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by: Burdette Dooley


Burdette Dooley
GPA 3.84

Delana Nivens

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Delana Nivens
Class Notes
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This 2 page Class Notes was uploaded by Burdette Dooley on Saturday October 3, 2015. The Class Notes belongs to CHEM 2300 at Armstrong Atlantic State University taught by Delana Nivens in Fall. Since its upload, it has received 70 views. For similar materials see /class/217880/chem-2300-armstrong-atlantic-state-university in Chemistry at Armstrong Atlantic State University.

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Date Created: 10/03/15
Dilution Calibration Curves Linear Range and Linear Regression Instructor notes DI water is more acid than tap water use indicator that doesn t change color in the pH range of 5 7 don t use MR BB and BG are better The purpose of this laboratory experiment is to practice making dilutions to learn to use the Spec20 to use graphing programs to draw a calibration curve to add error bars and to perform a linear regression analysis Procedure 1 Turn on the Spec20 and allow it to warm up for 20 minutes 2 Obtain approximately 50 mL of the stock dye solution Note the dye name molecular weight and color 3 Calculate the molarity of the dye in the stock solution based on the given amount on the reagent bottle Qualitative Data Obtaining the UV VIS spectrum of the dye l Pipette 200 mL of stock dye solution into a 10000 mL volumetric and dilute up to the mark Calculate the concentration of the dye 2 Obtain a set of cuvettes make sure they are clean and free of scratches 3 Using distilled water in a cuvette as a blank wipe it clean with KimWipes place into the Genesis20 spectrometer and press the 0 Abs button to set the blank sample to zero 4 Put the diluted dye into another cuvette wipe with Kimwipes and place into the Genesis20 5 Measure the absorbance of the diluted dye sample at 340 nm then remove the dye cuvette and set it aside Put the blank back into the instrument 6 In 10 nm increments repeat step 35 until you cover the entire range of 340700 nm on the Genesis20 or until the dye absorbance drastically increases then decreases again You must reset the blank sample to zero each time you change the wavelength 7 Find the wavelength where the dye exhibits maximum absorbance minimum transmittance 8 Set the wavelength of the Genesis20 to the maximum absorbance wavelength and reset the blank sample to zero See section below on the detection limit now Before continuing Quantitative data Preparing a calibration curve 1 Obtain N3 mL of the unknown dye solution 2 Obtain a perfectly clean and dry cuvette 3 Measure the unknown s absorbance at the chosen hmax 4 You should use the same cuvette for all quantitative measurements and measure from most dilute sample to most concentrated sample with triple rinsing in between each concentration Why You must also make sure that the cuvette is always facing the same direction when placed into the instrument Why 5 YOU ARE FIGURING OUT YOUR OWN DILUTIONS YOU ARE GUESSING YOUR INSTRUCTOR IS NOT GOING TO TELL YOU HOW MUCH TO USE TRY TO VISUALLY MATCH THE COLOR To make the best calibration curve at least 5 different dilutions of stock dye solution must be measured There must be at least 2 concentrations absorbencies lower than the unknown and 2 higher than the unknown The data points must also follow a linear relationship and cover as wide a range as possible You should make a rough graph of the data in your notebook as you perform the experiment so that you don t make unnecessary dilutions and waste time For example your dilutions might be 010 M 020 M 040 M 080 M and 160M and the unknown may be determined to be 055 M from the graph 6 Samples having Absorbencies of less than 0005 or greater than 16 should not be used if it can be avoided The linearity of your curve will be affected Why 7 Make your dilutions from the stock dye solution You may use your 200 mL dilution from the qualitative section of this experiment if it s absorbance falls into the correct range just measure it again after setting the blank to zero at Xmax 8 To minimize error you should make all dilutions from same stock solution If your calculations lead you to need an amount smaller than the smallest volumetric pipette 01 mL we have you are advised to dilute the original stock solution and then make all dilutions from the diluted stock solution For example dilute the stock solution by 50 and use twice as much in each dilution 9 You should measure 3 different aliquots from each dilution to allow you to calculate the average and standard deviation Estimating the detection limit should be done at various points along the experiment 1 You should take at least 5 measurements of the blank solution at the absorbance maxima over the course of the experiment After setting the blank to zero in Qualitative step 8 do not re set the blank for the remainder of the experiment Just measure the blank to get an idea of how much your blank might drift 0r uctuate over the course of the experiment Calculations and Laboratory Write Up Qualitative Section 1 Graph Absorbance versus wavelength using Excel You must choose XY scatter for your Cha1t Type Note the Absorbance maximum on the graph Using Beer s Law to calculate a the molar absorptivity for your dye at 7mm A 8 be assuming a path length of 1 cm and the concentration that you calculated Don t forget the correct units on a Quantitative SectionUsing Excel Spreadsheet 2 Subtract the average blank measurement from all other measured absorbances 3 Calculate the average absorbance for each concentration as well as the standard deviation in the y direction and the uncertainty in the volumes in the x direction figure out how to propagate the error for concentration 4 Follow the example in chapter 4 of your textbook for the rest of the steps They have also been outlined below


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