EXP 17

• A primary standard is a solid substance which is used for the direct standardization of a given solution. The volume of the given solution required to react with the dissolved standard gives a measure of the Normality (N) of the solution.
• The standardization of a solution of acid is usually accomplished by determining experimentally the volume of the acid which is equivalent to a known weight of pure basic substance.

  The most common standards for the direct determination of the normality of an acid are:

  • Sodium carbonate (Na2CO3)
  • Calcium carbonate (CaCO3)
  • Mercuric oxide (HgO)
  • Tri-hydroxymethylamino–methane.
• the standardization of a solution of an alkali is usually made by determining experimentally the volume of the base equivalent to a known weight of pure acid. Although any one of a large number of solid organic acids can be used as a primary standard for Sodium

  hydroxide (NaOH) solutions, only four (4) are in common use.
• • Potassium acid phthalate or Potassium biphthalate, (KHC8H4O4)
  • Sulfamic acid
  • Benzoic acid, (C6H5COOH)
  • Potassium acid iodate or Potassium biiodate.
• Standard solutions or titrants are usually added by means of a Burette apparatus, and the act of adding or measuring the volume of the titrant used is called “titration”.
• In performing volumetric analysis, an “indicator” must be added to the analyte.
• An indicator is a chemical reagent which changes color at/or very near the end-point in the titration where equivalent quantities of analyte and titrant have reacted.
• The theoretical point or end-point at which equivalent amounts of each have reacted is also called the “stoichiometric point” or “equivalence point”.
• At the end-point, the titration is stopped, at which there occurs a sudden change in some property of the reaction mixture – a change which is made apparent by the use of the indicators or electrochemically reactive substances.
• strong acid is titrated with a strong alkali (acidimetry)
• strong alkali is titrated with a strong acid (alkalimetry)
• Methyl orange (M.O.); Methyl Red (M.R.); or Phenolphthalein indicators.