Buffers and Buffering Capacity Learning Goals 1. 2. Investigate the buffering ability of a solution containing both a weak acid and its conjugate base. Prepare several buffered solutions and test their buffering capacity. Abstract The solutions to be investigated in this experiment are buffers. Buffers are composed of weak acids or bases and the salts of weak acids and bases that are called their conjugates. For example, acetic acid is a weak acid and in a dilute solution a small number of the acetic acid molecules dissociate to produce acetate ions and protons, H+. The acetate ions are the conjugate base of the acetic acid. Buffer solutions are made up of an acid-base pair composed of either a weak acid and its conjugate base or a weak base and its conjugate acid. For example the following are acid-base pairs from which a wide variety of Buffers can be made: Table 1. Examples of Acid-Base pairs Acid Acetic acid CH3COO CH3COOH monobasic Phosphate ion H2PO4 Acetate ion De-protonation (loss of Hydrogen cation) reaction Base - NH4+ CH3COOH - - dibasic Phosphate ion <=> CH3COO- H H2PO4 <=> HPO4 2- - +H + + 2- monobasic Citrate ion C3H5O(COOH)3 <=> C3H5O(COOH)2(COO)- + H+ C3H5O(COOH)2(COO)- Ammonium ion HPO4 Citric acid C3H5O(COOH)3 Ammonia NH3 NH + <=> NH + H+ 4 3 To be effective as a buffer (i.e., having the pH and Buffering Capacity required) the solution must be composed of the correct acid-base pair and their concentrations must be in the exact ratio that will give the desired pH value. The prepared buffer solution also needs to have a sufficient concentration of both the acid and the base so that the solution will have the strength to neutralize a given amount of a strong acid or base. This strength of the buffer solution is called its Buffering Capacity. So a Buffer solution has the following three requirements: 1. It is made up of the correct acid-base pair to get close to the pH value needed. 2. It has the acid and base concentrations in the exact ratio for the exact pH value. 3. It has sufficient concentration of the acid-base pair to produce the required Buffering Capacity. A pH meter is a valuable tool that can be used to test the pH of prepared solutions and gain a better understanding of buffers and their buffering abilities. Using the understanding of the chemistry of buffered systems, buffer solutions can be prepared that have a specific pH value and buffering capacity (the strength of the buffer solution). Buffered systems consist of either a weak acid and a salt with the same anion as the weak acid, for acidic buffer values, pH < 7, or a weak base and the salt of that weak base for basic buffer values, pH > 7. For example: A solution containing a relatively large concentration of a weak acid, i.e. Formic acid (HOOCH), and the corresponding salt of that weak acid, Sodium formate (NaOOCH), is a buffer. As the weak acid and its conjugate base are dissolved into the solution the following reactions occur: HOOCH (aq) NaOOCH <=> (aq) H+ -----> (aq) Na+ + OOCH(aq) (de-protonation) (aq) + OOCH- (aq ) (dissolution) The addition of the common anions from the acid and its salt establishes an equilibrium that has a high concentration of formic acid and its conjugate base, the formate ion, as shown below. HOOCH (aq) <= > H+ (aq) + OOCH- (aq) This is the acid-base pair that makes up the buffer system. If strong acids are added to the buffered solution, the strong acid will react and be neutralized by the conjugate base present in the buffer. This drives the equilibrium of the above reaction to the right. If strong bases are added to the buffered solution, the strong base will react and be neutralized by the weak acid present in the buffer. This drives the equilibrium of the above reaction to the left. Therefore, the addition of acid or base to this buffered solution will not cause a large change in pH, but will cause a change in the concentrations of both the weak acid and conjugate base. Your text (Moore, Stanitski, Jurs) has an excellent explanation of Buffer calculations. See Chapter 17,Section 1, Pages 805-816. Prelab Assignment In your lab notebook, prepare the following information: 1. A brief (2-3 sentence) introduction to the lab. 2. A table of safety information including the chemicals used in the lab and their safety handling precautions. This information can be obtained from the MSDS safety sheets. 3. Pick a pH (9.0, 7.0, or 3.0) and using your knowledge of equilibrium, pH, Ka and Kb values, choose a buffer pair from the acids, bases and salts provided in Table 2 (below). Determine the correct molar ratio of acid or base to salt needed for the selected pH. Then determine the mass (for solids) or volume (for liquids) of the chemicals needed to make 100 mL of the buffer with a Buffering Capacity so that 50 mL of the buffer is capable of neutralizing at least 2 mL of either 1 M HCl or 1 M NaOH without letting the pH change by more that 1 pH unit. See your text pages 815-816. You may also need to look up the solubility of each chemical. See the MSDS forms for this information. Here is an example of a solved buffer calculation and here is the same problem solved using the Henderson-Hasselbalch equation. Include the above calculations with your Pre-lab to be handed to your TA at the beginning of the lab. You will not be allowed to work in the lab without these Buffer calculations. Table 2. Available Acid-Base pairs for Preparation of buffer solutions in this experiment Chemical Type Formula Molecular weight Ka pKa Kb pKb Acetic Acid solution 1.0M weak acid HC2H3O2 60.05 1.8 x 10-5 4.75 ---- --- conjugate base of above acid NaC2H3O2 82.03 ------ --- ---- --- Sodium phosphate monobasic, monohydrate weak acid NaH2PO4*H2O 137.99 6.2 x 10-8 7.21 ---- --- Sodium phosphate, dibasic heptahydrate conjugate base of above acid 268.07 4.7 x 10-13 12.32 ---- ---- 210.14 7.5 x 10-4 3.128 ---- --- Na3 C6 H5 O7 2H2O 294.11 ------ --- ---- --- weak base NH4OH 17.3 5.9x10-10 9.24 1.7 x 10-5 4.76 conjugate acid of above base NH4Cl 53.5 ------ --- ---- --- Sodium acetate anhydrous Citric acid monohydrate Sodium citrate dihydrate Ammonia solution 2.0 M Ammonium Chloride . Na2 HPO4 7H2O . weak acid C6 H 8 O 7 H 2 0 . conjugate base of above acid Chemicals and supplies Chemicals Liquids: Distilled Water 1.0 M Acetic acid (HC2H3O2) 1.0 M Hydrochloric acid (HCl) 1.0 M Sodium hydroxide (NaOH) 2.0 M Ammonia solution Glassware and Supplies (NH4OH) • • • • • pH meter 10 mL plastic syringes 1 mL plastic syringes 100 mL volumetric magnetic stirrer and stir bar Solids: Sodium acetate, anhydrous (NaC2H3O2) Citric acid, monohydrate (C6H8O7H2O) Sodium phosphate, monobasic dihydrate (NaH2PO42H 2O) Sodium phosphate, dibasic heptahydrate (Na2HPO47H2 O) Sodium citrate, dihydrate (C6H5O7Na32H 2O) Ammonium chloride (NH4Cl) Procedure: Part A. Preparing an Acetic acid/Acetate Buffer 1. 2. 3. 4. 5. 6. 7. 8. Weigh 4.1g of solid Sodium acetate, anhydrous into a 250mL beaker and dissolve it in 50 mL of 1M Acetic acid and 50 ml of water. Determine the concentration of Acetic acid, the concentration of Acetate ion, and the pH of the solution. Divide this solution equally between two beakers labeled B1 and B2. Prepare two other beakers with 50 ml of deionized water in each. Label these beakers D1 and D2. Assemble the stirrer apparatus and the pH meter, with electrode. Place the electrode in the solution labeled D1, measure and record the pH. Using a 1 mL syringe, add 0.2 mL of 1.0 M HCl solution, stir, and measure and record the pH. Repeat the addition of 0.2 mL more of HCl with stirring, measuring and recording until the pH changes dramatically (±2 pH units). Repeat the above experiment using the beaker labeled D2, only this time use 1.0 M NaOH instead of the HCl. Repeat with the beaker labeled B1 (the buffered solution) and add 0.2 mL of the 1.0M HCl solution until the pH makes a dramatic change (at least ±2 pH units). Repeat using the beaker labeled B2 using 1.0 M NaOH. Part B. Preparing a Buffered Solution of Specific pH and Buffering Capacity 1. Prepare 100 mL of your assigned buffer solution, either pH 9.0, pH 7.0 or pH 3.0, based on your Pre-Lab calculation. 2. Test the buffering capacity of your prepared buffer by following the strong acid/strong base addition procedure used in the Part A with the acetic/acetate buffer. Record the volume of acid or base and the corresponding change in the pH. Post Lab Assignment Include the following information in a lab report to give to your TA. 1. Graph pH vs. volume of acid (or base) added to each of the four solutions from the Acetic acid/Acetate buffer system. 2. How many moles of strong acid and/or base were added to the Acetic acid/Acetate buffer system before the Buffering capacity was reached (change of ±1 pH unit)? 3. Graph pH vs. volume of acid (or base) added to your assigned buffer solution. What was the buffering capacity of this buffer system? 4. Show all equations for the calculations used in the preparation your assigned buffer solution Rubric for Buffers and Buffering Capacity: Claim(s): Statement(s), derived from evidence, using scientific reasoning. (15 points total) Summary of claims: Clearly state the major conclusions or claims for this experiment: What buffering capacity was achieved with the acetic acid/acetate buffer? With the other buffering system selected? Evidence: Scientific data that supports the claim. The data needs to be appropriate and sufficient to support the claim. Submission of the Analog to Target and Designing Experiments Worksheets are required. (30 points total) Introduction: Provide background information to put the experiment in context: What is a buffer and what is its purpose? What is meant by “buffering capacity”? Reasoning: Scientific explanations that use evidence and appropriate chemistry concepts to construct claims. (30 points total) Discussion: Explain how the evidence you presented supports your claim. Show calculation of moles acid/base added to the acetate buffer system. Show calculations for how to prepare your specific buffer and its experimentally determined buffer capacity. Explain any noted errors that occurred in lab and how that affected your results. Procedure: In a narrative (not numbered directions) summarize the major steps of the experiment. You may reference parts of this procedure by providing the title and web address. Results: Organize and present all the data you collected. Include graphs of pH vs. volume of acid/base added to D1, D2, B1 & B2. Include graphs of pH vs. volume acid/base added to your specific buffer. Copyright (c) 2014, the ICN Team.