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1、Detection of Ions in Solutions Using Acid/Base Chemistry: A Quality Control TestObjective: This lab focuses on the detection of ions using titration as an analysis tool. You will standardize NaOH and HCl solutions so that you know the exact concentration and then prepare samples of common household
2、items in order to determine the amount of calcium in Tang, Mg(OH)2 in Milk of Magnesia, etc. You will learn to prepare samples of a specified concentration, learn about acids and bases through the use of titrations and learn how to detect endpoints using different indicators. You will become adept a
3、t measuring pH with both a pH meter and indicator paper. You will then conduct quality control testers and determine if the label on a bottle of over-the-counter product actually contains the percentage of compound that it advertises. What is a Titration?A titration is an analytical procedure used t
4、o determine the concentration of a sample by reacting it with a standard solution. One type of titration uses a neutralization reaction, in which an acid and a base react to produce a salt and water.In equation 1, the acid is HCl (called hydrochloric acid) and the base is NaOH (called sodium hydroxi
5、de). When the acid and base react, they form NaCl (sodium chloride), which is also known as table salt. The titration proceeds until the equivalence point is reached, where the number of moles of acid is equal to the number of moles of base. This point is usually marked by observing a color change i
6、n an added indicator.In a titration, the standard solution goes in a buret, which is a piece of glassware used to measure the volume of solvent to approximately 0.1 mL of accuracy. The solution that you are titrating goes in an Erlenmeyer flask, which should be large enough to accommodate both your
7、sample and the standard solution you are adding.What is an Indicator and What is it Used For?An indicator is any substance in solution that changes its color as it reacts with either an acid or a base. Selecting the proper indicator is important because each indicator changes its color over a partic
8、ular range of pH values. Indicators are either weak acids or weak bases. For example, phenolphthalein is a weak acid (which we will represent as HIn). In aqueous solution, the phenolphthalein dissociates slightly, forming an equilibrium.An equilibrium occurs when the amount of reactants and the amou
9、nt of products are constant. When a system is in equilibrium, it will stay there until something changes the conditions. A famous French chemist, named Le Chatelier, developed a way to predict how changes in equilibrium affect the system. Le Chateliers principle states that when an equilibrium is di
10、sturbed by applying stress, the equilibrium will shift to relieve the stress. In an acidic solution, there is an excess of H3O+ ions so the equilibrium will shift to the left and favor the formation of HIn, thus we observe a clear solution. In basic solution, there is an excess of OH- ions that reac
11、t with the H3O+ ions to form water. This shifts the equilibrium to the right because water is being formed and H3O+ ions are being removed, thus we observe a pink solution. We can use this color change to determine when the end of the titration has been achieved.Table 1 lists common indicators and t
12、he pH range over which they change colors. Measuring pH: How to Calibrate a pH MeterTable 1: Table of IndicatorsIndicatorColor change interval (pH)AcidBasethymol blue1.2 - 2.8redyellow methyl orange3.1 - 4.4redyellow methyl red4.4 - 6.2redyellow chlorophenol red5.4 - 6.8yellowRed bromothymol blue6.2
13、 -7.6yellowBlue phenol red6.4 - 8.0yellowRed thymol blue8.0 - 9.6yellowBlue Phenolphthalein8.0 - 10.0colorlessRed alizarin yellow10.0 -12.0yellowgreenpH is a measure of acidity or basicity. An acid has a pH less than 7, a neutral compound (like water) has a pH near 7, and a base has a pH from 7-14.
14、pH can be measured using either litmus (or indicator) paper, which changes color based on the acidity of a solution, or by using a pH meter. A pH meter is a more accurate means of measuring pH because it can be calibrated to measure one tenth of a pH unit, whereas the indicator paper only measures t
15、o one pH unit. A pH meter uses an electrode to measure the pH of a solution. The electrode is stored in distilled water in order to keep it at a neutral pH. To calibrate the pH meter:1. Remove the electrode from the distilled water and place it in pH 4 buffer, which is pink. Make sure the electrode
16、is completely covered in buffer and swirl the solution around. 2. Set the pH meter to pH 4 and then rinse the electrode with distilled water to remove any excess solution. 3. Place the electrode in pH 10 buffer (which is blue) and swirl it around in the solution. 4. Set the pH meter to pH 10 and rinse the electrode, returning it to the distilled water once you are finished. The pH meter should now be calibrated to me
