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CH 103: ACID-BASE TITRATIONS • An analyte is the substance that is being analyzed. For example, the concentration of glucose in blood is commonly analyzed by diabetics. Glucose is the analyte. • A titrant is a solution of reagent that reacts with the analyte. The concentration of this reagent is accurately and precisely known. • In a titration, incremental volumes of titrant are added to the analyte until the reaction is complete. • A buret is often used to measure the volume of titrant added to the analyte. REQUIREMENTS OF A TITRATION 1. The reaction must be stoichiometric. For example, the net ionic equation for the reaction of potassium hydrogen phthalate (KHP, HOOC-C6H4-COOK) and sodium hydroxide (NaOH) is quantitative. Exactly 1 mole of KHP reacts with exactly 1 mole of NaOH. HOOC-C6H4-COO- + OH- → -OOC-C6H4-COO- + H2O 2. The reaction should be rapid. 3. The reaction should be specific; that is, there should be no competing reactions. Systematic error caused by interferences must be eliminated or reduced. For example, HOOC-C6H4-COO- should be the only acid in the above reaction. 4. There should be a marked change when the reaction is complete. For example, there is a marked increase in pH when the above reaction is complete. This increase in pH can be observed with an indicator that changes color immediately after the reaction between the titrant and analyte is completed. EQUIVALENCE POINT, END POINT, AND INDICATORS • The equivalence point occurs when the volume of titrant added to the analyte is the exact stoichiometric amount that is needed to bring the reaction to completion. • The end point occurs when the indicator changes color. • We want to measure the equivalence point. We actually measure the end point. We need to select an indicator that has the same end point as the equivalence point. • The indicator is added to the analyte. It is NOT added to the titrant. EQUIVALENCE POINT, END POINT, AND INDICATORS Titration using Phenolphthalein as an Indicator • When do you stop adding titrant to the analyte? • At the end point. EQUIVALENCE POINT, END POINT, AND INDICATORS • The titration of 0.100 M acetic acid (CH3COOH) with 0.100 M NaOH has an equivalence point at pH 8.73. What indicator should you use for this reaction? • Phenolphthalein. STANDARDIZATION • Today we will standardize NaOH against potassium hydrogen phthalate (KHP, HOOC-C6H4-COOK). • KHP is a primary standard. • For example, 25.71 mL of a NaOH solution are used to titrate 1.095 g of KHP to a phenolphthalein end point. The molar mass of KHP is 204.2 g/mol. One mole of KHP reacts with 1 mole of NaOH. What is the molarity (M) of this NaOH solution? • Next week you will use your NaOH solution to determine the molecular weight of an unknown acid. • This NaOH solution is a secondary standard. REQUIREMENTS OF A PRIMARY STANDARD 1. A primary standard should be 100.00% pure; although a 0.01% to 0.02% impurity is tolerable if it is accurately known. 2. A primary standard should be stable at drying temperatures, and it should be stable indefinitely at room temperature. (A primary standard is always dried before weighing, unless it is a hydrate.) 3. It should be readily available. 4. It should have a relatively large formula weight. Therefore, a relatively large mass of it will be weighed for titration. This will reduce error. • Explain this last point. TITRATION OF A STRONG ACID WITH A STRONG BASE • • • • • Where is the equivalence point? At pH 7. Where is the end point? Between pH 8.0 and 9.5. Is there a significant difference between the volume of titrant used to reach the equivalence and end points? • No. Therefore, phenolphthalein is an appropriate indicator for this titration. TITRATION OF A STRONG BASE WITH A STRONG ACID • Where is the equivalence point? • At pH 7. • Could phenolphthalein be used as an indicator for this titration? • Yes. TITRATION OF A WEAK ACID WITH A STRONG BASE • Where is the equivalence point? TITRATION OF A WEAK BASE WITH A STRONG ACID • Where is the equivalence point? STANDARD DEVIATION • Standard deviation is a measure of precision. It is used to quantify the agreement between repeated measurements of the same sample. SAFETY • Give at least 1 safety concern for the following procedure. • Using HOOC-C6H4-COOK, NaOH, phenolphthalein, and an unknown acid. • These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in your eye, immediately flush with water. • Your laboratory manual has an extensive list of safety procedures. Read and understand this section. • Ask your instructor if you ever have any questions about safety. SOURCES • Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc. • Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company. • Traverso M. 2006. Titration using Phenolphthalein as an Indicator. Available: www.chemistry.wustl.edu/.../AcidBase/phph.htm [accessed 14 September 2006].