ANACHEM_Volumetric Analysis

• Titration is a method of determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration.
• Standard solution is a solution of known concentration prepared by dissolving a known weight of solute to make a specific volume.  It is also called titrant.
• Analyte is a solution of unknown concentration or the substance in a sample whose concentration must be determined.
• Equivalence point is the point in titration at which an added titrant is stoichiometrically equal to the number of moles of analyte present in the sample.
• End point is the point in titration at which there is an observable physical change associated with the condition of equivalence. 
• Indicators -  substances added to the analyte in very small quantities which will indicate the equivalence point with a significant change in color
• Cresol red: red acid color, yellow alkaline color
• Thymol blue: red acid color, yellow alkaline color
• Methyl orange: red acid color, yellow alkaline color
• Phenolphthalein: colorless acid, pink alkaline color
• HCl (Hydrochloric Acid) vs NaOH (Sodium Hydroxide)

  • Indicator: phenolphthalein
  • End point: pink
• HCl vs Na2CO3 (Sodium Carbonate)

  • Indicator: methyl orange
  • end point: light orange
• KMnO4 (Potassium Permanganate)

  • Indicator: none
  • End point: pale pink
• K2Cr2O7 (Potassium Dichromate)

  • Indicator: Potassium Ferrocyanide
  • End point: light blue
• Na2S2O3 (Sodium Thiosulfate)

  • Indicator: starch solution
  • End point: light blue
• Primary standard is an extremely pure compound that serves as a reference material in titrimetric analysis
• Requirements to be considered as primary standard:

  • Purity - it should be available in highly pure form.
  • Atmospheric stability - it should be stable at 110˚C- 120˚C.
  • Should have a high molecular mass so that weighing errors are minimized
  • Has no water of hydration
  • Readily available at a modest cost•
  • Reasonably soluble in the titration medium
• Examples of primary standards:

  • sodium carbonate (Na2CO3)
  • potassium hydrogen phthalate (KHC8H4O4)
  • potassium hydrogen iodate (KH(IO3)2)
  • potassium dichromate (K2Cr2O7)
• Secondary standard is a less pure compound but its purity can be established by careful analysis.

  • It must be standardized first before use because a secondary standard solution is not in its stable form.
  • The concentration of the dissolved solute in a secondary standard has not been determined from the weight of the compound dissolved but by reaction of a volume of the solution against a measured volume of a primary standard solution.
• Examples of secondary standard:

  • sodium hydroxide (NaOH)
  • potassium hydroxide (KOH)
  • hydrochloric acid (HCl)
  • potassium permanganate (KMnO4)
• methods used in establishing the concentration of the titrant:

  • direct method
  • standardization
• Direct method -   by carefully weighing a quantity of the primary standard and adding a suitable solvent to make up a known volume of solution in a volumetric flask
• Standardization –  a method by which the titrant to be standardized is used to titrate a weighed quantity of a primary standard, or a weighed quantity of a secondary standard or a measured volume of another standard solution.
• sources of error in titrimetric operations:

  1. loss of sample
  2. contamination
  3. faulty mixing
  4. impurities in primary standard
  5. errors in weighing
  6. errors in reading burets
  7. use of wrong indicators
  8. poor drainage from buret or pipet
  9. wrong concentration
• Acid-base titration  is a quantitative determination of the concentration of an unknown acid or base by exactly neutralizing the unknown acid or base with an acid or base of known concentration.
• Acidimetry is a method of determining the concentration of an acidic substance where a basic solution of known concentration is used to titrate an acid solution of unknown concentration.  In this case, the standard solution or the titrant is the base while the analyte is the acid.
• Alkalimetry is a method of determining the concentration of a basic substance where an acidic solution of known concentration is used to titrate a basic solution of unknown concentration.  In this case, the standard solution or the titrant is the acid while the analyte is the base.   
• steps in solving titration problems

  • Write the balanced equation
  • Identify the number of moles of known/ standard solution
  • Use mole ratio to solve for moles of unknown
  • Solve for unknown
• Back titration is a method where the concentration of an analyte is determined by reacting it with a known amount of excess titrant.  The excess titrant is then titrated with another, second titrant.
• standard solution: HCl

  primary standard: Na2CO3 (sodium carbonate)
• standard solution: NaOH

  primary standard: KHC8H4O4, H2C2O4
• standard solution: KMnO4

  primary solution: sodium oxalate
• standard solution: K2Cr2O7 (potassium dichromate)

  primary standard: iron wire
• standard solution: I2

  primary standard: arsenic oxide
• standard solution: Na2S2O3 (sodium thiosulfate)

  primary standard: potassium iodate
• standard solution: KCNS (potassium thiocyanate)

  primary standard: silver nitrate