CHEMISTRY 59-320 ANALYTICAL CHEMISTRY Fall - 2010 Lecture 3 Principle of operation (3) • The QCM-Z, based on impedance analysis, provides unique information on changes in the mass properties of adsorbed or deposited layers on a quartz crystal. The mass change is proportional to the change in resonance frequency. Tolerances (allowable deviations) Weighting in a Vacuum is the Most Accurate • Weights of objects in air can be corrected to the weight in vacuum by • Wvac= Wair + Wair((0.0012/Do)(0.0012/Dw)) • Wvac = weight in vacuum, g • Wair = weight in air, g • Do = density of object • Dw = density of weights • 0.0012 = density of air Buoyancy Correction Buoyancy: Upward force exerted on an object in a liquid or gaseous fluid. An object weighed in air appears lighter than its actual mass by an amount equal to the mass of air that it displaces. Where da is the density of air (0.001 2 g/mL near 1 bar and 25°C),11 dw is the density of the calibration weights (typically 8.0 g/mL), and d is the density of the object being weighed. 2.4 Burets Parallax Operating a buret • • • • • • • • Wash buret with new solution. Eliminate air bubble before use. Drain liquid slowly. Deliver fraction of a drop near end point. Read bottom of concave meniscus. Estimate reading to 1/10 of a division. Avoid parallax. Account for graduation thickness in readings. Position the black field just below the meniscus. Avoid parallax error by reading at eye level. Place the flask on a white background. Place the buret tip in the neck of the flask while your swirl. Proper technique for titration. 2.5 Volumetric Flasks A volumetric flask is calibrated to contain a particular volume of solution at 20°C when the bottom of the meniscus is adjusted to the center of the mark on the neck of the flask The proper usage of volumetric flask • Glass is notorious for adsorbing traces of chemicals—especially cations. Adsorption is the process in which a substance sticks to a surface. (In contrast, absorption is the process in which a substance is taken inside another, as water is taken into a sponge.) • For critical work, you should acid wash glassware to replace low concentrations of cations on the surface with H+. 2.6 Pipets and Syringes . Pipets deliver known volumes of liquid. 1. Transfer pipet 2. Measuring pipet 2.7 Filtration Liquid from which a substance precipitates crystallizes is called the mother liquor. Liquid that passes through the filter is called filtrate. 2.8 Drying desiccator 2.9 Calibration of volumetric glassware Calibration: the process of measuring the actual quantity of mass, volume, and so on, that corresponds to an indicated quantity on the scale of the instrument. Correction for thermal expansion c' c d' d Temperature Dependence of Molarity • The Molarity of a solution is temperature dependent, therefore when preparing or standardizing solutions you have to record the temperature of solutions. • Conversion Formula: • Mnew temp = Mold tempx(Dnew temp/Dold temp) • See examples in Text 2.10 Introduction to Microsoft Excel This is a problem from s2-3.