Determination of Mn in Steel (Standard Addition)

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Presentation transcript:

Determination of Mn in Steel (Standard Addition) Sample Calculations of Determination of Mn in Steel (Standard Addition) Vikram Sharma

Initial Values Find the mass of your steel for each trial, in this calculation it will be .5091g, in a 100.00mL flask Mn Standard concentration (Cs) = .1025 mg/mL Absorbance data  Absorbance Blank 0.031 Sample 0.198 Sample+5mL 0.631 Sample+10mL 1.082

Why are we using the standard addition calculation? We have to use the standard addition calculation because we have to take into account the matrix effect Matrix effect- the change in the analytical signal (absorbance in this case) because of all the stuff other than the analyte (Mn) in the solution.

Part I, 2 equation 2 unknowns Use aliquots 2 and 3 1.082 Sample+10mL 0.631 Sample+5mL 0.198 Sample 0.031 Blank Absorbance Remember to subtract your blank!!!

Defining Variables Ax,T=Corrected Absorbance of aliquot 2 = .167 Ax+s,T=Corrected Absorbance of aliquot 3 = .600 Vs=Volume Standard .1025 mg/mL Mn= .00500 L Vx=Volume Unknown with conc. Cx = .02000 L Cx= Concentration Mn in sample

The Numbers… Cx = [.167/(.600-.167)]* [(.00500L*.1025mg/mL)/.02000L] Cx= 0.00988 mg/mL Cx*100mL=.988mg .988mg/1000mg/g=.000988g Mn .000988gMn/.5091gSample*100%= .194%

Part II, Graphical Procedure The X axis equation Xn=[S]i*(Vs/Vo) Where [S]i= Original Conc. Std Mn (.1025 mg/mL) Vs = Volume standard (0, 5, 10 mL) Vo = Volume unknown (20 mL)

Graphical Procedure The Y axis equation Yn=IS+X*(V/Vo) Where: IS+X= Absorbance value V = Total volume (50 mL) Vo = Volume unknown (20 mL)

Graphical analysis X values Y Values Sample 0.4175 Sample+5mL 0.025625 0.4175 Sample+5mL 0.025625 1.5 Sample+10mL 0.05125 2.6275

Find the conc. of unknown, [X]f [X]f is the magnitude of the X-intercept value Therefore Y=0, so find X in the calculated equation from the least squares line: y=43.1[X]f+.410 0=43.1[X]f+.410 -.410/43.1= [X]f [X]f =|-.00951|=.00951mg/mL

Finding %(wt/wt) [X]f *100mL=.951mg .951mg/1000mg/g=.000951g Mn .000951gMn/.5091gSample*100%= .187% Compare .194% to .187%

Question In the graphing procedure, is the x intercept supposed to be positive or negative? Negative. If it’s positive, you screwed up.