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# PRINCIPLES BIOCHEM CHEM 3375

Texas State

GPA 3.65

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This 15 page Class Notes was uploaded by Valentina Kozey on Wednesday September 23, 2015. The Class Notes belongs to CHEM 3375 at Texas State University taught by L. Watkins in Fall. Since its upload, it has received 10 views. For similar materials see /class/212901/chem-3375-texas-state-university in Chemistry and Biochemistry at Texas State University.

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Date Created: 09/23/15

How to Interpret Titration Curves nd the equivalence point it is the steepest part of the curve where the pH rises the fastest the equivalence point can be used to determine the equivalent weight molar mass of the acid nd the mid point located in the center of the buffer region geometrically halfway between the equivalence point and the beginning of the titration sometimes it is a little more complicated than this see the example the midpoint determines the pKa of the acid How to Interpret Titration Curves things to do rst graph your data as seen in the next slide make sure you turn on the major and minor tick marks on both axes double click on the axis and click on the patterns tab there is enough precision in the tick marks you should have at least 1 mL or smaller for the minor tick mark on the xaxis you should have at least 02 pH units or smaller for the minor tick mark on the yaxis Two Different Methods there are two methods of analysis that will be shown geometric method requires a ruler a pencil and the titration graph 1st derivative method requires a spreadsheet and some formula entries gives you cool graphs with the 1st derivative pointing to the equivalence and mid points scores you brownie points with the instructors pick your method either will work A Typical Titration Curve Weak Acid Titration Curve 12 7 i mid point 10 E 39 equlvalence i oint 8 p 6 E 4 E 2 i i i i i i i i i i i i i i i i i i i i 0 10 20 30 40 50 Buret Volume m L Find the Equivalence Point Geometric method 1 using a ruler Neak Acid Titration Curve draw lines that follow the at more horizontal part of the curve equivalence point 2 draw a line that follows the at 4 more vertical part zwmm Hofthecurve 0 10 20 3O 40 5O Buret Volume m L Find the Equivalence Point Geometric method 3 using a ruler ak Acid Titration Curve measure the distance between the top intersection and the equlvalence bottom intersection point 4 the geometric center of this line 4 segment is the 2 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 0 1O 20 30 4O 5O Buret Volume m L Find the Equivalence Point Geometric method 12 10 Weak Acid Titration Curve 5 draw a vertical 7 line from the equivalence point 7 to the Xaxis equivalence point 6 Where the line crosses the XaXis is the volume at the equivalence point 287 mL in this case Buret Volume m L Find the Mid Point Geometric method H tion Curve 1 if there is a steep rise in the p at the beginning of the graph draw a line that follows the steep part of the curve mid point 2 ii ii i I 10 20 3O 4O 5O Buret Volume mL Find the Mid Point Geometric method tion Curve 2 using a ruler measure the distance between the far left and right intersections 1o 39 equivalence point 3 the geometric center 3 between these points is 4 the mid point 2 l l l l l l l l l l l l l l l l l l l l l x x x l J 10 20 30 40 5O Buret Volume mL Find the Mid Point Geometric method 4 draw a horizontal line from the rtion Curve mid point to the yaXis 39 equivalence point 5 Where the line crosses the XaXis is the volume at the equivalence point 2llllxllll HHHHHHH I I I 10 20 pH 72 1n this case Buret Volume mL mid point 5O How to Interpret Titration Curves nd the equivalence point make sure you subtract the initial buret volume in this case the initial buret volume was 107 mL true equiv pt 287 mL 107 mL 2763 mL the 3 is the indicate the limit of the signi cant gures calculate the equivalent weight molar mass equiv wt acid massNaOH concequiv pt equiv wt 4302 mg01139 M2763mL equiv wt 136699 137 gmol How to Interpret Titration Curves nd the mid point mid pt 72 pKa 0f the acid For you Excel Aficionados equivalence point use the rst derivative d pH d V01 the spike in the graph points to the equiv pt mid point reverse the axes for the pH curve 0 x axis pH values yaxis Vol values use the rst derivative d V01 d pH the spike in the graph points to the mid point use extra columns in the spreadsheet to make these calcs 1st deriv d pH d V01 pH2 pH1V012 V011 1st deriv d V01 d pH V012 V011 pH2 le 0 or just the rst 1st deriv 1 Find the Equivalence Point derivative method Weak Acid Titration Curve 4oi 7 7 1 identify volume H W 6 value at the peak F W 5 1stderiv I 285 mL in this case a 4 a 5 773 6 772 4 W1 21W 0 0 1O 20 3O 4O 50 Buret Volume mL 1st Derivative m L 391 Buret Volume m L Find the Mid Point derivative method Weak Acid Titration Curve 1 identify pH value at the peak pH 73 in this case 35 3 30 25E 20 15 10 mL 1st deriv

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