ANCH WEEK 13

• Titration Methods


  are based on determining the quantity of a reagent of known concentration that is required to react completely with the analyte.
• Volumetric Titrations 


  the volume of a standard reagent is the measured quantity.
• Coulometric Titrations 


  the quantity of charge required to complete a reaction with the analyte is the measured quantity.
• Gravimetric Titrations 


  the mass of the reagent is measured instead of its volume
• Terms used in volumetric titrations: standard solution or a standard titrant


  is a reagent of known concentration that is used to carry out a volumetric titration.
• Terms used in volumetric titrations: standard solution or a standard titrant


  It is sometimes necessary to add an excess of the standard titrant and then determine the excess amount by back-titration with a second standard titrant.

  For example, the amount of phosphate in a sample can be determined by adding a measured excess of standard silver nitrate to a solution of the sample, which leads to the formation of insoluble silver phosphate:
• Terms used in volumetric titrations: standard solution or a standard titrant


  The excess silver nitrate is then back-titrated with a standard solution of potassium thiocyanate:

  • The amount of silver nitrate is chemically equivalent to the amount of phosphate ion plus the amount of thiocyanate used for the back-titration.

  • The amount of phosphate is then the difference between the amount of silver nitrate and the amount of thiocyanate.
• Equivalence Point 


  in a titration is a theoretical point reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample.
• Endpoint


  We cannot determine the equivalence point of a titration experimentally. Instead, we can only estimate its position by observing some physical change associated with the condition of chemical equivalence. The position of this change is called the _
• Equivalence Point and Endpoint


  We try very hard to ensure that any volume or mass difference between the equivalence point and the end point is small. Such differences do exist, however, as a result of inadequacies in the physical changes and in our ability to observe them. The difference in volume or mass between the equivalence point and the end point is the titration error.
• Indicators 


  are often added to the analyte solution to produce an observable physical change (signaling the end point) at or near the equivalence point.
• Indicators


  Large changes in the relative concentration of analyte or titrant occur in the equivalence-point region. These concentration changes cause the indicator to change in appearance
• Primary Standards


  is a highly purified compound that serves as a reference material in titrations and in other analytical methods.
• Important Requirements for a primary standard


  1. High purity. Established methods for confirming purity should be available.
  2. Atmospheric stability.
  3. Absence of hydrate water so that the composition of the solid does not change with variations in humidity.
  4. Modest cost.
  5. Reasonable solubility in the titration medium.
  6. Reasonably large molar mass so that the relative error associated with weighing the standard is minimized.
• Secondary Standard 


  is a compound whose purity has been determined by chemical analysis.
• Secondary Standard 


  serves as the working standard material for titrations and for many other analyses.
• IDEAL STANDARD SOLUTION FOR A TITRIMETRIC METHOD


  1. be sufficiently stable so that it is necessary to determine its concentration only once;
  2. react rapidly with the analyte so that the time required between additions of reagent is minimized
  3. react more or less completely with the analyte so that satisfactory end points are realized:
  4. undergo a selective reaction with the analyte that can be described by a balanced equation.
• Standardization


  the concentration of a volumetric solution is determined by titrating it against a carefully measured quantity of a primary or secondary standard or an exactly known volume of another standard solution.
• VOLUMETRIC CALCULATIONS 


  As we indicated in Section 4B-1, we can express the concentration of solutions in several ways.
• Symbol: c


  molar concentration
• Symbol: CN


  normal concentration
• Molar concentration 


  c , is the number of moles of reagent contained in one liter of solution
• Normal concentration


  CN , is the number of equivalents of reagent in the same volume.
• Primary Standards



  The accuracy of a method critically depends on the properties of __
• Titration Methods



  The reagent may be a standard solution of a chemical or an electric current of known magnitude.
• VOLUMETRIC CALCULATIONS 


  For the standard solutions used in most titrations, either molar concentration, c, or normal concentration, CN, is usually used.
• Titration Error


  The difference in volume or mass between the equivalence point and the end point
• Terms used in volumetric titrations: standard solution or a standard titrant



  The titration is performed by slowly adding a standard solution from a burette or other liquid-dispensing device to a solution of the analyte until the reaction between the two is judged complete.
• Terms used in volumetric titrations: standard solution or a standard titrant



  The volume or mass of reagent needed to complete the titration is determined from the difference between the initial and final readings