lab tech?

• Water


  Has equal amounts of H+ and OH- ions
• Dissociation of water


  H2O → H+ + OH-

  
• pH


  Measure of the amount of hydrogen ions (H+) in a solution
• Neutral solution


  [H+] = [OH-] = 1 × 10-7
• Acidic solution


  [H+] > [OH-], and [H+] > 1 × 10-7

  
• Basic solution


  [H+] < [OH-], and [H+] < 1 × 10-7
• pH + pOH


  = 14
• pH scale


  Defined from 0 - 14
• pH values can be less than 0, or greater than 14
• A negative pH is possible for high molarity acidic solutions
• Strong acids


  Completely dissociate in water to produce H3O+ and anion

  
• Strong acids


  • HCl, HBr, HI, HClO3, HClO4, HNO3, H2SO4
• Weak acids


  Partially dissociate in water

  
• Weak acids


  • Aspirin (acetylsalicylic acid), citric acid, ascorbic acid, vinegar (acetic acid)
• Strong bases


  Completely dissociate to form OH- ions

  
• Strong bases


  • NaOH, KOH, CsOH, Ca(OH)2
• Weak bases


  Partially dissociate in solution
• Weak bases


  • Ammonia, baking soda (sodium bicarbonate, NaHCO3), pyridine, amine
• pH of common solutions

  • Baking Soda (8.5), Wine (2.8 - 3.8), Blood (7.3 - 7.5), Vinegar (2.4 - 3.4), Milk (6.6 - 7.6), Soft Drinks (2 - 4), Apple Juice (3.7), Gastric Juices (1 - 3)
• Buffer solutions


  Resist change in H+ concentration upon addition of an acid or base

  
• Buffer solution


  Mixture of weak acid (HA) and its conjugate base (A-)

  
• pH paper


  Saturated with a chemical indicator that changes colour at certain pH values
• pH meter


  Directly measures the hydrogen ion concentration
• pH meter


  • Consists of a glass electrode, reference electrode, and potentiometer

  
• Titration


  Process for obtaining quantitative information about a reactant by the controlled addition of one reactant of known concentration (standard solution) to the other reactant until the reaction is complete
• Equivalence point


  Point at which all of the reagent has reacted with all of the standard added
• End point


  Point at which the colour change of the titrated solution is observed

  
• Performing a titration


  1. Clean and fill a buret with standard solution (titrant) and mount in buret clamp

  2. Record the initial buret reading (volume) to 2 decimal places

  3. Add measured volume of reactant to an Erlenmeyer flask and add indicator

  4. Add titrant to the Erlenmeyer flask slowly, swirling/stirring constantly

  5. As end point approaches, add titrant one drop at a time, washing down the sides of the flask with water

  6. End point is reached when colour stays for 30 seconds or more

  7. Record the final buret reading to 2 decimal places

  
• Acid-base indicators


  Organic dyes that change colour near the equivalence point
• Bottle


  Container used in experiment
• Performing a Titration


  1. Frequently wash down the sides of the flask with water

  2. End point is reached when colour stays for 30 seconds or more

  3. Allow about a minute from last addition of titrant before reading the final position of the meniscus, to allow the burette to drain down

  4. Record the final buret reading to 2 decimal places

  
• Watch the video on performing an acid base titration and determining the end point using phenolphthalein indicator

  
• Indicator


  • A colour change caused by an indicator determines the end point of the reaction
  • Acid-base indicators are organic dyes that change colour near the equivalence point
  • The colour change is rapid and is reversible
  • Indicators show pH-dependent colour changes
  • The molecular form of the indicator has a different colour than the ionic form
• Indicators


  • Methyl Red
  • Bromothymol Blue
  • Phenolphthalein
• Phenolphthalein


  A common indicator
• Potentiometric Titration


  One method for determining the equivalence point of an acid–base titration is to use a pH electrode to monitor the change in pH during the titration

  
• Potentiometric Titration


  1. Record initial volume of titrant and initial pH of analyte

  2. Add small aliquots of titrant, record burette reading and pH after each addition

  3. Continue titration well after end point is observed

  4. Create a titration curve by plotting pH vs volume of titrant

  
• Titration Curve


  • The equivalence point is the inflection point on the curve, where the slope is steepest
  • Occurs over a small volume of titrant but a large range of pH values

  
• pH at Equivalence Point

  • Strong Acid with Strong Base: 7 (neutral)
  • Weak Acid with Strong Base: ~7-10 (basic)
  • Strong Acid with Weak Base: ~4-7 (acidic)
• Acid-Base Titrations


  1. Neutralization reaction occurs: acid + base → salt + water

  2. Concentration of acid or base can be determined by titrating with known concentration of base or acid