Standard solutions-PHAY0002

• Whats the difference between the end point and equivalence point?

  • Equivalence Point: theoretical point where the exact amount of titrant has been added to react completely with the analyte (the substance being analyzed) according to the chemical equation.
  • End Point: This is the practical point in the titration where you see the indicator change color, signaling that the equivalence point has likely been reached. It’s the point you actually observe.
• Whats titration error?


  The difference between the equivalence and end point - ideally it should be as small as possible
• what are standard solutions 


  Solutions with known concentrations.
• What are the requirements for substances to be used as primary standards?


  Readily obtainable in a state of very high purity (> 99.99% m/m).

  • Stable to heat so they may be dried. This allows moisture absorbed on storage to be

  removed.

  • Do not absorb/react with water or carbon dioxide or anything else from the air.

  • Have a high molecular mass so that weighing errors are small.

  • Readily soluble in solvent for titration.

  • Easily tested for impurities
• Why are hydrated susbtances not used as standards?


  as they can easily lose their water of hydration and hence the

  exact composition is uncertain.
• What are secondary standards?


  The concentration of standards that are determined by titration against suitable primary standards.
• What forms when you react an acid and a base?


  Salt and water
• Give 2 examples of primary standard acids?


  Benzoic acid and potassium hydrogen phthalate
• Give an examples of primary standard base?

  Sodium carbonate
• What are complexometric reactions?


  Reactions involving metal ions and complexing agents.
• What titrant is used in complexometric reactions?


  EDTA
• What ratio does EDTA form with the metal ions?


  1:1
• State the steps of a typical titration?


  1.A known mass or volume of sample is measured into a conical flask. In the case of a solid sample it would be dissolved in water or other suitable reagent.

  2. An indicator is added.

  3. Titrant is added from a burette until the indicator changes colour - the end-point.

  4. From the volume of titrant used the amount of substance is calculated.
• What equation do you use to calcuate the conc of from an indirect titration?


  a and b are the moles

  
• Why do back titrations happen?


  some substances react too slowly so will not be suitable to do a direct titration
• State the steps of a back titration?

  1. A known excess of a standard solution (‘A’) is added to the sample and allowed to react.
  2. The amount of excess reagent A remaining after the reaction is then determined by titration with another standard solution (‘B’).
  3. The difference between the amount of reagent A determined by titration and the original amount gives the amount used by the sample
• Why isn't water used in non-aqueous titrations?


  Because if you dissolve amine in water and is titrated with a strong acid the lone pair on the nitrogen will react with the H+ but the proton could also react with the lone pair on the oxygen in the water forming H3O.

  Therefore the proton has a choice to react with the nitrogen or the oxygen there wont be a sharp end point and therefore difficult to detect when the equivalence point has been reached.
• What are amphiprotic solvents?


  Solvents that can both donate and accept protons.

  2H2O ⇌ H3O+ + OH-

  2NH3 ⇌ NH4+ + NH2-

  2CH3COOH ⇌ CH3COOH2+ + CH3COO-
• What are aprotic solvents?


  Solvents that have no acidic or basic properties
• What are the advantages of using classical analytical techniques?

  • Robust methods that can be carried out very precisely

  • Fairly cheap, little specialist equipment is required

  • The methods do not depend on instrument calibration (but they do require pure substances for standards…)
• What are the disadvantages of using classical analytical techniques?


  • Require relatively large amounts of sample

  • Time consuming and depend upon the skill of the analyst

  • often unsuitable for complex mixtures of ingredients in formulated medicines

  •
• How do you calculate percentage purity?

  (Amount of drug/ Dried mass of sample ) x 100
• How do you calculate loss on drying?


  (Difference in mass /mass of sample) x 100