Etiket Arşivleri: endpoint

Principles of Titration and Errors ( Dr. A. Amsavel )

Principles of Titration and errors

By Dr. A. Amsavel

Introduction

Volumetric analysis
 Simple and easy
 Fast and can be done on site
 Less expensive
 Estimation of content or Assay
 Precise and accurate
 Depends on method and specificity

Requirements of a Titration Reaction

Reaction must complete by 99.9 % so that < 0.1 % (or 1 ppt) remains unreacted
Rxn must be rapid
Titration needs to be performed in a reasonable time period
The stoichiometry must be well defined, and known
It can be predicted from equilibrium constants
A method must be available to determine the equivalence point

Types of Titration

1) Precipitation
– A(aq) + B(aq) = AB(s)
2) Acid-Base rxn
– H+ + OH ̄ = H2O (strong acids or bases)
– HA + OH ̄ = H2O + A ̄ (weak acids)
– A ̄ + H+ = H2O + HA (weak bases)
3) Complexation rxn
– Zn2+ + 4NH3 = Zn(NH3)42+
4) Redox rxn (oxidation-reduction)
– Fe2+ + Ce4+ = Fe3+ + Ce3+

Standards

• Measurements are made with reference to standards
– The accuracy of a result is only as good as the quality and accuracy of the standards used
– A standard is a reference material whose purity and composition are well known and well defined
• Primary Standards
– Used as titrants or used to standardize titrants
– Requirements
• Usually solid to make it easier to weigh
• Easy to obtain, purify and store, and easy to dry
• Inert in the atmosphere
• High formula weight so that it can be weighed with high precision

Endpoint Detection

It is critical, to know the completion of reaction / determination
1) Visual indicators:
• Observe a colour change or precipitation at the endpoint.
– Rxn progress checked by addition of external or self indicator
2) Photometry:
• Use an instrument to follow the colour change or precipitation
3) Electrochemistry:
• Potentiometry – measure voltage change ( pH electrode)
• Amperometry – measure change in current between electrodes in solution
• Conductance – measure conductivity changes of solution
Later two used for coloured, turbid, end point accurate

Acid-base titration

 Neutralization titration
 Neutralization Indicators
 Indicators & mixed indicators
 Neutralization curve
 Non-aqueous titration


Titration of Sodium Carbonate

  • Carbonate Chemistry

  • CO2 in atmosphere and dissolved in water

  • Major global buffering system

  • Industrial sources

–limestone:  CaCO3(s) + heat à CaO(s) + CO2(g)

–trona (Na2CO3) deposits

  • Sodium Carbonate As a Base

  • Commercially important

–source or base or carbonate for industrial processes

–washing soda (automatic dishwashers)

  • Carbonate is a moderately strong base

  • Titration of Na2CO3 with HCl

  • Titration of Sodium Carbonate

  • Derivative Plots

  • Determination of Carbonate in a Sample

  • Effects of Carbonate Equilibria on Titration of Carbonate

  • Shift in Phenolphthalein Endpoint

  • Boiling to Enhance Second Endpoint

  • Effect of CO2 Absorption on Phenophthalein Endpoint

  • Reaction of CO2 with carbonate solutions

CO2(g) + H2O(l) + CO32-(aq) à 2HCO3-(aq)

–Let 1 mmol CO2 react with 1 mmol CO32-

–For titration with HCl to BCG endpoint to form H2CO3

  • Initial 1 mmol CO32- would require 2 mmol HCl

  • Resultant 2 mmol HCO3- requires 2 mmol HCl

  • Net effects

–Volume of HCl require to reach phenolphthalein end decreases

  • This effect may be avoided by excluding air from the titration system.

–Volume required to reach bromcresol green endpoint is not affected.

  • Recommendation

–Use BCG endpoint to calculate amount of carbonate in your sample

  • Effect of Absorbed CO2 on Titration of Na2CO3 with HCl

  • Boiling to Enhance Visual Endpoint in Titration of Na2CO3 with HCl

  • Summary: Titration of Sodium Carbonate

  • Titration of Na2CO3 with strong acid yields two equivalence points

  • At phenolphthalein end point

–Na2CO3 à NaHCO3

  • At bromcresol green (or methyl red) end point

–All carbonate is converted to H2CO3

  • pH of carbonate solutions may be unstable due to exchange of CO2 with atmosphere.


Laboratory‎ > ‎Acid – Base Titration

An acids-base titration is used to determine the unknown concentration of an acid or base by monitoring its neutralization reaction with a standard of known concentration in the presence of an indicator. For example, a base of unknown concentration may be titrated with an acid standard of known concentration or the reverse. The endpoint is the point of the acid-base titration in which just enough of the standard solution has been added to completely neutralize the solution of unknown concentration. Indicators are used to signal this point of the titration, and are generally selected according to the type of titration (strength of acid and based used in the titration). When the endpoint of the titration has been reached, one drop of standard solution past the endpoint will result in a color change of the indicator.

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