EXPERIMENT 2: ACID/BASE TITRATION. Each person will do this laboratory individually. Individual written reports are required.

Transcription

1 EXPERIMENT 2: ACID/BASE TITRATION Each person will do this laboratory individually. Individual written reports are required. OVERVIEW. Acid/base titration, relying on a color change of the indicator, is a basic example of a quantitative measurement technique. In brief, the base is gradually titrated into an acidic solution. When the point is reached where the concentration of the base is matched with the concentration of the acid as 1:1, the solution starts turning basic. In response to this development the indicator turns pink. Having registered the color change, the experimentalist can determine the unknown acid concentration based on the known base concentration (or the other way around determine the unknown base concentration based on the known acid concentration). The laboratory provides a chance to practice the use of standard equipment such as pipets, volumetric flasks, balances, etc. KHP* solution known concentration titration (standardization)** NaOH solution unknown concentration NaOH solution known concentration HA solution unknown concentration titration Report determined HA concentration Scheme 1. General layout of the acid/base titration experiment. Question to think about: why a two-step procedure is used (standardization step, followed by actual titration step) instead of a conceivable one-step procedure? Consult lectures/text for an answer. * KHP is potassium (K) hydrogen (H) phthalate (P) (chemical formula C 6 H 4 (COO) 2 KH, molecular weight = g/mol) ** standardization step is executed multiple times first as a crude scout titration, then three more times as an actual quantitative measurement. 1

2 Standardization Sodium hydroxide, a strong base, reacts with potassium hydrogen phthalate as follows NaOH + KHP Na + + K + + P 2- + H 2 O (1) where P 2- is the phthalate ion. At the point when sufficient sodium hydroxide has been added to react with all the KHP in each sample, a slight excess of sodium hydroxide will react with phenolphthalein indicator, HIn as follows NaOH + HIn (Colorless) Na + + In - (Pink) + H 2 O (2) The change from colorless to pink signals that sufficient sodium hydroxide has been added to react with all the KHP in the sample. This point in a titration is called the end point. By measuring the amount of KHP added to the sample and the volume of sodium hydroxide required to react with all the KHP in the sample, it is possible to calculate the concentration of the sodium hydroxide solution. This process is called standardization; i.e. it is said that the sodium hydroxide solution is standardized. Titration of the Unknown The logic is exactly the same as in the standardization step above. The acid/base reaction can be symbolically represented as NaOH + HA Na + + A - + H 2 O (3) where A - is the ion of unknown nature. EXPERIMENTAL PROCEDURE Solutions provided 1. Sodium hydroxide (40 %). A solution containing 40 % (by weight) of NaOH. 2. Phenolphthalein indicator. A solution containing 0.2 % phenolphthalein in 90 % ethanol. 3. Unknown. Supplied individually to each student. Solutions you must prepare (NB always use distilled water) 4. Diluted sodium hydroxide (expect the concentration to be approx. 0.1 M). This is the solution to be standardized. Recipe: use a graduated cylinder to add about 6.6 ml of 40 % sodium hydroxide solution to a 1-L bottle. Then add approximately 1 L of distilled water and mix the solution thoroughly. 2

3 5. Potassium hydrogen phthalate. This is the solution to be used as a standard. Recipe: weigh approximately 5 g of the KHP that you dried last week, accurate to the nearest 0.1 mg. (NB it is unnecessary to try to weigh exactly 5.00 g, but it is very important that you know exactly nearest 0.1 mg how much you weigh). Transfer the solid to a clean but not necessarily dry 250-mL volumetric flask. Use a wash bottle to wash any solid from the sides of the flask into the main body of fluid at the bottom of the flask. Swirl the solution to dissolve the salt and dilute to volume (250 ml) using distilled water. Mix the solution thoroughly by inverting the flask a minimum of ten times. 6. Diluted solution of the Unknown. This is the solution to be analyzed. Recipe: dilute your unknown to volume (250 ml) with distilled water. Mix the solution thoroughly, and set it aside to be used later. Standardization of sodium hydroxide Buret in a holder Step 1. Fill buret with NaOH solution. Recipe: rinse a 250-mL beaker with several small volumes of the 0.1 M sodium hydroxide solution. Label the beaker (NaOH). Transfer about 50 ml of the solution to the rinsed beaker; then rinse a clean buret with several small volumes (5 to 10 ml) of the solution, being careful to run portions of each rinse solution through the tip of the buret. Then tilt the buret slightly and pour the sodium hydroxide solution down one side until the buret is filled above the first calibration line at the top. Put the buret in a buret holder and deliver solution into a container until the tip is filled with solution and the top meniscus is in the calibrated range of the buret (the solution that ends up in the container is a waste). NB. Do not try to adjust the volume to exactly zero. Erlenmeyer flask Step 2. Fill Erlenmeyer flasks with KHP solution. Recipe: rinse a clean 250-mL beaker with three small volumes (5 ml) of the KHP solution and then pour about 100 ml of the solution into the beaker. Label the beaker (KHP). Rinse a clean 25-mL transfer pipet with several 5-mL volumes of the KHP solution. Then rinse one clean 250-mL Erlenmeyer flask with distilled water and pipet ml of the KHP solution into it. NB It is not necessary to dry the flask after rinsing. Add three drops of phenolphthalein indicator solution to the flask containing the KHP solution. 3

4 Step 3. Scout titration (standardization). Recipe: record the initial buret reading. Select one of the Erlenmeyer flasks with KHP solution. Start adding the titrant (NaOH solution) from the buret continuously at a moderate rate while swirling the solution in the flask. Stop the flow of titrant when the solution changes from colorless to pink. Record the final buret reading and calculate the titrant volume difference from start to finish. This scout volume provides an estimate which allows you to conduct subsequent titrations with better accuracy. After scout titration is completed, prepare for the next step. For this purpose empty the Erlenmeyer flask containing the products of the scout titration and rinse it with several small volumes of distilled water. Then prepare three (3) Erlenmeyer flasks with fresh KHP solution (use the same procedure as described in Step 2). Step 4. Accurate titration (standardization). Recipe: refill the buret as described above. Then drain the solution from the buret until the top meniscus is below the first calibration mark and record the initial volume. NB Scale markings on burets can be read more accurately than volume levels can be adjusted. Accordingly, do not attempt to adjust volume levels to exactly zero. Instead, bring the fluid level to within the calibration scale of the buret, let the fluid drain from the sides, and then read the volume. Select one of the Erlenmeyer flasks with fresh KHP solution and start delivering titrant slowly until you are within ~3 ml of the endpoint volume (as estimated from the scout titration). Beginning from this point, the titrant should be added very slowly until the pink color begins to spread throughout the solution. At this point, use a wash bottle to wash any droplets from the sides of the flask into the main body of the solution. Then add titrant dropwise until the entire solution turns and remains faint pink for several seconds. Record the final volume of the titrant in the buret. The entire routine described in step 4 should be repeated three times (for three flasks containing fresh KHP solution) and the results should be carefully recorded in the logbook. Titration of the Unknown. Use the same procedure as outlined in steps 1-4 above (the only difference is that you are now using the solution of the Unknown instead of 4

5 the KHP). Remember to execute one scout titration followed by at least three accurate titrations. DATA PROCESSING AND REPORTING Concentrations The titration end-point is reached when the molar content of base in solution becomes equal to the molar content of acid. Consider, for example, the standardization step. The end point is reached when c KHP [mol/l] V KHP [L] = c NaOH [mol/l] V NaOH [L] (4). Here c KHP and c NaOH are molar concentrations expressed in the units of mol/l (mol/l M), while V KHP and V NaOH are the volumes of the respective solutions. Equation (4) provides the basis for treatment of the experimental data, allowing for determination of c NaOH. NB When using Equation (4), think carefully about the exact definition of the volumes V KHP and V NaOH. The data from the titration of the Unknown should be analyzed in the same fashion. Determine and report the concentration of the diluted solution of the Unknown (i.e. the solution of the Unknown with the volume 250 ml). Errors In this laboratory the errors are estimated solely on the basis of reproducibility of the results from the repeat measurements. Accordingly, the results should be reported as: ts c NaOH ± (5) N where: c NaOH is the average from multiple determinations of c NaOH s is the sample standard deviation of c NaOH (Equation 6-4 in the text) t is the so-called Student s coefficient (Table 7-3 in the text; we suggest to use 95% confidence level) N is the number of repeat measurements The results for concentration of the Unknown should be reported in the same form. 5

6 Report The general guidelines for writing a laboratory report can be found in the course packet. The main body of the results should be presented in a form of tables. For standardization step, the table should include V KHP, V NaOH, c KHP, and c NaOH. The equivalent table should also be generated for the titration of the Unknown. NB The determined concentration of the Unknown, in the same form as shown in Equation 5, should be included in the body of the report and also displayed on the title page. In addition to the standard information that needs to be included in the laboratory report (see course packet), please address explicitly the following points: (a) Why it is necessary to measure volumes very carefully when preparing the KHP solution (solution 5), but NOT necessary to measure volumes too carefully when preparing the dilute NaOH solution (solution 4)? (b) Why in step 2 the pipet and the beaker should be rinsed with the KHP solution (not with distilled water), whereas the Erlenmeyer flasks should be rinsed with distilled water (not with the KHP solution)? (c) Give at least two reasons why it is better to read the initial volume of a buret than to adjust the volume to some round value such as zero. Please, include the answers at the end of the Results and Discussion section in your written report. 6