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NOTES - Introduction Page 1 of 2 What is Analytical...

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Unformatted text preview: Introduction Page 1 of 2 What is Analytical Chemistry? The branch of chemistry dealing with the identification and quantitation of species in a given sample -- Qualitative (What) -- Quantitative (How much) Chemical Analysis "Wet" Methods Instrumental Methods Gravimetric Separation Volumetric Optical E l e c t r o c h e m i c a l Measuring is not the only thing Analytical Chemists do ¾ Develop new methodologies and instruments ¾ Proper sampling, sample handling and preparation ¾ Calibration and standardization ¾ Data interpretation, classification, fitting Introduction Page 2 of 2 Examples of "Wet" Methods : 1. Ag + + Cl-----> AgCl Mass (Gravimetry) 2. H + + - OH ----> H 2 O Volume (Titrimetry) 3. Unknown + dye ----> Color Qualitative Anal. http://www.cbs.com/primetime/csi/ Examples of Instrumental Methods : Optical : F e 3+ + Ligand ----> Fe(L) 3 3+ Ö Colorimetric Determination Electrochemical : P b 2+ + 2e-----> Pb (At Indicator Electrode) Ö Application of Potential yields Charge Chromatography : Size-Exclusion Chromatography ---> Protein Fractionation and Identification Ö Separation allows Specific analysis Measurement and Errors Page 1 of 13 Measurement and Errors Every measurement has associated with it an uncertainty . • Measuring the width of a room with a tape measure may be fine, but using a tape measure to obtain where the center of a 1 cm rod is, would not be. (See accuracy and precision later.) Rule of Thumb Ö Always estimate the measured value to 10% of the distance between scale divisions. Example: A ruler with 1mm divisions should be read to within 0.1mm or value ± 0.1mm. Of course this is subjective, but the error should incorporate the uncertainty. ) When we start manipulating numbers, when should we keep or throw away part of the answer? Measurement and Errors Page 2 of 13 Significant Figures- minimum number of digits necessary to write a value in scientific notation without loss of accuracy. ) 103620900 Q: How many significant figures? A: HUH?!! Zero Rules 1. Significant if in the middle of two sig digits 2. Significant if to the left of a decimal point and to the right of a number 3. Significant in scientific notation NUMBER #Sig Figs (s.f.) 103620900. 9 3.765 x 10-5 4 3.7650 x 10-5 5 0.00003675 4 Measurement and Errors Page 3 of 13 How do we determine the # of significant figures when we add, subtract, multiply, divide, etc.? Add or subtract 1.6392 (five sf) 46.231 (five sf) 47.870 2 <--- the 2 is not significant the correct answer is 47.870 Multiply or divide 20.962 (five sf) x 1.050 (four sf) 22.01 01 <--- the 01 is not significant the correct answer is 22.01 (four sf) Rounding 12.815 12.815 + 3.60 + 3.61 16.41 5 16.42 5 answer is 16.42 answer is 16.42 Measurement and Errors Page 4 of 13 THE RULES!...
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