Titration of Acids and Bases

Transcription

1 Exercise 3 Page 1 Illinois Central College CHEMISTRY 132 Laboratory Section: Titration of Acids and Bases Name: Equipment Objective 1-25 ml burette 1-pH electrode 1-50 and ml beaker 1-stir plate and stir bar The objective of this experiment is to determine the concentration of an unknown acid and to illustrate the importance of titration curves and their derivatives. Background When neutralization is carried out using carefully measured amounts of acid and base, the procedure is called titration. One reagent (acid or base) is slowly added to the other until the endpoint is reached, that is, the point where the acid and base have exactly neutralized one another. The endpoint can be detected using an acid-base indicator (an organic compound that appears one color in acid and another in base) or can be determined instrumentally. The purpose of the titration is to determine the concentration of a solution of unknown strength. This solution is measured against a known quantity of the opposite reagent until the endpoint is reached. The known quantity can be a carefully weughed sample of a pure compound or a carefully measured volume of solution of prcisely known concentration. The concentrations of acids and bases are often expressed in Normalities (N) rather than Molarities (M), where the Normality of a solution is defined as the number of "equivalents" of acid or base present per liter. In this respect, an equivalent is either 1 mol H +1 (for an acid) or 1 mol OH -1 (for a base). So an acid with two hydrogen ions to donate such as H 2 SO 4 has a Normality twice as great as its Molarity since each mol of H 2 SO 4 provides two "equivalents" of acid. Thus, the Normality of the solution more accurately reflects its true acidity (or basicity) in terms of the total number of H +1 (or OH -1 ) ions available. Since it would require 1 equivalent of acid (1 mol H +1 ) to neutralize 1 equivalent of base (1 mol OH -1 ) we can show that the following relationship exists at the endpoint of a titration. Normality(acid) x Volume(acid) = Normality(base) x Volume(base) In this experiment, you will be given a sample of acid of unknown concentration and titrate it against a standard solution of M NaOH. We will follow the ph of the titration through the endpoint using the LabWorks Interface. The first derivative of this "titration curve" will indicate the volume of base used to reach the endpoint allowing you to calculate the "Normality" of the unknown acid.

2 Exercise 3 Page 2 Procedure Calibration of the ph probe 1. The ph probe used in this experiment must not be allowed to dry out, so when not in use, it should soak in distilled water or a ph 7 buffer. 2. Connect your ph probe to the ph1 terminal of the interface. Obtain approximately 30 ml of a ph 7 buffer and place it in a 50 ml beaker with a magnetic stir bar. Place the beaker on a stir plate and set the speed so that the solution is very gently mixed. 3. Using a ring stand and a clamp, lower the ph probe into the solution so that the glass sphere at the end of the probe is completely submerged. Be careful that the stir bar does not strike the tip of the ph probe as the glass is very fragile. 4. From the opening window of LabWorks, select the Calibrate button. In the next window select ph1. The new dialogue box will indicate the voltage reading (in millivolts) from the ph probe and prompt you to enter the ph of your calibration solution. Once the voltage reading has settled down, enter 7 for the calibration ph. That's it...your probe is calibrated. Titration 1. Fill a ml burette with M NaOH, making certain that the tip of the burette is filled with solution and free of air bubbles. Clamp the burette to a ring stand and set it aside for the moment. 2. Obtain a ml sample of one of the unknown acids (A, B, C, or D) in a 150 ml beaker. Add approximately 40.0 ml distilled water, place the magnetic stir bar in the beaker, and place it on the stir plate (stirring gently). 3. Position the ring stand holding the ph probe such that the probe tip is submerged in your acid solution toward one side of the beaker. Position the other ring stand holding the burette such that the NaOH can be dispensed into the acid solution. 4. From the LabWorks window, click the Design button on the toolbar and choose the EZ Program option button. 5. Drop the menu choices for the "Read" window in line (3.). Select Keyboard as you input and "ml of base added" as your label. 6. Drop the menu choices for the "Read" window in line (4.) and select ph1 as your input choice. The computer will automatically label this input as ph1.

3 Exercise 3 Page 3 7. Select the Aquire button on the toolbar, and click the Start button to begin the program. You should now see a User Input dialogue box. We will begin with "0" ml of base added, so simply click OK or press <Enter>. The computer will now read the ph of your solution. 8. Before you input anything else, add 0.5 ml of the NaOH and allow it to stir for seconds. Type in 0.5 for the User Input and click OK. 9. Continue this procedure in 0.5 ml increments, always inputting the total volume of NaOH added to that point. 10.As we approach the equivalence point, the ph will begin to rise. Decrease your increments to 0.1 ml at each addition. (But remember, you are inputting the total amount of NaOH used to that point.) 11.Once the upper plateau of the titration curve begins, you can increase your increments back to 0.5 ml. 12.Continue the process until the upper plateau is well established. When you have added your last increment of base, choose Cancel in the User Input dialogue box and choose Yes when prompted to end the program. 13.Click the Save Data button and assign a file name to your data, such as, Trial Carefully place your ph probe back into the ph 7 buffer, discard your first sample and obtain another ml sample of the same unknown acid. Add approximately 40 ml distilled water and, as before, place it on the stir plate with the stir bar, ph probe and your re-filled burette. 15.Click on the Start button to clear your Trial 1 data and repeat the entire titration process. 16. Save your data as 'Trial 2' and repeat for a third trial. Data Analysis 1. After you have saved your data for trial 3, click the Analyze button and load the data from Trial 1 into the spreadsheet. 2. Columns A and B should contain the "ml of base added" and the corresponding ph's. Highlight Column C and choose Column Setup from the Menu Bar. Enter "First Derivative" for the label of Column C and enter the formula Deriv(b,a). Be sure to set the column to 3 decimal places. (This formula will take the first derivative of the ph values with respect to ml of base added. In essence, this will represent the values of the slope of our titration curve at any given point.)

4 Exercise 3 Page 4 3. Choose Graph Setup from the Menu Bar, plotting Column A on the x-axis, Column B on the y1 axis and Column C on the y2 axis. 4. The "spike" in the first derivative plot indicates the ml of base added at the endpoint (where the slope of the titration curve is at its maximum). Estimate this value to the nearest 0.01 ml and record the value on your Report Sheet. 5. Repeat steps 1-4 for your data from Trial 2 and Trial Calculate the Normality of the Unknown Acid for all three trials. Report the average value, and the %R.A.D for the three trials. 7. Print the Graph and Data Table for only one of the three trials and attach it to your report sheet. Exit the program.

5 Exercise 3 Page 5 Illinois Central College CHEMISTRY 132 Laboratory Section: Name: REPORT SHEET Titration of Acids and Bases Acid Sample Used (A, B, C,or D) Normality of standard NaOH N Titration Data Trial 1 Trial 2 Trial 3 Volume of acid used Volume of base at endpoint Normality of acid Average Normality Calculations: Trial 1: %R.A.D. Trial 2: Trial 3: Average Normality: %R.A.D.:

6 Exercise 3 Page 6 Questions 1. What mass of NaOH pellets would be needed to prepare 300 ml of a N NaOH solution? 2. How many milliliters of M NaOH would be needed to neutralize 45.0 ml of M H 2 SO 4? (Note that the concentrations are given in Molarities and not Normalities) 3. Concentrated HCl is 37% HCl by mass. Its density is 1.19 g/ml. What is the Molarity of concentrated HCl? 4. Why did your titration curves flatten out at the top?

7 Exercise 3 Page 7 Illinois Central College CHEMISTRY 132 Laboratory Section Name: PRELAB: Exp. 3 Titration of Acids and Bases 1. A sample of ml of an acid of unknown concentration requires exactly ml of N NaOH for complete neutralization. What is the Normality of the acid? 2. Given a bottle of NaOH pellets, how would you prepare one liter of M NaOH? 3. What is the Normality of M NaOH? 4. What is the Normality of M H 3 PO 4?

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