Expt 3 Direct vs. Back Titration PDF

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Expt 3 Direct vs. Back Titration Expt 3 Direct vs. Back Titration Expt 3 Direct vs. Back Titration...

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3. Direct Titration vs. Back Titration for the Determination of Vitamin C in Tablets. I. Objective The purpose of this experiment is to determine the amount of ascorbic acid (the active ingredient) in commercial Vitamin C tablets. You will determine the mass percentage of ascorbic acid in Vitamin C tablets by using titration. You will use a new type of titration, named back titration using the standardized solutions 0.1 M NaOH and 0.1 M HCl you have prepared in the previous lab. You will monitor the titration with a pH electrode during the direct and back titration. You will observe how pH changes slowly during most of the reaction and rapidly near the equivalence point. You will be able to locate the equivalence point of the acid-base reaction more accurately by computing the first and second derivatives of the titration curve.

II. Required Reading 1. Harris, D. C. Acid-Base Titrations. In Quantitative Chemical Analysis, 9th ed.; W. H. Freeman & Company, New York, N.Y., 2015; pp 233-264.

III. Introduction Vitamin C, also known as ascorbic acid (Figure 1) is an essential antioxidant for human nutrition. It is a water-soluble compound found in many fruits and vegetables. Lack of Vitamin C can lead to a deficiency disease, named scurvy. The use of analytical methods such as acid-base or redox titrations for the determination of the of quantity ascorbic acid in supplementary tablets is a common quality control test.

Figure 1. Structure of Ascorbic Acid.

A back titration is a type of titration that determines the concentration of an unknown using an excess amount of a compound with a known concentration. It is typically used for samples that are not easily soluble, or when the acid-base reaction occurs very slowly. Other reason could be the analyte may contain impurities that interfere with direct titration. To overcome these problems, an excess of a standard reagent is added to the solution. This excess reagent is then titrated with a second standard reagent of known concentration. In this lab experiment, you will perform a direct and back titration to determine the ascorbic acid in tablets. During the back titration, an excess of a standardized solution of NaOH (of known concentration) is added to the sample and completely reacts with the analyte (Fig. 2). The excess of the standard solution is determined by titrating using a standardized solution of HCl.

Figure 2. Progress of a back titration.

IV. Lab Equipment 1. Volumetric pipette, 25 mL. 2. Erlenmeyer flasks, 125-250 mL. 3. Burette, 50 mL. 4. Measuring cylinder, 50 mL.

V. Reagents 1. Potassium hydrogen phthalate (KHP), Primary standard grade. 2. pH buffer solutions. 3. Standardized HCl stock. 4. Standardized NaOH stock. 5. Vitamin C tablets or powder. 6. Phenolphthalein indicator solution.

VI. Procedure Your instructor will provide you with tablets of Vitamin C. The tablet needs to be dissolved with hot water and diluted with deionized water to a volumetric flask up to the mark. You will first perform a rough direct titration with phenolphthalein indicator to estimate the endpoint. Then you will perform a direct titration accurately using the pH meter. Finally, you will perform a back titration using the pH meter by preparing the sample and an excess of 10 mL of NaOH at a new Erlenmeyer flask.

VI. A. Preparation of Vitamin C tablets 1. Weigh out 1 tablet of Vitamin C accurately using an analytical balance and dissolve the sample in a beaker with hot water using a stirring rod. Transfer quantitatively to a 100 mL volumetric flask.

VI. B. Direct Titration for Vitamin C 2. For the direct titration, pipette accurately 25.00 mL of the solution in a beaker/Erlenmeyer flask and add 50.0 mL of DI water with a measuring cylinder. You will first perform a rough titration to determine the approximate endpoint. 3. Repeat the procedure at this time using the pH meter to read the initial pH of the solution. Add 0.1 M NaOH in increments of 2 mL and read the pH. As the pH changes more rapidly, and when you are within 4 mL of the expected equivalence point, add titrant in 0.5-mL aliquots. Be sure to record the volume and pH after each increment. The equivalence point has the most rapid change in pH. Continue to take pH measurements,

after reaching the endpoint. Add five more 1-mL aliquots of titrant and record the pH after each. Discard of any waste in the waste container.

VI. C. Back Titration for Vitamin C 4. Pipette accurately 25.00 mL of the solution of powdered vitamin C in a beaker/Erlenmeyer flask and add 50.0 mL of DI water using a measuring cylinder. Add more than sufficient amount of NaOH to the vitamin C sample (~10 mL excess) and titrate the excess NaOH with the 0.1 M HCl standard solution using a pH meter. Discard any wastes in the waste container.

VII. Data Analysis for the Titration Curve 1. Plot your experimental data as pH versus volume of standard NaOH added. Mark on your graph where the phenolphthalein color change was observed. 2. Following the example in Figures 11-5 and 11-6 of the Harris textbook, compute the first derivative (the slope, ΔpH/ΔV) for each experimental data point you collected. From your graph, estimate the equivalence volume as accurately as you can. 3. Following the example in Figure 11-6, compute the second derivative (the slope of the slope, Δ(slope)/ΔV). Prepare a graph like Figure 11-7 in the textbook and locate the equivalence volume as accurately as you can. 4. Go back to your graph from step 1 and mark where the indicator color changes were observed. Compare the phenolphthalein end point to the end point estimated from the first and second derivatives.

VIII. Calculations 1. From the direct titration, determine the equivalence point using the 1st and 2nd derivatives. 2. Calculate the percentage of ascorbic acid per vit. C tablet. 3. Calculate the number of milligrams of ascorbic acid contained in each tablet and compare with the label. 4. Also determine the Ka for ascorbic acid and compare it with literature values.

5. From the back titration, determine the equivalence point and calculate the mass percentage of ascorbic acid and the milligrams per tablet. 6. Tabulate your results. 7. Compare these results with those obtained from the direct titration.

IX. Question to Answer 1. Why is a rough titration carried out? 2. The indicator phenolphthalein is colorless in an acidic solution and is red when the solution becomes basic. The transition range is at pH 8.0 - 9.6. Which direction of titration, HCl by NaOH or NaOH by HCl, would give better precision and accuracy? Or does it really matter?...