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Lecture

2b. Standard.docx

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Department
Chemistry
Course
CHM217H1
Professor
Stone
Semester
Fall

Description
CHM217 Section 2 How Pure is Pure? Standards & Calibration (Chapter 1 and 7.1 & 11.7)  Pure is a very relative term  Bottled natural spring H2O o 0% lead (it is a lie, cannot proof) o All can do is to say: if there is lead in there, it is below a certain threshold concentration, beyond which cannot get an accurate measurement  If we get new instrument, 10X better then limit of detection go down by 10X, then the guideline of allowed chemicals in water go down by factor of 10 o Therefore, 0% lead  used the best instrument screened it; and cannot see anything  When doing the lead test o Can buy a lead standard for trace element analysis; [] of the standard is certified to 3-4 sig. fig Standards and Reference Materials  All analytical methods require standard  Example o Determination of physical properties (i.e melting point)  Archimedes: purity of gold  Melting point, boiling point  need a refined standard o Determination of reaction stoiciometry  Take known amount of materials; react them together, and see the product o Characterization of instrument response  How do u known pH electrode give u is 7? How to have pH 7 system to measure against?  Methods of obtaining pure substances can also be used as methods of analysis  Define different types of standard, depending on availability, purity, and complexity o Primary standard o Secondary standard o Certified reference material (CRM) Primary standards  ―A substance of high purity and stable chemical and physical properties that is designated or generally accepted as being determined without reference to other standards‖  E.g. o Metallic silver ~ check purity o Potassium hydrogen phthalate (KHP) ~ weak acid, readily made, generated as a solid, high level of purity, further refined by re- crystallization o 4-amino pyridine ~ used as base  Criteria defining a primary standard: o Easy to obtain, purify, dry, preserve (mostly solid) o Unaltered in air during weighing (moisture, O2, CO2) o Testable for impurities (<0.1-0.2% w/w) o High molar mass (i.e. weigh sample, bigger mass   High # of sig. fig  high level of CL  accurate o Readily soluble o Undergo stoichiometric, instantaneous reaction (go quickly, completely) Secondary standards  A substance of high purity which must, none the less, be standardized with respect to a primary standard  i.e. most mineral acids, sodium hydroxide (p44, 43) o Typically hygroscopic or air reactive o Concentration changes slowly over time o May not be as pure as primary standard Certified Reference Materials  A material having 1 or more property values certified by a procedure and for which each certified value has a stated uncertainty at a specified confidence level  Key concept is traceability to some well defined standard of unit o E.g. mass  standard kg  When buy a standard, comes w certificate instrument should be able to trace back too o Must specify mythology as well as property values (e.g. concentration)  Mythology: will get different results using different method  E.g. standard 2n solution, Ca2+ and silicate in mile powder, Carbamate pesticides in tomato juice Determining purity 1. Precipitation 2. Recrystalization 3. Distillation / Fractional Distillation (more selective) 4. Refining Macroscale determination of purity 1. Titration (titrimetry) o Titration: not a purification technique 2. Precipitation (gravimetry) 3. Elemental analysis (CHNO) o Reagent ~ cheaper o Chemical for analytic things ~ more expensive Volumetric analysis: Titration (chapter 7)  ―A volumetric method in which reagent of known concentration is added to a known volume of another reagent‖  Titrant: reagent of known [] added from a burette  Titrant (titrand): substance being titrated an aliquot i.e. precisely known volume  Equivalence point: point (volume) at which the reaction is complete i.e. stoichiometry amount of each reagent  End point: point (volume) at which the reaction is observed to be complete Acid – Base Titrations chapter 9 and 10 skip blood pH  Result is a salt and mostly water i.e. HCl + NaOH  NaCl + H2O  Titration curves End point indicatior: table indicators P215  H In (aq) H +(aq) In -(aq) [ ][ ] [ ]  ( ) ( ) [ ] [ ]  At equivalent point [In-] = [HIn]  pH = pKa  Poly protonate element: o H3PO4: the 3 H+ is hard to dissociate, there is 3 pKa nd o H2SO4: 2 H+ to lost, the pK(In) should be increase, or it cannot see the 2 H+ lost  All indicator will give some kind of end point error  End point we see is not the equivalent point of the titration  systematic error  We can make it worse by adding large amount of indicator to see color change better  You will titrate indicator + sample  Once adding indicator, you are separating it from equivalent point  In acid base titrations, use pH electrode or potentiometric method  Potentiometric method o No indicator o Ion selective electrode for titrand or titrant o Standardize reagent against primary standard o Similar methodology to pH titration Precipitation (often Argentimetric: use silver or Argentimetric) -10  Typically used for chloride Cl-(aq) + Ag+ (ag)  AgCl (s) Ksp = [Ag+] [Cl-] = 1.8 x 10  Silver with any Halide (F, Cl, Br, I), the end point is showing the white color, shimmering o Since the color is hard to see and hard to get the accurate end point, therefore, we need to detect the excess trace of Ag+ o We need the point at which stop forming AgCl o We need reagent to aid end point observation Mohr’s method (direct titration)  This method determines the chloride ion concentration of a solution by titration with silver nitrate  As the silver nitrate (AgNO3) solution is slowly added, a precipitate of silver chloride forms.  Ag (aq) + Cl (aq) → AgCl(s)  The end point of the ti2-ation occurs when all the chloride ions are precipitated  Add indicator: K 2rO 4 faint lemon yellow color  Then additional Ag+ react with the chromate ions of the indicator, potassium chromate, to form a red-brown precipitate of silver chromate  Ag+(aq) + 2 CrO4 (aq) → [Ag (CrO4 ) ]- red brown color 2  The pH of the sample solutions should be between 6.5 and 10, or affecting the accuracy of the end point.  At higher pH (basic), silver ions may be removed by precipitation with hydroxide ions (OH-)  At low pH (acid), chromate ions may be removed by an acid-base reaction to form hydrogen chromate ions or dichromate ions  If the solutions are acidic, the gravimetric method or Volhard‘s method should be used  Problem with Cr ions: oxidize, toxic, form other ppt Volhard’s method (indirect titration)  Add excess st
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