TITRIMETRIC (LABORATORY)

• Titrimetric Analysis

  • refers to quantitative chemical analysis carried out by determining the volume of a solution of accurate known concentration which is required to react quantitatively with the solution of the substance to be determined.
• Titrimetric Analysis

  • Also known as Volumetric Analysis
• Titrimetric Analysis

  • Titrimetric analysis, formerly known as the volumetric analysis, refers to quantitative chemical analysis carried out by determining the volume of a solution of accurate known concentration which is required to react quantitatively with the solution of the substance to be determined.
• Standard Solution

  • is a solution of accurate known strength
• Standard Solution

  • The weight of the substance is to be determined is calculated from the volume of the standard solution used.
  • The process of adding the standard solution until the reaction just complete is termed as Titration and the substances to be determined is Titrant.
• Indicator

  • It gives a clear visual change (either a change in color or the formation of turbidity) in the solution being titrated.
• Indicator

  • Also knows as the auxillary reagent
• End point

  • is the point at which the visual change of a solution occurs.
• Ideal titration:

  • There must be a simple reaction which can be expressed by a chemical equation, the substance to be determined should react completely with the reagent in stoichiometric or equivalent portion.
  • There must be marked change in the free energy to alter chemical or physical properties of the solution at the equivalence point.
  • The reaction should practically instantaneous or rapid. In some cases, a catalyst is added to increase the speed of the reaction.
  • An indicator should be available which should sharply define the end pint of the reaction.
• Types of Titrimetric Analysis:

  • Neutralization Reactions (Acidemetry and Alkalimetry)
  • Complex Formation Reactions
  • Precipitation Reactions
  • Oxidation-Reduction Reactions
• Neutralization Reactions (Acidemetry and Alkalimetry)

  • These include the titration of bases with a standard acid and the titration of acids with a standard base
• Neutralization Reactions

  • These include the titration of bases with a standard acid (alkalimetry) and the titration of acids with a standard base (acidemetry).
• Neutralization Reactions (Acidemetry and Alkalimetry)

  • It involves the combination of hydronium (H3O+) and hydroxyl ions (OH-) to form water.
• Complex Formation Reactions

  • These depend upon the combination of ions, other than hydronium and hydroxyl ions, to form soluble, slightly dissociated ions or compound, as in the titration of metal ions with an organic complexion agent such as ethylene diaminetetra acetic acid (EDTA).
• Precipitation Reactions

  • These depend upon the combination of ions to form a simple precipitate as the titration of silver ion with a solution of chloride ion.
• Oxidation-Reduction Reactions

  • These includes all reactions involving change in oxidation number or transfer of electrons among the reacting substances.
• Oxidation-Reduction Reactions

  • The standard solutions are either oxidizing or reducing agents.
• Common oxidizing agents:

  • KMnO4
  • K2Cr2O7
  • Ce(SO2)2
  • I2
  • KIO3
  • KBrO3
• Frequently used reducing agents

  • Fe2+
  • Sn2+
• Compounds

  • Na2S2O3
  • As2O3
  • HgNO3
  • CrCl2
  • TiCl3 or Ti2(SO4)3
• Standard Solutions

  • Contains a known weight of the reagent in a definite volume of solution, concentrations are expressed in terms of molarity and normality
• Standard Solution

  • Normal Solution is a solution containing one equivalent of a defined species per liter according to the specified reaction and a molar solution containing one mole of a defined species per liter.
• Preparation of Standard Solution

  • The preparation of standard solution can be made easier when the material to be dissolved is of known purity.
  • A careful weighed or measured volume of sample is usually transferred to a graduated flask and diluted to an exact volume.
  • It is undesirable to use solution of low concentration that one or two drops of reagents are required to see a change in color of the indicator used.
  • The use of concentrated solution is also not feasible.
  • Commonly used are prepared solution of 0.1N concentration.
• Steps for preparing standard solution:

  • Weigh the pure solid on a balance
  • Transfer the solid into the volumetric flask using a clean, dry funnel
  • Rinse any remaining solid particles into the flask using deionised water.
  • Half full the flask with deionised water, stopper and swirl vigorouslt to dissolve the solid.
  • Add deionised water up to the calibration line on the neck of the flask. The bottom of the meniscus of the solution should be on the mark when viewed at eye level.
  • Stopper and shake the solution to ensure an even concentration throughout.
• Standardization of solutions requires:

  • The primary standard must be readily available.
  • The weight and/or volume of the standard used should not be too small. Never use a volume less than 10mL.
  • Back tirations during standardization must be avoided because the process increases the possibility of errors.
  • Standardization must be performed in 3 trials and the results should agree within 0.1-0.2%.
• Primary Standard Substances

  • Should be easily obtainable and easy to purify, to dry and to preserve in a pure form.
  • It should remain stable I air during weighing, that is, it should be nonhygroscopic (moist), not oxidized by air not affected by CO2.
  • It must be of definite, known composition.
• Primary Standard Substances

  • It should be capable of being tested for impurities by qualitative and other tests of known sensitivity. (the total amount of impurities should not in general exceed 0.01-0.02%)
  • It should have a high equivalent weight so that the weighing errors may be negligible.
  • It should be readily soluble under the condition in which it is employed.
• Methods of Performing Titrations:

  • Direct Method
  • Indirect Method
  • Back Titration Method
  • Titer Method
• Direct Method

  • Dead-Stop End Point
  • The amount of standard solution added is just up to end.
• Direct Method Formula:

  • meq analyte = meq titrant
• Indirect Method

  • A measured amount of A is added in excess of that theoretically added to react with B.
  • Then the excess A is titrated to end point.
• Indirect Method Formula:

  • meq analyte = meq excess - meq titrant
• Back Titration Method

  • When the end point is overstepped or an excess of the titrant has been added unintentionally, the process of back titration is carried out.
  • The second titrant, which has the same nature as the analyte is used to get back the end point of titration.
• Back Titration Method Formula

  • meq analyte = meq excess - meq backtitrant
• Titer Method

  • The weight of the sample equivalent to 1mL of a reagent is called TITER.
  • It is used to describe the strength of a solution in terms of another reagent.
• Titer Method Formula

  • Titer = weight in mg / weight in mL = NEW
• Acid-Base Titration/ Neutralization Reactions

  • The equivalent of an acid is a mass of it which contains 1.008 (more accurately 1.0078)of replaceable hydrogen.
  • The equivalent of a monoprotic acid, such as HCl, HI, HClO4 of HOAc, identical with the mole.
  • A normal solution of a monoprotic acid will therefore contain one mole per liter of solution.
  • The equivalent of a diprotic acid (example: H2SO4 or H2C2O4) of triprotic acid (Example H3PO4) is likewise 1/2 or 1/3 respectively of the mole.
• Acid-Base Titration/ Neutralization Reactions

  • The equivalent of a base is that mass of it which contains one replaceable hydroxyl group, that is 17.008g or ionizable; 17.008g hydroxyl are equivalent to 1.008g hydrogen.
  • The equivalents of NaOH and KOH are the mole of Ca(OH)2, Sr(OH)2, and the Ba(OH)2 half a mole.
  • Salt of strong bases and weak acids possess alkaline reaction in aqueous solution because of hydrolysis.
• Acid-Base Titration/ Neutralization Reactions

  • A mole of Na2CO3 with methyl orange as indicator, reacts with two mole of HCl hence is equivalent to half a mole.
  • Sodium tetraborate under similar conditions, also reacts with two mole HCl and its equivalent is likewise half a mole.
• Preparation of Standard Acids and Bases

  • Standard solutions of acids are usually prepared from HCl and H2SO4 .
  • Both of these are commerically available as concentrated solutions, concentrated HCl is about 10.5-12M and concentrated H2SO4 is about 18M.
  • By suitable dilutions, solution of any desired approximate strength may be readily prepared.
  • HCl is generally preferred since most chlorides are water soluble.