of acetic acid in the original unknown solution along with the 90% confidence interval for this result. Assume a density of 1.00 g/mL for the acetic acid unknown. Clean-up. After Part I. of the experiment is completed, drain any remaining solution from the buret. Rinse each buret with deionized water. Then, leaving the stopcock open, return the inverted burets to the buret clamp. SAVE YOUR REMAINING NaOH FOR PART II OF THIS EXPERIMENT! PART I. DATA ANALYSIS Volumetric Analysis Part I. DATA ANALYSIS How many grams of pure, solid NaOH are required to make 500 mL of 0.15M NaOH solution? . How many grams of a solution that is 50% by weight NaOH is required to make 500 mL of 0.15M NaOH solution? g. The density of a 50% solution of NaOH is 1.525 g/mL. What volume of a solution that is 50% by weight NaOH is required to make 500 mL of 0.15M NaOH solution? Why does weighing by difference eliminate systematic balance errors? For each of your 3 acceptable trials (not the cursory one) in standardizing your NaOH against KHP, what are the mass in grams of KHP for each of the three samples to one thousandth of a gram, e.g. 0.507g? What is the volume in mL of NaOH required to reach the endpoint for each of the KHP samples and enter the volume to a hundredth of a mL, e.g. 15.24 mL? Calculate the values for each of your three trials for the molarity of your NaOH solution. Place your molarity entries in the order corresponding with the masses of KHP and the volumes of NaOH required for the trials. Calculate the value for the average of the three trials determining the molarity of your NaOH solution. Calculate the value for the standard deviation of the average molarity. Calculate the value for the 90% confidence limits for the molarity of your NaOH solution. Why does it not matter how much water you added when dissolving the acid or when carrying out the titration? What was the letter of your unknown sample of acetic acid? What volume in mL did you use for each of the three acceptable trial samples of your unknown solution of acetic acid? What was the volume of standardized NaOH solution that was required for each trial determining the amount of acetic acid in your unknown? Calculate the molarity of your three acetic acid trials using the volume of standardized NaOH solution required for each and the average molarity of the NaOH solution from the standardization trials with KHP. Calculate the value for the average of the three trials determining the molarity of your unknown acetic acid solution. Calculate the value for the standard deviation of the three trials for the molarity of your unknown acetic acid solution. Volumetric Analysis PART I. DATA ANALYSIS Calculate the value for the 90% confidence limits for the molarity of your unknown acetic acid solution. Volumetric Analysis Part II. Procedures PART II. PROCEDURES Neutralization Capacity of Commercial Antacid Tablets A. Preparing the Hydrochloric Acid Solution In this procedure the secondary standard NaOH solution will be used to standardize an HCl solution to form a tertiary standard. 1. You will prepare 200 mL of ~0.6 M HCl from a 6.0 M stock solution in a 500 mL Erlenmeyer flask. Calculate how much 6.0 M HCl you will need and roughly how much deionized water will be required. Place about 80% of the required deionized water into the 500 mL Erlenmeyer flask. Measure the calculated amount of 6.0 M HCl using a graduated cylinder and transfer this volume to the flask. Now add deionized water to the 200 mL mark on the Erlenmeyer flask. Cover the flask with Parafilm and mix well. You have now successfully prepared the acid solution. Next, you will determine the true concentration of your acid. Question C: You always add acid to water, and never the reverse. Why? B. Standardizing the Acid Against Sodium Hydroxide 2. Accurately transfer 5.00 mL of your prepared hydrochloric acid solution using a clean volumetric pipet into a clean Erlenmeyer flask and add approximately 20 mL of deionized water to the flask to allow you to swirl the contents easily. Rinse and fill a 25 mL buret with your standard NaOH solution. Make three or four samples. Add 2-3 drops of phenolphthalein indicator to the flask and titrate to the endpoint as described in step 9 of Part I. Record the final volume of base. Repeat this titration at least two more times. You will need to reuse one of your Erlenmeyer flasks. Calculate the actual concentration of your hydrochloric acid solution using the concentration of base you determined in Part I for each trial. 3. Calculate the average concentration of your acid and the relative and standard deviation of your data. C. Antacid Analysis 4. Prepare 2 burets for your antacid titration. Condition and fill a 25 mL buret with standard acid. Make sure the buret is thoroughly washed since it may have previously contained base. Condition and fill a 25 mL buret with your standard base. You may label the buret holder in order to be sure to remember which buret contains acid and which contains base. Record the initial buret readings. 5. Obtain antacid tablets from one of the brands provided in the laboratory. Record the masses of the tablets in your notebook. Crush the first antacid tablet using a mortar and pestle. Suggested masses to be used for analysis are indicated on the antacid containers. Record the mass of the crushed sample in your laboratory notebook. Quantitatively transfer the sample to a clean 125 mL Erlenmeyer flask. Volumetric Analysis PART II. DATA ANALYSIS 6. Add approximately 25 mL of deionized water to the flask. Carefully and accurately dispense 12.5 mL of your standard acid into the 125 mL flask using your buret. Next clamp the flask and heat the contents with a hot plate. Boil the solution for 5 minutes. The sample may not completely dissolve during this process due to the presence of "fillers", but any base will have reacted with the excess HCl. Using your alkacid paper, check to be sure that all base has reacted, and that there is excess acid present. If the solution is not acidic, then accurately add another 12.5 mL of acid and boil for another 5 minutes. Question D: Write a balanced chemical equation that describes the reaction if the antacid contains hydroxide ion. Question E: Write a balanced chemical equation that describes the reaction if the antacid contains carbonate ion. 7. Cool the solution to room temperature by carefully immersing the flask in a container of tap water. Add 5 drops of methyl orange/xylene cyanole FF indicator (MOXC) to the solution. Titrate the sample with your NaOH standard. The indicator MOXC goes from red (acid), through grey, to green (base). The grey color is the end-point. The end point is not as obvious as that of phenolphthalein. If you add too much base, you will need to back-titrate with the standard HCl. That is, you will have to add a carefully measured volume of standard HCl to return to the red color. You can then titrate again to the grey end-point using the standard base. 8. Repeat the experiment using the leftover mass from the brand you selected. Report the neutralization capacity (millimoles of HCl neutralized) of the antacid tablet you used. Calculate the mass and percentage of hydroxide or "equivalent hydroxide" per gram of antacid tablet. 9. Calculate an average, relative deviation, and standard deviation. Report the results in your notebook. Be sure to show all calculations. Clean-up. Drain any remaining solution from the burets. All waste from this experiment can be poured down the drain. Rinse each buret with deionized water and return to the buret shelf. PART II. DATA ANALYSIS What volume of 6 M HCl is needed to prepare 200 mL of 0.6 M HCl? What volume in mL of your HCl solution did you use for each of the three acceptable trial samples for the standardization? Your volume of HCl should be approximately 5 mL. What was the volume of standardized NaOH solution that was required for each trial for the standardization of HCl? Volumetric Analysis Part II. DATA ANALYSIS Calculate and report the molarity of your three HCl standardization trials using the volume of standardized NaOH solution required for each and the average molarity of the NaOH solution from the standardization trials with KHP. Report the value you calculated for the average of the three trials determining the molarity of your HCl solution. Report the value you calculated for the standard deviation of the three trials for the molarity of your HCl solution. Report the value you calculated for the relative deviation, in parts per thousand, of your HCl solution. Report your measured masses of the antacid tablets. For each of the three trials you performed for the analysis of an antacid tablet, what was the mass of the crushed portion of the tablet that you dissolved in your standardized HCl? For each of the three trials you performed for the analysis of an antacid tablet, what volume (in mL) of tertiary standard HCl did you use to dissolve the samples? Report the total volume of acid that you used to dissolve the samples, including any additions of acid after the first one needed to make the solution acidic, to the nearest 0.02 mL. For each of the three trials of the analysis of the antacid, what was the volume of standardized NaOH required to titrate the residual standardized HCl after it reacted with the antacid? Based upon the molarities of the standardized HCl and standardized NaOH, and the volumes of each added to analyze the antacid samples, calculate the MILLIMOLES of HCl per MILLIGRAM of sample. Using the equivalent millimoles HCl/mg sample and the mass of the tablet, calculate the equivalent millimoles HCl neutralized per tablet for each of your three trials. Using the equivalent millimoles HCl/mg sample, the stoichiometry of reaction of HCl with NaOH, and the molar mass of the hydroxide ion, report the gram equivalent OH- per gram of antacid for each of your three trials. Convert the gram equivalent OH-/gram antacid to % equivalent OH- for each of your trials. Report the value you calculated for the average of the three trials for % equivalent OH-. Report the value you calculated for the standard deviation of the three trials for the % equivalent OH-. Volumetric Analysis PART II. DATA ANALYSIS Report the value you calculated for the relative deviation, in parts per thousand, of your % equivalent OH-. Write a summary of this experiment in a paragraph or two describing the goal, procedures undertaken, results, and sources of error in the experiment.