User:Douglas M. Fox/Notebook/AU CHEM-571 F2011 Lab Support/2012/02/07

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Project 3: Solution Preparations Main project page
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Project Objectives

Standardization of NaOH (titrant) & HCl stock solutions
Preparation of 2.7 mM chlorauric acid solution
Preparation of buffer solutions

Today's Tasks

Preparation of 50 mM Tris (8.2 pH) buffer

  • Placed 1.211 g Tris base (121.14 g/mol) in 200 mL volumetric flask
  • Rinsed weighing dish & transfered solution to flask
  • Added 4.430 mL 0.948 M HCl using automatic micropipet (100 - 1000 μL)
  • Filled with deionized water & inverted 10 times
  • pH = 8.39 as measured by VWR SympHony meter


Analysis

pH discrepency

  • inaccurate calibration (unlikely; new standards used & readings were stable)
  • [HCl] of stock actually lower than 0.948 M? (unlikely; potentiometrically determined)
  • Base impurities in glassware? (unlikely; clean & dry glassware with no visible residue)
  • Too much Tris added? (unlikely; precision balance used & mass was stable)
  • Not enough HCl added (probable)
    • micropipet used was not previously calibrated (instrument error)
    • technique may have been poor, including incomplete time to draw up solution (human error)


Other buffers needed

  • 7 - 7.5 pH dihydrogen phosphate - hydrogen phosphate (50 - 100 mM)
  • 8.2 pH Tris - TrisH+ (50 - 100 mM) in saline (0.9% NaCl)



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