TITRATION

• Titration

  • A technique for determining the concentration of a solution by measuring the volume of one solution needed to completely react with another solution.
• Titration process involves addition of solution of known a burette to the measured volume concentration from of analyte.
• THE PRINCIPLE of TITRATION:

  It is based on the complete chemical reaction between the analyte and the titrant of known concentration.
• ANALYTE + TITRANT -> PRODUCT
• Volumetric titrimetry

  • involves measuring the volume of a solution of known concentration that is needed to react essentially completely with the analyte
• Gravimetric titrimetry

  • differs only in that the mass of the reagent is measured instead of its volume
• Coulometric titrimetry

  • the reagent is a constant direct electrical current of known magnitude that consumes the analyte
• Standard solution (aka. standard titrant)

  • a reagent of known concentration that is used to carry out a titrimetric analysis
• A titration is performed by adding a standard solution from a buret or other liquid-dispensing device to a solution of the analyte until the reaction between the two is judged complete.
• The volume of reagent needed to complete the titration is determined from the difference between the initial and final volume readings.
• Equivalence point

  • is reached when the amount of added titrant is chemically equivalent to the amount of analyte in the sample
• Equivalence Point

  • the point in a titration when the amount of added standard reagent is exactly equivalent to the amount of analyte
• End point

  • the point in a titration when a physical change occurs that is associated with the condition of the chemical equivalence
• Indicators

  • are often added to the analyte solution to produce an observable physical change at or near the equivalence point
• INDICATORS (Physical Change)

  • the appearance or disappearance of a color
  • the change in color
  • the appearance or disappearance of turbidity
• Typical setup for carrying out a Titration

  The apparatus consists of:

  • buret, buret stand, and clamp with a white porcelain base to provide an appropriate background for viewing indicator changes
  • wide-mouth precisely known volume of Erlenmeyer flask containing the solution to be titrated
  • The solution is normally delivered into the flask using a pipette
• Detail of the Buret Graduations

  • Normally, the buret is filled with titrant solution to within 1 or 2 mL of the zero position at the top.
  • The initial volume of the buret is read to the nearest ± 0.01 mL.
  • There's a reference point on the meniscus and the proper position of the eye for reading.
• Before the titration begins.

  • The solution to be titrated, an acid in this example, is placed in the flask and the indicator is added.
  • The indicator in this case which is phenolphthalein, turns pink in basic solution.
• During titration.

  • The titrant is added to the flask with swirling until the color of the indicator persists.
  • In the initial region of the titration, titrant may be added rather rapidly, but as the end point is approached, increasingly smaller portions are added; at the end point, less than half a drop of titrant should cause the indicator to change color.
• The end point is achieved when the barely perceptible pink color of phenolphthalein persists.
• The final reading of the buret is made at the end point, and the volume of base delivered in the titration is calculated from the difference between the initial and final buret readings.
• The solution turns a deep pink color, when the end point has been exceeded.
• Titration error (Et)

  • the difference in volume or mass between the equivalence point and the end point
  • Et = Vep– Veq
  • Vep: the actual volume of reagent required to reach the end point
  • Veq: the theoretical volume to reach the equivalence point
• Primary Standards

  • a highly purified compound that serves as a reference material in all volumetric and mass titrimetric methods.
  • The accuracy of a method is critically dependent on the properties of this compound
• Important requirements for a primary standard are:

  1. High purity
  2. Atmospheric stability
  3. Absence of hydrate water so that the composition of the solid does not change with variation in relative humidity
  4. Ready availability at 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

  • a compound whose purity has been established by chemically analysis
  • serves as the reference material for a titrimetric method
• ESTABLISHING THE CONCENTRATION OF STANDARD SOLUTIONS

  • The accuracy of a titrimetric method can be no better than the accuracy of the concentration of the standard solution used in the titration.
• Two basic methods are used to establish the concentration of such solutions:

  • Direct method
  • Standardization
• Direct Method

  • in which a carefully weighed quantity of a primary standard is dissolved in a suitable solvent and diluted to a known volume in a volumetric flask
• Standardization

  • in which the titrant to be standardized is used to titrate
• STANDARDIZATION

  • (1) a weighed quantity of a primary standard
  • (2) a weighed quantity of a secondary standard
  • (3) a measured volume of another standard solution
• EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS

  • The concentrations of standard solutions are generally expressed in units of either molarity c or normality N.
• EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS

  • Molarity gives the number of moles of reagent contained in one liter of solution
• EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS

  • Normality gives the number of equivalents of reagent in the same volume.
• EXPRESSING THE CONCENTRATION OF STANDARD SOLUTIONS

  • M= moles/L
  • N= equivalents/L
• SOME USEFUL ALGEBRAIC RELATIONSHIPS (VOL)

  • Most volumetric calculations are based on two pairs of fundamental equations that are derived from definitions of millimole, mole, and molar concentration.
• SOME USEFUL ALGEBRAIC RELATIONSHIPS(VOL)

  
  

  1st Pair

  Amount (mol) = mass (grams)/molar mass (gm/mole)

  Amount (mmol) = mass (grams)/mmolar mass (gm/mmole)
• SOME USEFUL ALGEBRAIC RELATIONSHIPS (VOL)

  
  

  2nd Pair from the definition of molar concentration:

  amount (mol) = volume (L) x concentration (mole/L)

  amount (mmol) = volume (mL) x concentration (mmole/mL)
• SOME USEFUL ALGEBRAIC RELATIONSHIPS (MASS)

  • Weight or Gravimetric Titrimetry – Mass of titrant is measured
  • Weight molality (MW) : the number of moles of reagent in 1 Kg Solution
• SOME USEFUL ALGEBRAIC RELATIONSHIPS (MASS)

  Weight Molality= Mole / solution (Kg)

  
  

  Ex. 0.1 MW NaCl

  = 0.1 mol of the NaCl in 1 Kg of solution

  = 0.1 mmol of the NaCl in 1 g of solution