Lab 2 Procedures Explanation Document

Lab 2 – Statistical Evaluation of Acid-Base Indicators I. PURPOSE & OVERVIEW The purpose of this lab is to practice obtaining reproducible results with your buret, to expose you to chemical indicators (particularly pH indicators), and to provide you with data on which to practice your statistical skills. You will employ the concepts of mean, standard deviation, Grubbs test, and t test. You will compare the accuracy of different indicators in locating the end point in the titration of the base “Tris” with hydrochloric acid: (HOCH2)3CNH2 + H+ → (HOCH2)3CNH+ tris(hydroxymethyl)aminomethane “Tris” (121.136 g/mol) Titrate a primary standard base, tris(hydroxymethyl)aminomethane (“tris” or “tham” for short) with hydrochloric acid to determine its concentration. HCl solutions cannot be made to be a precise concentration, so they must be standardized against a known standard. In this experiment, you will standardize an HCl solution with one of several indicators assigned to you by your instructor. However, note that you will only need to detect the end point with your indicator, not the equivalence point (what is the difference?). Results from the entire class will allow you to determine which indicators give endpoints that are statistically different and which are the same. II. REAGENTS PROVIDED BY THE STOCKROOM Get more as needed, but don’t take more than you can use in one day—do not store the solution in your drawer from one day to the next. All students will use the same solution from a single batch, and the total amount is limited, so don’t be profligate. (pKa = 8.1). This can be stored in a vessel in your desiccator. If you use it all, you can get more. Indicators available in dropper bottles for the titration are: Bromothymol blue: blue (pH 7.6) − yellow (pH 6.0) (end point-disappearance of green) Bromocresol green: blue (pH 5.4) − yellow (pH 3.8) (end point-appearance of green) Be aware that this indicator has a particularly difficult end point Methyl orange: yellow (pH 4.4) −red/pink (pH 3.1) (end point-first orange appearance) Methyl red: yellow (pH 6.0) −red/pink (pH 4.8) (end point is disappearance of orange) Your TA or instructor will tell you which indicator you should use. 1 III. PROCEDURE Perform the following procedure with the indicator assigned to you. 1. a) Calculate the mass required to react with 35 mL of 0.10 M HCl. (You may use the first tab of the Lab 2 report spreadsheet to do this.) b) Weigh and dispense this amount of Tris into a 250-mL Erlenmeyer flask. 2. a) Rinse your buret with three small portions (3–5 mL) of the 0.1 M HCl. Tilt and rotate the buret so that the liquid washes the walls; drain the liquid through the stopcock. Discard the washings into a waste beaker that will later be put in the appropriate ULM container. b) Fill the buret with 0.1 M HCl to the 0.00 mL mark, allow the liquid to settle, and record the reading to the nearest 0.01 mL. c) Add ~20 mL of HCl from the buret to the flask and swirl to dissolve the tris. Put a magnetic stir bar in the flask and place the flask on a stir plate. Stir the solution at a speed that will provide smooth, continuous mixing without splashing. Add 2–4 drops of indicator and titrate to the end point. If any titrant splashes onto the side of the flask, rinse it into the solution in the flask. 3. Do a “rough” titration. For this titration, allow the titrant to flow out of the buret rapidly to within 5–10 mL of where you expect the end point to be, then slow to a rate where the titrant is coming out in drops. Even so, you will likely overshoot the end point, but this will let you determine about where the end point will be on subsequent titrations. Note: this titration should not require more than 50 mL of titrant. 4. a) Calculate how much tris will be required to cause each succeeding titration to require ~35mL of HCl. Weigh this amount of tris into a clean flask. b) Refill your buret and record the reading. Repeat the titration in step 3, but this time use only 1 drop at a time near the end point. When you are very near the end point, use less than a drop at a time. To do this, carefully suspend a fraction of a drop from the burette tip and touch it to the inside wall of the flask. Carefully tilt the flask so that the bulk solution overtakes the droplet and then swirl the flask to mix the drop back into the solution. Titrate until you can just see the indicator change color. (If you are color blind, you might still be able to detect a change, depending on which indicator you have. Talk to your TA.) Record the total volume of HCl required to reach the end point to the nearest 0.01 mL. Keep this titrated solution as a “comparison solution” and try to match it with each subsequent titration. c) Correct the volume reading using your buret correction and also calculate the true mass of tris with the buoyancy equation from lab 1 (density of tris = 1.327 g/mL). d) Calculate the molarity of HCl. e) Do at least 6 titrations in addition to the first rough one. 2 Helpful tips: • Placing a small piece of white paper between the stir plate and flask may help you see the color changes. • If your spin bar spins too fast, it will splash material on the sides of the flask that will not be part of the titration process 5. a) Repeat the titration to obtain at least 6 good measurements of the HCl molarity. b) Don’t forget your glassware volume corrections. c) Use the Grubbs test in Section 4-4 (Harris) to find any outliers in your data. d) Report your retained values, their mean, their standard deviation, and the relative standard deviation on the “Report” tab. IV. DATA ANALYSIS Post your results to the Google document for the class as soon as possible. Select the tab corresponding to the indicator you used and post your data for each run along with your name. When all of the class data has been posted to the google document, import the class data to your lab report (making sure to put the data for each indicator on the appropriate tab). Do the Grubbs Test on the dataset for each student to determine which data points to keep. Calculate the average, standard deviation, and relative standard deviation and record the number of valid trials for each student’s data in the appropriate place in your spreadsheet. For the calculations in the rest of this lab, refer to the calculations map at the end of the lab report. (When your spreadsheet references an equation, it is also referring to this map). Complete the rest of the calculations on each indicator page and then pool this data in the appropriate cells on the “2. Stat. Comparison” page. The first test you will do is to compare the standard deviations associated with each indicator to that of each of the other indicators to determine if you can use the standard t-test or an alternate t-test. The normal t-test is only valid when the standard deviations of the two populations you are comparing are relatively similar, as determined by the F test. The F shows whether two standard deviations are significantly different from each other. F is the quotient of the squares of the standard deviations: s12 s22 Put the larger standard deviation in the numerator so that F is always greater than or equal to 1. If Fcalc > Ftable, the difference in standard deviations is significant. A partial F-test table is shown below, but you will be using the “FINV” function in Excel to calculate the table value for your data. The syntax for this function (how you use it) is =FINV(probability,DoF1,DoF2), where the probability for a 95% confidence interval is 0.05, and DoF1 and DoF2 are the degrees of freedom associated with the larger and smaller standard deviations, respectively. Make sure not to mix up DoF1 and DoF2, because the order you put them in matters. Fcalculated = 3 Table 2. Critical values of F=s21/s22 at the 95% confidence level Degrees of freedom for s2 2 3 4 5 6 7 8 9 10 ∞ Degrees of freedom for s1 2 19.0 9.55 6.94 5.79 5.14 4.74 4.46 4.26 4.10 3.00 3 19.2 9.28 6.59 5.41 4.76 4.35 4.07 3.86 3.71 2.60 4 19.2 9.12 6.39 5.19 4.53 4.12 3.84 3.63 3.48 2.37 5 19.3 9.01 6.26 5.05 4.39 3.97 3.69 3.48 3.33 2.21 6 19.3 8.94 6.16 4.95 4.28 3.87 3.58 3.37 3.22 2.10 7 19.4 8.89 6.09 4.88 4.21 3.79 3.50 3.29 3.14 2.21 8 19.4 8.84 6.04 4.82 4.15 3.73 3.44 3.23 3.07 1.94 9 19.4 8.81 6.00 4.77 4.10 3.68 3.39 3.18 3.02 1.88 10 19.4 8.79 5.96 4.74 4.06 3.64 3.35 3.14 2.98 1.83 inf 19.5 8.53 5.63 4.36 3.67 3.23 2.93 2.71 2.54 1.00 If the standard deviations are statistically similar, you can use the normal t-test equation (Equations 4 and 5) to determine if the two endpoints are the same or different. If the standard deviations are significantly different, use Equations 6 and 7 for the t test. Equation 7 usually gives a non-integer result, which you can round to the nearest integer. Hint: entering the equations for the alternate t-test can be time consuming. This process can be made much faster by doing parts of the calculation in other empty cells. For each t-test comparison, calculate the t value from the table for the 95% confidence interval using the TINV function in Excel. The syntax for this equation is =TINV(probability,DoF), where the DoF are the degrees of freedom and the probability is again 0.05. Repeat the t test for all the indicators to each other to test whether or not any of the results are significantly different. 4